Neurological manifestations of acute intermittent porphyria

Abstract

Acute intermittent porphyria (AIP) is an inherited metabolic disease due to a deficiency of the hydroxymethylbilane synthase in the haem biosynthesis. It manifests with occasional neurovisceral crises due to overproduction of porphyrin precursors such as aminolaevulinic acid (ALA) which is released from the liver to the circulation. The majority of the acute attacks manifest as a combination of abdominal pain, mild mental symptoms and autonomic dysfunction mainly due to vagal insufficiency. However, both acute peripheral neuropathy and encephalopathy may develop if an acute attack proceeds especially due to administration of porphyrinogenic drugs. Acute porphyric neuropathy is predominantly motor and associates with a history of abdominal pain and dysautonomia, CNS involvement and mild hepatopathy. Other features include preservation of achilles reflexes while global hyporeflexia and neuropathic or myalgic pain. The pathogenesis of porphyric neuropathy is complex but overproduction of ALA via direct neurotoxicity, oxidative damage, and modification of glutamatergic release may initiate the neuronal damage. Acute encephalopathy manifests as a combination of mental symptoms, seizures, SIADH, but rarely focal CNS deficits. Posterior reversible encephalopathy syndrome (PRES), which has been found in patients' MRI during an acute attack with severe encephalopathy, could explain the pathogenesis of encephalopathy and seizures in AIP. Neurological manifestations are unspecific and careful interpretation of abnormal excretion of porphyrin precursors should be done before the symptoms can be related to inherited acute porphyrias and not to secondary porphyrinuria. Currently the prognosis of neuropathy and encephalopathy in AIP is good even in severe attacks, but physicians should be aware of a potentially fatal outcome of the disease.