ON THE EAR - A REVIEW

DASA 1858 EFFECTS OF OVERPRESSURE

ON THE EAR - ^{A REVIEW}

Frederic

G .

Hirsch, M.D.

Technic a1 P r ogr e s s Report on

Contract No. DA-49-146-XZ -372

This work, an aspect of investigations dealing with the Biological Effects of Blast from Bombs, was supported

by

the Defense Atomic Support Agency of

the Department of Defense.

(Reproduction in whole o r in part is permitted for any purpose of the United States Government.

)

Lovelace Foundation f o r Medical Education and Research Albuquerque, New Mexico

November 1966

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DISCLAIMER

This report was prepared as an account of work sponsored by an

agency of the United States Government. Neither the United States

Government nor any agency Thereof, nor any of their employees,

makes any warranty, express or implied, or assumes any legal

liability or responsibility for the accuracy, completeness, or

usefulness of any information, apparatus, product, or process

disclosed, or represents that its use would not infringe privately

owned rights. Reference herein to any specific commercial product,

process, or service by trade name, trademark, manufacturer, or

otherwise does not necessarily constitute or imply its endorsement,

recommendation, or favoring by the United States Government or any

agency thereof. The views and opinions of authors expressed herein

do not necessarily state or reflect those of the United States

Government or any agency thereof.

DISCLAIMER

Portions of this document may be illegible in

electronic image products. Images are produced

from the best available original document.

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. FOREWORD

T h i s r e p o r t , which i s p r i m a r i l y a review ^of published m a t e r i a l con- cerning the effects of b l a s t on the human and the m a m m a l i a n e a r , a l s o i n c o r p o r a t e s s o m e previously unpublished data concerning the impact of:

age on the vulnerability of the tympanic m e m b r a n e to b l a s t o v e r p r e s s u r e s , and s o m e f u r t h e r m a t e r i a l on i n t r a s p e c i e s scaling.

An a t t e m p t has been made t o c o r r e l a t e s u c h quantitative data as,&an be found in the l i t e r a t u r e , but this h a s not been found possible of a c c o m - plishment in a completely s a t i s f a c t o r y way. By r e s c a l i n g s o m e previously published m a t e r i a l in v i e of available data, and incor porating s o m e un- published m a t e r i a l a v a l f o r - t h e threshold of e a r d r u m r u p t u r e in m a n is e s t i m a t e d to be 5 p s i , add the "skort"-rising o v e r p r e s s u r e r e q u i r e d f o r r u p t u r e of human e a r d r u m s i s 15 psi.

h a z a r d s and identify populations at r i s k , a s well a s those r e s p o n s i b l e f o r providing s a f e working environments and s i m i l a r problems in the field of e nv i r onme ntal he a 1 th

.

T h i s information will be of use t o those whose t a s k it i s t o assess

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- ACKNOWLEDGMENTS

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T h e a u t h o r w i s h e s to acknowledge the considerable a s s i s t a n c e in the p r e p a r a t i o n of this r e p o r t which w a s r e n d e r e d by the following people:

t o D r . Donald R . Richmond of the Lovelace Foundation, f o r p e r m i s s i o n t o include s o m e of his unpublished data; t o M r . Roy H. R e i d e r , of the Los Alamos Scientific Laboratory, f o r providing d a t a on s o m e r e c e n t in-

d u s t r i a l explosions; t o Mr-. I. -Gerald Bowen, of the Lovelace Foundation, and M r . Luke J. Vortman-of the Sandia Corporation, f o r t h e i r help in p r e s s u r e -di alculations; t o Dr. Thomas L. Chiffelle, of the Lovelace o r a s s i s t a n c e in p r e p a r i n g and i n t e r p r e t i n g histological of tympanic m e m b r a n e s ; t o D r . Thomas Shipman, of the Los Alamos Sci-entific L a b o r a t o r y , and-Dr. Donald E. Kilgore J r . , of the Lovelace Clinic, f o r making clinical m a t e r i a l available; t o M r . Robert A. Smith, of the Lovelace Foundation, fo,r the p r e p a r a t i o n of

i l l u s t r a t i v e m a t e r i a l ; to Mr. Walter Scheurle, of the Lovelace Foundation, f o r the t r a n s l a t i o n of m a t e r i a l ; t o M r s . L a u r a M. C a r r a s c o and M r s . Ruth P.

Lloyd f o r s e c r e t a r i a l and e d i t o r i a l a s s i s t a n c e ; t o the Defense Atomic Support Agency f o r c o n t r a c t support; t o Colonel G e r r i t L. Hekhuis, USAF, MC,

Defense Atomic Support Agency, Department of Defense and Lt.

Colonel Edmund L. Fountain, USA, VC, Defense Atomic Support

Agency, D e p a r t m of Defense, f o r t h e i r i n t e r e s t and s u p p o r t ; and finally t o D r . Clayton S . P r e s i d e n t - D i r e c t o r of the Lovelace Foundation, whose v e r y g r e a t

v e r y much t o whatever m e r i t r e s i d e s in this r e p o r t .

1 i n t e r e s t and valuable suggestions contributed

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E F F E C T S O F OVERPRESSURE ON THE EAR

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^{A REVIEW}

1. INTRODUCTION

F o r m a n y y e a r s physicians have been a w a r e t h a t t h e e a r s of humans and o t h e r m a m m a l s have a n e s p e c i a l .vulnerability t o i n j u r y when exposed t o o v e r p r e s s u r e s generated' by explosions and the m u z z l e b l a s t of guns. By the y e a r 1900 t h e r e a p p e a r e d in the m e d i c a l l i t e r a t u r e t h i r - t e e n a r t i c l e s on t h e subject, beginning with t h e r e p o r t of G r e e n i n 1872.

The m o s t c o m p r e h e n s i v e ' t r e a t m e n t w a s that of C a s t e x i n 1893.

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Since 1:hat t i m e , w e l l over one hundred p a p e r s have been published on b l a s t i n j u r y of the ear, t h e i r number incyeasing during periods when a m a j o r a r m e d cain- flict w a s being waged and diminishing during peaceful t i m e s ; although, when- e v e r m a j o r explosions have o c c u r r e d , renewed i n t e r e s t h a s followed. 3, ^'** 5

A

review of t h a t which h a s been w r i t t e n concerning b l a s t injury of the e a r r e v e a l s that, with s o m e i m p o r t a n t exceptions, the r e p o r t s d e a l m o s t l y with clinical aonsiderations, and t h a t quantitative d a t a relating a v e r

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p r e s s u r e s t o incidence o r d e g r e e of i n j u r y a r e lacking. T h e s e exceptions date back quite a few y e a r s , however, t o the w o r k of Zalewski in 1906.6 T h i s study, which will be d e a l t with m o r e fully l a t e r on, was f o r a l m o s t f o r t y y e a r s the only one of its kind until the w o r k of-Zuckerman7, 8 colleagues during World W a r I1 w a s done.

published his s t u d i e s which have contributed so much t o a n understanding of the r e s p o n s e of the otic s t r u c t u r e s t o b l a s t waves.

and -1 1 Also about t h i s time, P e r l m a n

In the r e c e n t past; the Comparative Environmental Biology De- p a r t m e n t of the Lovelace Foundation f o r Medical Education and R e s e a r c h has had a n opportunity t o obtain s o m e quantitative data on a n u m b e r of m a m m a l i a n s p e c i e s during the full-scale wea ons t e s t s , its shock tube s t u d i e s , and s o m e of its other activities.

lz-zpl

R,ecently, studies of the vulnerability of the e a r t o b l a s t i n j u r y have been r e s u m e d in England by Golden and C l a r e . 22

It would a p p e a r useful and t i m e l y to p r e p a s e a review of what i n known about otic b l a s t trauma i n view of r e c e n t advances in explosives technology, the development of new ,types of ordnance which have higher m u z z l e velocities and consequently higher m u z z l e b l a s t ove r p r e s s u r e s

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and f o r p u r p o s e s of Civil Defense planning. T h i s communication r e p r e - s e n t s s u c h a n undertaking.

