Melanocytic Proliferations of the Uveal Tract

The uveal tract is the pigmented vascularized soft tissue coat of the eye. It consists of the iris, ciliary body, and choroid. Melanocytic lesions can occur throughout the uveal tract. These invclude nevi, and what may be their malignantly transofrmed courterpart, melanomas. This chapter discusses the clinical findings, histopathological features, molecular findings, differential diagnosis, prognosis and treatment of these lesions.

Uveal Melanocytic Nevus

The uveal melanocytic nevus is the most common primary intraocular tumor in adults. It is most often situated in the choroid but may also be found in the iris ( Fig. 23.1 ), ciliary body, or optic disc. Estimates for its frequency range from 10% to 20% of Caucasians older than 50 years. If small melanocytic iris lesions were included, 30% to 50% of eyes would have at least one uveal melanocytic nevus. A uveal melanocytic nevus may be a precursor for uveal melanoma. However, that is a rare event. The rate of for uveal melanocytic nevi to undergo malignant transformation has been estimated at approximately 1 per 6000 to 15,000 per year.

Clinical Findings

A choroidal nevus is usually flat and has sharp borders (see Fig. 23.1A ). Patients with uveal nevi are usually asymptomatic. Secondary retinal changes can develop overlying the nevus and are typically a sign of chronicity. However, visual loss may result from a macular choroidal nevus. The typical iris nevus appears as a localized melanotic stromal lesion of the iris. It can involve any portion of the iris from the pupillary margin to the iris root. The typical iris nevus is 3 mm or less in diameter and 0.5 mm or less in thickness. Uveal nevi are followed closely in the clinic for signs of growth or potential malignant transformation.

Histopathologic Features

Uveal melanocytic nevi are usually not biopsied or excised. Knowledge of their pathologic features comes from incidental lesions found in eyes removed for other reasons or autopsy studies. On rare occasion an iris nevus may also be biopsied for reassurance (i.e., to exclude melanoma) (see Fig. 23.1B ). Similar to melanocytic nevi at other sites, a range of cytologic features has been reported, ranging from small epithelioid to fusiform. The amount of melanin pigment may vary greatly from none to heavy. Heavily pigmented blue nevus–like melanocytic proliferations of the uveal tract are often referred to as melanocytoma. Many uveal nevi are composed of a mixture of cell types. It has been reported that melanocytic nevi of the iris, in contrast to nevi of the posterior uvea, tend to contain large epithelioid multinucleated cells, as well as melanocytes with nuclear pseudoinclusions.