Hand-Held Goldmann-Type Tonometers
Perkins Tonometer
Perkins tonometer (Fig. 6.8) uses same prisms as Goldmann but is counterbalanced so that tonometry is performed in any position (Fig. 6.9). The prism is illuminated by battery powered bulbs. The force on the prisms is adjusted manually. Being portable it is practical when measuring IOP in infants / children and for use in operating rooms.
Fig. 6.8: Perkins tonometer
Draeger Tonometer
Draeger tonometer is similar to Perkins but uses different set of prisms and operates with a motor adjusting the force on these prisms.
Mackay-Marg Tonometer
Basic concept: Force is required to keep the flat
plate of a plunger flush with a surrounding sleeve against the pressure of corneal deformation. Tonometer incorporates a 1.5 mm diameter plunger affixed to a rigid spring that extends 10 μm beyond the plane of surrounding rubber sleeve. Movement of plunger is electronically monitored by a transducer and recorded on a moving paper strip. When the tonometer is placed against cornea, the tracing that represents the force applied to the plunger begins to rise. At 1.5 mm of corneal area applanation, tracing reaches a peak and the force applied = IOP + force required to deform the cornea. At 3 mm flattening, force required to deform cornea is transferred from plunger to surrounding sleeve, creating a dip in tracing corresponding to IOP. Flattening of >3 mm of area gives artificial elevation of IOP. It is accurate in eyes with scarred, edematous and irregular corneas.
Other Mackay-Marg-type Tonometers: CAT 100 Applanation and Biotronic Tonometers
They have an internal logic program which automatically selects the acceptable measurement and 3 or more good IOP readings are averaged and displayed on screen.
Tonopen
Tonopen (Fig. 6.10) is a portable and battery operated tonometer. It has the same principle as that of Mackay-Marg tonometer. The tip has a strain gauge that is activated when in contact with cornea. The built-in microprocessor logic circuit senses a trough force and records until an acceptable measurement is achieved. Four to ten such measurements are averaged to give a final IOP which is displayed.
The probe tip is applied perpendicularly to cornea until it is just indented. An audible click indicates that the measurement is acceptable. The process is repeated 4-10 times until a beep indicates a statistically valid average reading. Pneumatonometer
Pneumatonometer or pneumatic tonometer is like Mackay-Marg tonometer. It has a core sensing mechanism for measuring IOP while force required to bend the cornea is transferred to surrounding structure. The sensor is a air pressure like electronically controlled plunger in Mackay-Marg tonometer. It can also be used for continuous monitoring of IOP. It gives significantly higher IOP estimates.
Constant Force Applanation Tonometry Maklakov Applanation Tonometer With Maklakov applanation tonometer IOP is estimated by measuring the area of cornea flattened by a known weight. It consists of a dumb-bell-shaped metal cylinder with flat end plates of polished glass on either end with a diameter of 10 mm. Tonometers weighing 5, 7.5, 10, and 15 gm are used to measure the IOP. Cross- action wire handle to support instrument on the cornea is used. A thin layer of dye is spread onto the bottom of either end plate and the instrument is brought in contact with anesthetized cornea in supine position for 1 second. A circular white imprint on end plate corresponds to the area of corneal flattening. Area is measured and IOP is read from conversion table in the column corresponding to the weight used.
Noncontact Tonometer
Noncontact tonometer (NCT) was introduced by Grolman. A puff of room air creates a Fig. 6.10: Tonometry with tonopen
constant force that momentarily flattens the cornea. The time from an internal reference point to the moment of flattening is measured and converted to IOP. The corneal apex is deformed by a jet of air. The force of air jet which is generated by a solenoid activated piston increases linearly over time.
Fig. 6.11: Tonometry with noncontact tonometer
Original NCT has 3 subsystems:
1. Alignment system: It aligns patient’s eye in 3 dimensions.
2. Optoelectronic applanation monitoring system: It comprises transmitter, receiver and detector, and timer.
a. Transmitter directs a collimated beam of light at corneal apex.
b. Receiver and detector accept only parallel coaxial rays of light reflected from cornea.
c. Timer measures from an internal reference to the point of peak light intensity.
3. Pneumatic system: It generates a puff of room air directed against cornea.
When the reflected light is at peak intensity, the cornea is presumed to be flattened. The time elapsed is directly related to the force of jet necessary to flatten the cornea and correspon- dingly to IOP. NCT is accurate if IOP is nearly
normal, accuracy decreases with increase in IOP and in eyes with abnormal cornea or poor fixation. New NCT, Pulsair is a portable hand held tonometer.
Devices under Investigation
Flush fitting silastic gel contact lens instrumented with strain gauges that measures changes in meridional angle of corneoscleral junction caused by variations in IOP. A similar device using a pressure transducer is made in form of a cylindrical guard ring applanation tonometer. A scleral gauge is embedded in an encircling scleral band to measure the distension of globe. An instrument using suction cups for recording IOP up to 1 hour in supine position is under investigation.