During ophthalmoscopy, I noticed a brown lesion on the retina of one eye. It looks flat, has no colouration other than brown and has well defined sharp margins. What should I do?

This can be a difficult differential diagnosis. When I see this type of lesion in practice the question in my head is: Is this a choroidal naevus (pigment clump-forming what is often referred to as a mole) or a choroidal melanoma? It a tough question but we can use the following to guide us:

Symptoms are not helpful. Choroidal naevi, are typically found on routine retinal examination and are usually asymptomatic. However, they can be associated with symptoms such as central and peripheral vision loss secondary to subretinal fluid, cystoid retinal oedema or, rarely, choroidal neovascularization.

Choroidal melanoma also tends to be asymptomatic, although it is more likely to be symptomatic than a choroidal naevus. Symptoms of a choroidal melanoma may include decreased vision, flashes or floaters.

Signs are useful. If it is a choroidal naevus, it will:

• Have clearly defined margins
• Be flat or slightly elevated
• Remain stable in size over time.

If it is a choroidal melanoma it will have:

• Indiscrete margins
• An irregular or oblong shape
• Abruptly elevated edges (difficult to see with a direct ophthalmoscope).

The following features are shared by choroidal naevi and choroidal melanomas and therefore cannot be relied upon to aid differential diagnosis:

• Size
• Colour, which may be pigmented or non-pigmented
• Location
• Associated dormant features, such as overlying retinal pigment epithelial alterations and drusen; and suspicious features, including subretinal fluid and orange pigment.

Growth over one to two years is a convincing characteristic of an active melanoma. However, it is ideal to detect choroidal melanoma before the recognition of growth, as documented growth imparts an almost eightfold greater risk for metastasis. On the other hand, slow growth of 0.5 mm over many years or decades may simply reflect the natural progression of a benign choroidal naevus.

This is a condition, which many eye specialists will rarely see in a career, and it takes skill and experience to make this differential diagnosis. Unfortunately, choroidal naevi can convert to choroidal melanomas over time.

If the lesion has all of the choroidal naevi characteristics listed above and none of the choroidal melanoma characteristics then a practitioner can be confident in monitoring the lesion over time and looking for a change. During the first year, the patient should be monitored twice; subsequently, they should be evaluated annually as long as the naevus remains stable. Although the link between UV light exposure and choroidal melanoma has not been proved, sunglasses could possibly reduce ocular melanoma risk.

If there is any doubt, a misdiagnosis can have catastrophic results and therefore the patient deserves a referral to an ophthalmologist.