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The porphyrias are a heterogeneous group of disorders in which one or more of seven enzymes of haem biosynthesis show reduced activities due to inherited genetic abnormalities (table and fig) or secondary enzyme inhibition. Of particular concern to the neurologist are the four genetic neuropsychiatric porphyrias, in order of decreasing prevalence: acute intermittent porphyria, variegate porphyria, hereditary coproporphyria, and the extremely rare Doss porphyria. The overall prevalence of symptomatic acute porphyria is about 1 in 50,000. Relevant causes of secondary porphyria include chronic lead poisoning, hereditary tyrosinaemia, hexachlorbenzene poisoning, and liver disease including alcoholism and iron overload.
The hallmark of active or symptomatic porphyria is increased excretion of porphobilinogen (PBG) and amino-laevulinic acid (ALA). As only 10% of individuals who inherit the genetic defect develop the overt clinical features with raised excretion profiles, a normal excretion of these metabolites does not exclude inheritance of the defect but does exclude porphyria as a cause of the present symptoms. In between attacks the excretion of ALA and PBG may be normal …