In summary, the most characteristic features of porphyric neuropathy are the relapsing nature, acute onset, abdominal pain, psychotic symptoms, predominant motor neuropathy, often with an early bibrachial distribution of weakness, truncal sensory loss, and tachycardia. Rarely, the neuropathy develops without other symptoms.
The pathologic findings in the peripheral nervous system vary according to the stage of the illness at which death occurs. In the first few days, the myelinated fibers may appear entirely normal, despite almost complete paralysis. If symptoms had been present for weeks, degeneration of both axons and myelin sheaths are found in most of the peripheral nerves. The relation between the abnormality of porphyrin biosynthesis in the liver and nervous dysfunction has never been explained satisfactorily.
The diagnosis is confirmed by the demonstration of large amounts of porphobilinogen and Δ-aminolevulinic acid in the urine. The urine turns dark when standing as a consequence of the formation of porphobilin, an oxidation product of porphobilinogen.
Treatment The use of intravenous glucose and intravenous hematin (4 mg/kg daily for 3 to 14 days) is recommended as the most effective therapy. Other aspects of treatment include respiratory support, use of beta-blocking agents (labetalol) if tachycardia and hypertension are severe, continued intravenous glucose to suppress the heme biosynthetic pathway, and pyridoxine (100 mg bid) on the supposition that vitamin B6 depletion has occurred. Attempted prevention is of the utmost importance, since attacks can be precipitated by the aforementioned drugs as well as numerous others that are porphyrinogenic.
As indicated in Chap. 41, the peripheral nerves may be affected by a wide variety of toxins including metals, drugs, organophosphates, and industrial solvents. As a rule, the neuropathies induced by these agents fall into the subacute and chronic categories (to be discussed further on). However, certain drugs—notably triorthocresyl phosphate (TOCP) and other organophosphates, thallium and rarely, arsenic—produce a polyneuropathy that may be fatal in a matter of days. It should be stressed that organophosphate neuropathy can be identified in almost all instances by the severe cholinergic effects that are apparent immediately after exposure. Severe and permanent motor paralysis is caused by TOCP; this ultimately proves to be a result of involvement of both upper and lower motor neurons.