Understanding Nevi

What Is A Choroidal Nevus

A choroidal nevus is a flat, benign pigmented area that appears in the back of the eye and is basically an eye freckle. If your doctor refers to a lesion in your eye that needs to be tracked, she is most likely talking about a choroidal nevus.

The accumulation of pigmented cells that makes up a nevus occurs in a very small percentage of the population. Estimates put occurence at 2%1,2,3 on the low end and 10%-13%4 on the high end. Wills Eye Hospital, which sees a vere large number of cases, states, "Choroidal nevus is the most common intraocular tumor, occurring in about seven percent of adults." Suffice it to say, approximately 1 in 10 people have these freckles.

They are never present at birth, though. Pigmentation appearing to be a choroidal nevus in infancy is usually from choroidal neurofibroma, seen as a part of systemic neurofibromatosis. Nevi of the choroid can be seen before puberty but they are unusual. They are also very rare in African Americans.
Figure 1. Two examples of flat, round nevi.
Choroidal nevi are slate-gray and relatively flat (less than 2mm thickness), although there is significant size overlap between small melanomas and larger nevi. Like choroidalal melanomas (i.e. full-blown eye cancers), they also may show overlying drusen or lipofuscin (figure 2).

A nevus almost never causes any symptoms. Very rarely, a choroidal nevus may leak fluid or be linked to the growth of abnormal blood vessels. These events can lead to retinal detachment, flashing lights and vision loss.

The lesions are typically discovered on routine eye examination by an experienced eye doctor using indirect ophthalmoscopy and with the help of tests such as ultrasonography, photographic imaging and ocular coherence tomography. Although it may be congenital, choroidal nevus is not typically observed in small children. It usually is first detected in adulthood and can be flat or minimally elevated and pigmented or nonpigmented (amelanotic).

Are Nevi Cancerous?

Choroidal nevi carry a slight potential, like with skin moles, to evolve into melanoma. Statistically, of every 500 choroidal nevi, one will undergo malignant transformation if followed for 10 years.

There are a number of risk factors for such melanoma transformations, the most important of which appears to be initial thickness of greater than 2 mm. Carol Shields at Wills Eye Hospital has done a lot of work in this area and, along with her husband and Wills colleagues and an analysis of 1,300 cases, identified five factors associated with risk of growth of small choroidal lesions. These factors were:
1) tumor thickness greater than 2.0 mm
 2) subretinal fluid
 3) visual symptoms
 4) orange pigment
 5) posterior tumor margin touching the disc

These five factors can be eaily remember with the mnemonic:
To Find Small Ocular Melanoma 
Thickness, Fluid, Symptoms, Orange pigment, Optic Disc Margin

Lesion growth was observed in only 4% of patients with none of these risk factors, 36% of those with a single risk factor and in over half of those with three or more factors. The small-tumor observational study conducted by the Collaborative Ocular Melanoma Study (COMS) Group identified similar risk factors: 1) greater apical tumor thickness, 2) larger initial basal diameter, 3) presence of orange pigment, 4) absence of drusen, and 5) absence of retinal pigment epithelial change adjacent to the tumor. These last two factors confirm the clinical observation that the presence of drusen and pigment alterations are indicators of chronicity, and therefore are more likely to be seen over dormant, benign lesions. 

Imaging characteristics may also be helpful in evaluating the potential for malignancy. The presence of an internal quiet zone on B-scan ultrasonography and hot spots on fluorescein angiography have been shown to be predictors of tumor growth.The role of optical coherence tomography in evaluating choroidal lesions is also being explored. Gabriella Espinoza, MD, and colleagues have proposed that OCT may be valuable in differentiating between active subretinal fluid, where the retina overlying the lesion is elevated but otherwise normal in appearance, and chronic changes such as retinal thinning or intraretinal cysts. In their small series, they found a correlation between an active subretinal fluid on OCT and documented tumor growth.6
Whether choroidal melanomas (i.e. full blown OM) originate only from nevi is unclear. Abramson, et al. have pointed out though that patients with flat pigmented, untreated nevi followed for more than 20 years have developed melanomas resultant of the previously dormant nevus lesion.

Whereas choroidal melanomas tend to grow relatively rapidly, choroidal nevi may enlarge slowly over a period of several years. In younger patients, this slow, non-malignant growth is more common and does tend to stabilize with age.

Nevi need to be continually monitored for any signs of change and a trained ocular oncologist will work with you to set up a smart surveillance schedule.

Other Resources

Scientific paper authored by Carol and Jerry Shields of Wills Eye Hospital:

In an analysis of 2514 choroidal nevi, factors predictive of growth into melanoma included greater thickness, subretinal fluid, symptoms, orange pigment, margin near disc, and 2 new features: ultrasonographic hollowness and absence of halo.

Overview from Amy Schefler of Retina Consultants of Houston:

1. Augsburger JJ, Correa ZM, Trichopoulos N, Shaikh A. Size overlap between benign melanocytic choroidal nevi and choroidal malignant melanomas. Invest Ophthalmol Vis Sci. 2008 Jul;49(7):2823-8.
2. Kaiserman I, Kaiserman N, Pe’er J. Long term ultrasonic follow up of choroidal naevi and their transformation to melanomas. Br J Ophthalmol. 2006 Aug;90(8):994-8.
3. Mashayekhi A, Siu S, Shields CL, Shields JA. Slow enlargement of choroidal nevi: a long-term follow-up study. Ophthalmology. 2011 Feb;118(2):382-8.
4. Schefler A. Retina Consultants of Houston. All about choroidal nevi.
5. Weidmayer S. Choroidal Melanoma is a Life Sentence.