The porphyrias are caused by inherited deficiencies of enzymes involved in heme synthesis.75 Heme is used in the bone marrow to make hemoglobin and in the liver to make cytochrome P-450 enzymes. The hepatic synthesis of heme is largely regulated by d-aminolevulinic acid (ALA) synthase, an inducible enzyme.
The symptoms and signs of the porphyrias result from the accumulation of toxic metabolites. They are classified either as erythropoietic and hepatic or on the basis of the specific enzyme deficiency.
Seizures and other neurologic manifestations only occur in the hepatic group, in which the porphyrin precursors ALA and porphobilinogen (PBG) accumulate.76 Both compounds have been implicated as directly neurotoxic.77
Three acute hepatic porphyrias, all autosomal dominant, are associated with seizures:78
HCP and VP are much less common than AIP. They present similarly except that HCP and VP are also associated with cutaneous photosensitivity.
Patients with AIP have approximately 50% reduction of PBG deaminase activity.75 This condition is particularly prevalent in Northern Europe, especially Finland.79 When the demand for hepatic heme synthesis is increased, the reduced PBG deaminase activity leads to the accumulation of ALA and PBG in the liver, plasma, and urine.78 Some of the PBG and ALA is converted to porphyrin.
AIP can be precipitated by:80
Clinical manifestations can persist for days to weeks and may be preceded by nonspecific symptoms.76 The most common acute symptom is abdominal pain, often severe enough to require narcotic analgesia. The pain is sometimes accompanied by nausea, vomiting, ileus, and constipation.
Sympathetically mediated signs include tachycardia, hypertension, sweating, and tremors. Sleep disturbance, anxiety, delirium, hallucinations, depressed mood, and paranoid delusions may be present.81,82 Lethargy and coma occur rarely. Peripheral motor neuropathies may lead to foot or wrist drops.
Seizures associated with AIP occur either as a direct neurologic manifestation of the condition or from hyponatremia, which may result from SIADH, vomiting, diarrhea, or poor oral intake. Some studies suggest that focal and generalized seizures occur in nearly one-third of pediatric cases and up to 20% of adult cases.83-85 By contrast, a study of patients registered at the National Porphyria Center in Sweden found that the lifetime prevalence of AIP-associated seizures was 2.2% of all those with known AIP and 5.1% of all those with manifest AIP.86
The diagnosis may be suspected based on the clinical presentation and may be confirmed by the presence of photosensitive porphyrins in the urine and reduced monopyrrole PBG deaminase in red blood cells.81,87
After a definitive diagnosis, treatment of acute attacks consists of
Drugs known to precipitate AIP must be avoided,78,90 particularly when treating seizures, pain, and acute anxiety attacks. Hahn et al. used a cell-culture model of primary chicken embryo liver cells, which maintain intact heme synthesis and regulation, to study the effects of several of the more recently approved antiepileptic drugs on porphyrin accumulation.91 Based on the results, they recommended vigabatrin or gabapentin, for which success was reported anecdotally,92 but not felbamate, lamotrigine, or tiagabine, as possible treatments for seizures in patients with porphyria.
Similar animal studies have suggested that patients with acute porphyrias may be at greater risk for developing porphyric attacks when treated with certain medications:
|Greater risk||Less risk|
|tramadol||hydrocodone, oxycodone, morphine93|
|bupropion, nefazodone||fluoxetine, paroxetine|
|diazepam, midazolam, triazolam||low doses of lorazepam, oxazepam94|
Complete recovery from attacks is the rule, although neuropathic deficits may take months to resolve.
Reviewed and revised April 2004 by Steven C. Schachter, MD, epilepsy.com Editorial Board.
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