Pressures in the juxtacanalicular tissue and Schlemm's canal in monkeys

Abstract

A micropuncture technique involving the use of microcannulas with tip diameters less than 5 μm was used to measure the pressure in Schlemm's canal and in the meshwork at distances approximately 7 and 14 μm from the inner wall of Schlemm's canal. In one set of experiments where the spontaneous intraocular pressure (IOP) was 12·2 ± 0·5 cmH 2O and the Schlemm's canal pressure (P Sc wwas 7·6 ± 0·7 cmH 2O, the pressure at 7 μm from the inner wall of Schlemm's canal was found to be 8·9 ± 0·7 cmH 2O and at a distance of 14 μm, 11·0 ± 0·5 cmH 2O—that is, 1·3 ± 0·2 and 3·4 ± 0·3 cmH 2O respectively, higher than the P Sc. In another set of experiments, the spontaneous IOP and P Sc were also measured and then the IOP was increased by means of an external reservoir and measured once again. Spontaneous IOP was 16·0 ± 1·3 cmH 2O and the P Sc was 11·5 ± 1·4 cmH 2O before the IOP was increased. After the IOP was increased to 20·2 ± 1·2 cmH 2O, the P Sc was 11·7 ± 1·6 cmH 2O. When the microcannula was introduced into the juxtacanalicular tissue to locations at about 7 and 14 μm from the inner wall of Schlemm's canal the pressure measured at 7 μm was 16·9 ± 1·3 and at 14 μm it was 18·9 ± 1·4 cmH 2O—that is, 5·2 ± 0·8 and 7·2 ± 1·0 cmH 2O respectively, higher than the P Sc. The results indicate that at the spontaneous IOP about 75% of the resistance between the anterior chamber and Schlemm's canal is located within 14 μm from the canal with some 50% being located within the region 7 and 14 μm from the canal. After a small increase in IOP, the tissue causing most of the outflow resistance became relocated to a region within 7 μm from the canal.