porphyria

n.

one of a group of rare disorders due to inborn errors of metabolism in which there are deficiencies in the enzymes involved in the biosynthesis of haem. The accumulation of the enzyme's substrate gives rise to symptoms of the disorder. The defect may be primarily in the liver (hepatic porphyria) or in the bone marrow (erythropoietic porphyria) or both. The prominent features include the excretion of porphyrins and their derivatives in the urine, which may change colour on standing (see porphobilinogen); sensitivity of the skin to sunlight causing chronic inflammation or blistering; inflammation of the nerves (neuritis); mental disturbances; and attacks of abdominal pain. The commonest porphyria is porphyria cutanea tarda, which affects up to 1 in 5000 people in some countries. It is a hereditary or acquired hepatic porphyria in which light-exposed areas of the skin become blistered and fragile. Acute intermittent porphyria is a hereditary hepatic porphyria characterized by recurrent attacks of acute abdominal pain, severe constipation, and psychotic behaviour. Factors triggering attacks include alcohol and many drugs.