Acute intermittent porphyria (AIP) is a rare metabolic disorder affecting the production of heme resulting from a deficiency of the porphobilinogen deaminase. It is the most common of the acute porphyrias. Signs and symptoms
The clinical presentation of AIP is highly variable and non-specific. The patients are typically asymptomatic, with most gene carriers having no family history because the condition had remained latent for several generations. The syndrome marked by acute attacks affects only 10% of gene carriers. The mean age at diagnosis is 33 years old. Like other porphyrias, AIP is more likely to present in women. A distinguishing feature of AIP that separates it from other porphyrias is the absence of photosensitive cutaneous symptoms that occur in addition to acute attacks. Acute attacks
AIP is one of the four porphyrias that presents as an acute attack. 90% of affected individuals never experience an acute attack and are asymptomatic, while an estimated 5% of affected individuals experience repeat attacks. Attacks are most common in young adult women and are rare before puberty or after menopause. Severe acute attacks may require hospitalization. Patients usually experience symptoms in attacks that last from several hours to a few days. Between attacks, patients are asymptomatic.The most frequent presenting symptoms are abdominal pain and tachycardia. The abdominal pain is typically severe, colicky, poorly localized, and often associated with pain in back and legs. Patients may also present with vomiting and constipation, but diarrhea is unusual.
The penetrance of AIP is incomplete with only 10% of gene carriers experiencing acute attacks suggesting role for other modifying genes or environment.The exact mechanism of acute attacks is not clear. The most favored hypothesis is that porpholobilinogen buildup causes a toxic effects on neurons. The autonomic and peripheral nervous system are more vulnerable than the central nervous system because they are not protected by the blood-brain barrier. This explains findings such as abdominal pain and tachycardia. Some individuals may be more likely to develop paresis based on increased susceptibility of neurons to toxins. Genetics
Inheritance
AIP has an autosomal dominant pattern of inheritance. The dominance pattern is a result of partial deficiencies from the heme biosynthesis enzymes, hydroxymethylbilane synthase  (HMBS;  EC  2.5.1.61). Due to the rarity of this disease it is difficult to estimate the prevalence of AIP and the inclusion criteria differs widely among studies causing varying statistics. AIP has a low penetrance when considering the general population, but within families the penetrance increases. This is indicative of another interaction affecting the inheritance pattern. It is speculated that this pattern is due to AIP propensity caused by the inheritance of an additional gene mutation, HMBS, in addition to other genetic and environmental influences. There have been 421 HMBS mutations that have been linked to AIP. This suggests that AIP inheritance instead follows an oligogenic or polygenic inheritance pattern.