Chapter 18 Toxic disorders of the upper motor neuron system

Lathyrism and konzo are two similar self-limiting cortical motor neuron disorders characterized by spastic disability of the lower extremities and, in severe cases, with disability in the upper extremities and even (in konzo only) pseudobulbar dysfunction. They are caused by prolonged and almost exclusive dietary dependence on grass pea (lathyrism) or insufficiently processed bitter cassava (konzo), with poor nutritional state a significant risk factor and excessive exercise an apparent precipitating factor. Only lathyrism has been partially modeled in laboratory primates. Experimental studies suggest these toxic neurodegenerative disorders are primary neuronopathies dominated by bilateral involvement of upper motor neurons and their axonal projections. Those cortical motor neurons with the longest axonal projections seem to be most vulnerable, possibly because they have the largest dendritic tree and available glutamate receptors that are likely targets of the culpable grass pea neurotoxin (BOAA) and the neurotoxic cassava metabolites (SCN, OCN ti]Onset of lathyrism and konzo is often abrupt, and some degree of clinical improvement is characteristic before the clinical signs stabilize into a pure form of spastic para/ tetraparesis. Clinical progression may occur with continued intake of the offending foodstuff. Prolonged lower-level intake of cassava has been associated with distinct neurological syndromes in adults, notably an ataxic (myelo)neuropathy for which OCN is a likely etiological factor.