Hamad Hejazi ¹ , David Farrell ²

1. Newcastle upon Tyne Hospitals NHS Foundation Trust, England, UK
2. Southern Health and Social Care Trust, Northern Ireland, UK

Key points

• OCT gives a cross-sectional, microscopic image of the retina in minutes and now underpins the diagnosis and monitoring of most macular disease.
• Read every scan in the same order: quality, contour, layers, fluid, interfaces.
• Most scans you will meet show one of a few patterns: macular oedema, epiretinal membrane, drusen, subretinal fluid or a macular hole.
• A scan supports a clinical assessment but never replaces it. Sudden loss or distortion of central vision needs same-day ophthalmic review.

Introduction

Optical coherence tomography (OCT) is now as routine in eye clinics as the ECG is on the wards. It uses near-infrared light to produce a cross-sectional image of the retina at micrometre resolution, without contact and in a few minutes (1). OCT images and reports now appear in referral letters, discharge summaries and clinic notes for patients with diabetes, age-related macular degeneration and neurological disease, yet few foundation doctors are taught to read them. This article offers a simple, repeatable approach.

What an OCT shows

Think of OCT as an ultrasound B-scan made with light instead of sound. Reflected light from each retinal layer is measured and displayed as a slice through the retina, usually through the fovea. Reflective tissue appears bright (hyperreflective) and fluid appears dark.

From the inner surface outwards, the main layers are the retinal nerve fibre layer, ganglion cell layer, inner plexiform layer, inner nuclear layer, outer plexiform layer and outer nuclear layer (the photoreceptor nuclei), followed by the ellipsoid zone (a bright band reflecting photoreceptor integrity) and the retinal pigment epithelium (RPE) with Bruch’s membrane (2). At the centre of the macula the inner layers part to form the foveal pit.

A five-step approach

1. Check the scan. Confirm the right patient and eye, adequate signal strength and a scan centred on the fovea. Blinks and segmentation errors (misplaced layer boundaries) can mimic thickening.

2. Look at the contour. Is there a normal foveal pit? Loss of the pit suggests oedema or a surface membrane.

3. Follow the layers. Are they continuous and in order? Breaks in the ellipsoid zone suggest photoreceptor damage and thinning suggests atrophy.

4. Look for fluid. Dark spaces may lie within the retina (intraretinal), beneath it (subretinal) or beneath the RPE (pigment epithelial detachment).

5. Check the interfaces. On the surface, look for a membrane or vitreous traction. Below, look for RPE elevations.

Common patterns

• Macular oedema: rounded dark cystic spaces in a thickened retina with loss of the foveal pit. Seen in diabetes, retinal vein occlusion and after eye surgery.
• Epiretinal membrane: a thin bright line on the retinal surface with wrinkling and a flattened pit. It can cause distortion and blur.
• Drusen: small dome-shaped bumps beneath the RPE, the hallmark of early age-related macular degeneration.
• Subretinal fluid: a dark space between the neurosensory retina and the RPE. It is seen in neovascular (‘wet’) age-related macular degeneration and central serous chorioretinopathy. New fluid in a patient with new symptoms is urgent.
• Full-thickness macular hole: a gap through all retinal layers at the fovea, usually with central distortion or a blind spot.
• OCT is not limited to the macula. Scans of the optic nerve head measure retinal nerve fibre layer thickness, which falls in glaucoma and after optic neuritis and is increasingly used in neurology.

Pitfalls

• A normal scan does not exclude disease. It samples a small area, can be degraded by cataract or blood, and does not show retinal detachment, many optic nerve problems or visual pathway disease.
• Artefact is common. Poor signal, motion and segmentation errors can create false thickness readings, so look at the image, not only the number.
• Match the scan to the patient. A striking scan in an asymptomatic patient, or a normal scan in a patient with sudden visual loss, both need clinical judgement.

When to escalate

Contact ophthalmology the same day for sudden loss of vision, new central distortion or a new central blind spot (particularly in someone with known macular degeneration), new subretinal or intraretinal fluid with new symptoms, and any flashes, floaters or a curtain over the vision. Do not wait for an OCT to be arranged first.

Conclusion

Reading an OCT does not require subspecialty training. A fixed routine, awareness of the common patterns and respect for its limits will let you understand most reports, communicate better with ophthalmology colleagues and recognise the patient who cannot wait.

References

1. Huang D, Swanson EA, Lin CP, et al. Optical coherence tomography. Science. 1991;254(5035):1178-81.

2. Staurenghi G, Sadda S, Chakravarthy U, Spaide RF. Proposed lexicon for anatomic landmarks in normal posterior segment spectral-domain optical coherence tomography: the IN•OCT consensus. Ophthalmology. 2014;121(8):1572-8.