A 48-Year-Old Man With Abdominal Pain and Vomiting: Osmosis USMLE Study Question of the Week

January 20, 2017

Answer: D. Porphobilinogen deaminase

Abdominal pain, vomiting, paranoia, and anxiety are consistent with acute intermittent porphyria, which is the second most common form of porphyria after porphyria cutanea tarda. Acute intermittent porphyria is an autosomal dominant metabolic disorder that affects the production of heme, which is the oxygen-binding prosthetic group of hemoglobin. Acute intermittent porphyria is characterized by a deficiency of the enzyme porphobilinogen deaminase, which is also called hydroxymethylbilane synthase. Without porphobilinogen deaminase, heme synthesis cannot finish, and the metabolite porphobilinogen accumulates in the cytoplasm.

Cytochrome P450 inducers, such as alcohol, can precipitate acute attacks in patients with acute intermittent porphyria because they stimulate cytochrome inducers, which, in turn, increase the synthesis of heme proteins. This leads to a faster buildup of porphobilinogen, thus exacerbating the symptoms. Patients are also commonly misdiagnosed with psychiatric diseases. Subsequent treatment with antipsychotics can increase the accumulation of porphobilinogen, thus aggravating the disease enough to make it fatal.

Most patients with acute intermittent porphyria develop nonspecific symptoms, such as recurrent or acute abdominal pain, peripheral neuropathy, and psychological disorders (ie, anxiety, paranoia). Specific areas of focus should include a detailed family history, in order to identify an autosomal dominant inheritance pattern.

Major Takeaway: Acute intermittent porphyria is an autosomal dominant disorder characterized by a deficiency of hydroxymethylbilane synthase. Cytochrome P450 inducers (ie, alcohol, rifampin, and phenytoin) are related with the exacerbation of symptoms.

For more on acute intermittent porphyria, read here.

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