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Acute intermittent porphyria is an uncommon genetic disorder. The condition results from a deficiency in the metabolism of porphyrins, a class of biochemical compounds which includes the oxygen-carrying molecule heme. Patients with acute intermittent porphyria can suffer seizures and abnormal brain function, but the cause of these neurological symptoms is not clear. Some researchers favor the notion that the metabolic defect affects brain metabolism as well, while others believe that the disorder affects the blood vessels of the brain, depriving many brain regions of adequate blood flow. Unfortunately, many of the investigations to uncover the cause of these neurological symptoms have been conducted using autopsy material; there are reasons to suspect that the dead patients are not a representative sample. With the advent of magnetic resonance imaging (MRI), however, it has become possible to look into the living brain with high resolution. In a recent case, a 20-year-old woman with acute intermittent porphyria suffered an attack which included seizures and hallucinations. Magnetic resonance images obtained during the attack revealed areas of increased signal intensity over large areas of the brain. These regions, which appear as bright areas in the MR image, returned to normal appearance after treatment successfully resolved the symptoms. The increased signal intensity visible on the images during the patient's attack does not reveal the cause of the abnormality with certainty. However, the appearance of the image is similar to that which is observed for abnormalities of the blood vessels, such as those which occur in disorders such as systemic lupus erythematosus, vasculitis, and hypertensive encephalopathy. Therefore, the observations made on this patient support the notion that the neurological symptoms that occur during attacks of acute intermittent porphyria may be due to abnormalities of the central nervous system circulation. (Consumer Summary produced by Reliance Medical Information, Inc.)