This chapter discusses the upper and lower motor neuron disorders. Motor neuron disorders can be classified according to the structure involved, the age at onset, the course of the disease, the primary site of involvement, and the genetic evidence. In amyotrophic lateral sclerosis (ALS), there is clinical and pathological involvement of both upper and lower motor neurons, usually with sparing of the oculomotor nuclei and the spinal Onuf nucleus, which supplies the anal sphincter muscles. The clinical hallmark of ALS is the coexistence of neurogenic atrophy, weakness, and fasciculations caused by lower motor neuron degeneration, together with hyperactive or incongruously present deep tendon reflexes, pyramidal tract signs, and increased muscle tone caused by corticospinal tract involvement. Although the classic clinical presentation leaves little doubt about the diagnosis, certain ALS cases can be difficult to diagnose, particularly at an early stage. The differential diagnosis of ALS includes, physical causes, immune disorders, toxins, infections, metabolic disorders, enzyme deficiencies, and other neurological disorders.
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