with congenital myotonic dystrophy have marked body and facial hypotonia.
They usually do not appear alert. Arthrogryposis tends to be more common
in the lower extremities than in the upper extremities (Figure 165.1).
The distal joints are more involved than the proximal joints. The only
distinguishing facial feature is temporal muscle atrophy, but this feature
is seldom present at birth. The head is usually large. Magnetic resonance
imaging of the brain may show ventricular dilation. The prognosis is poor.
Mental retardation is usually noted as the patients get older. Congenital
myotonic dystrophy is diagnosed by demonstrating the presence of myotonia
in the mother and is confirmed by DNA testing. The disorder is caused
by expansion in the number of trinucleotide repeats at chromosome region
19q13.3. It is an autosomal dominant disorder but the neonatal form only
occurs if the mother is the affected parent.
Figure 165.1.— Distal arthrogryposis in a neonate
with congenital myotonic dystrophy.