Measurement of the optical density of the optic nerve head I: the pallor of the papilla in artificial ocular hypertension.

The pallor of the optic disc during acute raise of intraocular pressure can be shown on the negatives of orthochromatic film and is measured by a photoscanner. When raising intraocular pressure from normal values to ablut 40 mm Hg, ten young persons showed an increase in optical density from 0.28 log units +/- 0.04 SEM to 0.38 log units +/- 0.04 SEM. When raising IOP to 65 mm Hg, the optical density was 0.43 log units +/- 0.04; with 110 mm Hg the optical density was 0.44 log units +/- 0.04 SEM. Above values of 40 mm Hg IOP, the pallor does not remain constant but may also decrease in some cases. The higher the intraocular pressure, the more contradictory the response. The phenomenon may be shown on various parts of the rim simultaneously. The amount of maximum pallor is directly related to the value of optical density with normal intraocular pressure. After restoring IOP to normal values, optical density decreases again, the amount of decrease being related to initial value. The contradictory response with higher IOP may be due to autoregulation of the capillaries of the optic nerve head.