Selective sparing of sartorius and biceps femoris muscles on MRI in post-polio syndrome: a case report of two cases

BACKGROUND: Post-polio syndrome (PPS) is characterized by progressive muscle weakness and atrophy decades after acute poliomyelitis. While muscle involvement is typically asymmetric and patchy, specific patterns of selective muscle sparing have not been well characterized. CASE PRESENTATION: We report two male patients (ages 36 and 69 years) with a remote history of paralytic poliomyelitis who presented with progressive lower limb weakness and atrophy, fulfilling the March of Dimes diagnostic criteria for PPS. Detailed clinical examination revealed asymmetric motor deficits with right-sided predominance. Comprehensive electrophysiological studies demonstrated chronic neurogenic changes without evidence of active denervation or upper motor neuron involvement. Muscle MRI of the thighs revealed a progressive pattern of selective muscle sparing: Patient 1 demonstrated unilateral preservation of the sartorius and biceps femoris in the newly affected left thigh, with end-stage fatty obliteration of these muscles in the previously affected right thigh; Patient 2 demonstrated bilateral preservation of these muscles, representing an earlier stage of the same pathophysiological process. CONCLUSIONS: This observation suggests that post-polio muscular degeneration follows a hierarchic and progressive pattern. The sartorius and biceps femoris exhibit relative resilience-delayed rather than absolute resistance to degeneration-possibly reflecting unique anatomical, developmental, or pathophysiological protective factors. This MRI pattern may serve as a dynamic imaging clue to support PPS diagnosis and stage assessment, though validation in larger cohorts is required.