Prevalence and correlates of apathy in myotonic dystrophy type 1

Apathy in DM1 has long been acknowledged in clinical practice. However, a major drawback is that the concept has been only sparsely explored in previous specific studies. This study aimed to determine the prevalence of apathy in myotonic dystrophy (DM1), to compare it with facioscapulohumeral dystrophy (FSHD) patients and normal healthy controls, and explore its relationship to psychopathological features and cognitive function.

Levels of apathy in 38 DM1 patients with adult phenotypes were compared with 19 patients with FSHD and 20 matched controls. Patient participants were consecutively recruited, regarding their interdisciplinary annual evaluation at the neuromuscular pathology reference center (Institute of Myology, Paris, France), within an 18-month period. Additional measurements included motor disability, fatigue, depression, anxiety, and cognitive abilities. Inter-group comparisons were performed using non-parametric Kruskal-Wallis tests and Mann-Whitney U Tests. Intra-group comparisons were carried out with the Wilcoxon Signed rank and Friedman tests. Also, Spearman's correlations were used to assess the strength of linear relationships between pairs of variables. The significance level was set at 0.05.

Global score of apathy was significantly higher in DM1 patients than in FSHD patients (p < 0.01) and in controls (p < 0.001). Sixteen of 38 DM1 patients (39.5 %) met the criterion for apathy, contrasting with only 4 of the 19 (21.1 %) FSHD patients. No control subject was apathetic. Moreover, apathy in DM1 patients was negatively correlated to MMSE (r = -.46, p < .05) and Stroop Word (r = -.55, p < .01) scores, but not with age, educational level, disease duration, CTG repeats, motor functional disability, fatigue, depression, and anxiety.

Apathy is a frequent symptom in DM1 (almost 40 %). It is more prevalent than in a similarly disabled group of patients with FSHD and in controls. Results also show that apathy in DM1 is independent of the psychopathological domain, fatigue, age, and motor disability, but associated to general cognitive status. These results altogether could suggest a central cause for apathy in DM1 rather than an adjustment process to cope with the progressive and debilitating nature of the disease. Data emphasize the importance to evaluate this symptom in routine clinical management of DM1 patients.