Upper motor neuron lesion vs Lower motor neuron lesion
How to tell Upper motor neuron lesion and Lower motor neuron lesion apart when a board question makes them look alike.
UMN lesion: spastic weakness, hyperreflexia, upgoing Babinski, no atrophy; LMN lesion: flaccid weakness, hyporeflexia, atrophy, fasciculations.
Side by side
Weakness with spasticity, hyperreflexia, increased tone, and Babinski sign.
Weakness with flaccidity, hyporeflexia, fasciculations, and muscle atrophy.
Corticospinal tract injury removes inhibitory control over spinal reflex arcs.
Anterior horn cell, nerve root, peripheral nerve, or motor neuron injury denervates muscle.
Babinski and clonus support UMN involvement.
Fasciculations and marked atrophy support LMN involvement.
The trap
Weakness alone never localizes the lesion. The high-yield traps: ALS gives both UMN and LMN signs in the same limb without sensory loss; a spinal cord lesion gives LMN signs at the level of the injury (anterior horn destruction) and UMN signs below it; a cortical or capsular stroke gives pure contralateral UMN signs but may be flaccid for hours to days during spinal shock before evolving to spastic.
Watch it explained
Test it nowThe repair move
Do not label weakness until you check tone, reflexes, atrophy/fasciculations, and whether UMN plus LMN signs coexist.
Try a free question
CoreA patient has leg weakness with spasticity, increased tone, brisk reflexes, ankle clonus, and an upgoing plantar response. Which lesion type is most likely?
Show answer and explanation
Answer: A. Upper motor neuron lesion. Spasticity, hyperreflexia, clonus, and Babinski sign point to an upper motor neuron lesion.
Why the other choices are tempting
- B. LMN lesions cause flaccidity, hyporeflexia, atrophy, and fasciculations.
- C. NMJ disorders cause fatigable weakness without UMN signs.
- D. Myopathies cause proximal weakness with preserved sensation and no Babinski.
- E. Sensory neuropathy causes sensory loss and reduced reflexes.
Flashcards
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What is the fastest separator for Upper motor neuron lesion vs Lower motor neuron lesion?
UMN lesion: spastic weakness, hyperreflexia, upgoing Babinski, no atrophy; LMN lesion: flaccid weakness, hyporeflexia, atrophy, fasciculations.
What clue should make you choose Upper motor neuron lesion?
Weakness with spasticity, hyperreflexia, increased tone, and Babinski sign.
Quick answers
What is the fastest way to tell Upper motor neuron lesion from Lower motor neuron lesion?
UMN lesion: spastic weakness, hyperreflexia, upgoing Babinski, no atrophy; LMN lesion: flaccid weakness, hyporeflexia, atrophy, fasciculations.
What is the key clue for Upper motor neuron lesion?
Weakness with spasticity, hyperreflexia, increased tone, and Babinski sign.
What is the key clue for Lower motor neuron lesion?
Weakness with flaccidity, hyporeflexia, fasciculations, and muscle atrophy.
What is the common trap with Upper motor neuron lesion vs Lower motor neuron lesion?
Weakness alone never localizes the lesion. The high-yield traps: ALS gives both UMN and LMN signs in the same limb without sensory loss; a spinal cord lesion gives LMN signs at the level of the injury (anterior horn destruction) and UMN signs below it; a cortical or capsular stroke gives pure contralateral UMN signs but may be flaccid for hours to days during spinal shock before evolving to spastic.