An accurate tonometer has been described for measuring the pressure within the human eye, and within various other body cavities (1,2). It has an exact theory, can be used in any orientation, and is sufficiently fast and gentle so that it does not require anesthesia for application to the human cornea. The probe of this unit is pressed against the eye and, in essence, if the cornea is momentarily flattened to slightly beyond a pressure sensitive spot on the tip of the flat end of the probe, then the only force sensed will be the intraocular pressure. However, satisfactory results were not obtained on application to sclera. This has now been demonstrated to depend in part on the adjustment of the degree of flatness of the probe end.
The pressure-sensitive spot in the centre of the probe is a rod the end of which is coplanar with the surrounding plate, and which activates a differential transformers (2). For reasons of cleanliness, the unit is encased in a thin sterilized cover before use. If the output of the instrument as a function of pressure in a test cavity is plotted (Fig. 1), one sees that the result depends on the initial perfection of the plunger setting. Fig. 1 shows the result of application of the tonometor to a condom at the base of a water column, hut similar curves are obtained with excised rabbit eyes and with rabbit eyes in vivo. If the plunger initially projects beyond the base plate then the cornea and/or the rubber covering will effectively increase the area of the plunger until increasing pressure moulds them down into full contact with the projection. Similarly, the rubber covering can sustain some of the forge if the plunger is initially recessed, until it has been fully moulded down into the recess. This leads to curves that are concave downward and upward respectively, as shown. Corneal readings are usually lower than scleral in the former case, and vice versa. This indicates the solera to be the less elastic. All the curves eventually become parallel when just the actual area of the plunger is effective. It can be seen that, for a 1 5-mm. diameter plunger, a 5u initial projection is ideal. A tonometer when adjusted this way indicates its plunger to be projecting 5u when pressed into contact with a flat glass plate. This may apply only to a given method of construction in which irregularity can extend the plunger surface above and below the surrounding plate, thus leaving the average almost coplanar.
Fig. 1. Response of Mackay-Marg tonometer with initial plunger
extension as parameter. Curves concave upward result if the
plunger is initially recessed, as shown by the sketch in the
lower right corner, and the curve is concave downward if the
plunger is initially extended as shown by the sketch in the upper
left corner
A unit tested by the water column measurement to be linear through the origin is often found to give the same reading when applied to the cornea or to the sclera in man and rabbits. However, in a significant number of human subjects scleral and corneal readings sometimes do not agree. Tests were made on a dozen subjects using two tonorneters (a type manufactured by Biotronics, Financial Center Building, Oakland, California) simultaneously on a single eye, one tonometer being applied to the temporal sclera and the other to the nasal sclera, or one to the cornea and one to the sclera. These readings were often identical, but not always so. This is probably due to the irregular and somewhat compressible and viscoelastic structure of the sclera. We have noticed occasional unreliable readings in similar larger instruments applied to the abdomen due to inhomogeneities (fat lumps) in the wall, presumably for similar reasons. A larger plunger might help here.
Scleral application was desired so that it would be possible to look into the eye while pressing. Since the end of the probe need be only about 5 mm. in diameter, it can instead be applied to the side of the cornea near the limbus and still leave room to observe the eye through an ophthalmoscope. Raising the pressure within the eye to diastolic by pressing will cause the appearance of appreciable pulsations in the blood vessels, and further increase in pressure to systolic will cause the disappearance of pulse due to collapsing of these vessels. This tonometer accurately records the pressure within the eye during such a procedure. It is observed that different regions may start and stop pulsating at somewhat different pressures. A discussion of these pressures has appeared elsewhere, and the interference with the state of normality that measurement produces (3). A clinical criterion of insufficiency on one side is often taken to be a 25 per cent difference in systolic pressure between the two eyes. The use of a less-exact tonometer and spring-produced pressure is sometimes termed ophthalmodynamometry. A survey of values will be reported separately, but it can be stated here that, of the first half-dozen subjects tested, the diastolic and systolic pressures fell very roughly in the ranges of 50 and 100 mm. of mercury. In all such observations it is found that pressure does not distort the cornea sufficiently to make ophthalmoscopic observation impossible past the de-centred tonometer. Alternatively, one can record the pressure at which the subject's vision disappears. One can also apply the tonometer centered on the cornea with steadily increasing pressure, and in the record the points of maximum oscillation and cessation of oscillation can be taken as the diastolic and systolic pressures, respectively. However, it is not clear to which vessels these readings apply. A suction method (4) can be used to forge contact between the present tonometer and the eye to reduce the effect of extraocular vessels, if visual inspection is not being used.
The pressures in the blood vessels are usually considerably larger than the intraocular pressures observed in tonometry. Because of the eventual parallelism of the curves in Fig. 1, pressure indication errors due to lack of coplanarity tend to be constant in the absolute rather than the percentage sense, and thus adjustment is less critical than for tonometry. It has been found that with the Mackay-Marg tonometer when anesthetics are not used a somewhat greater variability in pressure occurs if anesthetics are used. The operator must select a relaxed instant so that the pressure, which will be accurately measured, is not artificially raised by squinting, etc. The presence of corneal bending forces puts a dip in the recorded pressure vs. time curve that is an excellent indicator of the proper degree of oorneal flattening, and hence shows where on the graph the reading should be made. With the best possible centering of the probe on a rabbit eye, the pressure corresponding to this trough will be raised by less than 0 5 mm. mercury due to the observation process. Accidental de-centering of the probe by 1.5 mm. will almost cause the loss of the characteristic tonogram form, and the actual pressure in that ease will be raised by 4-5 mm. mercury. (All pressures in these experiments were measured with a low compliance capacitance manometer coupled to the eye through a hollow needle.) Holding the probe at an improper angle as it approaches the eye exercises a smaller effect. For angles up to 10° away from the normal to the surface of the eye the pressure within the eye will be raised by less than 1 mm. mercury at the trough point where the reading is to be taken. The effect of an error in application is probably less with a human eye, though the clip in a rabbit tonogram is often less pronounced than in man, thus implying their corneas to be the less stiff. All known errors raise the indication so high pressures are not missed.
We thank Mr. J. Gould for help in making some of the observations. This work was supported by Aerospace Medical Laboratory Contract AF33(616) 7664.
R.S. MACKAY
E. MARG
R. OECHSLI
School of Optometry, University of California, Berkeley, 4.
1. Mackay, R.S., Marg, E., and Oechsli, R., Science,
131, 1668 (1960).
2. Marg, E., Mackay, E.S., and Oechsli, R., Vision Res.,
1, 379 (1962).
3. Duke-Elder, W.S., Textbook of Ophthalmology,
1, 420 (H. Kimpton Co., London, 1932).
4. Yamamori, A., Jap. J. Ophthal., 4,
175 (1960).