Painless Progressive Weakness With Sensory Sparing: Does the EMG Prove ALS?
64-year-old man reports seven months of painless, progressive right foot drop followed by left foot slap and clumsy hands. He has cramps but no numbness. Examination shows asymmetric distal weakness and atrophy of thenar and interossei, widespread fasciculations, brisk knee reflexes, and an extensor plantar response on the right. Vibration and pinprick are intact. Nerve conduction studies show normal median, ulnar, and sural sensory responses; peroneal CMAP is 0.8 mV and tibial CMAP 2.1 mV. Needle EMG shows active denervation in tibialis anterior, gastrocnemius, first dorsal interosseous, and thoracic paraspinals, with chronic reinnervation and no conduction block. He asks whether the study proves ALS and whether he should start riluzole today.
1. Does the electrophysiologic pattern confirm ALS? Explain your reasoning and identify the most important alternatives that this pattern does not exclude.
Read the worked answer
No. The study strongly supports widespread lower motor neuron degeneration: active denervation in leg, hand, and thoracic paraspinal muscles with chronic reinnervation, plus normal sensory responses and no focal conduction block. That pattern is consistent with ALS but is not diagnostic by itself. ALS remains a clinical diagnosis requiring upper and lower motor neuron findings in multiple regions, progression, and exclusion of mimics. The normal sensory studies and absence of conduction block argue against CIDP and multifocal motor neuropathy, but do not exclude cervical or lumbar spondylotic radiculopathy, Kennedy disease, post-polio syndrome, HIV-related neuropathy, lead toxicity, or a paraneoplastic process. Thoracic paraspinal denervation raises concern but can occur with other widespread processes. I would not start riluzole solely on the EMG; I would confirm the clinical syndrome, review progression, and complete mimic testing first.
2. What additional testing and clinical review would you order before giving a diagnosis, and what findings would make you reconsider?
Read the worked answer
I would obtain cervical and lumbar MRI because asymmetric distal weakness with hyperreflexia can reflect combined spondylotic disease, and I would check CK, TSH, vitamin B12, methylmalonic acid, SPEP/free light chains, HIV, and lead only if exposure is plausible. Anti-GM1 is reasonable if multifocal motor neuropathy is considered, though the absence of conduction block makes it less likely. I would review family history for Kennedy disease and assess bulbar function, respiratory capacity, weight, and falls. I would reconsider ALS if there is substantial sensory loss, a persistent focal conduction block, marked asymmetry that remains confined to one limb, improvement over time, or purely upper motor neuron signs. I would also involve neurology and ALS multidisciplinary care before prescribing riluzole or discussing prognosis.
3. If the sensory responses were abnormal and EMG showed conduction block, how would that alter your interpretation and immediate management?
Read the worked answer
It would argue strongly against ALS. Sensory abnormalities plus conduction block suggest an acquired demyelinating polyneuropathy such as CIDP, multifocal motor neuropathy with sensory involvement, or a plexopathy or radiculoplexus process. I would repeat nerve conduction studies with temperature control, study multiple limbs, assess conduction block criteria, and consider CSF, MRI of the plexus, and an immunotherapy trial. Management would shift to treating an inflammatory neuropathy and avoiding ALS-specific counseling until the alternative is excluded.