Severe abdominal episodes that mimic a dozen other diagnoses. A condition where knowing the genetic cause doesn't just explain the pain — it tells you exactly what medications and triggers to avoid.

Acute intermittent porphyria (AIP) is a rare autosomal dominant disorder of heme biosynthesis characterized by acute neurovisceral attacks triggered by specific drugs, hormonal changes, fasting, stress, or illness. Prevalence is approximately 1 in 75,000, though exact prevalence is uncertain due to underdiagnosis. Acute attacks feature severe abdominal pain (often colicky), nausea, vomiting, constipation, tachycardia, hypertension, and neuropsychiatric symptoms including confusion, anxiety, depression, and psychosis. Attacks can progress to dangerous complications including respiratory failure from neuromuscular involvement. Penetrance is remarkably low: only approximately 0.5–1% of germline HMBS variant carriers develop clinical attacks; approximately 20% penetrance is observed in known families with previous cases. Attacks are recurrent once initiated, with stress, menses, fasting, or medication triggering recurrence for decades. Many carriers never develop an attack and remain asymptomatic. The disease has been described as 'the little imitator' because acute presentation can mimic acute abdomen, psychiatric crisis, seizure, or other medical emergencies.

HMBS encodes hydroxymethylbilane synthase (porphobilinogen deaminase), the third enzyme in the heme biosynthesis pathway. It catalyzes the condensation of four molecules of porphobilinogen (PBG) into hydroxymethylbilane (HMB), a critical step in heme synthesis. Loss-of-function HMBS variants cause reduced enzyme activity, permitting accumulation of upstream pathway intermediates (δ-aminolevulinic acid, ALA, and porphobilinogen, PBG), which are neurotoxic. Elevated urine ALA and PBG are diagnostic for acute porphyria. More than 400 HMBS variants have been identified, including nonsense, missense, and splice-site mutations causing variable degrees of enzyme deficiency. In asymptomatic carriers (the majority), enzyme levels are sufficient to prevent attack unless triggered by factors that increase heme demand (e.g., drugs that induce cytochrome P450 enzymes, leading to increased demand for heme cofactor).

An HMBS pathogenic variant identification has major implications for lifestyle and medication safety. Confirmed AIP carriers must avoid 'porphyrinogenic' drugs including barbiturates (phenobarbitone, pentobarbital), sulfonamides, oral contraceptives (though hormone-specific subsets may be safer), NSAIDs, and many others; comprehensive drug lists are maintained at porphyria.org and debrisoquine.org. Carriers must maintain adequate calorie intake (fasting is a trigger), manage stress, and receive education about attack recognition. Acute attacks are treated with IV dextrose or IV hemin (hematin), the latter being highly effective at downregulating ALA synthase and terminating attacks. Givosiran (Givlaari), an RNA interference therapeutic targeting ALAS1 (the rate-limiting heme synthesis enzyme), was approved in 2019 and reduces annualized attack frequency from 12.5 to 3.2 attacks per year in phase 3 trials—a major therapeutic advance. For women, hormonal management during pregnancy and menopause requires careful planning.