This is the most common cause of oral pigmentation. The gingivae are particularly affected (Fig. 21.1). The inner aspect of the lips is typically spared. Intraoral pigmentation is commoner in those with a dark skin. However, light-skinned individuals have up to an average of 30 cutaneous local pigmented lesions of various types and occasionally one will be present in the mouth.

Inflammation interferes with the synthesis of melanin by melanocytes and its transfer to keratinocytes. Melanosomes escape from the epithelium into the underlying connective tissue where they are taken up by macrophages, a process known as melanin ‘drop out’. Accumulation of this subepithelial melanin gives rise to post-inflammatory pigmentation. The process is commoner in dark-skinned races. Intraorally, the most common inflammatory condition to become pigmented is lichen planus (Fig. 21.2) and it can become very dark. Pigmentation may also develop in other inflamed sites and scars. This type of pigmentation is also seen diffusely through the mouth of heavy cigarette smokers.

Fragments of amalgam frequently become embedded in the oral mucosa and form the most common pigmented patches which may simulate melanomas clinically. They are usually close to the dental arch and typically 5  mm or more across (Fig. 21.3). Dense tattoos may be radiopaque.

Peutz–Jeghers syndrome, a rare heritable disease, is characterised by multiple mucocutaneous pigmented macules and intestinal polyposis. The pigmented macules typically develop during the first decade of life and can be widespread, affecting the hands and feet and perioral skin as well as the oral mucosa. Cutaneous macules usually fade after puberty but the oral macules persist (Fig. 21.5).

Clinically, the oral macules resemble other melanotic macules and affect the lips, buccal mucosa, tongue and palate.

The intestinal polyps mainly affect the small intestine, are regarded as hamartomatous and rarely undergo malignant change. Colon cancer is a hazard but the risk for breast and gynecological cancers is particularly high. Multiple oral melanotic macules may therefore indicate the need for cancer screening.

Histologically, the oral macules show slight acanthosis and melanin in the basal layer. The melanocytes may have elongated dendritic processes.

The genetic locus of this syndrome is the mutated gene LKB1(STK11) on chromosome 19p. This was the first gene to be found that predisposes to cancer by disabling protein kinase activity.