Epidemiology And Classification Of Dm1

The incidence of DM1 is estimated to be 1 in 8000 births and its worldwide prevalence ranges from 2.1 to 14.3/100 000 inhabitants.1 Based on the age of onset and on its clinical features, DM1 can be divided into three forms: congenital, classical, and minimal, which may occur in the same kindred.

Congenital DM1 presents at birth or during the first year of life in a severe form. It is characterised by neonatal hypotonia, facial diplegia, joint contractures, frequent and often fatal respiratory failure, feeding difficulties, and developmental delay. The risk of dying from congenital DM1 in the neonatal period is high.1 Patients who survive exhibit non-progressive psychomotor retardation and may subsequently exhibit the features of the adult-type, classical form of DM1.

In the classical form, which is the most common, symptoms become evident between the second and the fourth decade of life, showing a slow progression over time (table 36.1). The key feature of the disease is myotonia, which is characterised by delayed relaxation after muscular contraction (fig 36.1A,B); progressive muscular weakness (dystrophy) and wasting are also typical findings; facial, axial, semi-distal, and distal compartments are predominantly involved. DM1 is, however, a multisystem disorder; indeed, affected patients can manifest abnormalities of other organs and systems including the eye (cataract), the endocrine system (diabetes, thyroid dysfunction, hypogonadism), the central nervous system (cognitive impairment, mental retardation, attention disorders), the gastrointestinal system (dysphagia, constipation, gallbladder stones, pseudoobstruction), and the heart (table 36.2).

Minimal DM1 begins later in life, usually after 50 years of age, with a very mild degree of muscle weakness and myotonia or only cataracts, associated with a normal lifespan.

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