Primary acquired melanosis (PAM) is patchy, flat, acquired conjunctival pigmentation.  Histologically, PAM may have one of two main patterns:

(1) PAM without atypia is a histologic term denoting either increased pigmentation within the epithelium without hyperplasia of melanocytes or hyperplasia of the basal melanocytes lacking cytologic atypia. 

(2) PAM with atypia denotes atypical melanocytes which may involve or replace the epithelium with spindle or epithelial cells.  PAM without atypia does not usually progress to melanoma, whereas almost half of the cases of PAM with atypia eventually progressed to melanoma.