Differentiation of central from peripheral type of motor neuron lesion

1. Big picture

This topic is one of the most important localization topics in neurology. In the exam, when a patient has weakness, the first question is not “what disease is it?” but:

Is this weakness central/upper motor neuron type, or peripheral/lower motor neuron type?

A central motor neuron lesion means damage of the upper motor neuron (UMN) pathway: motor cortex, corticospinal tract, corticobulbar tract, internal capsule, brainstem, or spinal cord pyramidal tract.

A peripheral motor neuron lesion means damage of the lower motor neuron (LMN) system: anterior horn cell, cranial nerve motor nucleus, anterior root, plexus, peripheral motor nerve.

The core exam rule:

UMN lesion → spasticity, hyperreflexia, pyramidal signs, mild disuse atrophy, no fasciculations. LMN lesion → flaccidity/hypotonia, hyporeflexia/areflexia, marked focal atrophy, fasciculations, no pyramidal signs.