Anterior Horn Cell Disease in Adulthood: Unmasking Spinal Muscular Atrophy Type 4.

Spinal muscular atrophy (SMA) type 4 is a rare, adult-onset motor neuron disorder characterized by slowly progressive proximal weakness with preserved ambulation. Its indolent clinical course and nonspecific early manifestations frequently result in prolonged diagnostic delays. We present the complete diagnostic evaluation of a 33-year-old male patient with a multi-year history of painless, progressive lower extremity weakness. Initial laboratory testing revealed an isolated elevation of creatine kinase (CK) and vitamin D deficiency, with otherwise normal metabolic, endocrine, and inflammatory studies. Persistent CK elevation despite vitamin repletion prompted a neuromuscular referral. Electrodiagnostic testing demonstrated normal nerve conduction studies with electromyographic evidence of widespread chronic active denervation and, critically, normal paraspinal musculature. This pattern strongly supported the localization of the pathology to the anterior horn cells. Subsequent genetic testing confirmed a homozygous deletion of exon 7 in the SMN1 gene, establishing the definitive diagnosis of SMA type 4. This case underscores the importance of a systematic diagnostic approach integrating serial CK monitoring, electrodiagnostic localization, and genetic confirmation in evaluating adult-onset proximal weakness, particularly in the current era of disease-modifying therapies.