Which finding is characteristic of an upper motor neuron lesion but not typical of a lower motor neuron lesion?

Which finding is characteristic of an upper motor neuron lesion but not typical of a lower motor neuron lesion?

• A. Hyperreflexia
• B. Fasciculations
• C. Absent reflexes
• D. Reduced tone

Answer: (A)

When upper motor neurons are damaged, the brain’s dampening influence on spinal reflexes is lost. That makes reflexes run hot, so they become hyperactive, and muscle tone tends to rise over time (spasticity). Lower motor neuron lesions hit the final common pathway to the muscle, so the reflex arc itself is impaired, leading to diminished or absent reflexes, reduced tone (hypotonia), and sometimes spontaneous fasciculations as motor units become unstable. Among the options, the sign that best fits an upper motor neuron lesion is hyperreflexia. Fasciculations are a hallmark of lower motor neuron damage, absent reflexes are more typical of LMN injury, and reduced tone is also characteristic of LMN lesions.

When upper motor neurons are damaged, the brain’s dampening influence on spinal reflexes is lost. That makes reflexes run hot, so they become hyperactive, and muscle tone tends to rise over time (spasticity). Lower motor neuron lesions hit the final common pathway to the muscle, so the reflex arc itself is impaired, leading to diminished or absent reflexes, reduced tone (hypotonia), and sometimes spontaneous fasciculations as motor units become unstable.

Among the options, the sign that best fits an upper motor neuron lesion is hyperreflexia. Fasciculations are a hallmark of lower motor neuron damage, absent reflexes are more typical of LMN injury, and reduced tone is also characteristic of LMN lesions.