A simple manometer for the continuous measurement of intra-ocular pressure.

THE adequate measurement of the intra-ocular pressure, both clinically and experimentally, has remained a baffling problem. The tonometer has obvious disadvantages whiclh have been summarized by Duke-Elder (1932). Chief amongst these, in the opinion of the present authors, is the possibility of self-deception, since the variations in the results of repeated determinations are often quite large and there is always a danger that the investigator will choose unconsciously the value expected. Manometric methods involve the insertion of a needle into the anterior chamber; this of itself may upset the normal phvsiology of the eye, although unequivocal evidence that it does so is lacking. Moreover the investigator is ignorant of the pressure within the eye before inserting the needle, so that to prevent loss or gain of fluid by the eye, he must arbitrarily choose a certain pressure of saline within the needle to minimize these changes An incorrect choice may disturb the normal mechanism of production and drainage of aqueous humour, and there is thus some uncertainty regarding the physiological condition of the eye on this count and greater uncertainty regarding the true value of the intra-ocular pressure in the eye before the needle was inserted, since the pressure imposed on the eye by the saline in the needle may persist for some time. Again, the mechanical registration of changes in the intra-ocular pressure requires that fluids either leave the eye or enter it; this may be largely cut down by a compensatory device in which the excursions of a minute bubble of air inthe recording system are kept to a minimum; the operation of such a compensator is, however, tiresome, and a membrane-type manometer, in which very small changes in the volume of fluid in the sy$tem are magnified into large excursions of a reflected beam of light, is preferable. The instrument'to be described here is of this type and gives a deflection of some 3 to 4 in. of a light spot on a scale, 5 feet away, in response to a rise in pressure of 10 cm. H20; the fluid loss involved in this reflection is only 8 mm. corresponding to some 2 to 3 minutes' production of aqueous humour if present estimates of this rate are acceptable; it is sufficiently sensitive to show the normal pulsation corresponding to the arterial systole.