Diagnostic Pitfalls in Adult-Onset Spinal Muscular Atrophy Type 4: When Incidental Lumbar Stenosis Confounds the Clinical Picture.

Spinal muscular atrophy type 4 (SMA‑4) represents a rare, late‑onset motor neuronopathy with insidious proximal limb weakness and preserved walking ability. Its slow progression and vague initial symptoms often cause years of diagnostic uncertainty. We describe a 38‑year‑old man who experienced slowly worsening, fatigue‑related proximal leg weakness over three years. Initial suspicion fell on a metabolic myopathy, and later, lumbar spine magnetic resonance imaging (MRI) showed multilevel degenerative changes with canal stenosis, findings that were initially attributed to the cause but did not align with the clinical picture. Crucially, the patient's younger brother had already received a genetic diagnosis of SMA‑4. Nerve conduction studies were normal. Needle electromyography (EMG) revealed widespread chronic partial denervation with reinnervation, yet the paraspinal muscles remained electrically silent, a pattern that localizes the lesion to the anterior horn cell. Genetic testing subsequently confirmed homozygous deletion of SMN1 exon 7. This report highlights how incidental imaging abnormalities can confound the diagnostic process and underscores the importance of systematic clinical‑electrodiagnostic‑genetic correlation in adult‑onset proximal weakness, particularly in the era of available disease‑modifying therapies.