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2 . 0 PHYSICAL, ANATOMICAL AND PHYSIOLOGICAL CONSIDERATIONS The vulnerability of the e a r t o blast o v e r p r e s s u r e s is readily a p -

It is generated when air molecules a r e rapidly and preciated when one considers that a blast wave is physically the same phenom- enon as a sound wave.

violently .compressed s u c h as occurs when a solid o r liquid substance is suddenly changed into a gas.

occurs i n t i m e s of 1 x 10-5 seconds o r less, depending on the chemical nature of the substance. The resultant compression c a n be of many hun- d r e d s o r thousands of pounds to the s q u a r e inch at the source.

s o u r c e , the molecules a r e forced outward for s o m e distance before com- mencing t o oscillate. Once oscillation has occurred, a wave is propagated radially. A s it i s propagated through the air, the peak o v e r p r e s s u r e and its velocity fall off at a rate which is f a s t e r than a n i n v e r s e s q u a r e of the d i s - tance relationship, s o that at distances of s e v e r a l hundreds of feet the over- p r e s s u r e will have diminished to a magnitude of s e v e r a l atmospheres. One definition of a shock pulse can be given as a sound wave of g r e a t initial con- densation and g r e a t initial velocity. "

s y s t e m which is built especially f o r the reception of sound will receive b l a s t o v e r p r e s s u r e s equally well.

In a n explosion, this change in physical state

N e a r the

It follows, therefore, that a n organ

The e a r has evolved as a n organ s y s t e m f o r the transduction of sound waves into nerve impulses which a r e e l e c t r i c a l i n nature.

developed a n ability t o respond t o a limited band of frequencies which f o r the human may be regarded as lying between 20 Hz and 2 0 , 0 0 0 Hz. It has developed a high o r d e r of sensitivity. It responds t o signals which have a n energy level as low as

five billionths of a n atmosphere, o r whose f o r c e c a u s e s a n excursion of the e a r d r u m

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^a istance which is l e s s than the d i a m e t e r of a single hydrogen molecule. 9-1q 23 The drumhead cannot respond faithfully t o pulses which have periods of l e s s than 0. 3 milliseconds, but it attempts to do s o by m a k - ing a single l a r g e excursion which corresponds to that occasioned by a

sound wave of high audible frequency.9-1 It is this excursion which mediates the trauma t o the e a r .

It has

Watts/cm2, o r whose p r e s s u r e is about one

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The m a m m a l i a n e a r is divided anatomically and physiologically into the external, middle, and inner e a r s . Each of these plays a different r o l e i n the overall function of the auditory apparatus.

The external edr is a sound-gathering device which has two m a i n functions: the f i r s t is that of amplification; and the second is as a direction- sensing device;

affected albeit i n different ways.

F r o m the standpoint of blast t r a u m a , both functions a r e

Golden and Clare22 have found in t h e i r studies that, when a shock pulse is generated in the air at a distance f r o m a n e a r , t h e r e is a n i n c r e a s e of about 0.2 percent in the o v e r p r e s s u r e at the s i t e of the tympanic membrane over that which is m e a s u r e d in the air a t the s a m e distance f r o m the s o u r c e of the pulse. FIGURE 1 shows t h e i r experimental a r r a n g e m e n t and a typical

set of values f o r the r e s p e c t i v e o v e r p r e s s u r e s .

When a n e a r h a s been significantly injured in s u c h a way t h a t a

.

hearing loss r e s u l t s , t h e r e a l s o o c c u r s a n i n t e r f e r e n c e with s t e r e o g n o s i s , which is the ability of a n a n i m a l t o locate the origin of sounds i n its environ- ment. Part of this ability depends on differential loudness i n one e a r as opposed t o t h e other, and the r e s t de?ends on the reception of sounds at slightly different times i n e a c h ear, so that a phase difference between the two ears is occasioned. T h i s phase difference is mediated and i n t e r p r e t e d by the nervous s y s t e m in a complex way, the d e t a i l s of which a r e not g e r - m a n e t o the p r e s e n t consideration.

injury t o a n ear, both loudness and phase a r e affected, s i n c e a n a l t e r e d impedance h a s been introduced into one s i d e of the s y s t e m .

Suffice it t o s a y t h a t when t h e r e is

Before leaving the consideration of the e x t e r n a l ear, something should be s a i d about the influence of c e r u m i n o u s , o r wax, plugs on t h e r e s p o n s e of a n ear t o b l a s t waves. T h e s e are not always p r e s e n t in the externzl auditory canal, but their incidence is quite high. Most often a plug which has a d i a m e t e r sufficient t o occlude the c r o s s s e c t i o n of the canal will not be sufficiently long t o f i l l the length of it, s o t h a t a n air s p a c e exists between it and the e a r d r u m .

be impacted a g a i n s t the d r u m .

L e s s frequently, one end of a plug will

When a ceruminous plug of the first s o r t is p r e s e n t , it will dimin- i s h the gain function of the c a n a l and will a c t as a n attenuator of a b l a s t wave b e c a u s e , being both c o m p r e s s i b l e and displaceable, it a b s o r b s s o m e

of the e n e r g y of the shock pulse. T h a t plugs of this s o r t a r e effective i n reducing trauma t o the e a r by b l a s t has been r e m a r k e d by s e v e r a l of the a u t h o r s who have w r i t t e n on t h e subject.9, 24-26

S o m e t i m e s , however, the p r e s e n c e of a ceruminous plug will behave in a m a n n e r which m a k e s a n e a r i n j u r y w o r s e .

the plug is v e r y c l o s e t o the e a r d r u m , o r actually is in contact with it, and if tbe displacement of it by the shock pulse is sufficient, then the plug behaves as a r a m r o d causing not only a n extensive r u p t u r e of the d r u m but a l s o a displacement of the e a r o s s i c l e s . C 0 1 l e d g e ~ ~ has r e - m a r k e d about t h i s , and e x p e r i e n c e with dogs at the Nevada T e s t Site

If the d e e p end of

w a s confirmatory. 12

.

In the latter case, a plug was placed in one e a r of e a c h e x p e r i - It w a s m a d e of cotton and liquid r u b b e r which solidified

T h e s e plugs completely filled the e x t e r n a l m e n t a l animal.

a f t e r exposure t o the air.

auditory canals. The r e l a t i v e l y "slowtt -ris ing, but v e r y "longtt-duration o v e r p r e s s u r e , m e a s u r e d n e a r t h e exposure locatioris inside underground s h e l t e r s , apparently acted f o r sufficient t i m e t o push the plugs into the middle ear causing obliteration of the e a r d r u m s and dislocation of the os8 i c l e s

.

E x p e r i e n c e with e x p o s u r e of a n i m a l s t o high-explosive detonations

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on the one hand, and o v e r p r e s s u r e s generated i n shock tubes on the o t h e r , h a s shown that f o r relatively "low" overlpressures t h e r e is l i t t l e d i s p l a c e - m e n t of a ceruminous plug when the duration of the o v e r p r e s s u r e is l l s h o r t l l as i n t h e c a s e of s o m e high-explosive detonations. When the duration 0.f a n o v e r p r e s s u r e of similar magnitude is r e l a t i v e l y "long,

s o m e of the shock tube-generated p u l s e s , the plugs a r e frequently displaced m o r e deeply into the canal. So whether lor not a ceruminous plug in t h e ex- t e r n a l auditory c a n a l p r o t e c t s the d r u m 'or m a k e s matters w o r s e depends on a v a r i e t y of things: the d m a t i o n of the o v e r p r e s s u r e , whether t h e r e is a s p a c e between the plug and the e a r d r u m , and the c h a r a c t e r of the plug; :i. e.

