A 5-month-old male infant is referred for further evaluation, as he is yet to achieve head control. Careful questioning reveals a weak cry and weak movements during the neonatal period. Now, he is always 'floppy' and unable to roll over unaided. The mother also says that he has difficulty feeding, and has recurrent episodes of choking. There is no history of abnormal movements or convulsions, and his social development is age-compatible. The medical history is significant for a single hospitalization at 2 months due to a severe respiratory tract infection. This was managed with intravenous antibiotics, and the infant discharged after 1 week. He was born via an uncomplicated normal vaginal delivery, following an unremarkable antenatal history. His parents are not related, and there is no family history of inherited disorders. All vaccines are up to date. His basic bloodwork, including a complete blood count, electrolyte assay, and liver and renal profiles, shows no abnormalities.
Creatine phosphokinase levels are within normal limits.
Nerve conduction studies show reduced motor nerve conduction velocities and a remarkably low M-wave amplitude in both median nerves and posterior tibial nerves. Electromyography reveals fibrillations and fasciculations at rest and an increased mean duration and amplitude of motor units.
Magnetic resonance imaging of the brain reveals no structural defects.
Genetic studies reveal a homozygous mutation of the SMN1 gene.