Glaucoma is one of the leading causes of blindness, despite its infrequency of occurrence. The most effective means of detecting early glaucoma in humans is the fundus examination, which is too difficult to perform in animals. What is the most effective test for experimental animals? It must be an objective test. Tonometry seems to be an effective way of determining a glaucomatous condition in animals, and it is easy to use, has, less need of anesthesia, and is gentle to the eyes. In this paper, I give a short history of the principles of some kinds of tonometers, and I recommend the Tono-Pen as an adequate.

To select the optimum type of tonometers for laboratory animals, we tried to record intraocular pressure of Crj: CD-1 (ICR) mice and Crj: CD (SD) rats using a Keeler pulsair, a Topcon tonometer CT-30, a Perkins hand-held applanation tonometer, an Alcon applanation pneumatonograph, and a Tono-Pen XL.

The Keeler pulsair did not show focuses in the operation zone on mice and could focus in the operation zone but not work automatically on rats. The Topcon tonometer CT-30 did not show the alignment points on mice and could not control two points within the inner alignment circle on rats. The Perkins hand-held applanation tonometer did not show half circles of fluorescence on both rodents. The Alcon applanation pneumatonograph was unusable because of the difficulty in immobiliging the animals and the size of the sensor tip on both animals. Although the Tono-Pen XL did not show data with less than 5% error on mice, it could show data with less than 5% error on rats. We perform the repeated observations for 5 days on 5 rats using the same observer. It took 45 to 90 minutes to complete testing of both eyes of 5 rats (5 times/rat/day). The data varied widely within and between animals and days. Furthermore, the value (total mean value 36.6±8.8 mmHg) was somewhat higher than that in references.

We conclude that these tonometers cannot be used for recording intraocular pressure in mice and rats.

Five models of tonometers being used currently in human ophthalmological clinics were tested for applicability in the study with rabbits.

Ten male Japanese white rabbits aged 13 weeks were used. The tonometers tested were 3 models of the contact type (ALCON, TONO-PEN and PERKINS) and 2 models of the non-contact type (TOPCON and KEELER), which the Japanese Society of Comparative Ophthalmology arranged for the cooperative study group headed by Dr. Ueda of JRA.

The intraocular pressure (IOP) was measured on the corneal surface of both eyes, 3 times a day, in the morning, early afternoon and evening for 3 successive days, using each of the 5 tonometers.

All tonometers except PERKINS could be used for measurement of rabbit IOP. From a practical point of view, TONO-PEN and KEELER were the most conveniently handled, while ALCON required much skill to manipulate. TOPCON was of little practical value because of its inconvenient shape.

The results of statistical analyses of IOP data obtained by the 4 useful models of tonometers suggested the existence of a circadian rhythm of IOP in rabbits. The recording with the 3 tonometers other than TONO-PEN showed a circadian rhythm with the same tendency to elevation in the evening during the 3-day observation period. TONO-PEN recorded no consistent pattern during the observation period.

If the circadian rhythm in IOP really exists in rabbits, the irregular recording with TONO-PEN may indicate some problem in reliability of this model. Of the non-contact type tonometers tested, TOPCON and KEELER provided fairly stable data, but lower IOP values than those reported in the literature. In addition, the shape of the TOPCON model and the function of the KEELER model made than inconvenient to use on rabbits.

We concluded that the ALCON model is the best of the tonometers tested in the present experiments from the viewpoint of reliability of data, although it has some problems in operation.

To determine the optimal tonometer for dogs, the Keeler pulsair, Perkins hand-held applanation tonometer MKII, Tono-PenXL and Alcon applanation pneumatonograph (AlconPTG) were compared using male beagles. The primary emphasis was centered on determining 1) the characteristics of the four tonometers and if they are applicable to dogs, 2) if variation of intraocular pressure (IOP) is observed among multiple operators, 3) if variation of IOP is observed by a single operator when measured over five days, 4) if there are any variations of IOP in different age groups of dogs, and 5) if there are any correlation among tonometers.

Tono-PenXL and AlconPTG were applicable to dogs, and Tono-PenXL was superior in handling when IOP was measured in the animal room. The variation in IOP obtained by five operators using these two tonometers was minimal and no statistically significant difference was noted in either tonometer. The coefficient of variation of IOP over five days was smaller in Tono-PenXL than in AlconPTG, indicating that IOP determined by Tono-PenXL was more stable in repeated measurements. IOP obtained by Tono-PenXL showed no statistically significant difference among 9, 32 and 45-month-old dogs. Statistically significant positive correlation (r=0.550, p<0.001) was observed between Tono-PenXL and AlconPTG.

We concluded that of four tonometers tested, Tono-PenXL was the most applicable to dogs.

