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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 microns was used to measure the pressure in Schlemm's canal and in the meshwork at distances approximately 7 and 14 microns 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) was 7.6 +/- 0.7 cmH2O, the pressure at 7 microns from the inner wall of Schlemm's canal was found to be 8.9 +/- 0.7 cmH2O and at a distance of 14 microns, 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 microns from the inner wall of Schlemm's canal the pressure measured at 7 microns was 16.9 +/- 1.3 and at 14 microns 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 microns from the canal with some 50% being located within the region 7 and 14 microns from the canal. After a small increase in IOP, the tissue causing most of the outflow resistance became relocated to a region within 7 microns from the canal.
Insights
This study measured pressure in Schlemm's canal and surrounding tissues, revealing that most resistance to aqueous humor outflow occurs near the canal. Increased intraocular pressure shifted this resistance closer to Schlemm's canal.
Area of Science:
- Ophthalmology
- Fluid Dynamics
- Biomedical Engineering
Background:
- Intraocular pressure (IOP) regulation is crucial for ocular health.
- Understanding the pressure gradients within the anterior eye is key to identifying mechanisms of aqueous humor outflow resistance.
- The juxtacanalicular tissue and Schlemm's canal are critical sites for regulating IOP.
Purpose of the Study:
- To precisely measure pressure within Schlemm's canal (PSc) and the adjacent meshwork at varying distances from the inner wall.
- To investigate how changes in intraocular pressure affect pressure distribution and resistance in the outflow pathway.
- To localize the region of highest resistance to aqueous humor outflow.
Main Methods:
- Utilized a micropuncture technique with microcannulas (tip diameter < 5 microns) for precise pressure measurements.
- Measured pressures in Schlemm's canal and at 7 and 14 microns from its inner wall under spontaneous and elevated IOP conditions.
- Correlated pressure readings with known intraocular pressure (IOP) and Schlemm's canal pressure (PSc).
Main Results:
- Under spontaneous IOP (12.2 cmH2O), pressures at 7 and 14 microns were significantly higher than PSc (7.6 cmH2O), indicating substantial resistance in this region.
- With elevated IOP (20.2 cmH2O), pressures at 7 and 14 microns were also elevated relative to PSc (11.5 cmH2O), but the distribution of resistance shifted.
- Approximately 75% of outflow resistance was located within 14 microns of Schlemm's canal, with 50% between 7 and 14 microns at baseline IOP.
Conclusions:
- The juxtacanalicular tissue, particularly the region 7-14 microns from Schlemm's canal, is a major site of resistance to aqueous humor outflow at normal IOP.
- Elevating IOP appears to alter the location of peak resistance, potentially concentrating it closer to Schlemm's canal (within 7 microns).
- These findings provide critical insights into the biomechanics of aqueous humor outflow and IOP regulation.