Amyotrophic lateral sclerosis and degenerative cervical myelopathy: phenotype-based diagnostic pitfalls, investigative mismatch, and practical clinical reasoning.

Differentiating amyotrophic lateral sclerosis (ALS) from degenerative cervical myelopathy (DCM) remains difficult because the two disorders can converge clinically while diverging biologically. ALS is a progressive motor neuron disease, whereas DCM is a potentially treatable compressive myelopathy; however, both may present with upper-limb weakness, hand wasting, hyperreflexia, gait disturbance, and cervical MRI abnormalities. This narrative review examines ALS-DCM overlap through the concept of explanatory sufficiency: whether the available clinical, imaging, and electrophysiological evidence adequately explains the whole syndrome rather than a single visible abnormality. We synthesize evidence on phenotype-specific overlap, MRI-clinical mismatch, EMG/NCS distribution, somatosensory and motor evoked potentials, Gold Coast diagnostic criteria, primary lateral sclerosis, and coexistence of motor neuron disease with structural cervical pathology. The review emphasizes that MRI is indispensable but not self-interpreting, EMG/NCS is most useful when interpreted by distribution rather than positivity alone, and SEPs/MEPs can add a functional cord-conduction layer when MRI and examination are discordant. We also provide action-oriented clinical warning signs for common overlap scenarios. Rather than offering a rigid algorithm, this review proposes a clinically driven reasoning framework that helps distinguish ALS, DCM, radiculopathy, and coexistence while reducing premature diagnostic closure in neuro-spine practice.