,

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as i n the c a s e of

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l its density, m a s s , and composition. I

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The e a r d r u m , o r tympanic m e m b r a n e , s e p a r a t e s the e x t e r n a l I ear f r o m the middle e a r .

the attention of the clinicians who have concerned t h e m s e l v e s with b l a s t i n j u r i e s of the e a r .

It is not, however, the m o s t i m p o r t a n t l e s i o n a s s o c i a t e d with b l a s t t r a u m a . Indeed, as will be subsequently developed, it is probably b e t t e r f o r the d r u m t o give way t o the o v e r p r e s s u r e than fori it t o r e m a i n intact.

It is t h i s s t r u c t u r e which h a s ,engaged m o s t of This is due t o the f a c t that it is frequently ruptured.

The d r u m and i t a a s s o c i a t e d o s s i c l e s a c t i n c o n c e r t t o t r a n s f e r a c o u s t i c a l e n e r g y f r o m the e x t e r n a l e a r l t o the i n n e r e a r , which is the s i t e of the a c t u a l transduction of the mechanical e n e r g y t o the e l e c t r i c a l e n e r g y that constitutes a n e r v e impulse. T h i s s y s t e m functions a s a n impedance matching device

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and as s u c h contributes a n amount of gain t o the s i g n a l s received at the tympanic m e m b r a n e . It should be empha- s i z e d that frequency, wave f o r m , and rdlative amplitude a r e not a l t e r e d by middle e a r s t r u c t u r e s - but that t h e r e is a concentration of s 3 u t d p r e s - s u r e on the oval window. 2 3 ~ 2 7 ~ 2 8 ~ 2 9

The adult human tympanic merdbrane h a s a n a r e a of about 70 sq. mm.

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which is s o m e 22 t i m e s g r e a t e r than the a r e a of the oval win- dow. This difference in a r e a c a u s e s a n l i n c r e a s e i n sound p r e s s u r e a t the oval window andt s e r v e s to m a k e ZIP f o r the r e l a t i v e difference between the i n e r t i a of the a i r and that of a fluid

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in this c a s e , the endolymph of the inner e a r . It is the motion of the endolymph which actually s t i m u l a t e s the-sensing organ in t h e inner e a r .

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1 . The n a t u r a l r e s o n a n t frequencylof t h e middle e a r s t r u c t u r e s is between 1200 and 1700 Hz; however, the / l i g a m e n t s and m u s c l e s attached t o the o s s i c u l a r % chain s e r v e t o highly dampen the s y s t e m .

of which is composed of the stapedius m u s c l e and its ligaments.

this one which a c t s t o limit the vibration] of the s t a p e s when intense signixls a r e being received.

m e n t s , which s e r v e t o l i m i t the vibration of the e a r d r u m . It is of i n t e r e s t t o note t h a t , due t o the m a n n e r of construction of the l i g a m e n t s , the

limitation of amplitude excursion is l a r g e l y effective when positive pulses

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T h e r e a r e two d a m p e r s in the middle e a r , the m o s t important It is The other is the t d s o r tympani m u s c l e and its l i g a -

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a r e placed on the e a r d r u m and less on the negative excursions.

T h e s e musculo-tendinous d a m p e r s a r e controlled by a n involun-

t a r y r e f l e x arc. ^~

s t r i k i n g the otic s t r u c t u r e s which g e n e r a t e s feedback.

i n t u r n , a c t i v a t e s the r e f l e x arc i n a n analgous m a n n e r t o a n e l e c t r o n i c automatic volume controI c i r c u i t .

f o r t h e r e f l e x t o become operative and effectual.

too long a time f o r the b r a c i n g a c t i o n of the stapedius and t e n s o r tympani m u s c l e s t o provide a n y protection a g a i n s t any but a I1slowt1-rising, r e l a - tively weak b l a s t wave.

intensity b l a s t wave a s s a u l t s the e a r under conditions w h e r e high ambient noise levels have existed p r i o r t o its a r r i v a l . P e r l m a n , 1 1 Ireland,

3o

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o t h e r a u t h o r & have noticed t h a t r u p t u r e of the d r u m is m o r e common when a n explosion o c c u r s i n a quiet environment than it is i n a noisy one.

seems that the middle e a r is quite a different t a r g e t under t h e s e two c i r c u m - s t a n c e s .

Its s t i m u l u s is a n e x c e s s i v e amplitude of vibration The feedback, It r e q u i r e s about 5 -1 0 milliseconds

T h i s is, of c o u r s e ,

It does provide s o m e help, however, when a tllowvl-

It

B e c a u s e of the manner i n which the m a l l e u s is linked t o the incus, inward displacement is g r e a t e r than outward displacement. The incudo- m a l l e o l a r joint is ^so c o n s t r u c t e d t h a t when the m a l l e u s is subjected t o a n inward f o r c e th.3 two bones move f o r w a r d as one. However, when t h e ex- c u r s i o n ' p f t h e bones is outward, a s e p a r a t i o n of the m a l l e u s f r o m the incus o c c u r s , thus allowing a g r e a t e r movement of tht2 m a l l e u s v i s - a - v i s the incus. ¹¹

T h i s f e a t u r e of the incudo-malleolar joint s e r v e s as a protective device t o the i n n e r e a r .

by a l f s h a r p ' ' - r i s i n g , positive p r e s s u r e peak of "short" duration which is followed by a "longer," negative phase, it becomes a p p a r e n t that a longer excursion of the o s s i c u l a r chain c a n r e s u l t f r o m the negative phase of t h e b l a s t wave t h a n f r o m the positive one, depending on how *lfast1! a n equal- izing p r e s s u r e c a n e n t e r the middle e a r via the eustachian tube.

follows that the displacement of the endolymph c a n be of a "longer" d u r a - tion during t h e negative p b f s e , thus prolonging the stimulation of the hearing organ. P e r l m a n

that a n overload is applied t o t h i s organ, the Organ of C o r t i , is of i m p o r - t a n c e i n the d e g r e e of damage that ensues. By permitting a longer e x c u r - s i o n of the m a l l e u s , which is attached t o the d r u m , without requiring the s a m e e x c u r s i o n of the i n t e r m e d i a t e bone (the incus) the movement of the s t a p e s , which t r a n s m i t s the motion of the endolymph, is diminished, thus affording s o m e d e g r e e of protection. He is a l s o of the opinion t h a t if the excursion of the d r u m and o s s i c u l a r s y s t e m t o the negative phase could be reduced f u r t h e r o r even eliminated, a g r e a t e r d e g r e e of protection t o the i n n e r e a r would r e s u l t . It is t h i s s o r t of thing t h a t o c c u r s when the d r u m gives way in f a c e of the positive o v e r p r e s s u r e of a blast wave. The d r u m having l o s t its integrity, t h e r e r e s u l t s a reduced ability f o r the con- ductive s y s t e m tu respond t o the negative phase, sothat the overload and its duration as applied t o t h e i n n e r e a r is l e s s

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how much l e s s is depend- ent on the s i z e of the perforation. It is this that was meant when it was pointed out e a r l i e r t h a t r u p t u r e of the d r u m can be r e g a r d e d a s m o r e helpful than catastrophic. However, s i n c e the maximum o v e r p r e s s u r e and its r i s e ti= govern the n a t u r e of the ensuing negative phase, the f o r m e r m u s t be r e g a r d e d as of over-riding importance.

When one r e c a l l s t h a t a b l a s t wave is c h a r a c t e r i z e d

It a l s o

f e e l s t h a t i n all probability the length of t i m e

-

3.0 QUANTITATIVE CONSIDERATIONS O F EARDRUM RUPTURE I

4 T h e r e are a n u m b e r of f a c t o r s , the interplay of which m a k e s the vulnerability of the tympanic m e m b r a n e tot r u p t u r e a v a r i a b l e thing.

m a y be divided into physical and anatomico-physiological f a c t o r s .

