Case of paediatric neuromuscular disease with a surprising clinical outcome: time to challenge the dogma?

Marie Wright (Specialist Registrar): A 4-month-old female infant was referred with a 4-week history of tachypnoea and faltering growth. She was the first child of healthy unrelated parents, and was conceived following two early miscarriages. Noteworthy family history included the unexplained sudden deaths of her mother’s sibling aged 8 months and a 12-year-old first cousin. The pregnancy was complicated by intrauterine growth restriction, and birth weight at 37 weeks’ gestation was 2.5 kg (second centile). She had no respiratory compromise in the neonatal period, and initially established good weight gain and met developmental milestones age appropriately. At 12 weeks old, she was observed by her parents to be tachypnoeic. Her work of breathing gradually increased over the next month, but was not associated with cough, stridor or systemic symptoms. Simultaneously, her weight drifted to below the 0.4th centile.At first presentation, the infant appeared malnourished but was alert and interactive. She had marked respiratory distress and a paradoxical breathing pattern with indrawing of the abdomen on inspiration, but respiratory examination was otherwise unremarkable. Muscle strength and tone were normal, but she had positional talipes and correctable flexion deformities of her fingers.Oxygen saturation was 95% in air, and gas exchange was normal on capillary blood gas analysis (pH 7.44, pCO2 4.66 kPa, HCO3 24 mmol/L, BE −0.2 mmol/L). ECG and echocardiogram were normal. X-ray showed an elevated right hemidiaphragm (figure 1), and unilateral diaphragmatic paralysis was confirmed on fluoroscopic ultrasound. Rhinovirus and Adenovirus were detected on nasopharyngeal aspirate viral PCR.Figure 1 Chest X-ray at 4 months of age.Andy Bush (Professor of Respiratory Paediatrics):  These clinical features are suggestive of a progressive neuromuscular disorder with relative preservation of limb muscle function and early respiratory muscle involvement. There are several possible causes; the likeliest diagnosis is spinal muscular atrophy with respiratory distress type 1 …