This case series reports three new cases of cystic benign melanosis. All three patients had darkly pigmented skin, two were black and one was Native American. The condition was unilateral in one case (case 1), and bilateral in two cases with marked asymmetry. The lesion was of childhood onset in case one, and the other two cases began during adulthood. All three cases displayed slowly growing pigmented lesions concerning for malignancy that prompted biopsy.
Hutchinson et al reported black twin sisters with bilateral light brown pigmentation of the bulbar conjunctiva just outside the limbus7
. Electron microscopy showed melanocytes in the substantia propria and melanin granules in the cytoplasm of epithelial cells. None of the five total reported specimens of CBM contained nesting cells or nevus cells.
CBM and cystic nevus share several features (). Similar to CBM, cysts are a common feature of conjunctival nevi2,4
. Approximately 50 to 65% of conjunctival nevi contain cysts2,9
. Both CBM and nevi may be elevated, and often occur in childhood. CBM and cystic nevi may grow over time, but nevi classically grow during times of hormonal stimulation4
CBM and the cystic nevus are distinctly different entities. While both contain epithelial lined cysts and share other clinical features, they differ clinically in that CBM has less well-defined edges and may involve the cornea epithelium. Additionally, the pigment in CBM is from secondary melanosis of the basilar keratinocytes from epithelial melanocytes, while the pigment in a melanocytic nevus is from hamartomatous proliferation of epithelial melanocytes with deposition of melanin into neighboring keratinocytes. Immunohistochemical stains for S-100 were performed in all three of our cases and failed to demonstrate any melanocytic proliferations, confirming that the lesions represent secondary pigmentation of the epithelium and not an increase in melanocytic cells. Perhaps most importantly, CBM does not contain nevus cells.
CBM also shares some clinical features with PAM (). Both entities may present in middle-aged patients, and both display poorly defined edges and potential involvement of the cornea epithelium6
. CBM should be rather easily differentiated from PAM, as CBM is elevated and inherently contains cysts.
Case two requires special discussion due to the surgical history. The melanosis and cysts may have been prompted by incisions from cataract or strabismus surgery, but there are more cysts than would be expected in a surgical wound. Furthermore, the location of the melanosis and cysts are most dense in the superior and inferior bulbar conjunctiva, while the strabismus surgery involved only horizontal muscles. The cataract extraction likely involved a large superior scleral tunnel. Regardless, this case of cystic benign melanosis is quite unusual and was histopatholically similar to the other cases. Jahnle et al reported a similar finding in a black patient with racial melanosis and a unilateral pigmented conjunctival mass that clinically simulated melanoma8
. That patient presented six years after extracapsular cataract extraction with a fornix based flap. Histology revealed substantia propria cysts associated with melanocytic hyperpigmentation of the basilar cell layer of the epithelium as well as hyperpigmentation of the inclusion cyst lining.
CBM is a secondary melanosis with pigmented cystic inclusions that may be highly asymmetric. Prior inflammation of epithelial and goblet cells may cause sequestration into the substantia propria which subsequently forms cysts. Less likely, CBM could represent nevi where all nesting cells have regressed leaving behind substantia propria cysts and melanocytic pigmentation.
CBM is most analogous to benign racial melanosis with the unusual addition of subepithelial cysts. With increasing awareness of this unique entity and its differences from other brown pigmented conjunctival lesions, more knowledge regarding the natural history, clinical behavior, and heredity will be gained. Nonetheless, our cases indicate that CBM has no malignant potential and does not warrant further treatment after establishment of the diagnosis.