White lesions may be transient or persistent. Transient white patches are either due to Candida infection or are very occasionally seen in systemic lupus erythematosus. Oral candidiasis in adults is seen in seriously ill or immunocompromised patients, or following therapy with broad-spectrum antibiotics or inhaled steroids.

Local causes include mechanical, irritative or chemical trauma from drugs (e.g. aspirin).

Leucoplakia describes white patches for which no local cause can be found. It is associated with alcohol and (particularly) smoking, and is regarded as a premalignant condition. A biopsy should always be undertaken; histology shows alteration in the keratinization and dysplasia of the epithelium; aneuploidy is associated with a very high risk of cancer. Treatment is unsatisfactory. Isotretinoin possibly reduces disease progression. Oral lichen planus presents as white striae.

Leprosy (Hansen’s disease)

Leprosy usually involves the skin, and the clinical features depend on the body’s immune response to the organism Mycobacterium leprae.

Indeterminate leprosy is the commonest clinical type, especially in children. This presents as hypopigmented or erythematous circular macules with occasional mild anaesthesia and scaling. This may resolve spontaneously or progress to one of the other types. Biopsy reveals a perineural granulomatous infiltrate and scant acid-fast bacilli.

Tuberculoid leprosy presents with a few hypopigmented or erythematous plaques with an active erythematous, raised rim. Lesions are usually markedly anaesthetic, dry and hairless reflecting the nerve damage. Nerves may be enlarged and palpable. Biopsy shows a granulomatous infiltrate centred on nerves, but no organisms.

Lepromatous leprosy presents with multiple inflammatory papules, plaques and nodules. Loss of the eyebrows (’madarosis’) and nasal stuffiness are common. Skin thickening and severe disfigurement may follow. Anaesthesia is much less prominent. Biopsy shows numerous acid-fast bacilli.