Diseases & Conditions


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Porphyrias


Porphyrias are a group of disorders caused by deficiencies of enzymes involved in the production of heme.

Heme is a chemical compound that contains iron and gives blood its red color. Heme is the key component of several important proteins in the body. One of the proteins is hemoglobin, which enables red blood cells to carry oxygen.

Heme is produced in the bone marrow and liver through a complex process regulated by eight different enzymes. The enzymes work one after another in separate steps that take the starting compound through several different intermediate compounds (heme precursors, also called porphyrins), finally producing heme. If there is a deficiency in one of these enzymes, certain heme precursors may accumulate. They may accumulate in the bone marrow or liver, appear in excess in the blood, and get excreted in the urine or stool. The accumulated heme precursors cause symptoms. The specific heme precursors that accumulate and the symptoms that develop depend on which enzyme is deficient.

Classifying Porphyrias

Porphyrias can be classified in several ways. Classification according to the specific enzyme deficiency is the most accurate. A simpler classification system distinguishes porphyrias that cause neurologic, mental, and abdominal symptoms (acute porphyrias) from those that cause skin photosensitivity (cutaneous porphyrias). A third classification system is based on whether the excess precursors originate primarily in the liver (hepatic porphyrias) or primarily in the bone marrow (erythropoietic porphyrias). Some porphyrias are classified into more than one of these categories.

Porphyrias are a number of different disorders, each caused by a deficiency in one of the heme production enzymes. Each enzyme deficiency is caused by damage to the gene (a mutation) responsible for the production of the enzyme in question. The damaged gene is almost always inherited from one of the parents or, rarely, both.

Porphyrias are commonly divided into two types: Acute Cutaneous

Acute porphyrias cause intermittent attacks of abdominal, mental, and neurologic symptoms. These attacks are typically triggered by prescription drugs (including oral contraceptives), alcohol, exposure to organic solvents, and other factors such as fasting, infections, or stress. The most common acute porphyria is acute intermittent porphyria. Others include variegate porphyria, hereditary coproporphyria, and the extremely rare delta-aminolevulinic acid dehydratase-deficiency porphyria. Some acute porphyrias also cause skin (cutaneous) symptoms.

Investigating the Family

In order to avoid exposure to substances that can precipitate acute porphyria, people need to know whether they carry the gene for a deficient enzyme. A child whose parent has an enzyme deficiency that can cause an acute porphyria should be tested well before puberty. The genes in the child's blood sample are analyzed for the enzyme. Older family members of a person with an enzyme deficiency should also be tested to confirm or reject the possibility that they are predisposed to developing an acute porphyria.

Cutaneous porphyrias cause symptoms involving the skin, usually when the skin is exposed to sunlight. In these porphyrias, certain porphyrins are deposited in the skin. When exposed to light and oxygen, these porphyrins generate a charged, unstable form of oxygen capable of damaging the skin. The skin becomes fragile and blistered. The most common cutaneous porphyria is porphyria cutanea tarda. Others include erythropoietic protoporphyria, and the extremely rare hepatoerythropoietic porphyria and congenital erythropoietic porphyria.

In many of the porphyrias of both types, the urine may take on a red or reddish brown discoloration. Sometimes the discoloration appears only after the urine has stood in light for about 30 minutes.

Source: The Merck Manual Home Edition