Basic OCT interpretation

Vishanna Balbirsingh

Optical Coherence Tomography (OCT) is a non-invasive imaging technique that provides high-resolution cross-sectional images of the retina. OCT uses light waves to capture detailed images, similar to ultrasound but with light instead of sound waves (1).

OCT can be particularly helpful in diagnosing macular holes, epiretinal membranes, vitreomacular traction, macular oedema and exudates and detachments of the retina.

Recall of the anatomy of the retina

The retina lines the posterior portion of the eye, except the optic nerve and contains 10 distinct layers of neurons (2). The layers from anterior to posterior are as follows:

  • Inner limiting membrane
  • Nerve fibre layer (RNFL)
  • Ganglion cell layer (GCL)
  • Inner plexiform layer
  • Inner nuclear layer
  • Middle limiting membrane
  • Outer plexiform layer
  • Outer nuclear membrane
  • External limiting membrane
  • Layer of photoreceptors; rods and cones
  • Retinal pigment epithelium (RPE)

The retina overlies a vascular layer known as the choroid which is partially visible under the RPE layer.

Within the retinal layers there are multiple cell types including

  • Rods
  • Cones
  • Retinal ganglion cells
  • Bipolar cells
  • Horizontal cells
  • Amacrine cells

Features of OCT interpretation

Retinal thickness can be measured from OCT images which is useful as increased thickness of the macula might be related to macular oedema, diabetic retinopathy, ischaemia or inflammation (3). Other features suggestive of macula oedema include cystoid spaces and intraretinal fluid, seen on the OCT scan as areas of hyperreflectivity (3).

Glaucomatous changes can be identified by measuring the thickness of the RNFL around the optic disc and can have corresponding changes in the GCL thickness of the macula (4).

Changes at the back of the eye due to age-related macular degeneration include the formation of drusen (extracellular deposits of waste material between the RPE and the inner layer of the choroid) and pockets of subretinal fluid can suggest wet age-related macular degeneration (5).

Approaching a basic OCT interpretation

Step 1: With an OCT Macula, identify the retinal layers. Recognize the normal anatomy: RNFL, GCL, IPL, INL, OPL, photoreceptor layer, RPE, and choroid.

Step 2: Look for abnormal fluid. Check for intraretinal fluid, subretinal fluid, or cystic spaces.

Step 3: Assess retinal thickness. Measure the macular thickness and look for signs of macular oedema, atrophy, or scarring.

Step 4: Look for structural abnormalities. Check for macular holes, drusen, or retinal breaks.

Step 5: Assess the optic nerve OCT. Evaluate the nerve fibre layer (RNFL) thickness for signs of glaucoma in combination with GCL thickness of the macula.

References

1.            Klein T, Huber R. High-speed OCT light sources and systems [Invited]. Biomed Opt Express [Internet]. 2017 Feb 1 [cited 2025 Jan 26];8(2):828. Available from: https://opg.optica.org/abstract.cfm?URI=boe-8-2-828

2.            Mahabadi N, Al Khalili Y. Neuroanatomy, Retina. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 [cited 2025 Jan 26]. Available from: http://www.ncbi.nlm.nih.gov/books/NBK545310/

3.            Sng CCA, Cheung CY, Man RE, Wong W, Lavanya R, Mitchell P, et al. Influence of diabetes on macular thickness measured using optical coherence tomography: the Singapore Indian Eye Study. Eye [Internet]. 2012 May [cited 2025 Jan 23];26(5):690–8. Available from: https://www.nature.com/articles/eye201211

4.            Hong RK, Kim JH, Toh G, Na KI, Seong M, Lee WJ. Diagnostic performance of wide-field optical coherence tomography angiography for high myopic glaucoma. Sci Rep [Internet]. 2024 Jan 3 [cited 2025 Jan 26];14(1):367. Available from: https://www.nature.com/articles/s41598-023-49542-y

5.            Diniz B, Rodger DC, Chavali VR, MacKay T, Lee SY, Stambolian D, et al. Drusen and RPE atrophy automated quantification by optical coherence tomography in an elderly population. Eye [Internet]. 2015 Feb [cited 2025 Jan 26];29(2):272–9. Available from: https://www.nature.com/articles/eye2014260

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