Neuromuscular and Neurologic System Review and Differential Diagnosis
Core concepts
This domain tests whether you can localize a lesion and separate neurologic from musculoskeletal and systemic mimics. Know upper versus lower motor neuron signs (spasticity, hyperreflexia, Babinski vs atrophy, fasciculations, hypotonia), the difference between cortical, subcortical, brainstem, cerebellar, and spinal cord syndromes, and the classic patterns of peripheral neuropathy (length-dependent, demyelinating vs axonal). Understand the corticospinal, dorsal column, and spinothalamic tracts and how dissociated sensory loss points to cord lesions. Recognize red flags: sudden onset, progressive deficit, bilateral symptoms, bowel/bladder change, saddle anesthesia, and cranial nerve involvement.
Traps
Questions often present a plausible musculoskeletal explanation for a neurologic problem (e.g., foot drop attributed to ankle weakness rather than L5 or peroneal nerve). Watch for distractors that confuse radiculopathy (dermatomal, myotomal, reflex changes) with peripheral neuropathy (stocking-glove, symmetric). Cerebellar vs sensory ataxia is a frequent discriminator: cerebellar worsens with eyes open and has dysmetria; sensory ataxia worsens with eyes closed and has positive Romberg. Remember that UMN signs can be absent acutely in spinal shock.
Rule of thumb
Localize first, then diagnose. Ask: which level (cortex, brainstem, cord, root, plexus, nerve, NMJ, muscle), which side, and which tracts or fibers? A single lesion should explain all findings; if it cannot, reconsider or suspect multifocal disease.
Worked example
Unilateral foot drop with numbness of the dorsum of the foot, preserved ankle inversion, and no back pain: localize to the common peroneal nerve at the fibular head rather than L5 root, since L5 radiculopathy would also weaken ankle inversion and hip abduction.