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  • Muscle weakness and atrophy

  • Absence of overt cognitive and sensory changes

  • Altered tendon reflexes—hyperactive if upper motor neurons (UMNs) are affected and either hypoactive or absent if lower motor neurons (LMNs) are involved

  • Babinski sign (if UMN is involved)

General Considerations

By convention, the term motor neuron disease encompasses disorders that predominantly or exclusively affect upper motor neurons, lower motor neurons, or both. By definition, sensory neurons are spared in these diseases. Motor neuron diseases can be acquired or inherited; however, the most common of these diseases in adults, amyotrophic lateral sclerosis (ALS), is typically sporadic and its cause is unknown (Tables 20–1 and 20–2).

Table 20–1.Acquired motor neuron diseases.
Table 20–2.Inherited motor neuron diseases.

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