Myotonic dystrophy type 1: 13 years of experience at a tertiary hospital. Clinical and epidemiological study and genotype-phenotype correlation

Introduction

The incidence of myotonic dystrophy type 1 (DM1), a disease with great phenotypic variety, in our region is unknown. This study aims to estimate the incidence of DM1 at our hospital (a reference centre in Aragon, Spain) and to identify the characteristics of our population (genotype-phenotype correlation).

Methods

Retrospective, descriptive study of 459 patients classified according to the number of CTG repeats, as follows: normal (5–35), premutation (36–50), protomutation (51–80), small expansions (81–150), intermediate expansions (151–1000), and large expansions (> 1000). Furthermore, according to clinical phenotype, patients were categorised as unaffected (5–50 CTG repeats), mild form or asymptomatic (51–150), classical form (151–1000), and severe form (> 1000).

Results

The incidence of DM1 was 20.61 cases per million person-years (95% CI, 19.59–21.63). An inverse correlation was observed between the number of CTG repeats and the age at genetic diagnosis (ρ = –0.547; 95% CI, –0.610 to –0.375; P < .001). CTG₅ was the most frequent polymorphic allele in healthy individuals. Of all patients with DM1, 28.3% presented the mild or asymptomatic form, 59.1% the classical form, and 12.6% the severe form. Inheritance was maternal in 35.1% of cases, paternal in 59.4%, and uncertain in 5.5%. In mild forms, frontal balding in men was the most prevalent phenotypic trait, as well as myotonia and cataracts, while in the classical form, ptosis, facial weakness, voice and pronunciation alterations, myotonia, and fatigue/sleepiness were most frequent.

Conclusions

The incidence of DM1 in Aragon is significant. Multidisciplinary study of the phenotype of patients with DM1 is key to early diagnosis and personalised management.