Myotonic dystrophy type 1 (myotonia dystrophia 1) is an autosomal dominant multisystem disorder caused by CTG expansion of the DMPK1 gene (dystrophia myotonica protein kinase) (OMIM #605377) resulting in anticipation, whereby offspring have greater repeat expansion and severity.  

Case Report: 

This full-term patient was born with severe hypotonia and respiratory depression. Brain MRI, EEG, Karyotype (46,XY) and FISH were normal. Whole Exome Sequencing (WES) revealed heterozygous NEB VUS, in trans, but histochemical and electron microscopic examinations of the muscle biopsy did not show nemaline rods.  EMG was performed and revealed myotonic discharges.  A genetic diagnosis of myotonic dystrophy type 1 was made, with over 200 CTG repeats in the DMPK1 gene. A week after tracheostomy and g-tube placement, oxcarbazepine was trialed, after which he was able to be weaned off ventilation and discharged to home. He made substantial progress over the next year.  

Neonatal muscle biopsies may not show nemaline rods due to being early or sampling error and thus further testing may be required. When evaluating severe congenital myopathies, testing for myotonic dystrophy should be considered.  Myotonia may be found in neonates as young as 6 days old characterized as briefer duration with waning repetitive small amplitudes between 50-100uV without the classic sound on a spectrum with complex repetitive discharges.  Severe congenital myotonic dystrophy patients may be born from an asymptomatic mother and the diagnosis may be missed by whole exome sequencing.