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Editors Selection IGR 23-4

Intraocular Pressure and Outflow: In-vivo measurement diameter Schlemm's canal

Comment by Arthur Sit on:

26148 Variation in Schlemm's canal diameter and location by ultrasound biomicroscopy, Irshad FA; Mayfield MS; Zurakowski D et al., Ophthalmology, 2010; 117: 916-920

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A possible role for Schlemm's canal in glaucoma pathogenesis has long been suspected. While it is far too large to produce any significant outflow resistance on its own, it may interact with the juxtacanalicular tissue of the trabecular meshwork¹or function dynamically to influence IOP.² It has been a site of interest for glaucoma surgery, either directly ( e.g., viscocanalostomy) or as a target for after bypassing or removing the trabecular meshwork ( e.g., goniotomy). The anatomy of Schlemm's canal is therefore of significant importance.

While previous studies of Schlemm's canal have utilized either light or electron microscopy specimens, Irshad et al. (732) utilized the novel approach of applying high-frequency 80MHz ultrasound biomicroscopy to perform in-vivo measurements. Measurements were performed at a single position in one eye (12 o'clock) of 94 patients. The authors found that the canal diameter measured in this fashion was smaller than previously published results using cadaver specimens. These results indicate the possibility that fixation and specimen preparation for light and electron microscopy affect the diameter of Schlemm's canal. This is an interesting result, but the significance of a smaller Schlemm's canal is unclear since it would still be far too large to generate any significant outflow resistance by itself.³

The significance of a smaller Schlemm's canal is unclear since it would still be far too large to generate any significant outflow resistance by itself

The authors also reported that the diameter of Schlemm's canal was smaller in myopes than in hyperopes, and the location varied with race and corneal thickness. Again, these are interesting findings but the significance remains to be demonstrated. For surgical planning, large individual variations would likely limit the utility of this information. As well, for angle procedures, this information is limited to ab externo approaches, such as those used for viscocanalostomy or canaloplasty. The information is of limited use for ab interno procedures such as goniotomy or Trabectome. There is also a possibility that the difference in Schlemm's canal size compared with cadaver specimens is due to measurement error. As the authors discuss, it is possible that Schlemm's canal exists in a partially collapsed state. Since the limit of resolution for the device is 25 µm, collapsed portions of the canal may be missed with high-frequency UBM.

In summary, the authors have reported some interesting findings that challenge our current understanding of the anatomy of Schlemm's canal. However, the work must be considered preliminary and further research is required to validate and understand the implications of this study.

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