THE
“FONTANOMETER”
Adaptation
of
the
Schiotz
Tonometer
for
the
Determination
of
Intracranial
Pressure
in the
Neonatal
and
Early
Periods
of
Infancy
By Leo M. Davidoff, M.D., and Max Chamlin, M.D.
1)epartments of Neurological Surgery and Ophthalmology, Albert Einstein College of Medicine of Yeshiva University, and the Monte/lore Hospital, New York City
1065
I
N AN EFFORT to discover a method forestimating intracranial pressure in the
newborn infant without resorting to spinal
or ventricular puncture, one of the authors
(L.M.D.) conceived of utilizing the principle
of the Schiotz tonometen, an instrument
cur-relltlv used for estimating intraocular
pres-sure.
SCHIOTZ TONOMETER
The Schiotz tonometer is an instrument of
standard weight which is applied to the cornea;
the area of application, designated the
foot-plate, consists of a concave metal surface made
to conform to the convexity of the average
con-neal curvature. In addition, a small plunger of
standard weight fits through an aperture in this
foot-plate and, by virtue of its weight on the
cornea! surface, it depresses the corneal surface a certain amount, depending on the intraocular pressure and the elasticity of the corneal
tis-sue. The greater the intraocular pressure, the
less the cornea is depressed by the plunger, and
the lower the reading on the tonometen scale. If the deviation of the indicator is too small for accurate reading on the tonometer, extra stand-ard weights are added to the plunger thus
af-fording higher readings on the tonometer scale,
with more accurate estimation of the
intraocu-lar pressure. The most accurate readings are obtained when the arrow on the tonometer registers between 3 and 7 on the scale. These
readings are then transposed into terms of
intra-ocular pressure in units of millimeters of
mer-cury by the use of a standard chart, the latest being the one adopted by the Committee on Standardization of Tonometers of the American Academy of Ophthalmology and
Otolaryngol-ogv in 1955.
If one considers the cornea as the wall of a
(Accepted May 20, 1959; submitted May 1.)
ADDRESS: (L.M.D.) Eastchester Road and Morris
closed balloon filled with fluid, then the open fontanelle consisting of skin and membraneous
bone covering the menmgeal and cerebral
tis-sues may be considered as the wall of a similar
closed balloon which should lend itself to
similar measurement of pressure.
DEVELOPMENT OF THE “FONTANOM ETER”
(FIG. I)
In order to adapt the foot-plate on the
tonometer (Fig. 2) to the scalp tissues over the
FIG. 1. The fontanometer with the plunger outside
the foot-plate.
Park Avenue, New York 61, New York.
1066 “FONTANOMETER”
Fic. 2. Schenlatic sketch showing construction of
the regular tonometer used for measuring
intra-ocular pressure. B) Concave foot-plate,
cross-see-tion. A) The plunger within it.
area of the fontanelle, it was obvious that the
application of the Schiotz tonometer,
con-structed to fit the curvature of the cornea,
would not be appropriate because the skin
over the fontanelle presents a surface described
by a much larger radius than that of the
con-neal curvature. Indeed, the surface at times
is practically flat. To obtain an accurate
ap-plication, therefore, a foot-plate which was
convex rather than concave was first made,
the weights and plunger being otherwise
un-changed. However, the readings were quite
inconstant, fluctuating widely with the angle
of application to the fontanelle surface, and
the convex foot-plate was therefore discarded.
Since the fontanelle skin is most often flat,
but may be convex or even somewhat concave
depending on the surrounding bony edges and
the intracranial pressure, it was decided to try
a foot-plate which had a flat surface (Fig. 3).
The size of the fontanelle was carefully
con-sidered. It was apparent that the foot-plate had
to be large enough to give sufficient surface to
afford an even application to the scalp
sun-face over the fontanelle. On the other hand, the
size would have to be limited by the area
of the fontanelle, exclusive of 2 or 3 mm of
the periphery of the open fontanelle, where the
tissues are supported by the neighboring hard
bony margins and are not as freely movable
by the pressure from within the skull. Since the
majority of open fontanelles have a width of
some 2 cm from bony m3rgin to bony margin,
it was decided to make the over-all width of the
foot-plate 8 mm so that the entire foot-plate
would rest on soft nonsupported skin and
scalp tissues of the fontanelle, and still leave a
rim of some 6 mm of free skin of the fontanelle
next to the bony edges.
The diameter of the plunger (Fig. 2) was
the next problem. Since the total tissue (skin,
galea, uncalcified membraneous bone and dura)
represents more thickness and volume per
square area than does the thin cornea, it was
believed that the tissue resistance would be
greater and that the plunger should
there-fore be wider than in the eye tonometer. It was
thought that this would cause a more definite
indentation in the scalp oven a broader area,
thus tending actually to overcome the
intra-cranial pressure rather than weighing only
against a small area of tissue resistance. In other
words, it was believed that a narrow plunger
would come in contact with so small an area
that, due to the scalp thickness and consequent
tissue resistance, there would not be enough
resilience to be influenced readily by the
intra-cranial pressure.
Another consideration was the width of the
outer rim of the plate (Fig. 3). It was thought
that this rim should be wide enough to permit
a good application to the scalp surface.
There-fore, since the over-all diameter of the
foot-plate had to be limited to 8 mm, an arbitrary
figure of 2 mm for the rim of foot-plate was
allowed, leaving a 4-mm width to the plunger
through the center of the foot-plate. This is
larger than the usual plunger in the eye
tonom-eter (Fig. 2) where it measures but 2.5 mm in
width. The spread of the same weight over a
larger area was naturally expected to influence
the interpretation of the readings of the
tonom-eter as related to the readings on the eye.
