No relevant excess prevalence of myotonic dystrophy type 2 in patients with suspected fibromyalgia syndrome
Introduction
Myotonic dystrophy type 2 (DM2) is a rare, dominantly inherited multisystem disorder, with proximal limb weakness, myotonia, pain, and cataract as the core clinical features [1], [2]. It is caused by an unstable CCTG repeat expansion in the CNBP (formerly ZNF9) gene on chromosome 3q21.3 [3]. Confirming the diagnosis by genetic testing is important for both genetic counselling as well as for cardiac monitoring because cardiac arrhythmias are common and may be prevented by implantation of a pacemaker [4].
DM2 seems to be underreported in the Netherlands. There are only 45 known genetically proven DM2 patients from 17 different families (16,600,000 Dutch inhabitants, a prevalence of 1:370,000). This contrasts with an estimated prevalence in Germany of 1:20,000 and even 1:1830 in Finland [5], [6]. DM2 is difficult to recognize, not only due to the unfamiliarity with the disorder by most clinicians but also because of the heterogeneous phenotype and the rather aspecific onset [2], [7]. The average delay between onset of symptoms and genetic confirmation of DM2 is 14 years [8].
DM2 shows some remarkable clinical similarities to fibromyalgia syndrome (FMS). FMS is a chronic disorder with an estimated prevalence of 2%, characterized by generalized musculoskeletal pain, and fatigue, and often with various associated symptoms [9], [10]. Striking similarities between the two conditions are widespread fluctuating pain, a high frequency of fatigue and gastrointestinal symptoms [2], [10], [11], [12], [13], [14], [15]. 4.4% of the Dutch DM2 patients were formerly diagnosed with FMS (unpublished data). In addition, a Finnish study reported that 2 out of 63 (3%) FMS patients had underlying DM2 as the explanation for their complaints [16]. We therefore hypothesized that there will be an excess frequency of DM2 in patients with suspected FMS.
Section snippets
Patients
The local ethical committee approved the cross-sectional study design, which was carried out according to the Helsinki Declaration. All patients provided informed consent. Between November 2011 and April 2014, all patients who were referred with suspected FMS to the outpatient Department of Rheumatology of the Sint Maartenskliniek, a regional and also tertiary referral centre [17], were invited for this study. Exclusion criteria were age <18 years and an established other diagnosis responsible
Results
Between November 2011 and April 2014, 398 patients with suspected FMS were included (Fig. 1, flowchart of inclusion). 95% were female, with a mean age of 42 years (Table 2). Based on the ICD-9, FMS was diagnosed in 96% by the treating rheumatologist. In 330 patients, the 2010 ACR criteria for FMS had been established, 304 patients met that criteria (92%). 281 patients (71%) were referred by the general practitioner; the remainder of the study population had been referred for a second opinion by
Discussion
In this large cross-sectional study in 398 patients with suspected FMS, only one patient was identified with the DM2 mutation (0.25%), so no relevant excess frequency of DM2 was found.
The strengths of our study are a large sample size, use of genetic testing as the golden standard, and a full rheumatologic evaluation of all patients. However, patient selection – given the tertiary function of the Sint Maartenskliniek – might not be representative for patients in general with (suspected) FMS.
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