EPISCLERAL VENOUS PRESSURE IN TONOGRAPHY * t BY ARTHUR

DURING the 11 years in which the clinical measurement of facility of outflow by means of tonography has been practised, various workers have sought to elucidate and correct the sources of error inherent in this method. One of the assumptions originally made concerning tonography was that the episcleral venous pressure was not disturbed during the measurement. Friedenwald (Grant, 1951) and Becker and Friedenwald (1953) drew attention to the possibility that changes in the episcleral venous pressure might constitute a source of error, and estimated that the weight of the tonometer might raise the intra-orbital pressure by almost 10 mm. Hg. The subsequent congestion of the orbital veins would then bring about a rise in the episcleral venous pressure, with consequent diminution of the pressure gradient from the anterior chamber to the episcleral veins. Failure to allow for this effect would cause the facility of outflow to be underestimated. The episcleral venous pressure in the normal eye has been measured by several workers and has been found to be in the region of 10 or 11 mm. Hg (Goldmann, 1950, 1951; Linner, 1955a; Linner, Rickenbach, and Werner, 1950; Lohlein and Weigelin, 1949; Rickenbach and Werner, 1950). The first measurement of the disturbance in episcleral venous pressure due to the application of the tonometer to the eye was reported by Linner (1955b). He measured the episcleral venous pressure before, during, and after the application of the tonometer to the eyes of recumbent subjects, and found that the application of the tonometer provoked a mean rise of 1 25 mm. Hg in the episcleral venous pressure. Linner suggested that allowance could be made for this by modifying Grant's formula for facility of outflow