T h e s e

I

It has b e e n established that t h e r e is a d i r e c t r e l a t i o n s h i p between the percentage of ruptured e a r d r m s and the maximum o v e r p r e s s u r e . 31

during the s e v e r a l full-scale weapons t e s t s , and by s t u d i e s on e x p e r i m e n t a l a n i m a l s subjected to high-explosive detonations and shock tube -generated o v e r p r e s s u r e s . 15

This has a l s o b e e n shown t o be the c a s e by! the e x p e r i e n c e s of White e t al. 14-20

- -

Although extensive s y s t e m a t i c s t u d i e s r e m a i n y e t t o be done, enough d a t a a r e at hand t o show t h a t t h e r e a r e additional p a r a m e t e r s which a r e probably of s o m e importance. T h e s e a r e t h e d u r a t i o n of the o v e r p r e s - s u r e , its r a t e of r i s e , and perhaps t h e duration and magnitude of the nega-.

tive phase which follows the positive

A

higher percentage of r u p t u r e s will be a s s o c i a t e d with a given than will be the c a s e when it is peak p r e s s u r e if i t s . r i s e t i m e is "fast,

relatively 'Islow. ^{' I} .For example, it w a s fopnd during the full-scale weap- ons t e s t s i n Nevada t h a t 5 0 p e r c e n t of dogs' e a r d r u m s r u p t u r e d when e x - posed t o a flslowll-rising-.overpressure of 3 1 . 2 p s i ; w h e r e a s , a rlfastll- r i s i n g pulse g e n e r a t e d i n a shock tube needed only t o r e a c h a l e v e l of 12 p s i in o r d e r t o produce the s a m e percentage of r u p t u r e s . 15,20

In the opinion of P e r l m a ~ ~ , ~ i f the lnegative p r e s s u r e which followai a f t e r t h e o v e r p r e s s u r e of a b l a s t wave is relatively l a r g e and prolonged, a d r u m which h a s not been ruptured by theipositive p r e s s u r e m a y be r u p - t u r e d during the negative phase, if its outward e x c u r s i o n is sufficiently l a r g e .

& .

%,

³²

ability- of a n e a r d r u m v a r i e s a l s o with its a g e a n whether o r not it has been subjected t o previous trauma o r d i s e a s e .

w a s applied t o the tympanic m e m b r a n e s of f r e s h c a d a v e r s , the p r e s s u r e r e q u i r e d t o c a u s e r u p t u r e v a r i e d o v e r a r a n g e of f r o m 5 . 4 t o 4 4 . 1 p s i ;

t h a t - t h e r e was aniage dependence was shown by the f a c t t h a t Many y e a r s ago' Zalewskl6 found that when a s t a t i c o v e r p r e s s u r e

3 p s i w a s r e q u i r e d t o produce r u p t u r e s i n c a d a v e r s life, the f i g u r e

this v a r i a t i o n with a g

2 0 p s i f o r the older a g e

c carefully studied, but t h e - a s s u m p t i o n c a n be explained by.

I l

reg$;c$yg the young m e m b r a n e 'as having m o r e e l a s t i c i t y t h a n a n o l d e r i

one.

Department of Pathology in which histologi,cal p r e p a r a t i o n s a r e being made:

of tympanic m e m b r a n e s removed f r o m both quite young and quite old Some c u r r e n t s t u d i e s a r e under way i n the Lovelace Foundation

7

I

a n i m a l s and humans who come t o autopsy.

a r a t i o n s have b e e n m a d e and studied, but a v e r y definite i m p r e s s i o n h a s e m e r g e d t h a t tne number and organization of t h e e l a s t i c f i b e r s i n the submucosal connective t i s s u e in the m a t e r i a l obtained f r o m the young d r u m is quite different when c o m p a r e d with the old ones. In t h e young d r u m they are m o r e abundant and organized; w h e r e a s , i n t h e old d r u m s they are m o r e s p a r s e and l e s s well organized.

the evaluation of much m o r e material than h a s yet become available, however, b e f o r e a definite s t a t e m e n t c a n be made.

So f a r , only a few s u c h p r e p -

4

It will r e q u i r e

The thickness and e l a s t i c i t y of a tympanic m e m b r a n e c a n a l s o be influenced by t h e a f t e r e f f e c t s of previous d i s e a s e o r injury.

which l e a v e s a s c a r i n its wake will, of c o u r s e , l e s s e n the d r u m ' s r e - s i s t a n c e t o blast.

chronic n a t u r e , which r e s u l t i n a thickened m e m b r a n e , will c a u s e the d r u m t o become m o r e r e s i s t a n t according t o P e r l m a n .

and f r o m t h a t of Blake e t al.

*

that t h e e a r d r u m s of the v a r i o u s mam- m a l i a n s p e c i e s m a n i f e s t differing d e g r e e s of vulnerability t o r u p t u r e by o v e r p r e s s u r e s . Zalewski found that in the c a s e of the dog, the range of p r e s s u r e s r e q u i r e d f o r r u p t u r e w a s between 9.1 and 2 2 . 8 p s i with a m e a n of 14. 9 psi. T h i s w a s in c o n t r a s t with the values obtained f o r the human c a d a v e r s he studied w h e r e the range w a s between 5 . 4 and 4 4 . 1 p s i with a m e a n value f o r all a g e s of 2 2 . 9 psi.

Anything Conversely, t h o s e pathologic conditions, usually of a

10

It h a s been known sirxe Zalewski's w o r k 6 w o r k w a s published in 1905

An opportunity f o r i n t e r s p e c i e s c o m p a r i s o n of t h i s point h a s been afforded o v e r t h e y e a r s t o t h e Comparative Environmental Biology Depart- m e n t of t h e Lovelace Foundation and, although a comprehensive, s y s t e m - atic study h a s not y e t been m a d e , t h e r e a r e enough d a t a at hand t o m a k e a few s t a t e m e n t s possible.

.

E x p o s u r e of young dogs t o n u c l e a r b l a s t a t t h e Nevada T e s t Site indicated t h a t t h e . m a x i m a 1 p r e s s u r e s f o r r u p t u r e of the e a r d r u m ranged f r o m 4. 1 t o 85.8 p s i , with 31.2 p s i being the s t a t i s t i c a l l y d e t e r m i n e d p r e s - s u r e r e q u i r e d f o r 50 p e r c e n t f a i l u r e of the m e m b r a n e .

p r e s s u r e without r u p t u r e w a s note2 to be 66.6 psi. The differences between t h e s e values and those of Zalewski possibly c a n be accounted f o r by r e c a l l - ing that his e x p e r i m e n t s w e r e p e r f o r m e d using a s t a t i c overloading of the d r u m ; w h e r e a s , t h e o v e r p r e s s u r e s i n the n u c l e a r t e s t s w e r e the r e s u l t of t h e detonation of a n explosive.

2o The highest

FIGURE

2 i l l u s t r a t e s what h a s been found in r e g a r d t o s p e c i e s differences.

of Blake, Douglas, Krohn and Z u c k e r m a n 8 and on swine f r o m Richmond.

It a p p e a r s that the e a r of t h e monkey is the m o s t b l a s t r e s i s t a n t of t h e s p e c i e s studied, and t h a t the ear of the goat is t h e m o s t vulnerable.

Included a r e s o m e d a t a on monkeys derived f r o m the studies 3 3

It is not possible a t t h i s t i m e t o a t t r i b u t e the difference i n v u l n e r -

.

ability t o any ,single f e a t u r e of the e a r s of t h e s e different s p e c i e s , s u c h as

7-

8

99.9 99.8

99 98

z)

Q c

3 90

LT a,

g51

0

E

6 0

Q,

'"I

⁵⁰

I t

I

Per cent Tympanic Membrane Rupture as a log function of maximal overpressure for various species. (F) = fast'lrising, (s) = "slow'lrising.

