Using OCT to establish differential diagnoses

The theory and practical applications of optical coherence tomography (OCT) was discussed by Professor Yit Yang during a Main Stage session on the Saturday of 100% Optical 2025 (1 March).

Yang, speaking on behalf of Practice Plus Group, told attendees that his session had a “retinal education purpose.”

Yang is a consultant ophthalmologist and vitreoretinal surgeon, and works with Practice Plus Group in ophthalmology services.

OCT is becoming increasingly important as hospital services move into the community, he said.

He outlined the main reasons for performing an OCT scan, noting that diagnosis is important to know whether a condition can cause blindness, as well as to inform a decision on the patient’s treatment.

OCT can also reveal whether a pathological process such as inflammation or infection is taking place, Yang said.

A sudden loss of vision could be either arteritic or non-arteritic, he said, adding that a referral to the stroke clinic could be required.

The 4x4 approach

Yang noted that there are four reasons for performing an OCT scan, four layers of the retina that should be considered by optometrists, and four levels of reporting that should come from the results.

The four reasons identified for performing an OCT scan were:

1. Opportunistic screening
2. When a patient is symptomatic, but with no suspicions of macular pathology
3. When a patient has suspicions of macular pathology
4. When the optometrist is monitoring a known macular problem.

Yang highlighted an instance of opportunistic screening that identified a case of amelanotic choroidal melanoma, noting that there had been no suspicions of macular pathology but that the scan was life-saving for the patient.

He also revealed a case of another patient, this time with a ruptured retinal artery leading to a macroaneurysm.

Another case saw a patient, who had presented with no symptoms in March, go on to be diagnosed with wet age-related macular degeneration (AMD) by July of the same year, Yang shared.

Discussing the layers of the retina, Yang noted that although there are 19, optometrists should not overcomplicate and should instead focus on four key layers in day-to-day practice.

The layers that should be focused on are:

1. The retinal pigment epithelium (RPE), sub-RPE and choroid layer
2. The sub-retinal space
3. The intra-retinal layer
4. The vitreoretinal interface.

The RPE band can be very thin in cases of dry AMD, while cases of drusen can see bumps and intraretinal fluid in the RPE band, he said.

Yang added that the contour of the choroid can also provide clues – a patch of dark on the choroid could signal dry AMD, for example.

Yang went on to explain the four levels of OCT interpretation:

1. Screening
2. Descriptive
3. Differential diagnoses
4. Definitive diagnosis.

Screening is primarily to check whether the area being scanned looks as expected and can be delegated to a junior member of staff, Yang said.

He added that the ‘descriptive’ level is when something abnormal is found.

At this stage, the location, the lesion, the elevation or thinning, which layer is affected and whether there is any extra substance, for example cysts, lipid, tissue or blood, should be identified.

Optometrists will need some awareness clinical ophthalmology in order to move on to the differential diagnosis stage, Yang said, adding that the fundus appearance can often rule out or increase the likelihood of certain conditions.

“We are stretching our knowledge of ophthalmology by considering all the possibilities,” he said.

He highlighted the case of a 58-year-old woman, with 6/9 visual acuity in her right eye and unexpected lesions that lit up via autofluorescence. Two months later, there was no change, and after one year the patient was diagnosed with acute exudative polymorphous vitelliform dystrophy.

It transpired that the woman had a history of melanoma in her leg, which had now recurred, Yang said.

He added that generally, the same symptoms could also be seen to suggest ovarian or testicular cancer.

Yang explained that certain layers suggest certain conditions – wet AMD in the RPE and sub-RPE layer, for example, or retinal angiomatous proliferation in the intra-retinal layer.

“You can use the layers to help you narrow down to those few conditions,” he said.

Yang added: “If you don’t have additional experience, it’s ok to stop at level three” - ie, with differential diagnoses and a referral.

“Often, you cannot make a definitive diagnosis based on an OCT scan alone,” he said.

“But even so, a descriptive report can be very useful, and you can produce a useful list of differential diagnoses to gauge the urgency.”

He added: “That’s the main thing – you don’t want the patient to miss any urgent treatment.”