Optical Coherence Tomography, or OCT, is a non-invasive imaging test that creates detailed, cross-sectional pictures of the back of your eye. It uses light waves to capture high-resolution images of the retina and the optic nerve. This technology allows eye care specialists to see the structures of the eye in microscopic detail, layer by layer, providing a way to monitor the delicate tissues that are responsible for vision.
The Optic Nerve and the OCT Scan
The optic nerve acts as the data cable connecting the eye to the brain, composed of over a million individual nerve fibers that transmit visual information. An OCT scan is designed to meticulously evaluate the health of this structure. The procedure itself is straightforward and painless for the patient.
You will be seated in front of the OCT machine, placing your chin on a supportive rest to keep your head still. You will look at a target light inside the machine while it scans your eye, which takes only a few seconds. The machine never touches your eye. In some cases, your pupils may be dilated with eye drops beforehand to provide a wider view of the retina, which can cause temporary light sensitivity and blurred vision for a few hours.
Visualizing a Healthy Optic Nerve
On an OCT scan, a healthy optic nerve often resembles a symmetrical, well-formed donut or bagel. The entire structure shown on the scan is the optic disc, which is the “donut,” and the small depression in its center is called the optic cup, or the “hole.” In a healthy eye, this central cup is small in relation to the overall size of the optic disc.
The “donut” itself is comprised of the neuroretinal rim, which should appear thick and uniformly shaped all the way around. This rim is made up of the retinal nerve fiber layer (RNFL), the collection of axons from retinal ganglion cells that form the optic nerve. A thick rim indicates a dense, healthy population of nerve fibers. The scan uses false colors to represent the different layers and their thicknesses, with warmer colors like red and yellow often showing thicker tissue and cooler colors like blue and green representing thinner areas.
The overall symmetry between a patient’s two eyes is also taken into account. While minor differences can be normal, the optic nerves in both eyes should have a similar appearance in size and shape. A healthy scan will show this consistent, donut-like structure with a small central cup and a thick, even rim.
Interpreting OCT Measurements and Colors
Beyond the initial visual assessment, an OCT scan provides precise quantitative data about the optic nerve’s structure. The primary measurements are the retinal nerve fiber layer (RNFL) thickness and the cup-to-disc ratio. The RNFL’s thickness is a direct indicator of the tissue’s health. A healthy adult’s average RNFL thickness is typically around 80 to 110 micrometers.
The cup-to-disc ratio (CDR) compares the diameter of the optic cup (the central depression) to the diameter of the entire optic disc. A smaller ratio, around 0.3 to 0.4, is considered normal, suggesting that the supportive neuroretinal rim is thick and intact. The OCT machine calculates these metrics, offering an objective assessment that can be tracked over time.
To make this data easy to interpret, OCT reports use a color-coding system. The machine compares the patient’s measurements to a built-in normative database, which contains data from thousands of healthy individuals of a similar age. Results that fall within the normal range are coded in green. Measurements that are borderline or suspiciously thin are coded in yellow, while those that fall outside normal limits are coded in red. A healthy report will predominantly feature measurements in the green zone.
How OCT Distinguishes Healthy from Unhealthy Nerves
A healthy OCT scan provides a clear baseline and can rule out the subtle, early signs of damage associated with optic nerve diseases. Its primary use is for detecting and managing glaucoma, a condition characterized by the progressive loss of retinal ganglion cells and their axons. This cell death leads to a thinning of the retinal nerve fiber layer (RNFL) and an enlargement of the optic cup.
An OCT can detect this RNFL thinning often years before a patient would notice any changes in their vision. For example, where a healthy nerve has a thick neuroretinal rim and a small cup-to-disc ratio, a glaucomatous nerve will show a thinner rim and an increased cup-to-disc ratio. The scan’s color-coded maps will highlight these areas of thinning in yellow or red.
By comparing scans over time, an eye care professional can track the rate of any changes. A stable, healthy OCT scan shows no significant deviation from the established baseline. A progressive thinning of the RNFL or an enlarging cup would indicate an active disease process, allowing for timely intervention to preserve vision.