Reduced Schlemm's Canal size observed by SD-OCT

Take-home

Spectral-domain optical coherence tomography measurements showed that Schlemm’s canal in glaucomatous eyes is 60% smaller than in normal eyes.

Dr. Kagemann

By Lynda Charters; Reviewed by Larry Kagemann, PhD

Pittsburgh—Use of spectral-domain optical coherence tomography (SD-OCT)—which enables non-invasive measurements of Schlemm’s canal in living human eyes—has found that the structure is 60% smaller in glaucomatous eyes than in normal eyes.

Larry Kagemann, PhD, and colleagues from the Glaucoma Imaging Group conducted a study in which they characterized and compared Schlemm’s canal in patients with glaucoma and without glaucoma.

Characterization of Schlemm’s canal

“If you randomly find a location in a volumetric scan containing a length of Schlemm’s canal, you can find any number you want and get any desired results,” said Dr. Kagemann, assistant professor of ophthalmology and bioengineering, University of Pittsburgh School of Medicine, Pittsburgh. “This cannot help but be biased.”

He demonstrated the considerable discrepancy in the size of Schlemm’s canal when the structure was measured manually in the same healthy eye from two slightly different locations. In one image the canal appeared quite large, but in the second image it appeared to be a slit.

“Spatial variation in Schlemm’s canal cross-sectional area is huge,” Dr. Kagemann said. “Its structure varies in size rapidly.”

The study under discussion included 10 normal healthy eyes and 9 eyes with primary open-angle glaucoma. The investigators used SD-OCT (Cirrus, Carl Zeiss Meditec) using an anterior segment 512 × 128 cube scan pattern.

In order to perform a meaningful comparison, a 1-mm section was obtained from a 4-mm scan. Two independent observers sampled 31 frames within the 1-mm segment.

Schlemm’s canal was measured and an average obtained of the representative tissue sample. This allowed quantitative measurements that eliminated the bias of choosing a single measurement location, Dr. Kagemann explained.

In all previous published studies on the same topic, the investigators chose a random sight for measurement from the slit-like opening in the scan. Dr. Kagemann contends that that type of measurement is “susceptible to unconscious bias.”

(Figure 1A) Schlemm's canal (SC, bottom, asterisks) can be isolated in sequential serial scans (top) for display in 3-D. The 3-D view emphasizes the rapid variation in SC size and position. Two cross-sectional views separated by only 31 µm (top left and right) highlight rapid change in SC size. (Figure 1B) in this OCT structures deep within, such as Schlemm's canal (left, white arrow), are visualized. Contrast enhancement improves visualization of its borders (right, white arrow) for measurement. (Images courtesy of Larry Kagemann, PhD)

Study findings

When Dr. Kagemann and his colleagues compared the Schlemm’s canal cross-sectional areas in the healthy and glaucomatous eyes, there was a relatively large difference between the healthy and glaucomatous Schlemm’s canals, he said.

The glaucomatous eyes had a Schlemm’s canal that was 60% smaller than the same structure in the healthy eyes.  The distribution of the sizes of the cross-sectional areas of Schlemm’s canal showed that, while there was considerable overlap in cross-sectional areas, there was a distinct separation between the two groups.

Another factor was that patients with normal eyes were significantly younger than those with glaucomatous eyes (41 ± 17 years versus 64 ± 9 years).

This may account for some of the differences in size, because aging may have had an effect on the size of Schlemm’s canal in older patients.

Characterization is not a trivial process and investigators need to evaluate Schlemm’s canal in far more than one slice, he said.

“In an average of 32 samples, we found an association between IOP and the Schlemm’s canal cross-sectional area,” Dr. Kagemann said. “However, we don’t know if that is a cause or an effect.”

It is possible that smaller tissue sizes are impeding aqueous outflow and causing the increasing pressure, or that the increasing pressure is pushing the trabecular meshwork into Schlemm’s canal and causing the decreased canal size. That is yet to be determined.

There is an approximate 60% difference in the Schlemm’s canal cross-sectional area between the glaucomatous and healthy eyes, Dr. Kagemann concluded.

Larry Kagemann, PhD

E: lek19@pitt.edu

Dr. Kagemann has no financial interest in the subject matter. This research was supported by grants from the Eye and Ear Research Foundation, Pittsburgh; National Institutes of Health; and Research to Prevent Blindness, New York.

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