S1 radiculopathy presenting as progressive unilateral calf hypertrophy.

Progressive unilateral calf enlargement is more commonly attributed to vascular, inflammatory, neoplastic, or lymphatic disease than to chronic denervation. We report a 55-year-old female recreational runner with progressive right calf enlargement and long-standing mild low back pain. Lumbar MRI demonstrated multilevel disc herniations, including a right-sided L5-S1 extrusion with compression of the right S1 nerve root. During the initial phase of calf symptoms, calf MRI showed mild asymmetric enlargement of the right posterior compartment with edema-like signal on fluid-sensitive sequences. At later-stage evaluation, 16 months after the initial calf MRI, ultrasound and follow-up calf MRI demonstrated significant interval progression to marked asymmetric enlargement of the right posterior compartment, predominantly involving the medial gastrocnemius and soleus; at a comparable anatomic level, the right soleus cross-sectional area had increased by approximately 58% relative to the prior calf MRI. Ultrasound showed mildly increased echogenicity with preserved fibrillar architecture, without increased color Doppler flow, focal mass, or architectural distortion. MRI confirmed preserved muscle architecture and absence of fatty replacement, consistent with true muscle hypertrophy rather than fatty pseudohypertrophy. Electromyography confirmed chronic right S1 radiculopathy. This case highlights that chronic S1 radiculopathy may present as progressive true calf hypertrophy without fatty replacement and that unilateral calf muscle enlargement with preserved architecture should prompt correlation with lumbar spine MRI and electrodiagnostic findings.