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What is Porphyria?
Porphyria is not a single disease but a group of at least eight disorders that differ considerably from each other. A common feature in all porphyrias is the accumulation in the body of "porphyrins" or "porphyrin precursors". Although these are normal body chemicals, they normally do not accumulate. Precisely which of these chemicals builds up depends upon the type of porphyria.
The clinical manifestations of the different types of porphyria are not the same. Forms of treatment also depend on the type of porphyria. Therefore, it is difficult to make general statements that apply to all these disorders.
The symptoms arise mostly from effects on the nervous system or the skin. Effects on the nervous system occur in the acute porphyrias. Proper diagnosis is often delayed because the symptoms are nonspecific. Skin manifestations can include burning, blistering and scarring of sun-exposed areas.
The terms "porphyrin" and "porphyria" are derived from the Greek word "porphyrus" meaning purple. Urine from some porphyria patients may be reddish in color due to the presence of excess porphyrins and related substances, and the urine may darken after standing in the light.
How does one get Porphyria?
In each type of porphyria there is a deficiency of a specific "enzyme". These enzymes are involved in the synthesis of heme, a substance important to many body functions and found in largest amounts in the bone marrow, red blood cells and the liver. Heme exists as hemoglobin in the bone marrow and red blood cells but has other functions in other tissues such as the liver. The type of porphyria present is determined by which enzyme is deficient. These enzyme deficiencies are usually inherited. Environmental factors, such as drugs, chemicals, diet and sun exposure can, depending on the type of porphyria, greatly influenced the severity of symptoms.
The inherited porphyrias are either autosomal dominant (inherited from one parent) or autosomal recessive (inherited from both parents). "Autosomal" genes always occur in pairs, with one coming from each parent. Individuals with an autosomal dominant form of porphyria have one abnormal gene paired with a normal one, and half of their offspring (on the average) will inherit the gene for the disease, while the other half will inherit the normal gene. Some of those who inherit the abnormal gene will develop symptoms. Individuals with an autosomal recessive type of porphyria have a pair of abnormal genes, and each of their children will inherit one abnormal gene for that disease, which will be paired with a normal gene from the other parent, and there will be no symptoms. If two carriers of the same abnormal recessive gene marry, approximately one fourth of their children will inherit two abnormal genes, and these children will develop symptoms of the disease. Because all porphyrias are uncommon, it is very unlikely that more than one type will occur in the same family, or that someone with one type of porphyria will go on to develop another.
What Treatment and Prevention are Available?
Treatment depends greatly on the type of porphyria and can be quite successful. Preventive measures, which include avoidance of certain drugs and alcohol, are also important in those who are identified as having inherited porphyria, even if they have never had symptoms. Asymptomatic carriers as well as patients who have symptoms should be educated about preventive measures.
Is Sunlight Always Harmful?
Sun sensitivity can occur in all but two types of porphyria. The degree of sensitivity to sunlight varies considerably. Patients with sun sensitivity have high levels of porphyrins in the blood plasma which, depending on the type of porphyria, have originated from the liver or the bone marrow. Ultraviolet light interacts with porphyrin in such a way as to damage skin tissue. Some treatments may help patients tolerate sun exposure even without lowering porphyrin levels. In some cases, treatment can lower porphyrin levels, and sunlight can be tolerated.
Diagnostic Testing for Porphyrias
There are many laboratory tests available for porphyrias, and it is often difficult to decide which should be chosen. Many of these tests are expensive. The results are often difficult to interpret. The tests vary in sensitivity and specificity. If a test is sensitive, it is unlikely to be falsely negative (i.e. fail to diagnose porphyria in a patient who has the disease). If a test is specific, it is unlikely to be falsely positive (i.e. diagnose porphyria in a patient who does not have porphyria). Certain tests are both sensitive and specific in patients who have symptoms that are suggestive of porphyria. When abdominal and neurological symptoms suggest an acute porphyria, the best screening tests are urinary ALA and PBG. When there are cutaneous symptoms that suggest porphyria, the best screening test is a plasma porphyrin determination. If one of these screening tests is abnormal, more extensive testing, including urinary, fecal and red blood cell porphyrins, are then indicated. Urinary, fecal, and red blood cell porphyrin measurements are not very useful for initial screening, because they lack either sensitivity or specificity and, therefore, are often difficult to interpret. Measurement of heme biosynthetic enzymes in red blood cells or lymphocytes is not appropriate for screening, unless it is part of a family study that is done after someone in the family is already known to have a particular enzyme deficiency. It is advisable to have testing performed by a laboratory that has expertise in the clinical aspects of porphyria and can provide a valid interpretation of the test results.
If testing has been performed in laboratories other than porphyria laboratories, consultation with a porphyria expert is advised before a final diagnosis of acute porphyrias is accepted.
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