Certain disorders cause the fairly selective loss of motor neurons, the most common of which is the idiopathic disorder called amyotrophic lateral sclerosis, motor neuron disease, or Lou Gehrig’s disease. This disorder causes motor neurons to degenerate over years, leading to progressive diffuse weakness. Both upper and lower motor neurons are usually lost, causing a mix of upper and lower motor neuron abnormalities on examination. Like most of the other idiopathic neurodegenerative conditions, it is unclear why only certain populations of neural cells are lost in the disorder while sparing the rest of them, at least initially. Late in the course of the disease, some of these patients will also develop diffuse neuronal loss of areas of the cerebral cortex, which may lead to a type of dementia.
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Introduction to Neurology 