To determine an applicable tonometer for measuring the intraocular pressure (IOP) in cynomolgus monkeys, the properties of six kinds of tonometer were compared. The IOP was measured in a total of 53 cynomolgus monkeys varying in age from 10 weeks to 20 years old, which were bred and kept at Tsukuba Primate Center for Medical Science, NIH Japan, using 6 commercially available tonometers: SCHIϕTZ, ALCON, TONO-PEN, PERKINS, TOPCON, and KEELER.

The mean IOP of clinically normal cynomolgus monkeys was 16.4±3.1 (range: 9.8-24.4) mm-Hg, using SCHIϕTZ, 16.1±3.9 (7.3-24.8) mm-Hg, using TONO-PEN and 15.2±4.3 (6-28) mm-Hg, using KEELER. Statistically significant correlations were obtained between IOP determined by TONO-PEN and SCHIϕTZ (r=0.771, p=0.0001), by TONO-PEN and KEELER (r=0.590, p=0.0001), and by SCHIϕTZ and KEELER (r=0.355, p=0.0006). In conclusion, it was judged that SCHIϕTZ, TONO-PEN and KEELER were applicable to the measurement of IOP in cynomolgus monkeys on the basis of their utilizability as well as the accuracy of results obtained.

In pilot studies conducted on Holstein-Friesian calves and adult cows with clinically normal eyes, we tested the measurement capabilities of the Perkins tonometer, the Keeler Pulsair tonometer, the Tono-Pen XL tonometer (TONO-PEN), and the Alcon Applanation Pneumatonograph (ALCON). These pilot studies showed that the TONO-PEN and the ALCON can be used for eye measurements in cattle. We subsequently measured 5 adults and 7 calves using the TONO-PEN and the ALCON.

Experiment 1: Measurements were taken by three persons (A, B, and C) in the sequence TONO-PEN (1)→ALCON→TONO-PEN (2).

Experiment 2: Measurements were taken by one person (A) in the sequence ALCON (1)→TONO-PEN→ALCON (2).

An average of 4 measurements with each tonometer were taken of the right and left eye alternately in both experiments. The average measurements taken by person A using the TONO-PEN (1) and the ALCON (1) were 23.4±5.9 mmHg and 12.1±3.5 mmHg, respectively, for adult cows and 17.2±4.0 mmHg and 10.9±3.5 mmHg, respectively, for calves. Measurements using the TONO-PEN were consistently higher than those using the ALCON in experiments 1 and 2. The values were more scattered in the TONO-PEN measurements and among the measurements taken by different individuals than in the measurements taken with the ALCON. The correlation of TONO-PEN and ALCON measurement values was low. At the time of rumination, measurements using the TONO-PEN were possible, but the rate of error was large. Measurements could not be taken by with ALCON.

In pilot studies of clinically normal eyes in adult horses, we tested the measurement capabilities of the Perkins tonometer, the Keeler Pulsair tonometer, the Tono-Pen XL tonometer (TONO-PEN), and the Alcon Applanation Pneumatonograph (ALCON). These pilot studies showed that the TONO-PEN and the ALCON can be used for eye measurements in horses. We therefore used the TONO-PEN and the ALCON in the following experiments.

Non-anesthetized horses:

Experiment 1:6 eyes of 3 adult thoroughbreds were measured. Measurements were taken by three persons (A, B and C) in the sequence TONO-PEN (1) →ALCON→TONO-PEN (2).

Experiment 2:8 eyes of 3 adult thoroughbreds and 1 heavy draught horse were measured. The measurements were taken by one person (A) in the sequence ALCON (1)→TONO-PEN→ALCON (2).

An average of 4 measurements with each tonometer were taken of the right and left eye alternately in both experiments. The average measurements taken by person A using the TONO-PEN (1) and the ALCON (1) were 29.6±6.2 mmHg and 15.1±3.0 mmHg, respectively.

In both experiments (1 and 2), measurements taken with the TONO-PEN were greater than those taken with the ALCON. The values were more scattered in the TONO-PEN measurements and among the measurements taken by different individuals than in the measurements taken with the ALCON. The correlation of the measurement values for TONO-PEN (2) and ALCON was 0.539.

Anesthetized horses:

8 eyes of 3 adult thoroughbreds and 1 heavy draught horse were measured. TONO-PEN and ALCON measurements were taken ten minutes after administration of xylazine 0.7mg/Kg IV. Both tonometer readings showed a decrease in eye pressure of about 32-33%.

Fluorescein angiography of the ocular fundus in rats was performed using a new portable ophthalmoscopic camera (Genesis; Kowa Optimed. Inc.). Seventeen Wistar male rats weighing 200-500 g, were used under general anesthesia. Ten percent fluorescein solution was injected intravenously into the external jugular vein with a disposable winged infusion set. The angiogram was subdivided into three major phases, i.e., the retinal arterial phase, the retinal arteriovenous phase, and the retinal venous phase. In these albino rats the choroidal phase could not be recorded. However, this technique of fluorescein angiography could be useful in assessing cinephotographic measurement of retinal circulation time in rats as well as in other mammals.