Comparison of perilimbal suction cup analysis and tonography in the diagnosis of glaucoma.

IN the past few years there has been increasing interest in the use of the pressure cup technique for the investigation of aqueous humour dynamics. Rosengren (1956) demonstrated a rise of intra-ocular pressure when a positive pressure was applied to the sclera overlying Schlemm's canal. He suggested that this rise of pressure was due to continued formation and accumulation of aqueous humour, the escape of which was prevented by occlusion of the outflow channels. The scleral pressure was exerted by a plastic cup with a flange fitting over the limbal region and held in position by vacuum. It has been shown that outflow of aqueous can be prevented by the application of a negative pressure of 50 mm. Hg in the cup, thereby creating a pressure of 28 mm. Hg on the sclera, provided that the intra-ocular pressure does not exceed 30 mm. Hg (Langham, 1963). From the induced rise of pressure several investigators have estimated the rate of aqueous formation. Such values have tended to be lower than with other methods. This may have been due to escape of aqueous as the intra-ocular pressure builds up, or to suppression of aqueous formation with rising intra-ocular pressure. It is also possible that the suction -cup flange may interfere with blood flow in the anterior segment and consequently affect aqueous formation. Considerable interest attaches to the features of the pressure decay curve following removal of the cup. From this and from the induced rise of pressure, Ericson (1958) claimed that it is possible to calculate the rate of aqueous formation, the coefficient of facility of outflow, rate of aqueous drainage, and the outflow venous pressure. Assuming that the intra-ocular pressure will decay at a rate dependent on the outflow facility of the eye, Galin, Baras, and Nano (1962) introduced the "percentage pressure decay" (P.P.D.) as an estimate of outflow resistance. The purpose of this paper is to report on a comparison between outflow values obtained by the suction cup technique and by tonography. In particular, it was intended to discover: (1) Whether the intra-ocular pressure fell exponentially to the level existing before application of the suction cup. (2) Whether there was any variation between the values of C calculated for different segments of the decay curve.