Variation in Schlemm's Canal Diameter and Location by Ultrasound Biomicroscopy

Purpose: To measure variations in diameter and location of Schlemm's canal in vivo by ultrasound biomicroscopy. Design: Prospective, single-institution, consecutive case series. Participants: Ninety-four patients with and without glaucoma. Methods: Under topical anesthesia, an 80-MHz iUltrasound probe (iScience Interventional, Inc., Menlo Park, CA) placed at the 12-o'clock position was used to measure the canal's diameter and its distance from both the anatomic limbus (corneoscleral junction) and the angle ( the base of the canal and the angle of iris insertion). Main Outcome Measures: Diameter and location of the canal were measured relative to gender, age, intraocular pressure, race, diagnosis, previous glaucoma surgery, pachymetry, refraction, lens type, axial length, and keratometry. Results: The average canal diameter was 121 mu m (+/- 45 mu m). The canal diameter in hyperopes was larger than the canal diameter in myopes (180 +/- 69 mu m vs. 122 +/- 45 mu m; P<0.001). The diameter of Schlemm's canal was smaller in patients with previous glaucoma surgery compared with patients without glaucoma surgery (98 +/- 20 mu m vs. 125 +/- 4 mu m; P<0.01). The mean distance between the angle and Schlemm's canal was found to be smaller in hyperopes than in myopes (281 vs. 335 mu m; P = 0.03). The location of the canal in black patients compared with white patients was found to be more posterior from the limbus (659 +/- 92 mu m vs. 624 +/- 73 mu m; P = 0.05). Similarly, canal location in patients with corneal thickness of more than 555 mu m was found to be more posterior to the limbus (702 vs. 625 mu m; P<0.01) compared with those with thinner corneas. Conclusions: When measured in vivo with ultrasound biomicroscopy, Schlemm's canal diameter was significantly smaller (121 mu m) than demonstrated in previous histopathologic studies.