How can you distinguish upper motor neuron signs from lower motor neuron signs during evaluation?

• A. LMN signs include spasticity and hyperreflexia; UMN signs include flaccidity, atrophy, and fasciculations.
• B. UMN signs include neuropathy; LMN signs include muscle tone abnormalities.
• C. UMN signs include spasticity and hyperreflexia; LMN signs include flaccidity, atrophy, and fasciculations.
• D. UMN signs include decreased reflexes; LMN signs include increased reflexes.

Answer: (C)

Distinguishing UMN from LMN signs rests on the pattern of motor impairment you observe with evaluation. An upper motor neuron lesion disrupts pathways that modulate reflexes and tone, so you see increased muscle tone and heightened reflex responses: spasticity is a velocity-dependent rise in tone, and hyperreflexia means exaggerated deep tendon reflexes. A lower motor neuron lesion, on the other hand, injures the final common pathway to the muscle, leading to reduced muscle activity: decreased or flaccid tone, muscle atrophy from loss of neural input, and fasciculations from motor unit instability or denervation.

So the pattern described—spasticity and hyperreflexia with UMN involvement, flaccidity, atrophy, and fasciculations with LMN involvement—best differentiates the two. The other statements mix signs in ways that don’t fit the physiology (for example, UMN lesions don’t typically cause decreased reflexes or peripheral neuropathies aren’t a direct UMN sign, and LMN signs aren’t characterized by increased reflexes).