Neuropathic pain in adult Myotonic Dystrophy type 1.  Presence of small and large fibre neuropathy?

Background: Myotonic Dystrophy type 1 (DM1) is an inherited neuromuscular disorder affecting multiple organs. There is an increasing awareness of chronic pain in DM1. In this cross-sectional study, we investigated symptoms of neuropathic pain and small and large fiber neuropathy in the adult form of DM1. We also studied if neuropathy was related to the number of CTG repeats, disease duration and other clinical DM1 symptoms, and investigated if skin biopsy tests for small fiber neuropathy differed in the DM1 group compared to reference values from healthy controls.Methods: 20 genetically verified DM1 adult patients were included in the study. Pain descriptions, neurologic examination and objective investigations of the peripheral nerve system by quantitative sensory testing, skin biopsies and neurography were conducted. Statistical analyses of group differences and frequencies were performed.Results: Six patients (30%) out of 20 patients with DM1 described neuropathic pain, and three of these had objective findings on both small and large fiber neuropathy, as well as clinically sensory findings. Together, large and/or small fibre neuropathy is present in 50% of the patients with DM1. The intra epidermal nerve fiber density was significantly lower (p= <0.001, Cohen`s d = 1.2) in the 20 patients with DM1 (mean 8.16, SD: 2.28) compared to a reference group (N = 106, mean 12.43, SD: 4.59). Patients with large fiber neuropathy had significantly lower muscle strength (p = 0.01, Cohen`s d = 1.6, mean difference 0.4, CI: 0.7 - 0.1) than patients without large fiber neuropathy.Conclusion: Symptoms of neuropathic pain was more frequent in patients with DM1 compared to the general population. Intra epidermal nerve fiber density was significantly lower in the DM1 group than in a sample of healthy controls. Neuropathy may be a mechanism of pain in DM1.