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Myotonic dystrophy is the most common myotonic disorder (Table 31-1). There are at least two genetically distinct forms of myotonic dystrophy: Dystrophica myotonia type 1 (DM1) and dystrophica myotonia type 2 (DM2), the later of which is also known as proximal myotonic myopathy (PROMM).
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MYOTONIC DYSTROPHY (DM1)
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DM1 in an autosomal dominant manner with a prevalence of 13.5 per 100,000.1–4 DM1 can present at any age, including infancy. Limb weakness begins distally in the extremities and can progress slowly to affect proximal muscles. Wrist flexors are often weaker than wrist extensors. The neck flexors, including the sternocleidomastoids, are also affected early. Atrophy and weakness of temporalis and other facial muscles as well as the jaw muscles giving rise to the characteristic “hatchet face” appearance (Fig. 31-1). Ptosis is often evident. Some patients develop dysarthria and dysphagia due to pharyngeal and lingual muscles involvement.
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Many patients do not complain or are not aware of their myotonia, although it is usually readily apparent on examination, particularly in the hands. Delayed relaxation of the fingers is seen following a forceful hand grip (action myotonia). The myotonia is lessened with repeated muscle contractions, a so-called warm-up phenomenon. Percussion of muscle groups, in particular of the thenar eminence or finger extensors also gives rise to delayed relaxation (percussion myotonia). Muscle reflexes are diminished, but sensory testing is normal.5 Adult patients with DM1 may have a mild reduction in cognitive abilities, while severe mental retardation is associated with congenital myotonic dystrophy.6,7
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Congenital myotonic dystrophy is much more severe than adult-onset DM1. Affected infants are invariably born to mothers with myotonic dystrophy.8,9 It is important to examine mothers of floppy infants, as they may not even be aware that they have the disorder. Pregnancy may be complicated by polyhydramnios and diminished fetal movements. Infants with congenital myotonic dystrophy have severe generalized weakness and hypotonia and may also have arthrogryposis. Clinical myotonia is not apparent in the neonatal period and may not be noticeable until about 5 years of age. However, myotonic discharges can be appreciated on electromyography (EMG) before the appearance of clinical myotonia. Many infants require ventilatory assistance due to ventilatory insufficiency. The mortality rate in infancy is approximately ...