To the Editor. —We read with considerable interest and some skepticism the article by Seiff and Shah1describing a mechanism of optic nerve sheath fenestration. They suggest that Bernoulli's principle might explain the reduction in optic nerve head swelling noted after surgery, and they support their suggestion with some experimental results. See also p 667. We have re-created the authors' hydraulic model in our laboratory, and have evaluated the rate of flow required to achieve pressure-lowering effects. Consideration was given to the diameter of the modeled fenestration: a large diameter decreases velocity; a small diameter increases resistance, thereby decreasing velocity. For water, we found that 2.5 mm gives maximum velocity. In our experiment, a 3-mm-diameter fenestration was used. The results, shown in Fig 1, are based on flow rate (in milliliters per minute), and, in Fig 2, on velocity calculations (in millimeters per minute) required by Bernoulli's formula. Flow