A choroidal nevus is a common, typically benign growth found inside the eye, often compared to a freckle or mole on the skin. This pigmented spot is an accumulation of melanocytes, which are the pigment-producing cells, located within the choroid layer of the eye. The choroid is the vascular layer that sits beneath the retina, supplying it with oxygen and nutrients. When detected, the finding can be alarming, but these lesions are extremely common, with estimates suggesting they occur in about one out of every ten people.
What is a Choroidal Nevus?
A choroidal nevus is essentially a flat patch of pigment cells nestled in the choroid, the extensive network of blood vessels situated between the retina and the outer white layer of the eye. These lesions usually appear as a slate-gray or brownish discoloration when viewed during a dilated eye examination.
Most nevi are flat or have only a minimal elevation, typically measuring less than 2 millimeters in thickness. The vast majority of choroidal nevi do not cause any symptoms like pain or vision changes. They are most often discovered incidentally during a routine eye check-up, especially in Caucasian individuals, where they are most prevalent.
The presence of a nevus is generally considered a pre-cancerous condition, analogous to a skin mole, where the risk of transformation to malignancy is very low. However, a small percentage of these nevi can evolve into a form of eye cancer called choroidal melanoma. Initial detection and characterization of the nevus are entirely dependent on the eye doctor’s examination.
Identifying Features of High-Risk Nevi
While most choroidal nevi remain stable, a small fraction will exhibit growth and transform into a choroidal melanoma. Ophthalmologists use standardized criteria to assess the nevus’s potential for malignant transformation, which helps determine the necessary monitoring schedule. The risk of transformation over five years is approximately 1% if it has no high-risk features, but the risk rises significantly with multiple factors.
These predictive features are summarized using the mnemonic “TFSOM-DIM,” which guides the clinician’s assessment of the lesion’s characteristics.
TFSOM-DIM Criteria
- Thickness greater than 2 millimeters, measured by ultrasound, as elevated lesions carry a higher risk.
- Subretinal Fluid, which is a sign of an active or leaking lesion, detected through imaging like Optical Coherence Tomography (OCT).
- Symptoms of vision loss, typically defined as visual acuity of 20/50 or worse, suggesting the lesion is affecting the overlying retina.
- Orange pigment, the accumulation of lipofuscin, a metabolic byproduct that suggests a high rate of cellular activity and growth.
- Melanoma acoustic hollowness on ultrasound, referring to a specific internal structure pattern associated with malignant tumors.
- Diameter greater than 5 millimeters, measured by fundus photography, as larger lesions are more likely to have already undergone transformation.
A nevus with three or more of these high-risk factors has a substantially increased likelihood of growth, necessitating a more aggressive monitoring strategy. The presence of these features does not confirm cancer but strongly indicates the need for specialized observation.
Long-Term Management and Monitoring
Once a choroidal nevus is identified, the management strategy focuses on observation to detect any sign of growth or transformation as early as possible. The frequency of follow-up visits is directly related to the initial risk assessment, with a low-risk nevus often being monitored annually. If the nevus presents with one or more high-risk features, the monitoring interval is often shortened to every four to six months.
Regular follow-up involves a multimodal imaging approach to capture subtle changes that may not be apparent on a standard eye exam. Fundus photography is used to create a permanent, high-resolution baseline record of the nevus’s size, shape, and color for future comparison. This allows for the precise measurement of any increase in its basal diameter.
Optical Coherence Tomography (OCT) is utilized to produce cross-sectional images of the retina and the nevus, which is particularly effective for detecting subretinal fluid or changes in the overlying retinal tissue. B-scan ultrasonography is the standard method for accurately measuring the nevus’s thickness and assessing its internal acoustic properties.
Treatment, such as plaque radiotherapy or proton beam irradiation, is not typically initiated unless objective evidence of growth or documented transformation into melanoma is observed. Since the majority of these lesions remain benign, the standard protocol is a vigilant, non-invasive surveillance to ensure patient safety while avoiding unnecessary treatment.