Data derived from references as numbered.

II

0.5- 0.2 -

0.

I 2 3 4 5 6 7 8 9 1 0 20 30 4050 70

100

I I 1 I I I I I I I I I I I I I I I F I G U R E 2.

A log-normal plot of overpressure versus percentage of tympanic membrane ruptures for various species.

(8. 5 psi) and greatest for the monkey (22 psi).

Overpressure, psi

It can be s e e n that the 50-percent value is least for the goat . .

the s t r u c t u r e of the tympanic m e m b r a n e , o r the anatomy of the e x t e r n a l e a r , s i n c e no intensive c o m p a r a t i v e s t u d i e s have been made. That t h e r e a r e wide v a r i a t i o n s i n the anatomy of the e a r among the mammals is ob- vious. Except for differences in the thickness of the s t r a t i f i e d c o l u m n a r epithelial l a y e r , t h e r e h a s not b e e n found any m a r k e d variation in the h i s - tology of the tympanic m e m b r a n e s f r o m a number of s p e c i e s which have been examined. The middle connective t i s s u e l a y e r s e e m s t o be the s a m e f o r all of the s p e c i e s examined.

ical p a r a m e t e r s , s u c h as r e s o n a n t frequency, e l a s t i c i t y , etc;

,

might well- show that the d r u m s of the v a r i o u s s p e c i e s of mammals a r e v e r y different in t h e i r dynamics, and they a r e different t a r g e t s m o r e because of t h i s , than because of differences in histologic s t r u c t u r e .

The m a m m a l of chief i n t e r e s t is, of c o u r s e , the human s p e c i e s ; Reference t o the w o r k of Zalewski h a s a l r e a d y been made.6

It is probable t h a t s o m e of the, biophys-

consequently, it is d e s i r a b l e t o know w h e r e it fits into the s c h e m e of

things. T h i s

is the most s y s t e m a t i c study which has been m a d e , a s well as the first.

His experiments d e a l t with the r e s i s t a n c e of c a d a v e r e a r s t o "slowly" ris- ing o v e r p r e s s u r e s .

-auditory c a n a l which had been s e a l e d t o prevent a i r escaping f r o m the e x - t e r n a l auditory meatus.

p r e s s u r e , s u c h a s a b l a s t wave. He found that 11 p e r c e n t of the c a d a v e r e a r d r u m s ruptured at a n o v e r p r e s s u r e which was l e s s than 1 a t m o s p h e r e (14. 7 p s i ) .

t u r e d , and at p r e s s u r e s g r e a t e r than 2 a t m o s p h e r e s the remaining 2 3 p e r - c e n t w e r e ruptured.

T h e s e he c r e a t e d by pumping air into the e x t e r n a l T h i s is different than t h a t of a "fast"-rising o v e r -

At o v e r p r e s s u r e s between 1 and 2 a t m o s p h e r e s 66 p e r c e n t r u p -

Blake, Douglas, Krohn, and Zuckerman8 a l s o studied the prob- l e m using c a d a v e r s .

they generated a shock wave by the bursting-diaphragm technique s u c h a s is used on a l a r g e r s c a l e in s o m e shock tubes. They found that the p r e s - s u r e r e q u i r e d t o r u p t u r e 50 p e r c e n t of the d r u m s was about 1 a t m o s p h e r e o r 14. 7 psi.

Instead of using a 'lslowlytl - r i s i n g o v e r p r e s s u r e ,

They a l s o studied the incidence of ruptured tympanic m e m b r a n e s amongst a i r r a i d c a s u a l t i e s .

in r u p t u r e of the e z r d r u m w a s between 2 and 4 psi. In t h e i r s a m p l e , how- e v e r , 37 p e r c e n t of people exposed t o b l a s t p r e s s u r e s in e x c e s s of 100 psi showed no damage to t h e i r tympanic m e m b r a n e s . They concluded that the lower limit of p r e s s u r e r e q u i r e d to r u p t u r e 50 p e r c e n t of e a r d r u m s was 15 psi and that the upper limit was 50 psi.

They found that the lowest p r e s s u r e resulting

Z u c k e r m a n 7 in a l a t e r r e p o r t concluded that over a range f r o m 6 psi to 70 psi e a r d r u m r u p t u r e m a y occur, but that the likelihood of s u c h a n event was much g r e a t e r a t the higher levels than a t the lower.

Data such a s we have just r e g a r d e d leave something t o be d e s i r e d in the way of p r e c i s i o n t o those who have a n i n t e r e s t in a s s e s s i n g casualty potentials.

p r e s s u r e below which damage i s not t o be expected and a p r e s s u r e value at T h e s e people like t o have n u m b e r s t o u s e , s u c h a s the t h r e s h o l d

10

1

which 50 p e r c e n t of-.an..expos d* DODUl

i

.A 1 tion c a n be-expected t o m a n i f e s t the effect.

n u m b e r s could not be culled f r o m the l i t e r a t u r e . -A5cordingly, a l i t e r a t u r e s e a r c h w a s undertaken with the r e s u l t that a n u m b e r o f g e r m a n e publications w e r e found.

b l a s t injury t o the e a r , -and no effort was m a d e ^$0. equate o v e r p r e s s u r e s *with e i t h e r the incidence o r extent of damage! In s o m e , however, r e f e r e n c e w a s m a d e t o a p a r t i c u l a r type of ordnance, a l p a r t i c u l a r type and weight of ex.- plosive, o r a measured-averpr1essure. By equating t h e s e things with other available d a t a dealing with explosions, scaling values f o r v a r i o u s explosives, and the known c h a r a c t e r i s t i c s of v a r i o u s ] types,of ordnance, it was possible t o m a k e s o m e approximations which when a s s e m b l e d s e e m e d o r d e r l y and reasonable. In doing thi's the help of pedple knowledgeable i n the field of' explosion phenomenology, such as M r . I!. G. Bowen of the Lovelace Founda- tion f o r Medical Education and Research! Albuquerque, New Mexico; Mr.

L. J. Vortman of Sandia Corporation, Albuquerque, New Mexico, and Mr.

R.

H. Reider of the Los Alamos Scientific L a b o r a t o r y , L o s Alamos, Ne*w Mexico, was needed. They a l l gave substantial a s s i s t a n c e which is g r a t e - fully acknowledge d.

T h e difficulties and uncertainties surrounding a n approach of this kind w e r e d i s c u s s e d by Zucgerman i n 1941. 7 The peak o v e r p r e s s u r e and its scaling produced

w e l l d e t e r m i n e d f o r given s e t of conditions, but s i n c e it is v e r y difficult t o d e t e r m i n e exactly which s e t of conditions apply to a given c a s e , s o m e e s t i - mation is required. So, recognizing the inherent deficiencies of this s o r t of a n approach, t h e following *data a r e presented.