How-ever, since all these readings were really
arbi-trary in terms of difference of tissues, tissue
ne-sistance, etc., as compared to the eye, the
adop-tion of a 4-mm diameter for the plunger was
also quite arbitrary.
With these factors decided, the Schiotz
tonometen was then modified. The weights were
kept very close to those of the standard
tonome-ter. Thus, the plunger with “5.5 gm weight”
actually weighed 5.43 gm. The new instrument,
hereafter referred to as the 0
(Fig. 1) weighed 17.24 gm exclusive of the
handle. This is quite close to the standard
to-0 Manufactured by and available at Matalene
Surgical Instrument Company, Inc., 125 East 46th
T F
Fic. 4. Enlarged view of the foot-plate of the fontanometer (F) as compared
to the original tonometer (T).
ARTICLES
1067Fic. 3. Schematic sketch of the
fontanom-eter. A) \Vider plunger. B) Fiat foot-plate
in cross-section.
nonleter requirement which is 16.5 gm (0.5
gm).
EXPERIMENTAL READINGS WITH THE
FONTANOMETER
Procedure
With the fontanometer, readings were taken
ill a number of newborn infants, since their
t_
-\ J,
fontanelles were large enough to allow such
readings. The technique consisted of holding
the infant in a sitting position, keeping the head
upright so as to place the skin over the
fon-tanelle in as horizontal a plane as possible.
While most of the infants did not have enough
hair to take into account, some with heavier
hair, or stiff curly hair as in some Negro
in-fants, had to have the hair smoothed down
with a little water in order to avoid some
in-consistancy of readings. It may even be
ad-visable to shave the area in some infants,
per-haps some time in advance of the readings.
The instrument was applied with the same
technique as that used in measuring
intra-ocular pressure, keeping the machine in as
vertical a position as possible (perpendicular
to the fontanelle skin). At all times it was
at-tempted to place the foot-plate in the
central-most portion of the fontanelle so as to avoid
proximity to the bony supported edges. In all
cases, three successive readings were taken
about 30 seconds apart. Some identifying
ana-tomic mark on the skin was noted so that the
successive readings could be repeated over
exactly the same portion of the fontanelle.
RESULTS
In a series of normal infants most of the
readings were between 4 and 6 on the
fon-tanometer scale with a 5.5-gm weight, the
average reading being 5.03. Constancy of
readings in the same individuals was also
mdi-( ‘ase IfO.VJ)it(ll Diagnosis
Fonianometer Equivalents of Fontanometer
______________-- Readings
. JJ’eig/it
Readings
(gni) ,nni Jig in rn 11,0
1. B.M.Il.C. #7598() Arrested iiydrocephalus I .5 55 ‘31 .6 429.7
. B.M.il.C.#8I64 Meiiingoeele 55 75 374.()
3. Moiitehore #9945 ilydrocephalus S‘2 .5.5 8 .0 38() .S
350 4. 755 1O.() 3.5.8 4’2 486.8 578.9
4. B.M.H.C. Meningitis 2.6 .5.5 26.4
42. I
359.0
.57’2.5
5. Montefiure Ilydrocephalus .0 7. .5
3.0 10.0 50.6 688.1
(direct reading) 4.0 10.0 43.4 590.
1068 “FONTANOMETER”
TABLE I
1)AT.k FROM FIVE ABNORMAl5 (.‘Asn.
(All cases showed clinical evidence (if increased intracranial pressure and correspondingly abnormally high readings with the fontaiwmeter.)
cation of reliability of readings. The normal
intracranial pressure as measured by the
cerebrospinal fluid pressure upon lumbar
puncture, with a patient horizontal and
re-laxed, is said to range between 80 and 200
mm of cerebrospinal fluid. This method, too,
is an indirect one, and may not represent the
actual state of intracranial pressure. It is
approximately within the same limits at all
ages under normal conditions. Ford’ gives
the range of normal pressures with this
method during childhood as 100 to 200 mm.
Merritt and Fremont-Smith2 give the range
as between 70 and 180 mm, without any
differences at various ages. However, Lups
and Hahn’ give the figures of 40 to 100 mm
for children and 60 to 180 mm for adults.
In any event a reading of 5.1
correspond-ing to 236 n-in-i of water is not very far from
the 180 or 200 mm described by the various
observers as the upper limit for the normal.
In five eases an opportunity was afforded
to apply the fontanometer to infants who
showed clinical evidence of disturbance of
intracranial pressure (Table I).
SUMMARY
The
“fontanometer”
is presented as anadaptation of the Schiotz tonometer to be
used for estimating the relative state of
intracranial pressure in the young infant. Its
standardization and more exact value will
be better understood only after a much
larger series of eases Ilas been studied, and
comparisons are made with direct,
simul-taneous measurement of intraventnicular
pressures with a manometer. In the
mean-time, despite the fact that definite
quantita-live values cannot be ascribed to the
read-ings at present, the instrument is offered as
a simple clinical method of estimating
in-tracranial pressure in the infant without
re-sorting to ventricular on spinal puncture.
Acknowledgment
Acknowledgment is made to Dr. Miguel
Martinez for his technical help in the
construe-tion of the adapted fontanometer, and in
prepa-nation of the schematic sketches in this paper.
REFERENCES
1. Ford, F. R. : Diseases of the Nervous
Sys-tern in Infancy, Childhood and
Adoles-cence, 3rd Ed. Springfield, Thomas, 1952,
p. 1181.
2. Merritt, H. H., and Frernont-Smith, F.:
The Cerebrospinal Fluid. Philadelphia,
Saunders, 1937, p. 330.
3. Lups, S., and Hahn, A. M. F. H. :
Cerebro-spinal Fluid. Amsterdam, Elsevier Pub.
