Melanotic lesions of the mucosa are extremely common and most of these are referred to as melanotic macules both clinically and on histopathology. The most common sites are lips (wet surface or vermilion), palatal mucosa and gingiva. Little change has occurred since this term was coined decades ago. I have often wondered: are these post-inflammatory hypermelanosis (PIH)? It is hard to say. Once the biopsy is done and a diagnosis of melanotic macule made, the patient is discharged and we never know whether the lesion then regressed which would support a PIH. One clinical feature that goes against a PIH is that some of them have been present for years, but those may be entirely different lesions altogether, and we are comparing apples and oranges.
They are commonly mislabeled as ephelis or lentigo by general and dermatopathologists. The typical appearance is of abundant pigment within the basal cells with minimal to no melanocytic hyperplasia, and melanophages in the lamina propria with “incontinent” pigment. There is usually minimal to no inflammation, but often vascular ectasia and sometimes fibrosis. I don’t think it is necessary to see inflammation to make a diagnosis of PIH since the pigmentation from PIH usually lingers for weeks or months, after inflammation has subsided, similar to what is seen on the skin I think.
Drug-induced pigmentation can look very similar, but more of that later.