PubMed1993
BACKGROUND: Abnormal pressure from the upper eye lid can cause a kink in the corneal vault along the lid margin. Depending on whether the lower or upper part of the lid exerts the higher pressure, the upper segment of the cornea acquires a prismatic effect which is base up or down, respectively. This causes a ghost image below or above the main image. In patients whose upper eye lid occasionally reaches down such that the kink traverses the pupillary area, the ghost image appears whenever they raise their lid above its usual position. The purpose of the present paper is to describe a retinoscopic phenomenon that allows an easy diagnosis of this condition. PATIENTS AND METHODS: About 20 patients 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 reflexes along a horizontal line at the border of the upper third of the cornea. This deformation was obvious only in a minority of the patients. The retinoscopic findings were more characteristic. With the streak horizontal, two or three light bands separated by dark intervals were seen in a "with movement", suggesting the impression of a Venetian blind being lowered or raised behind the pupil. Because of this impression, the author suggests the term ""Venetian blind phenomenon." DISCUSSION AND CONCLUSION: Monocular diplopia caused by abnormal lid pressure 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 effect of the upper cornea. Principally, this should result in four images; since, however, two of them overlay each other, only three separate images remain. In cases where the upper cornea is deflected backwards, 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.