A patient who has upper motor neuron syndrome is MOST likely to exhibit which of the following signs?

A patient who has upper motor neuron syndrome is MOST likely to exhibit which of the following signs?

• A. Clonus
• B. Severe muscular atrophy
• C. Fasciculations and fibrillations
• D. Hypoactive deep tendon reflexes

Answer: (A)

Clonus shows up when the brain can no longer properly regulate reflex circuits in the spinal cord, a hallmark of upper motor neuron involvement. When an upper motor neuron is damaged, there’s a loss of inhibitory control over the stretch reflex, making reflexes overly responsive. Clonus specifically is the repeated, rhythmic muscle contractions that occur with a sustained stretch, reflecting this hyperexcitability of the reflex arc. Other signs listed point more to lower motor neuron problems: severe muscle atrophy happens when motor neurons or their axons degenerate; fasciculations and fibrillations are spontaneous muscle fiber twitches seen with LMN loss; hypoactive deep tendon reflexes can occur with LMN injury or in the acute phase of UMN injury (spinal shock), but in chronic upper motor neuron syndrome the reflexes are typically exaggerated rather than diminished. So clonus best fits the pattern of upper motor neuron syndrome.

Clonus shows up when the brain can no longer properly regulate reflex circuits in the spinal cord, a hallmark of upper motor neuron involvement. When an upper motor neuron is damaged, there’s a loss of inhibitory control over the stretch reflex, making reflexes overly responsive. Clonus specifically is the repeated, rhythmic muscle contractions that occur with a sustained stretch, reflecting this hyperexcitability of the reflex arc.

Other signs listed point more to lower motor neuron problems: severe muscle atrophy happens when motor neurons or their axons degenerate; fasciculations and fibrillations are spontaneous muscle fiber twitches seen with LMN loss; hypoactive deep tendon reflexes can occur with LMN injury or in the acute phase of UMN injury (spinal shock), but in chronic upper motor neuron syndrome the reflexes are typically exaggerated rather than diminished.

So clonus best fits the pattern of upper motor neuron syndrome.