Consider the eye to be a camera. The camera has a system of lenses in front which focus light on a film or digital sensors on the back. In the same way the eye has a system of lenses in front which focus light onto a layer at the back called the Retina. This layer has living sensors to sense the light form and send a signal to the brain which interprets these signals as the image we see. The retina is connected to the brain though a living cable which is called the optic nerve.

Retina has a very complex structure having ten layers. The light detector cells (photoreceptors) of the retina are called rods and cones. Cones help more with daylight vision and color vision whereas rods are primarily responsible for night vision or vision in low light conditions.

The central part of the retina called the macula has the maximum light sensitivity and resolution.

So any disease process which distorts the retina or makes it thicker / thinner or reduces the functional ability of the cells of the retina gravely affect the vision.

To diagnose various retinal diseases certain tests are done such as Fluorescein Angiography and Optical Coherence Tomography. These tests and some common retinal diseases are described in various sections.

Fundus Fluorescein Angiography ( FFA )

Fluorescein Angiography is an investigation to further investigate the cause of the retinal disease. In this 3 ml of a water soluble fluorescent dye is injected into a vein on the patient's arm. As the dye reaches the blood vessels of the retina (takes only 10 seconds !! ), sequential photographs are taken using a sophisticated digital camera. The abnormal leakage of dye or absence of normal pattern of dye gives the doctor clues regarding the diagnosis and severity of the retinal disease.

This test is simple with generally no significant side-effects.

Optical Coherence Tomography (OCT )

OCT is a tool by which highly magnified photographs of the retina can be taken to study it's microscopic structure. The patient has to just sit in front of a machine for a few minutes and look at a target light while these special images are acquired. It also helps to measure the retinal thickness in microns. So the doctor can determine if the retina is getting thicker or thinner. Also which layers of the retina are getting more affected can be evaluated and the response to treatment can be judged by serial examinations.

Author: Dr. Rohan Chawla
Date: 27 November 2008