Acute intermittent porphyria (AIP) is a rare inherited metabolic disorder in which the body cannot properly produce heme, the iron-containing molecule that carries oxygen in your blood. The result is a buildup of toxic chemical precursors that cause episodes of severe abdominal pain, neurological problems, and psychiatric symptoms. Most people who carry the gene mutation never develop symptoms, but for those who do, attacks can be debilitating and sometimes dangerous.
How AIP Disrupts Heme Production
Your body builds heme through a chain of eight enzymatic steps, mostly in the liver. In AIP, the third enzyme in that chain, called hydroxymethylbilane synthase (HMBS), doesn’t work properly. Under normal conditions, the partial deficiency doesn’t cause problems because the enzyme still has enough activity to keep up with demand. But when something increases the liver’s need for heme, the weakened enzyme becomes a bottleneck.
When that happens, two chemical precursors that sit upstream of the defective enzyme, aminolevulinic acid (ALA) and porphobilinogen (PBG), accumulate dramatically. During an acute attack, urinary levels of these precursors rise to 10 to 20 times the normal upper limit. ALA is the substance most likely responsible for the nerve damage and pain that define the disease. It is directly toxic to nerve tissue, which explains why AIP symptoms are overwhelmingly neurological rather than, say, related to anemia or oxygen delivery.
Genetics and Who Gets Symptoms
AIP follows an autosomal dominant inheritance pattern, meaning you only need one copy of the faulty HMBS gene to be at risk. If one of your parents carries the mutation, you have a 50% chance of inheriting it. The gene variant is more common than most people realize. In northern European populations, roughly 1 in 1,800 people carry a pathogenic HMBS mutation.
The striking feature of AIP is its low penetrance. Most gene carriers never have an attack. In the general population of carriers, only about 1% ever develop symptoms. Within families where AIP has already been diagnosed, the numbers are higher but still modest: a French study of 253 families found penetrance of about 23%, while Finnish and Swedish data put the figure between 30% and 41%. Women are significantly more likely to become symptomatic than men, with penetrance reaching 41% to 50% for women in porphyria families. This gender gap is largely driven by hormonal fluctuations, particularly progesterone.
What an Acute Attack Feels Like
The hallmark symptom is severe abdominal pain. It often comes on suddenly, is poorly localized, and can mimic surgical emergencies like appendicitis or bowel obstruction. The pain is caused by nerve dysfunction in the gut rather than inflammation, so imaging and blood work often come back looking deceptively normal, which leads to frequent misdiagnosis.
Beyond the abdomen, attacks can produce a wide range of neurological and psychiatric symptoms. Peripheral neuropathy, which may start as weakness or tingling in the limbs, can progress to significant muscle weakness. Autonomic nervous system involvement causes rapid heart rate, high blood pressure, nausea, vomiting, and constipation. Some patients experience seizures, confusion, anxiety, or hallucinations. Bladder dysfunction and red or dark urine are also common during attacks.
Attacks typically last days to weeks. About one-third of women with AIP notice symptoms in the days before their period, when progesterone levels peak. The combination of abdominal pain, psychiatric symptoms, and normal-looking lab results means many patients cycle through emergency departments for years before getting the right diagnosis.
Common Triggers
Acute attacks rarely happen spontaneously. They’re usually set off by something that increases the liver’s demand for heme or stimulates the first enzyme in the heme pathway. The most well-established triggers include:
- Medications: Many common drugs are dangerous in AIP, including certain antibiotics, anticonvulsants, and oral contraceptives. Online databases (such as the drug safety lists maintained by porphyria foundations) help patients and doctors check whether a medication is safe.
- Hormonal changes: Progesterone is a potent inducer of the enzyme that kicks off heme production. Premenstrual hormonal shifts trigger attacks in roughly one-third of symptomatic women.
- Fasting or low calorie intake: Skipping meals, crash dieting, or any illness that reduces food intake can provoke an attack. Carbohydrate intake plays a protective role by suppressing the heme production pathway.
- Alcohol and smoking: Both increase demand on the liver’s heme-producing machinery.
- Infections and physical stress: Illness, surgery, and emotional stress are all recognized precipitants.
How AIP Is Diagnosed
The single most important test during a suspected attack is a urine PBG level. During an acute episode, PBG excretion jumps to 20 to 200 mg per day, compared to a normal range of 0 to about 4 mg per day. A level exceeding 10 mg per gram of creatinine (roughly five times the upper limit of normal) is diagnostic for an acute porphyria and does not occur in any other medical condition. This makes it a highly specific test when it’s actually ordered, but the challenge is that many doctors don’t think to order it.
A spot urine sample collected during symptoms is usually sufficient for initial testing. Quantitative measurement of both ALA and PBG in urine confirms the diagnosis. Between attacks, PBG levels may fall back toward normal in some patients, so testing during an episode is ideal. Genetic testing for HMBS mutations provides definitive confirmation and allows screening of family members who may be silent carriers. Identifying carriers matters because they can then avoid known triggers and prevent their first attack from ever occurring.
Treating an Acute Attack
The immediate goals during an attack are to reduce the toxic buildup of ALA and PBG and to manage pain. How aggressively this is done depends on the severity of the episode.
For mild attacks (pain without vomiting, weakness, seizures, or dangerous electrolyte changes), the first-line approach is carbohydrate loading. Eating carbohydrate-rich foods or drinking sugary fluids suppresses the enzyme that drives heme precursor production. If oral intake isn’t tolerated, intravenous glucose at 300 to 500 grams per day can be given. Once the patient can eat again, a high-carbohydrate diet should continue. This approach works, but it works slowly.
For moderate to severe attacks, intravenous hemin is the standard treatment. Hemin directly replaces the heme the body is trying to make, shutting down the overactive production pathway and rapidly lowering ALA and PBG levels. Treatment typically lasts 3 to 14 days, and its effectiveness depends heavily on how quickly it is started. Delays in administering hemin allow nerve damage to progress, so early recognition of an attack matters enormously.
Preventive Therapy
For patients who experience frequent, recurrent attacks, a newer option called givosiran has changed the treatment landscape. Approved by the FDA in 2019, it is a monthly subcutaneous injection that uses RNA interference technology to silence the gene for the enzyme that initiates heme precursor production in the liver. By reducing the activity of this enzyme at the genetic level, givosiran dramatically lowers circulating ALA and PBG. In clinical trials, patients saw ALA levels drop by about 94% and PBG levels by about 95% after three months of treatment, with reductions sustained over time. The therapy also reduced the need for emergency hemin infusions.
Long-Term Health Risks
AIP is not just a disease of acute episodes. Chronically elevated porphyrin precursors take a toll on the kidneys and liver over time, even between attacks.
Kidney disease associated with porphyria affects more than half of patients with symptomatic AIP. More than half of those with kidney involvement also develop high blood pressure. While only about 6.7% of symptomatic patients progress to end-stage kidney failure, that rate is substantially higher than in the general population. The damage appears to involve the kidney’s filtering tubules, and elevated PBG levels correlate with markers of ongoing kidney injury.
Liver cancer (hepatocellular carcinoma) is another established risk. Elevated urinary PBG has been directly linked to increased risk of primary liver cancer in AIP patients. Current guidelines recommend that patients with symptomatic AIP undergo regular liver imaging, typically starting around age 50, to catch any malignancy early. Standard liver blood tests like ALT and AST are not sensitive enough to detect early liver damage in this population, so imaging-based screening is essential.
These long-term risks reinforce why reducing ALA and PBG levels matters not just during attacks but on an ongoing basis, particularly for patients with persistently elevated precursors between episodes.