Congenital Nemaline Myopathy: The Value of Magnetic Resonance Imaging of Muscle

Nemaline myopathy is a clinically and genetically heterogenous disorder that may present from birth to late adulthood.Patients with nemaline myopathy exhibit weakness of proximal muscles (particularly neck flexors) and, in the case of younger children, facial, bulbar, and/or respiratory muscle weakness.Nemaline myopathy can be classified according to age of symptom onset and disease severity.1 It has been linked to nine genes: NEB, ACTA1, TPM2, TPM3, TNNT1, CFL2 and less commonly; KLH40, KLH41, and KBTBD13.The classic features seen on muscle biopsy include sarcoplasmic rodlike structures on microscopic study using Gomori trichrome stain.Electron microscopy also classically demonstrates electron-dense nemaline bodies associated with sarcomeric Z-lines and less commonly intranuclear inclusions.Skeletal muscle alpha-actin (ACTA1) is the principal actin isoform of skeletal muscle.It forms the thin filament of the sarcomeric unit and interacts most notably with myosin to generate muscle contraction.Mutations within ACTA1 are the second most common cause of nemaline myopathy (after NEB mutations) accounting for about 20%-30% of cases. 2 Most ACTA1 mutations are thought to be de novo, although autosomal dominant and recessive inheritance has been reported.2 We provide a clinical summary of an infant who presented with failure to thrive and tachypnea and who was later diagnosed with a nemaline myopathy.We illustrate the value of MRI in this case at assisting with the selection of muscle to biopsy.Genetic testing subsequently confirmed a mutation in an ACTA1 gene.Clinical follow-up was favourable as with adequate nutrition and bilevel positive airway pressure ventilation support the patient demonstrated appropriate gross motor development at his most recent clinical follow-up at 21 months old. CASEA 4-month-old boy presented to the emergency room for failure to thrive over the preceding 2 months.He was born at 37 weeks' gestational age to a healthy mother.Pregnancy was unremarkable.Antenatal ultrasounds were reassuring with no polyhydramnios.Delivery was via a planned Caesarean section with no resuscitation required.His birth weight was 3.32 kg (75th %ile).He established bottle-feeding without difficulty and was discharged home.His mother reported a gradual increase in fatigue with feeding, poor weight gain, and rapid breathing over at least 2 months.Physical examination showed an alert infant with tachypnea, mild intercostal indrawing, and abdominal breathing.No dysmorphic features were apparent.His weight was 4.59 kg

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