MONOCULAR DIPLOPIA CAUSED BY PRESSURE OF THE UPPER EYE LID ON THE CORNEA - DIAGNOSIS ON THE BASIS OF THE RETINOSCOPIC VENETIAN BLIND PHENOMENON

Background Abnormal pressure from the upper eye lid can cause a kink i n the corneal vault along the lid margin. Depending on whether the low er or upper part of the lid exerts the higher pressure, the upper segm ent of the cornea acquires a prismatic effect which is base up or down , respectively. This causes a ghost image below or above the main imag e. In patients whose upper eye lid occasionally reaches down such that the kink traverses the pupillary area, the ghost image appears whenev er they raise their lid above its usual position. The purpose of the p resent paper is to describe a retinoscopic phenomenon that allows an e asy diagnosis of this condition. Patients and methods About 20 patient s with a ghost image below or above the main image were examined with a Placido disc, with a photokeratometer, and with a streak retinoscope . Results Photokeratometry revealed a slight deformation of the ring r eflexes along a horizontal line at the border of the upper third of th e cornea. This deformation was obvious only in a minority of the patie nts. The retinoscopic findings were more characteristic. With the stre ak horizontal, two or three light bands separated by dark intervals we re seen in a ''with movement'', suggesting the impression of a Venetia n blind being lowered or raised behind the pupil. Because of this impr ession, the author suggests the term ''Venetian blind phenomenon''. Di scussion and conclusion Monocular diplopia caused by abnormal lid pres sure can be easily diagnosed by the ''Venetian blind phenomenon''. The optics can be explained as follows. Both the beams entering into, and emerging out of the patient's eye are being split by the prismatic ef fect of the upper cornea. Principally, this should result in four imag es; since, however, two of them overlay each other, only three separat e images remain. In cases where the upper cornea is deflected backward s, resulting in a prismatic effect base down, the examiner can see all three images. In cases where the, upper cornea is deflected forwards, resulting in a prismatic effect base up, the beams emerging from the patient's eye diverge, and the uppermost beam falls upon the examiner above his pupil, so that he can see only two of the three images.