Toxic Neuropathies

Toxic polyneuropathies are rare occurrences in children in North America. Toxic exposure to heavy metals and environmental toxins may be more common in other regions of the world. Expeditious diagnosis is critical to identify and remove the source of the toxicity and to establish treatment with agents such as penicillamine. Arsenic polyneuropathy is a sensorimotor neuropathy that may be axonal or, at times, predominantly demyelinating, simulating Guillain-Barre syndrome or CIDP. Gastrointestinal symptoms are common as well as tachycardia and hypotension. Mee's lines may be seen in nails along with other skin changes and alopecia. The diagnosis is established by obtaining levels of arsenic in blood, urine, hair, and nail samples.

Lead polyneuropathy is most commonly observed in children who have ingested old lead-based paint. Acute exposures cause lead encephalopathy more commonly. Clinical findings may include anorexia, nausea and vomiting, gastrointestinal disturbance, fatigue, clumsiness and ataxia, and occasionally cognitive impairment, seizures, mental status changes, papilledema, and coma. The weakness is predominantly in the lower limbs, but the upper limbs may be involved. Electrophysiologic studies show a primarily axonal degeneration affecting motor greater than sensory axons. A microcytic hypochromic anemia with basophilic stippling of red blood cells establishes the diagnosis. Lead lines may be evident in long bone films. Lead levels may or may not be elevated in urine and blood but levels of delta aminolevulinic acid are usually elevated in the urine.

Mercury poisoning may occur from the ingestion of mercuric salts, exposure to mercury vapor or use of topical ammonia mercury ointments. Patients present with a generalized encephalopathy, fatigue, and occasionally a skin rash. A predominantly distal motor axonal neuropathy occurs. DTRs may be absent and the gait is often ataxic. Sensory examination is often normal, although patients may complain of distal paresthesias. Electrophysiologic studies show motor axonal degeneration with normal sensory conduction studies.

Organophosphate poisoning may be due to exposure to insecticides or high-temperature lubricants or softeners used in the plastic industry. Patients present with an encephalopathy manifested by confusion and coma. In acute exposure cholinergic crisis, manifested by sweating, abdominal cramps, diarrhea, and constricted pupils, may be present. A predominantly motor polyneuropathy is a late effect. However, the disorder may present as a rapidly progressive polyneuropathy mimicking Guillain-Barre syndrome. Severe paralysis with respiratory failure requiring ventilatory support may occur and in this situation there may be a superimposed postsynaptic defect in neuromuscular transmission.

Glue-sniffing (N-Hexane) neuropathy may be seen in teenage recreational glue sniffers. Repeated use may cause symptoms and signs of a predominantly distal motor and sensory polyneuropathy which is predominantly demyelinating. Motor and sensory nerve conduction studies demonstrate moderate slowing.

Chemotherapeutic agents, in particular, vincristine, often produce a relatively pure motor axonal polyneuropathy. Severity is dose-dependent. Clinical findings include distal weakness, absent DTRs, and at times foot drop. The disorder is often readily apparent by clinical examination and electrophysiologic studies or nerve biopsy is usually not necessary. The neuropathy usually improves with discontinuation of the medication, although significant electrophysiologic abnormalities (reduced CMAP amplitudes and neuropathic recruitment) may persist. Vincristine may be particularly troublesome for children with HMSN.

Metabolic Neuropathies

Uremic neuropathy often occurs in children with end-stage renal disease. If clinical manifestations are present, they consist of a predominantly distal motor and sensory polyneuropathy with glove and stocking loss of sensation, loss of vibratory sense, and distal weakness, particularly involving peroneal innervated musculature. With successful renal transplantation, clinical findings and electrophysiologic abnormalities normalize. Diabetic polyneuropathy usually is a mixed motor and sensory polyneuropathy with both axonal changes and mild demyelination. The polyneuropathy is less common in children with diabetes mellitus, as compared with adults. The severity of the neuropathy may be related to the degree of glucose control (165).

 
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