(A)

(B)

(C)

Fig. 4-6  (A) Pneumotonometer tip on eye. (B) Pneumotonometer base unit with high IOP reading and real time recording. (C) Pneumotonometer base unit with IOP reading and real time recording.

close together results in decreasing IOP readings; this effect seems to be lost after two minutes.80

Non-contact tonometer

The non-contact tonometer applanates the cornea by a jet of air, so there is no direct contact between the device and the surface of the eye. This theoretically avoids the need to sterilize the instrument, but a recent study found the air puff produces a tear film aerosol that could potentially contain infectious material.81 The force of the air jet increases rapidly and linearly with time.The instrument also emits a collimated beam of light that is reflected from the central cornea and then received by a photocell. When an area of the cornea 3.6 mm in diameter is flattened, the light reflected to the photocell is at a maximum.The time required to produce the peak

reflection is directly related to the force of the air jet and thus to the counterbalancing IOP.82–84

The non-contact tonometer is useful for screening programs because it can be operated by non-medical personnel, it does not absolutely require topical anesthesia and there is no direct contact between the instrument and the eye. The IOP readings obtained with the non-contact tonometer correlate fairly well with readings taken by Goldmann tonometry, but differences of several millime-

ters of mercury are not unusual, particularly with pressures higher than the low 20s.85–88 The tonometer can be used without topical

anesthesia, but it is more accurate with anesthesia.The patient should be warned that the air puff can be startling, even after topical anesthetic.89 The non-contact tonometer measures IOP over very short intervals, so it is important to average a series of readings.90 The instrument has an internal calibration system. Several newer itera-

tions which have increased the popularity of this type of tonometry have appeared in recent years.91–94 The newer breed of units

seem to be more comfortable for patients as well as improving the

accuracy (at least as compared to Goldmann applanation tonometry (GAT)).95–97 One unit has software that allows indication and

measurement of pulse amplitude.98

However, not all studies have shown accuracy compared to Goldmann tonometry.99 In general, at least three but preferably four readings should be obtained on each eye.100 The accuracy of the non-contact tonometer in post-keratoplasty patients has been called into question.101

One interesting adaptation of the non-contact tonometer is in the new Reichert Ocular Response Analyzer™ (Reichert Ophthalmic Instruments, Depew, NY, USA).This device is basically an air puff tonometer that directs the air jet against the cornea and measures not one but two pressures at which applanation occurs – when the air jet flattens the cornea as the cornea is bent inward and as the air jet lessens in force and the cornea recovers (Fig. 4-7). The first is the resting intraocular pressure.The difference between the first and the second applanation pressure is called corneal hysteresis and is a measure of the viscous dampening and, hence, the biomechanical properties of the cornea. The biomechanical properties of the cornea are related to, but not the same as, corneal thickness and include elastic and viscous dampening attributes. It is thought that central corneal thickness is just one attribute that contributes to the biomechanical properties of the cornea.

Clinically, the IOPs as measured by the Ocular Response Analyzer (ORA) correlate well with Goldmann tonometry but, on average, measure a few millimeters higher since the device seems to be less dependent on central corneal thickness than the Goldmann applanation tonometer.102 Furthermore, while IOP varies over the 24-hour day, hysteresis seems to be stable.103 Congdon et al found that a ‘low’ hysteresis reading with the ORA correlates with progression of glaucoma, whereas thin central corneal thickness correlates with glaucoma damage.104 Not all studies have been impressed with the accuracy of this device.103 Whether the concept of corneal hysteresis, while showing promise based on early studies, will ultimately become of practical value in the management of glaucoma remains to be demonstrated.

The Ocuton™ tonometer

The Ocuton™ (Elektronik & Präzisionsbau Saalfeld GmbH, Jena, Germany) is a hand-held tonometer that works on the applanation principle using a probe that is so light that it is barely felt and, therefore, needs no anesthetic in most patients. It has been marketed in Europe for home tonometry (Fig. 4-8).The device is comparable to Goldmann tonometry but tends to read higher than the Goldmann tonometer when the cornea is thicker, and its accuracy may be