I t w a s r e g a r d e d as a-useful and i n t e r e s t i n g e x e r c i s e t o s e e if such

Most of t h e s e w e r e reports1 of- the clinical e x p e r i e n c e s with

any given explosive o r type of ordnance c a n be v e r y

In the m a t t e r of-the l e a s t , o v e r p r e s s u r e which c a n be a s s o c i a t e d with found that a p r e s s u e of 3. 9 p s i was the lowest that produced r u p t u r e in his studies. Machle," on the other hand, s t u d i e d forty-five gunnery i n s t r u c t o r s who had frequent exposures t o "fast"-rising o y z r p r e s s u r e s of 4 p s i and e n - countered fai r u m a t 6 psi. Shilling found no c a s e of r u p t u r e of t o p r e s s u r e s l e s s than 7 psi but his s u b j e c t s w e r e s i n g ' o v e r p r e s s u r e .

study.

about 5 psi is sufficient t o r u p t u r e - s o m e human t y m - t t h i s value c a n be r e g a r d e d as the threshold.

e a r d r u m r u p t u r e , .Zalewski6 found this number t o be 5 . 4 psi. P e r l m a n 10

Corey3& found 7 p s i to be the It a p p e a r s f r o m these data.that a "fast"-

When it c o m e s t o estimating t h e p r e

I-

^{s u r e r} i r e d t o r u p t u r e 50 p e r c e n t of human d r u m s , one is unable to find -any r e p o r t in the l i t e r a t u r e

. p r e s e n t i n g data which p e r m i t making a c a t e g o r i c a l statement. However.

= t h e r e a r e s o m e rep0 s which do p e r m i t I s o m e estimation. Henry 37 on 292 m e n who s u s t ned b l a s t injuries'do t h e i r e a r s .

p e r f o r a t i o n s ( o r 52 percent).' All t h e s e , patients w e r e injured' when they w e r e about 50 f e e t f r o m a n expl-osion of a landlmine, 500-lb. a e r i a l bomb, o r a

3-in. m o r t a r shell. Calculations indicate that the o v e r D r e s s u r e to which t h e s e people w e r e subjected was probably about 17 psi.38

on his experience with 75 m e n who sustained e a r i n j u r y a s a r e s u l t of e x - p o s u r e to the muzzle blast of a 37-mm. a n t i a r i c r a f t gun.

In t h e s e t h e r e w e r e 152

Vadala39 r e p o r t s F r o m the studies

11

of M u r r a y and Reid4' this gun has a muzzle blast o v e r p r e s s u r e of about 6 psi. In Vadala's m a t e r i a l he encountered 6 c a s e s of tympanic m e m b r a n e rupture, o r a n incidence of 8 percent. Reider41 a t Los Alamos has s o m e data f r o m two r e c e n t industrial explosions which indicate that 85 percent of 22 exposed d r u m s ruptured in one explosion where the peak o v e r p r e s s u r e was 30 psi. In the other explosion, where the o v e r p r e s s u r e reached 40 psi, all of the 6 exposed e a r d r u m s w e r e ruptured.

If one plots t h e s e percentages v e r s u s t h e i r associated o v e r p r e s s u r e as a log-normal plot, one obtains a s t r a i g h t line b st fit which is like those that have been obtained on experimental animaIs. f4-20 FIG U RE 3 shows

this plot compared with one p r e p a r e d by White e t al. f o r dogs exposed t o l l f a s t l t - r i s i n g o v e r p r e s s u r e s .

rising o v e r p r e s s u r e r e q u i r e d f o r r u p t u r e of 5 0 percent of human d r u m s is about 15 psi.

It would a p p e a r f r o m this that the probable "fastt1-

a

y.

0 c c

8

1

99.51

1

99 98

-

-

95

9 0

- -

-

80

-

70

-

60

-

50

-

40

-

30

-

20

-

I O

-

5 -

2-

I -

0.5

-

0 . 2 1

TOLERANCE OF EARDRUMS] TO

/

l l F A S f L R I S I N G O V E R P R E S S U R E S

V a d a l a (39)

d 7

Dog (Shock Tube

-

"Fast"-rising Pressure from White et al-CEX

65.4)

Combined D a t a )

Cr;.

I -+

Human' ( E s t i m a t e d from

1

I I I

I

^{I 1 1 1} I I I I I l l &

2 5 4 0 20 5 0 I O 0

Peak Pressure

I

( p s i )

i 0.1

I II

FIGURE 3.

I

A log-normal plot of the data on humans as derived from the references as .shown.

This i s compared with the data on dogs as presented by White in CEX 6 5 . ^4.1L5 13

4 . 0 HEARING LOSS CAUSED BY BLAST TRAUMA

One i m p o r t a n t r e s u l t of b l a s t injury t o t h e e a r is the amount of damage t o t h e Organ of C o r t i i n the i n n e r e a r c a u s e d by a n e x c e s s i v e m o - tion of the endolymph. Such damage c a n r e s u l t i n varying d e g r e e s of t e m p o r a r y o r p e r m a n e n t deafness.

whether o r not the e a r d r u m h a s r u p t u r e d has been r e m a r k e d about i n many of the r e p o r t s which have' been published i n t h e l i t e r a t u r e . For example, Bourgeois42 states c a t e g o r i c a l l y that when the e a r d r u m r u p - t u r e s the i n j u r y t o the i n n e r ear is l e s s , and the consequent deafness is l e s s g r a v e i n that it is l e s s a p t t o be p e 3 y a n e n t . Concurrence with t h i s opinion h a s been e x p r e s s e d by Henry,

Schenck,3l and others.

That the hearing l o s s is r e l a t e d ' t o

Colledge,24 Silcox and

When a b l a s t -wave has caused the e a r d r u m t o r u p t u r e , the a s - sociated hearing l o s s w i l l be of a mixed type; that is t o s a y , t h e r e will be a l o s s of a c u i t y ' f o r low tones as w e l l as f o r high tones. O r d i n a r i l y t h e low-tone l o s s will be of the o r d e r of 1 0 t o 30 db; w h e r e a s , the high- tone l o s s will be m o r e i n the n a t u r e of 40 t o 80 db.

providing t h e r e h a s been no dislocation of the o s s i c l e s a s s o c i a t e d with the r u p t u r e , because when this happens, t h e r e r e s u l t s a p e r m a n e n t and s e v e r e conductive d e a f n e s s , F r o m c l i n i c a l r e p o r t s it is apparently un- u s u a l f o r the o s s i c l e s t o be displaced by a "fast"-rising, l l s h o r t t l t i o n o v e r p r e s s u r e u n l e s s t h e o v e r p r e s s u r e is v e r y high. Ireland30 found no incidence of o s s i c u l a r f r a c t u r e o r dislocation in 317 c a s e s of b l a s t i n j u r y of the e g all due t o high-explosive detonations; however, B a r r o w and Rhoads encounter d s e v e r a l i n t h e i r study of 32 c a s e s of b l a s t injury of the e a r . Henry3f r e m a r k s in his study of 292 c a s e s that dislocation of the o s s i c l e s w a s uncommon.

T h i s will be t r u e

d u r a -

.

Henry a l s o pointed out that he w a s unable t o c o r r e l a t e the

' intensity of the i m m e d i z t e hearing l o s s with the s i z e of the perforation, and o t h e r a u t h o r s a r e found t o be in a g r e e m e n t .

r e p o r t s studied, no one w a s found who d i s a g r e e d with t h i s opinion.

Among the published B o e m e r 44 i n h i s study of 310 c a s e s of acbqstic t r a u m a found t h a t 78 p e r c e n t of the c a s e s of t e m p o r a r y deafness had n o t s u f f e r e d a . p e r f o r a t i o n of the d r u m and m a k e s the point that t h e s e w e r e the m o r e

s e v e r e l y deafened.

