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Editors Selection IGR 13-2

Anatomical Structures: Modified Zeimer episcleral venomanometer

Comment by Carol Toris on:

45641 A novel method for computerized measurement of episcleral venous pressure in humans, Sit AJ; Ekdawi NS; Malihi M et al., Experimental Eye Research, 2011; 92: 537-544

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An understanding of how a drug or surgical procedure reduces intraocular pressure requires an accurate assessment of all parameters of aqueous humor dynamics. One such parameter is episcleral venous pressure (EVP). It is measured in humans by venomanometry (Zeimer RC et al. Arch Ophthalmol 1983; 101: 1447-1449). No improvement to this method has been reported since 1983. Sit et al. (530) describe modifications to the Zeimer episcleral venomanometer. A video recorder was added to enable observation of the episcleral veins as the pressure applied to the veins was increased. The pressure was recorded and synchronized with the video stream. The width of a selected vessel was measured. The pressures were plotted against peak brightness, a measure of the degree of collapse of the vessel. The authors found that the pressures at 0%, 10% or 50% collapse were significantly different. The mean pressure of five healthy young volunteers (6.3 mmHg) at 0% collapse was considered to be EVP. There are a few weaknesses to the study. The handling of raw data is tedious. Once data are plotted, boundary lines are drawn between the periods of 'No Compression', 'Transition' and 'Complete Compression'. The placement of these lines is somewhat arbitrary and affects the final EVP value. Sit et al. suggest that prior publications using the 'half-blanched' method (50% collapse) may have overestimated EVP. However, using an EVP of 6.3 mmHg in the modified Goldmann equation could yield calculated uveoscleral outflows that would be very small and even negative. The true EVP remains unknown as direct pressure measurement is not possible in humans. Further improvements in the method and the resultant data analyses will be greatly anticipated. Clearly this study is a step in the right direction.

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