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 cmH2O and the Schlemm's canal pressure (PSc wwas 7·6 ± 0·7 cmH2O, the pressure at 7 μm from the inner wall of Schlemm's canal was found to be 8·9 ± 0·7 cmH2O and at a distance of 14 μm, 11·0 ± 0·5 cmH2O-that is, 1·3 ± 0·2 and 3·4 ± 0·3 cmH2O respectively, higher than the PSc. In another set of experiments, the spontaneous IOP and PSc 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 cmH2O and the PSc was 11·5 ± 1·4 cmH2O before the IOP was increased. After the IOP was increased to 20·2 ± 1·2 cmH2O, the PSc was 11·7 ± 1·6 cmH2O. 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 cmH2O-that is, 5·2 ± 0·8 and 7·2 ± 1·0 cmH2O respectively, higher than the PSc. 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. © 1992.