M u r r a y and Reid?' M a ~ h l e , ~ ~ and Bunch32 have emphasized that t h e r e is a n accumulative effect on hearing when multiple a c o u s t i c a l i n s u l t s a r e sustained by a n individual. It is a l s o apparent that the t i m e i n t e r v a l between incidents is of i m p o r t a n c e in that when they o c c u r with i n t e r v a l s s h o r t e r than the r e c o v e r y period, t h e permanent he ring l o s s is m o r e profound. M u r r a y and R e i d , 4 0 Silcox and Schenck?' and P e r l m a n 9 have all pointed out that t h e r e is a m a r k e d individual variation in susceptibility t o a c o u s t i c t r a u m a . It a p p e a r s t h a t susceptibility t o i n n e r e a r damage as evidenced by high-tone hearing l o s s and the t i m e r e q u i r e d f o r r e c o v e r y f r o m

A n u m b e r of studies of t r a u m a t i c d e a f n e s s , s u c h as t h o s e of

P

e

14

I

a n acoustic insult go hand in hand; that is t o s a y , the m o r e s u s c e p t i b l e i n n e r e a r s r e q u i r e longer f o r r e c o v e r y . 40

T h e r e a r e t h r e e types of hearing l o s s due t o b l a s t injury.

f i r s t of t h e s e is a n e u r o s e n s o r y type which involves audition of high tones, usually considered as those above 4000 Hz. This c a n be e i t h e r t e m p o r a r y o r p e r m a n e n t and c a n result whether o r , n o t t h e r e h a s been i n c u r r e d any damage t o the middle ear s t r u c t u r e s .

m a t i c episode, o r if t h e i n t e r v a l s between episodes have been long enough t o p e r m i t r e c o v e r y t o t h e i n n e r e a r s t r y c t u r e s , then the high-tone l o s s is t e m p o r a r y . On the o t h e r hand if multiple traumata of the i n n e r e a r a r e ex- perienced without adequate r e c o v e r y i n t e r v a l s , the loss w i l l be permanent.

t r a u m a t i c deafness w r i t t e n by C. C . * Bunch32 in 1937.- It c o n c e r n s a naval gunnery officer who had, in the c o u r s e of his duties, experienced periods of time when h i s e a r s w e r e exposed t o m u z z l e b l a s t o v e r p r e s s u r e s f r o m naval ordnance. He n e v e r suffered r u p t u r e of his e a r d r u m s so f a r as i n known.

of 36 y e a r s .

m o s t profound a t 4000 Hz.

as c h a r a c t e r i s t i c ,of c o c h l e a r damage and a l s o of s e n i l e degeneration of the i n n e r ear.

that the hearing l o s s c a n be on the l a t t e r b a s i s .

The second type of deafness a s s o c i a t e d with b l a s t i n j u r y is that which is obtained when a r u p t u r e of the b a r d r u m h a s o c c u r r e d .

Kilgore of the Lovelace Clinic h a s m a d e available d a t a on a 44-year old physicist who w a s involved i n a hydrogen-oxygen explosion and as a re- s u l t w a s exposed t o a "fast"-rising o v e d p r e s s u r e e s t i m a t e d by R e i d e r 4 ' t o b e n e a r 30 psi.

did not become infected and healed completely within s e v e r a l months.

FIGURE

5 is a n a u d i o g r a m showing the auditory acuity f o r his right e a r . The r e c o r d f o r his l e f t w a s virtually iddntical.

r u p t u r e , and the broken line is the r e c o r d m a d e following healing of the perforations. The mixed type of deafness is c l e a r l y apparent, as is the:

d e g r e e of recovery.

T h e

If t h e r e has been only a single t r a u -

A

c a s e in point is one taken frdm the comprehensive a r t i c l e on

I

The a u d i o g r a m depicted i n FIGURE-4 w a s taken on h i m at the a g e It shows a loss'of auditory acuity beginning at 2 0 0 Hz and is

This type oflaudiogram is r e g a r d e d by otologists When!one r e g a r d s the a g e o f this patient, it hardly s e e m s likely

,

I

Dr. D. E.

He suffered bilate.ra1 tympanic m e m b r a n e r u p t u r e s which

The solid line is p o s t -

I

The t h i r d type of hearing l o s s is that which r e s u l t s when the m i d -

* d i e e a r s t r u c t u r e s a r e i r r e v e r s i b l y d a d a g e d . , T h i s is i l l u s t r a t e d by a c a s e , a l s o taken f r o m Bunch>2 concern!ing a soLdier who was in a t r e n c h in World W a r I when a s h e l l exploded l e s s than t h i r t y f e e t away f r o m him.

He suffered obliteration of his e a r d r u m s and dislocation of the middle e a r o s s i c l e s as well.

s o m e twenty y e a r s a f t e r his i n j u r y which had o c c u r r e d a t the a g e of 2 5 y e a r s .

ductive deafness, a n d the fact that there, has been no r e s t o r a t i o n in twenty y e a r s e m p h a s i z e s the permanent nature1 of his disability.

His audiogram, which is sliown i n FIGURE 6 , w a s taken The d e p r e s s e d acuity f o r all frequencies is c h a r a c t e r i s t i c f o r con- ^I

I

0 0 0 00

0 0 0 4

0

6J

8

0 0 0

-

0

8

0 In 6J

In N

-

16

0 0 0

P

0 0 0

%I

(D

\

0

c

E

5 . 0 PREVENTION

OF

BLAST INJURY O F THE EAR

A s e a r c h f o r a s a t i s f a c t o r y method of preventing a c o u s t i c t r a u m a has been conducted by many people f o r many y e a r s .

by m o s t of t h e m h a s been t o r e c o m m e n d t h e u s e of s o m e type of ear plug which will attenuate a b l a s t wave ( o r loud sound) and d i s s i p a t e s o m e of its energy.

and thus give t h e i r w e a r e r s s u b s t a n t i a l d e g r e e s of protection. 40

,

4 4 - 4 g They all have a s s o c i a t e d with t h e i r u s e , however, s o m e d e g r e e of loss of auditory acuity which c a n be quite disadvantageous under c e r t a i n c i r c u r n - s t a n c e s .

The a p p r o a c h used

. -

\

All of the devices which have been designe.dswil1 d o t h e s e thin s

As

with a l l protective d e v i c e s , s u c h as "hard h a t s , ^It safety s h o e s , s a f e t y g l a s s e s , and o t h e r s , r e s i s t a n c e t o t h e i r u s e is p e r v e r s e l y encountered i n s o m e of the population at r i s k .

Some otologists have r e g a r d e d a s i m p l e cotton plug, constitilting a protective device,, as s a t i s f a c t o r y a s a m o r e e l a b o r a t e e a r plug.

and they s e e m t o be i n t h e m a j o r i t y , d e c r y the u s e of cotton plu s and point out with s o m e m e r i t the r e a s o n s f o r t h e i r

reviewing a substantial amount of the pertinent l i t e r a t u r e , one is forced t o cowzlude that the p r o p e r and faithful u s e of a n e a r defender i n a t r a u - m a t i c a l l y noisy environment, o r w h e r e a r e a l h a z a r d of b l a s t injury ex- ists, is w i s e and worthwhile. 32,34,40,47

O t h e r s , 4g After

The placement of gun c r e w m e m b e r s should c e r t a i n l y be m a d e with the t h r e a t of muzzle b l a s t in mind. Wherever the m u z z l e b l a s t o v e r -

p r e s s u r e approaches 4 psi, t h e c r e w m e m b e r s should be protected by ear defenders.

19

. .

. .

3. .

, .

. . ^~ . .

'. . . ^{. i , i:' ,} .

: . .

. .

6 . 0 DISCUSSION

F r o m what h a s been s a i d , it is e a s y t o a p p r e c i a t e why the ear is e s p e c i a l l y vulnerable to b l a s t o v e r p r e s s u r e s . Indeed, it is r e m a r k a b l e t h a t a s t r u c t u r e p o s s e s s e d of delicacy and sensitivity s u c h as the mam- m a l i a n ear c a n p r e s e n t as m u c h r e s i s t a n c e t o damage by blast as it does.

T h i s m u s t be due in p a r t t o the mitigative effect that r u p t u r e of t h e tympan- i c m e m b r a n e frequently e n t e r s into the p i c t u r e ; although as has been

pointed out, t h e r e a r e o t h e r i m p o r t a n t individual v a r i a b l e s such a s age, the previous health h i s t o r y of the e a r s , and the poorly understood dif- f e r e n c e s i n the susceptibility of individuals t o a c o u s t i c t r a u m a .

