An Optical Coherence Tomography (OCT) fundus exam is a non-invasive imaging test that provides a detailed, cross-sectional view of the back of your eye. It uses light waves to capture high-resolution pictures of the retina, the light-sensitive tissue lining the back of the eye. The technology is similar to an ultrasound, but it uses light instead of sound waves. This allows an ophthalmologist to see each of the retina’s distinct layers, mapping and measuring their thickness.
Purpose of an OCT Fundus Exam
An OCT exam is used to diagnose and manage a range of retinal conditions. A primary use is for age-related macular degeneration (AMD). The scan identifies drusen, which are small yellow deposits under the retina, and detects the fluid accumulation characteristic of “wet” AMD. This detailed view helps doctors determine the disease’s stage and the best course of action.
For individuals with diabetes, an OCT exam is used to detect diabetic retinopathy. It excels at identifying and quantifying diabetic macular edema, which is swelling in the macula. By measuring retinal thickness, doctors can track the severity of the edema and monitor how well a patient is responding to treatment.
The technology is also used in the management of glaucoma, a condition that causes progressive damage to the optic nerve. An OCT scan measures the thickness of the retinal nerve fiber layer (RNFL). By tracking changes in RNFL thickness and analyzing the optic nerve head, ophthalmologists can detect damage early, often before a person notices vision changes.
OCT imaging is also used to evaluate other issues at the back of the eye. It can visualize macular holes, which are small breaks in the macula, and epiretinal membranes, a layer of scar tissue on the retina’s surface. Conditions like central serous retinopathy, where fluid builds up under the retina, are also identified.
The Patient Experience
The process of getting an OCT scan is straightforward and comfortable. In some cases, the eye doctor may administer dilating eye drops before the test. These drops widen the pupil, providing a larger window to the back of the eye for a clearer scan. The drops cause temporary light sensitivity and blurred near vision, which subsides after a few hours.
During the exam, you will be seated in front of the OCT machine. You will be asked to place your chin and forehead into designated rests to keep your head still. You will then look at a small target light inside the machine to fix your gaze. The device scans each eye without touching it, and the process is painless, taking only a few seconds to complete.
If your pupils were dilated, you will be advised to wear sunglasses to help with light sensitivity until the effects of the drops wear off. It is recommended to have someone drive you home, as your vision may be too blurry for safe driving. Otherwise, you can resume your normal activities immediately.
Interpreting the Cross-Sectional Images
The diagnostic value of an OCT exam lies in the unique perspective it provides. Unlike a standard fundus photograph, which captures a flat, top-down view of the retina’s surface, an OCT provides a cross-sectional image. A useful analogy is to think of a fundus photo as a picture of the top of a decorated cake. An OCT image, in contrast, is like a slice cut from that cake, revealing the distinct layers of cake and filling inside.
This ability to visualize individual retinal layers is what makes the technology so informative. The retina is composed of multiple, highly organized layers of specialized cells, each with a specific function. An OCT scan allows a doctor to see and measure the thickness of these individual layers. This level of detail enables the identification of subtle abnormalities that would be invisible on a surface photograph.
On a scan of a healthy eye, the layers appear smooth and well-organized, with a dip at the fovea, the center of the macula responsible for sharp, detailed vision. When a disease is present, this orderly structure is disrupted. For instance, in a patient with wet AMD, the doctor might see dark, fluid-filled pockets that separate and distort the retinal layers. In a patient with a macular hole, the image will show a clear physical gap in the retinal tissue.