How can you distinguish upper motor neuron from lower motor neuron lesions clinically?

• A. UMN: flaccidity; LMN: spasticity
• B. UMN: hyporeflexia; LMN: hyperreflexia
• C. UMN: spasticity, hyperreflexia, Babinski; LMN: flaccidity, hyporeflexia, fasciculations
• D. UMN and LMN have identical presentations

Answer: (C)

The key idea is how the motor system changes with where the lesion lies. An upper motor neuron (UMN) lesion removes brain-supplied inhibition from the corticospinal pathways, leading to increased muscle tone (spasticity), exaggerated reflexes (hyperreflexia), and an abnormal plantar response (Babinski sign). A lower motor neuron (LMN) lesion disrupts the actual motor neurons or their axons, causing reduced muscle tone (flaccidity), diminished or absent reflexes (hyporeflexia), and signs of motor neuron loss such as fasciculations.

So, the best way to distinguish them clinically is by looking for spasticity, hyperreflexia, and Babinski sign for UMN involvement, versus flaccidity, hyporeflexia, and fasciculations for LMN involvement. Note that in some acute or unusual situations, UMN signs can be transiently reduced (spinal shock), but the classic pattern remains the most reliable discriminator.