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 microm (+/-45 microm). The canal diameter in hyperopes was larger than the canal diameter in myopes (180+/-69 microm vs. 122+/-45 microm; 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 microm vs. 125+/-4 microm; 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 microm; 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 microm vs. 624+/-73 microm; P = 0.05). Similarly, canal location in patients with corneal thickness of more than 555 microm was found to be more posterior to the limbus (702 vs. 625 microm; 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 microm) than demonstrated in previous histopathologic studies.