The identification of pigmented tissue within the oral cavity may present a diagnostic dilemma for the clinician. The manifestation of mucosal pigment is variable and can range from focal to diffuse macular coloration or from a small nodular growth to a large mass.1 Is the pigmentation of physiologic origin? Is it pathologic in etiology? Does it represent a malignant process? These are question
s that typically come to mind when a patient presents with oral mucosal pigmentation. The color, location, duration, distribution, and appearance of the pigmented lesion(s) may be of diagnostic importance. A thorough investigation of dental, medical, family and social histories, is also necessary to ensure accurate diagnosis. The presence of cutaneous pigmentation or other systemic signs and symptoms may be helpful in formulating a differential diagnosis. Clinical laboratory testing may also be beneficial. However, if the underlying cause of the pigmentation cannot be readily identified, a tissue biopsy is essential for definitive diagnosis.
A differential diagnosis for a clinically pigmented lesion may include an array of traumatic, reactive, neoplastic pathologies as well as pigmentation associated with systemic disease.1 Lesions or conditions of melanocytic, vascular, hematopoietic, and hemosiderotic origin may be given consideration. Genetic dysfunction may also manifest as oral pigmentation; Peutz-Jeghers syndrome is the prototypical example.1Where appropriate, exogenous substances including chemical coloring agents, amalgam, graphite, drug metabolites and chromogenic bacteria could also be considered.1 To this end, relatively common causes of mucosal coloration, including petechiae, purpura, ecchymoses, hematomas, vascular tumors and exogenous substances are not considered true pigmented lesions. In contrast, melanin, which is synthesized by melanocytes, is a true pigment and usually imparts a brown, blue or black appearance to the mucosa.2
Melanin pigmentation may be focal, multifocal or diffuse in its presentation. Melanin is the pigment derivative of tyrosine and is synthesized by melanocytes, which typically reside in the basal cell layer of the epithelium.2 In the skin, melanin is thought to be cytoprotective against the damaging effects of sunlight. The role of melanocytes in oral epithelium remains unclear. Unless a patient is of a race or ethnicity in which mucocutaneous pigmentation is physiologic, melanocytes are uncommonly observed in routine oral mucosal biopsies. Thus, in a Caucasian patient, oral melanocytic pigmentation may not always be of any significant clinical consequence but it is always considered pathologic in origin.
Pathologic melanin production within the oral mucosa may be associated with an array of etiologies. The most concerning of these are malignant melanoma and various systemic disorders, including adrenal insufficiency and Cushing disease (discussed below). Importantly, the oral manifestations of these potentially life-threatening disorders can mimic an array of idiopathic, reactive and benign neoplastic lesions. Thus, an understanding of the various disorders and substances that can contribute to oral mucosal coloration is essential for the appropriate evaluation, diagnosis and management of the patient. This article focuses on oral pathologies of melanocytic origin.