Clinical features of spinal muscular atrophy (SMA) type 3 (Kugelberg-Welander disease)

Spinal muscular atrophy type 3 (SMA3), also called Kugelberg-Welander SMA, typically presents with muscle fatigue, slowly progressive weakness and atrophy of lower limbs once they have already acquired independent ambulation. Visceral involvement frequent in type 1 and 2 subtypes is rare in SMA3. Hypotonia, hyperlaxity and absent osteo-tendinous reflexes are typical features. By definition, standing or walking without support is achieved but the vast majority of SMA3 patients lose ambulation with time. Lifespan is normal. In some classifications, an additional subtype is included in the mild end of the spectrum, namely spinal muscular atrophy type 4 (SMA4). In this rare subtype, symptoms begin in adulthood; patients remain ambulatory at least until the fifth decade and have a normal respiratory function. Molecular genetic testing is the gold standard tool for diagnosis of SMA. However, diagnosis in a child affected with SMA3 is often challenging because clinical presentation mimics a muscular dystrophy. Electrodiagnostic studies and muscle biopsy are useful tools for demonstrating the presence of denervation but sometimes may not show meaningful differences to help distinguish between SMA and myopathy. Recent specific therapies show promising results before severe neuronal degeneration and motor dysfunction is installed. Therefore, high suspicion should be maintained and genetic analysis performed early in the diagnostic process when facing patients with symmetric and prominent proximal weakness, especially if they present progressive motor impairment. © 2020 French Society of Pediatrics. Published by Elsevier Masson SAS. All rights reserved.