Which disease is classified as a mixed (upper and lower motor neuron) motor neuron disorder?

Amyotrophic lateral sclerosis (ALS) is classified as a mixed motor neuron disorder because it involves both upper and lower motor neuron degeneration and dysfunction. In ALS, there is a progressive degeneration of the motor neurons in the anterior horn of the spinal cord (lower motor neurons) as well as degeneration of the corticospinal tracts in the brain and spinal cord (upper motor neurons).

The clinical manifestations of ALS can include muscle weakness, atrophy, and fasciculations due to lower motor neuron involvement, as well as spasticity, increased muscle tone, and hyperreflexia due to upper motor neuron involvement. This combination of symptoms clearly distinguishes ALS from other motor neuron disorders that may only affect one type of neuron.

For example, spinal muscular atrophy primarily affects lower motor neurons and does not involve upper motor neuron pathways. Guillain-Barré syndrome is an acute polyneuropathy affecting peripheral nerves, leading to lower motor neuron symptoms without upper motor neuron involvement. Post-polio syndrome occurs in individuals who have recovered from polio and primarily exhibits signs related to lower motor neuron dysfunction. Thus, ALS stands out as the disorder exhibiting both upper and lower motor neuron pathology.