The pressures in the episcleral veins, Schlemm's canal and the trabecular meshwork in monkeys: effects of changes in intraocular pressure

Abstract

The pressures in the episcleral veins, Schlemm's canal and the trabecular meshwork were studied with a micropuncture technique using cannulas with tip diameters of less than 5 microns. The pressure in Schlemm's canal, Psc, was 14.3 +/- 1.0 cmH2O at spontaneous intraocular pressure, IOP, 19.2 +/- 0.9 cmH2O. The outflow pressure from the anterior chamber to Schlemm's canal was 4.9 +/- 0.7 cmH2O. The relationship between pressures was IOP = 0.73 Psc + 8.7. When the intraocular pressure was increased stepwise from the spontaneous level to 30 cmH2O there was an increase in pressure in Schlemm's canal of 1.7 +/- 0.6 cmH2O, (P less than 0.05). The total outflow resistance and the resistance between the anterior chamber and Schlemm's canal were 3.27 +/- 0.43 and 2.92 +/- 0.50 cmH2O min microliter-1 respectively for the intraocular pressure interval between the spontaneous pressure and a level 4-11 cmH2O higher. In the intraocular pressure range from 20 to 30 cmH2O the corresponding figures were 2.89 +/- 0.45 and 2.69 +/- 0.42 cmH2O min microliter-1 and for the pressure range 25-35 cmH2O, 2.48 +/- 0.58 and 2.31 +/- 0.59 cmH2O min microliter-1. The difference between the total outflow resistance and that between the anterior chamber and Schlemm's canal was about 10% of the total at intraocular pressures below 35 cmH2O. Stepwise increments in IOP increased the trabecular meshwork pressure by 0.88 cmH2O for each cmH2O increase in IOP in the interval of 30-50 cmH2O. The total outflow resistance and the resistance between the anterior chamber and the tip of the microcannula was 3.91 +/- 1.53 and 1.94 +/- 0.99 cmH2O min microliter-1 respectively for the intraocular pressure interval between the spontaneous pressure and 30 cmH2O. In the interval between 30 and 45 cmH2O the corresponding figures were 2.16 +/- 0.66 and 0.20 +/- 0.13 cmH2O min microliter-1. The episcleral venous pressure at the spontaneous intraocular pressure was 14.1 +/- 1.0 cmH2O in seven animals with minimal trauma, and 12.3 +/- 0.8 cmH2O in animals after cannulation of Schlemm's canal. The outflow pressure from the anterior chamber to the episcleral veins was 4.4 +/- 1.2 cmH2O in animals with minimal trauma and 7.1 +/- 0.8 cmH2O after cannulation of Schlemm's canal. The relationship between pressures was IOP = 0.68EVP + 11.(ABSTRACT TRUNCATED AT 400 WORDS)