What OCT sees
Retinal ganglion cells collect the signal from the photoreceptors and send it along their axons, which gather at the optic nerve. Glaucoma kills ganglion cells and their axons. OCT images the retina in cross section at a resolution of a few microns and measures two things: the thickness of the nerve fiber layer (the bundle of axons) in a circle around the optic nerve head, and the thickness of the ganglion cell layer at the macula, where the cells themselves are most concentrated. Both thin as glaucoma progresses, in patterns the software compares with a database of healthy eyes.
Why it detects glaucoma early
Visual field testing detects glaucoma only once enough ganglion cells are lost to affect sensitivity, which can be a third or more of them. OCT detects the thinning directly, often years earlier, in what is called preperimetric glaucoma. For a glaucoma suspect or a patient with ocular hypertension, OCT is the test most likely to reveal the moment damage begins, which is the moment to treat.
Tracking progression
Every scan is registered to the previous ones and the software plots the thickness over time, distinguishing true loss from measurement noise and from the slow thinning of normal aging. A statistically significant downward trend is progression, and it is the most objective evidence that treatment needs to be strengthened. Because the analysis depends on comparison, a good baseline set of scans at diagnosis is important.
The scan itself
You rest your chin on the instrument and look at a target while it scans, in a few seconds per eye. No drops are needed unless the pupil is very small or the lens cloudy. The results are available immediately and are reviewed with you.
Limits
OCT thickness has a floor: in advanced glaucoma, once the layer is very thin, the scan cannot measure further loss, and visual fields become the better monitor. High myopia, previous retinal disease and cataract can affect the scan and are taken into account. OCT is one part of the picture, read alongside pressure, fields and the examination.
Is this right for your eyes?
Nothing on this page is a diagnosis. Whether this is the right choice for you is decided by a full examination at one of our 4 offices: Pasadena, Huntington Beach, Woodland Hills and Beverly Hills.
About this content: This page was written with the assistance of AI tools and reviewed for medical accuracy by Dr. Bavand Youssefzadeh, DO. It is general information, not medical advice, and does not replace an eye examination. Individual results vary. Read our medical disclaimer.
