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F. A. Irshad, R. S. Ayyala, M. S. Mayfield, D. Zurakowski; Variation in Schlemm Canal Size and Location by Ultrasound Biomicroscopy. Invest. Ophthalmol. Vis. Sci. 2008;49(13):5091.
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Schlemm canal anatomy is becoming increasingly important in view of newer surgical techniques aimed at improving trabeculocanalicular outflow for glaucoma patients. Measuring its size has only been possible in postmortem eyes using microscopy; however, high-frequency ultrasound (iUltrasound) can measure canal size and location in a living eye. The purpose of this study is to measure the diameter and location of Schlemm's canal in vivo to investigate any significant differences among patients of different sex, race, age, diagnosis (glaucoma vs OHT vs healthy eyes), IOP, lens type (phakic vs psk), axial length, keratometry, previous glaucoma surgery, current glaucoma medications, and refractive error.
This is a prospective Tulane University Glaucoma Service study of 122 eyes (61 patients) with the permission of Tulane IRB. After obtaining informed consent and achieving anesthesia with one drop of Proparacaine, the 80 MHz iUltrasound machine was used to measure the diameter and location of the canal from the limbus and angle at the 12 o' clock position.
Mean age was 52.5 ± 20.8 years with average IOP 15.2 ± 5.0 mmHg. 72 Female eyes had an average diameter of 128 ± 51 microns located 363 ± 92 from the angle and 612 ± 85 from the limbus whereas 50 male eyes were 117 ± 36 in diameter at 326 ± 75 and 634 ± 83. Schlemm diameter was positively correlated with location from limbus and was greater for phakic lens type and smaller for patients with myope refraction (p<0.01). Univariate results indicated females (p = 0.028), African Americans (p<0.05), and myopes (p<0.001) have greater canal location from the angle. For location from limbus, there were no differences between males and females (p = .15), race (p = 0.47), or refractive error (p = 0.46).
Patient age, race, diagnosis, and IOP did not correlate with statistically significant canal size differences. Refraction was the only known predictor of canal size diameter. Phakic lens type resulted in greater diameter size compared to pseudophakics. Asian race and patients with glaucoma surgery and medications had significantly greater axial lengths and lower average keratometry. African Americans and myopes have more posterior canal location compared to others. Stronger correlations between patient age, race, and IOP may be possible with continued data collection to increase statistical power.
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