An e n t i r e l y different s e t of v a r i a b l e s a r e t h o s e a s s o c i a t e d with T o t h e s e m u s t be added all of t h o s e things a n y given explosion

-

^{such as} the m a x i m u m o v e r p r e s s u r e , the n a t u r e of its r i s e time, and its duration.

which collectively c a n be called "geometric positioning. ^{I t}

Where a n individual happens t o be when a n explosion o c c u r s , how he is oriented with r e s p e c t t o the d i r e c t i o n of propagation of the b l a s t wave, and whether environmental conditions a r e s u c h as t o afford shielding o r r e - flections of the o v e r p r e s s u r e will c a u s e m a r k e d differences in the magni- tude of e x p o s u r e i n different individuals who m a y be quite c l o s e t o one a n - o t h e r at the t i m e .

F o r all of-these r e a s o n s , it is v e r y difficult t o d e r i v e quantitative d a t a about the vulnerability of human e a r s f r o m the study of whzt h a s oc- c u r r e d t o a n exposed population. F u r t h e r , i n s o f a r as experimental studies of i n n e r e a r trauma a r e concerned, it has not been possible t o equate b v e r - p r e s s u r e v e r s u s damage due t o the e x t r a o r d i n a r i l y formidable difficulties a s s o c i a t e d with investi ations of i n n e r e a r function. T h i s h a s been pointed out by Wever and Brayg8 and m o r e r e c e n t l y by Vartanyan and Maruseva.49 The latter a l s o emphasize t h a t the frequency sensitivity range of the e a r of a n a n i m a l a r e highly s p e c i e s specific and that the m e c h a n i s m of c o c h l e a r function and n e u r a l organization i n the b r a i n v a r y .

polations of d a t a derived f r o m one s p e c i e s t o another a r e p r e c a r i o u s . Insofar as t h e middle e a r s t r u c t u r e s a r e concerned, it c a n proba-

F o r t h e s e r e a s o n s , e x t r a -

bly be safely s a i d that the t h r e s h o l d p r e s s u r e f o r damage t o middle e a r s t r u c t u r e s is about 5 psi and that o v e r p r e s s u r e n e a r 15 p s i will c a u s e the r u p t u r e of 50 p e r c e n t of the e a r d r u m s exposed t o them. However, except when the o s s i c l e s have been displaced, damage t o the middle e a r is of

secondary i m p o r t a n c e as c o m p a r e d with i n n e r e a r t r a u m a , especially i n s o - f a r as l o n g - t e r m and permanent disability a r e concerned.

n u n b e r s / l o s e s o m e of t h e i r significance and utility insofar a s they c a n be applied helpfully t o casualty a s s e s s m e n t and identifying conditions of hazard.

One c a n a s s u m e that obvious middle e a r t r a u m a is always a s s o c i a t e d with s o m e significant d e g r e e of inner e a r damage, but the c o n v e r s e is not a valid assumpti22. It h a s been shown by many, although perhaps no b e t t e r than by Bunch, that i n n e r e a r damage c a n v e r y often occur in the a b s e n c e of d e m o n s t r a b l e i n j u r y t o the middle e a r .

Thus, t h e s e

20

Compared with the other types of b l a s t trauma, s u c h as that t o the lung and t o those organs affected by the p r e s e n c e of air emboli in t h e i r v a s c u l a r e l e m e n t s , ’ i n j u r y t o the e a r s is distinctly a m a t t e r of s e c -

ondary importance. Even though the body of information concerning otic b l a s t t r a u m a m a y leave something t o be d e s i r e d concerning s o m e of i t s quantitative d e t a i l s , enough c a n be d i s c e r n e d f r o m reading t h a t which ‘has b e e n published on the subject t o enable a f a i r l y s a t i s f a c t o r y understanding of what happens and how it happens.

p r i a t e l y guide those who a r e concerned with environmental health and safety i n s o f a r as e a r s a r e concerned.

Enough c e r t a i n l y is at hand t o a p p r o -

0"

7.0 SUMMARY

T h e l i t e r a t u r e concerning b l a s t injury of the e a r has been r e - viewed.

d a t a t o be found. An effort h a s been m a d e t o a r r i v e at s o m e quantitative e s t i m a t e s relating b l a s t o v e r p r e s s u r e t o e a r injury. It a p p e a r s t h a t the threshold f o r r u p t u r e of the human e a r d r u m by a "fast"-rising o v e r p r e s

-

s u r e ' i s about 5 p s i and that the o v e r p r e s s u r e r e q u i r e d f o r r u p t u r e of 50 p e r c e n t of e a r d r u m s is n e a r 15 psi.

equate o v e r p r e s s u r e quantitatively with damage t o the i n n e r e a r . Some of the anatomical and physiological f a c t o r s involved i n otic trauma due t o b l a s t have been d i s c u s s e d , and the utility of e a r defenders has been con- s ide red.

In g e n e r a l , t h e r e is apparently little in the way of quantitative

It has not been found possible t o

1

+

.

0

0

22

1.

2.

3.

4.

5 .

6 .

7.

a.

9.

10.

11.

12.

REFERENCES

G r e e n e , J. O . , " C a s e s of Injury t o the Ear f r o m E x t e r n a l Vio- l e n c e , ^{I '} T r a n s . A m e r . Otol. SOC.

-

5: 88, 187>2.

Castex, A . , "Effects of Dynamite on the Ear,

"m. -

^{7: 307,}

1893.

R u s c a , F.

,

"Experimentelle Untersuchungen u e b e r die t r a u m a t i s c h e Druckwirkung d e r Explosionen, Z e i t s c h r . C h i r .

-

132: 315-374,

1915.

McReynolds, G. S . , F. R . Guilford aiid G. R . C h a s e , "Blast I n j u r i e s to the Ear, A r c h . Otolaryng.

-

50: 1-8, 1949.

Hofer, 1. and 0. Mauthner, Verletzungen d e s O h r e s bei k a t a s t r o p h a l e n Explosionen, J o s e p h Sofar, Wien, 1913. ( R e - viewed by A. Tweedie i n J. Laryngol. 28: 618, 1913. )

Zalewski, T.

,

"Experimentelle Untersuchungen u e b e r die R e s i s t - _- heitsfaehigkeit d e s T r o m m e l f e l l s , ^{I '} Z e i t s c h r . f . Ohren.

-

52: 109- 128, 1906.

Z u c k e r m a n , S . , "Rupture of the E a r D r u m s by Blast, ^{' I} Ministry of Home S e c u r i t y Report B. P. C. 148/W. S. 13, Oxford, England, 1941.

Blake, P. M . , J. W . B. Douglas, P. L. Krohn and S. Z u c k e r - m a n , "Rupture of E a r d r u m s by Blast, ^{' I} Medical R e s e a r c h - Council R e p o r t B. P. C. 43/179/W. S. 21, Oxford, England (undated). ^' P e r l m a n , H. B . , "Acoustic T r a u m a i n Man, ^{' I} A r c h . Otolaryng.

-

^34: 429-452, 1941.

_ -

P e r l m a n , H. B . , "Reaction of the Human Conduction Mechanism to Blast, ^{I '} Laryngoscope

-

55: 437-442, 1945.

P e r l m a n , H. B . , "The Effect of Explosions on the Acoustic Appa- 1942.

R o b e r t s , J. "E., C. S. White and T. L. Chiffelle, "Effects of O v e r p r e s s u r e in Group S h e l t e r s on Animals and D u m m i e s , ^{' I}

USAEC

Civil Effects T e s t Group Report, WT-798, Office of Technical S e r v i c e s

,

Depa r tment of Comme r c e , W a s hington, D. C . , September 1953.

. . *

ra'tus,

'5 -

47: 442-453,

23