We identified a higher frequency of pain and a greater number of pain sites in individuals in the LNA stage of DMD than in those in the Amb and ENA stages, and the pain interference on the mood was greater in the LNA group than other groups. Our results indicate the need for pain assessment in individuals with DMD, particularly in the LNA stage. In addition, pain assessment focusing on different clinical stages (Amb, ENA, and LNA) is required because pain location and aggravating/relieving factors differed depending on the stage of the disease.
The prevalence of pain in our study sample was 44.6% (66/148 participants), which is lower than the previously reported prevalence of 54–73.4% [6, 7, 9]. Previous studies on pain in neuromuscular patients included several motor neuron diseases other than DMD, such as spinal muscular atrophy and Becker muscular dystrophy [7, 9], or reported pain prevalence only for individuals with DMD ≥20 years of age [6]. This difference in study population might be related to the difference in pain prevalence.
To the best of our knowledge, only one previous study, by Lager et al.[7] has evaluated pain in the ambulatory and non-ambulatory clinical stages of progressive neuromuscular disease. The most frequently reported pain sites were the ‘neck and back’ in the non-ambulatory group, compared to the ‘legs’ in the ambulatory group, a finding which is similar to our study. They reported that the prevalence, intensity, and frequency of pain did not differ between the two groups, with a reported frequency of pain of ‘a few times a week.’ An increase in the frequency of pain in the non-ambulatory stage might not have been detected in their study because the sample size was small (55 participants), with mixed disease entities, including muscular dystrophy and spinal muscular atrophy.
In the assessment of pain quality in our study among patients with DMD, their pain quality were classified as nociceptive pain (resulting from activation of nociceptors innervating ligaments, small joints, muscle, and tendon) rather than neuropathic pain (resulting from a lesion or dysfunction of the peripheral or central nervous system) [15]. This finding underlines the fact that the pain in DMD patients could be mainly related to musculoskeletal conditions and this would be an important consideration because impairments in musculoskeletal structure and function are different according to the clinical stage of the disease: ankle plantar flexion contracture tends to begin in the Amb stage, hip and knee joint contractures tend to occur in the non-ambulatory stage [19, 20], and scoliosis also generally develops in the non-ambulatory stage[4].
Among participants in the Amb group in our study, calf pain was the typical pain reported, being aggravated by “standing, walking, and running” and relieved by “rest.” These results are similar to those of a previous study which reported that calf pain among patients in the early stage of DMD was related to prolonged daily toe walking, overuse syndrome (sprain and strain), and muscular fatigue due to an increase demand on the gastrocnemius-soleus muscle complex [21]. This pain could be managed by tailoring the intensity of ambulation and exercise, maintaining range of motion of the ankle.
DMD patients in the ENA group reported the knee as the most common site of pain, “transfer activity” as the most common aggravating factor, and “positional change” as the most common relieving factor. A potential hypothesis is that movement of the knee joint may be induced by transfer activity, resulting in pain provocation. Eventhough joint contracture itself does not cause the pain, pain occurs when the joint and its capsule are pushed to their end range [22]. This hypothesis suggests the need to prevent or alleviate joint contracture even in the non-ambulatory stage.
Individuals in the LNA group reported the lumbosacral region as the most common site of pain, with other sites including the chest-abdomen and buttocks (Figure 2). They also indicated “sitting” as the most common pain aggravating factor, with “positional change” as the most frequently used pain relieving method. Scoliosis and pelvic obliquity, which progress in the LNA period of the disease, are factors which negatively impact body alignment and posture and might lead to the pain described by these individuals. Moreover, the progression of muscle weakness makes it difficult for these individuals to correct their posture by themselves, further causing or worsening pain [22]. These explanations emphasize the importance of maintaining spine and pelvis alignment as well as developing an appropriate positioning program as the disease progresses to the LNA stage.
Study Limitations
The limitations of the study need to be acknowledged in the interpretation of results. Foremost, all participants were recruited from a single clinic at a tertiary hospital. The characteristics of pain may vary depending on how a particular clinic manages musculoskeletal conditions. Furthermore, because this was a cross-sectional study, we could not confirm the association between musculoskeletal problems (joint contractures and scoliosis) and pain; a longitudinal study should be conducted to investigate these associations.