Spinal muscular atrophy: SAQ

This infant presents the textbook picture of type 1 spinal muscular atrophy, or Werdnig-Hoffmann disease: the reduced fetal movements, the severe symmetric proximal weakness with absent reflexes and tongue fasciculations, the paradoxical breathing with a bell-shaped chest, and the weak cough with pooled secretions, all in a bright and socially engaged child whose intellect is spared. The diagnosis is the homozygous SMN1 deletion test, and the treatment is disease-modifying therapy started urgently, layered on multidisciplinary respiratory and nutritional support. ^[1]

Question 1 (10 marks)

Outline your diagnosis, the investigations you would request, and your immediate management over the first 24 hours. ^[1]

The diagnosis is type 1 spinal muscular atrophy. The discriminating pattern is the symmetric proximal weakness worse in the legs, the absent deep tendon reflexes, the tongue fasciculations, and the paradoxical breathing, all in a bright, socially engaged infant whose intellect and social interaction are spared. The contrast between the alert face and the limp body is the signature clue, and it separates the spinal cause from the cerebral causes of hypotonia that give developmental delay and a poorly social infant. ^[1]

The diagnosis is genetic and made by a single blood test for the homozygous deletion of SMN1 exon 7, present in over ninety-five percent of affected individuals, which confirms the diagnosis in the right clinical picture without further testing. I would order the SMN1 deletion test together with the SMN2 copy number, which predicts the type and the urgency of treatment. I would not delay for electromyography or muscle biopsy, because the genetic test makes them unnecessary in the typical case. I would send a creatine kinase to help exclude a muscular dystrophy if the picture were atypical, a sleep study and pulmonary function testing to quantify the respiratory reserve, and a swallowing assessment to guide nutrition. ^[1]

My immediate management protects the airway and the breathing while the diagnosis is confirmed. This infant has paradoxical breathing and a weak cough with pooled secretions, which signal impending ventilatory failure, and I remember that carbon dioxide retention precedes hypoxaemia in neuromuscular respiratory failure, so a normal oxygen saturation does not exclude it. I would involve the paediatric intensive care team early, suction the secretions, position the child, assess the swallow, and introduce cough-assist and non-invasive ventilation. I would secure nutrition with a nasogastric tube given the bulbar weakness, and I would contact the paediatric neuromuscular service immediately, because the disease-modifying therapy should begin as soon as the diagnosis is confirmed. ^[2]