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How to Read Your Eye Reports: OCT, ERG and Visual Field Results in Plain Language - Eye Stem Cell Center
November 08, 2026/ 0 comment
How to Read Your Eye Reports: OCT, ERG and Visual Field Results in Plain Language
Last updated: 8 November 2026 | Author: Prashant Tyagi (Stem Cell Biotechnology Specialist, 10+ years) and Dr. Pallavee Senior Consultant | Ophthalmologist & Eye Surgeon 22+ Years Experience
Short answer: An OCT report shows the structure of your retina or optic nerve, an ERG report shows how well your retina responds to light, and a visual field report shows which parts of your vision are working. Each report has numbers and colour maps, but you do not need to decode everything. Learn the few key terms below, compare your reports over time, and ask your ophthalmologist to explain what the changes mean for you.
Before you read: three simple rules
- Compare, do not judge one report alone. A single report is a snapshot. The trend across several reports tells you far more.
- Use the same machine where possible. Numbers from different brands of OCT or field machines are not directly comparable.
- Normal ranges are guides. Many people are healthy with values slightly outside the printed range, and some people with disease have values inside it. Your ophthalmologist interprets the report with your symptoms and examination.
Reading an OCT report
OCT (optical coherence tomography) uses light to take cross-section pictures of the retina or the optic nerve head. It measures thickness in microns and often shows colour maps, where green usually means within the normal range, yellow borderline and red below the normal range, although colours differ between machines.
| Term on the report | What it means | Why it matters |
|---|---|---|
| Central subfield thickness (macula) | Average retinal thickness at the centre of the macula | Rises with swelling and falls with thinning; used to track macular conditions |
| Macular volume | Total retinal volume in the macular area | Another way to follow swelling or loss over time |
| RNFL (retinal nerve fibre layer) thickness | Thickness of the nerve fibres around the optic disc | Thinning suggests loss of nerve fibres, as in glaucoma or optic atrophy |
| Ganglion cell layer thickness | Thickness of the layer containing cells that feed the optic nerve | Helps detect optic nerve and macular damage early |
| Ellipsoid zone (photoreceptor layer) | A line on the scan marking healthy photoreceptors | Its width and continuity are watched in inherited retinal disease |
| Cup-to-disc ratio | Size of the central cup compared with the optic disc | Helpful in glaucoma assessment |
| Signal strength / quality score | How good the scan was | A poor-quality scan can give misleading numbers |
For conditions like optic nerve atrophy, RNFL thickness is a key number. For Stargardt or other macular dystrophies and macular degeneration, the macular scan and layers matter more.
Reading an ERG report
An electroretinogram (ERG) records the tiny electrical signals the retina produces when it is stimulated by light. It tests function, not just structure. It is commonly used in retinitis pigmentosa and other inherited retinal conditions.
| Term on the report | What it means | Why it matters |
|---|---|---|
| Scotopic (dark-adapted) response | Response of the rods, which work in dim light | Often reduced first in retinitis pigmentosa |
| Photopic (light-adapted) response | Response of the cones, which work in bright light and colour | Shows how well daytime and colour vision cells function |
| a-wave | The first downward wave, from photoreceptors | Low values suggest photoreceptor dysfunction |
| b-wave | The upward wave that follows, from cells in the middle retina | Reflects signal transmission after the photoreceptors |
| Amplitude (in microvolts) | Height of the wave, meaning the strength of the response | Smaller than normal means weaker retinal function |
| Implicit time (in milliseconds) | How quickly the wave peaks | Delay suggests slower signal processing |
| 30 Hz flicker | A fast flashing test that mainly checks cones | Useful when other responses are very small |
| Multifocal ERG | Maps function across small areas of the central retina | Helps in macular disease |
An ERG with very small or “non-recordable” responses does not always mean total blindness, because the test shows overall retinal response. Ask your doctor to explain how it relates to your daily vision.
Reading a visual field report
A visual field test maps the area you can see while you look straight ahead. A grey-scale or colour map shows where sensitivity is reduced.
| Term on the report | What it means | Why it matters |
|---|---|---|
| Grey scale / total deviation map | Dark areas show lower sensitivity | Gives a quick picture of where vision is missing |
| Pattern deviation | Corrects for general effects such as cataract | Highlights localised damage |
| MD (mean deviation) | Overall difference from normal, in decibels; more negative means more loss | A single number to compare across visits |
| PSD (pattern standard deviation) | How uneven the field is | Higher values suggest localised defects |
| VFI (visual field index) | A percentage score of remaining field | Easy to follow over time |
| Fixation losses, false positives, false negatives | Reliability measures of how you performed the test | High values mean the test may be unreliable and might need repeating |
| 24-2 vs 10-2 test pattern | 24-2 covers a wide area; 10-2 focuses on the central area | 10-2 is often used in macular and advanced disease |
How the three reports fit together
| Report | Tells you | Example in practice |
|---|---|---|
| OCT | What the tissue looks like | Thin nerve fibre layer or thin macula |
| ERG | How well the retina responds | Reduced rod response |
| Visual field | What you can actually see | Narrowed side vision |
Changes in structure, function and field often move together but not always. That is why doctors use more than one test.
Want help understanding your reports?
Questions to ask about your reports
- Which numbers on my report matter most for my condition?
- Has anything changed since my last report, and by how much?
- Is the scan or test reliable, or should it be repeated?
- How often should I repeat each test?
- What do these results mean for reading, walking and daily tasks?
- Are these results relevant to any research option, and what is the evidence?
Stem cell therapy for eye disease is investigational for most conditions, results vary from patient to patient and no outcome can be guaranteed. A good review of your reports explains limits as clearly as possibilities.
Frequently asked questions
My report says “within normal limits” but my vision is poor. Why?
Some conditions affect function more than structure, or are in an area the test does not cover. Share your symptoms and ask whether other tests are needed.
Can I compare reports from different hospitals?
Be careful. Different machines use different scales, so ask your doctor to interpret them together.
How often should an OCT be repeated?
It depends on your condition. Many people repeat it every six to twelve months, but your specialist decides.
Does a low ERG mean I will go blind?
Not necessarily. ERG shows overall retinal function. Your doctor interprets it with your field, scans and symptoms.
What if the test was unreliable?
Visual field tests can be repeated. Fatigue and unfamiliarity often improve with practice.
Should I keep copies of my reports?
Yes. Keep digital and printed copies in date order. They are valuable for every future consultation.
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