A type IV spinal muscular atrophy with gastrocnemius pseudohypertrophy caused by SMN1 deletion: a case report and literature review

Spinal muscular atrophy (SMA) is a rare autosomal recessive genetic disorder characterized by severe neurological and muscular degeneration, often leading to severe disability or death. Its complex clinical manifestations frequently result in misdiagnosis or missed diagnosis. Type IV SMA primarily affects adults, rarely impacting normal lifespan or maximum motor capacity, and is clinically uncommon. However, SMA presenting with pseudohypertrophy of the gastrocnemius muscle and near-complete loss of motor function is extremely rare and has not been previously reported. We hereby report what we believe to be the first case of Type IV SMA caused by SMN1 deletion presenting with pseudohypertrophy of the gastrocnemius muscle. A 51-year-old man presented with progressive weakness in all four limbs over 16 years, which worsened over the past two months, leaving him unable to walk. Examination strongly suggested a diagnosis of SMA type IV. He was found to have a homozygous deletion of the SMN1 gene at exon 7, located on 5q13.2. Muscle strength in the biceps brachii and triceps brachii of both upper limbs was grade IV, with weakened grip strength in both hands. Grade III muscle strength in the quadriceps of both lower limbs, Grade IV muscle strength in the tibialis anterior, gastrocnemius, and extensor hallucis longus muscles, bilateral knee reflexes and Achilles reflexes (+), right Hoffmann’s sign (+), bilateral Babinski signs (+), and Gower’s sign (+). Multiple disc herniations with degeneration in the cervical spine. Multiple muscle atrophy of the thoracic wall. Electromyography of the limbs shows neurogenic changes. However, symptoms such as pseudohypertrophy of the gastrocnemius muscle and almost complete loss of motor function are rare, suggesting a poor prognosis. This rare case highlights that pseudohypertrophy of the gastrocnemius muscle may be a rare manifestation of type IV spinal muscular atrophy, and clinical differential diagnosis should be made with PMD and other diseases.