Exact nature of abnormality of heme biosynthesis unclear in AIP. Proposed block in porphobilinogen (PBG) deaminase activity in AIP; protoporphyrinogen oxidase in variegate form. Excessive urinary excretion of PBG, δ-aminolevulinic acid (ALA), several porphyrins.

Symptoms caused by interaction of genetic and environmental factors. Porphyric crises result most often from ingestion of drugs that adversely affect porphyrin metabolism (see below), especially barbiturates for sedation or general anesthesia. Attacks also attributed to menses, starvation, emotional stress, intercurrent infections, other drugs.

Episodic attacks: gastrointestinal (autonomic neuropathy), psychiatric, neurologic. Abdominal pain most common; alone or with neurologic or psychiatric disorder (conversion reaction, acute delirium, mood change, or acute or chronic psychosis). Neuropathic symptoms sometimes purely motor but almost always associated with abdominal pain. Rash may occur in variegate form (approximately 50%) but not in AIP; otherwise clinical features are indistinguishable.