The purpose of this study was to assess the prevalence of sarcopenia in patients with idiopathic DHS and whether loss of appendicular muscle mass is associated with loss of trunk muscle mass. Our data showed that sarcopenia was not associated with the onset of idiopathic DHS. This is the first report to accurately measure the prevalence of sarcopenia in DHS patients using the results of muscle strength and physical performance as well as muscle mass. The prevalence of sarcopenia in our control group showed a similar rate to the prevalence of the disease in the Asian population (5.5–25.7%) , which support the validity of our findings. Considering the blood parameters including serum albumin and total cholesterol in DHS patients, they were never undernourished. Based on our results, the prevalence of sarcopenia in DHS patients was not different from that in healthy individuals, while grip strength and walking speed were lower than in healthy individuals of the same age.
The AWGS 2019 algorithm  is the most commonly used diagnostic criteria in recent studies of sarcopenia in the Asian population. In this algorithm, the decreased SMI is a prerequisite for sarcopenia, and both the grip strength for muscle strength evaluation and the walking speed for physical function evaluation are used as diagnostic criteria. Although the DHS group exhibited decreased muscle strength and physical function, we believe the low 20% prevalence of sarcopenia in this study was due to the lack of decrease in SMI. Our prevalence rate is considerably different from a previous study by Eguchi et al  that reported 70%. Comparing the DHS patients between the two studies, there were no significant differences in race, height, weight, or BMI, and cervical malalignment. In 2010, The European Working Group on Sarcopenia in Older People (EWGSOP)  defined elderly people with reduced SMI but without reduced walking speed or muscle strength as pre-sarcopenia. An algorithm was developed specifically for the Asian population in 2019, and pre-sarcopenia was redefined as non-sarcopenia. The prevalence of sarcopenia in patients with DHS in the previous study  may be overdiagnosis, as it did not adopt the criteria of the AWGS 2019 algorithm .
We found that the decreased trunk muscle mass (mean value: 7.92 ± 0.53 kg/m2) was characteristic of DHS patients compared to healthy subjects. There was a significant correlation between SMI and trunk muscle mass in healthy subjects (r = 0.45, p < 0.05), but not in DHS patients (p = 0.55). The results show that DHS patients had low trunk muscle mass, even in those with high SMI. The musculoskeletal characteristics of DHS patients revealed a remarkable decrease in trunk muscle mass without the decrease in appendicular muscle mass that is characteristic of sarcopenia. Decreased trunk muscle mass has been reported to be associated with pain, spinal malalignment, and quality of life in patients with spinal disorders , and we believe that decreased trunk muscle mass is associated with the development of idiopathic DHS. We have previously reported that exercise interventions focused on the lower trunk and cervical muscles in patients with DHS yield good outcomes . From our findings, a focus on trunk muscle mass could be an option for conservative treatment of DHS patients. Although walking speed is correlated significantly with leg muscle strength , DHS patients did not demonstrate a significant correlation between leg muscle mass and walking speed. It is also interesting to note that the results of this study showed no significant difference in leg muscle mass between the healthy elderly and patients with DHS. It has been reported that maintaining the elevated position of the head is essential for optimizing input from the visual, vestibular, and somatosensory systems and maintaining systemic balance during exercise [24, 25]. In DHS patients, dropped head or reduced trunk function may contribute to decreased walking ability, regardless of leg muscle strength. Strength exercises aimed at increasing muscle mass in the lower extremities may not be effective in improving the reduced walking ability in patients with DHS.
Reports by Kyle et al  and Bahadori et al  show standard values for FFMI and FMI in non-Japanese populations, and these values for the elderly Japanese were examined in a cohort study of 4,500 people . An increasing FMI with aging represented by sarcopenic obesity is regarded as one of the problems in an aging society . There are no reports of FFMI and FMI in patients with orthopedic diseases, including DHS. Patients with DHS exhibited reduced FMI as opposed to age-related changes. FMI of female patients with DHS obtained in this study (mean 4.7 ± 1.7 kg/m2) was remarkably lower than that of healthy subjects in the previous study (mean 7.4 ± 2.6 kg/m2) . Furthermore, the FFMI-FMI ratio in the DHS group was significantly higher than that in control groups. Therefore, we believe that reduced body fat mass is an indicator of DHS. Adipose tissue, along with skeletal muscle, is depleted by various chronic inflammations . Fat loss in patients with DHS was not accompanied by reduced appendicular muscle mass. Although it is unclear why our findings differ slightly from the general patterns of age-related changes in body composition, trunk muscle and fat mass loss may occur in advance of DHS. We believe that DHS patients should be treated by focusing not only on muscle but also on fat. Adipose tissue has a rich composition of immune cells, and cytokines are important regulators of lipolysis . Further investigation into the association between body composition of DHS patients and inflammatory markers is warranted.
Our research has some limitations. Firstly, patients with idiopathic DHS only included those who visited a single institution; therefore, their external validity is low, and it is difficult to generalize the results of this study. A multicenter research is needed to consider regional characteristics. Secondly, there was an admission rate bias, because the DHS group only included patients who visited the hospital. The prevalence of sarcopenia in DHS might be underestimated. Thirdly, the BIA method was used to evaluate body composition in this study. The most reliable tool for assessing body composition is the dual energy X-ray absorptiometry (DXA) method. The BIA method has a slightly lower accuracy in evaluating muscle mass. However, the BIA method provides a portable and inexpensive means for assessment without radiation exposure, which was the most suitable and practical method when considering the ethical aspect of radiation exposure to the control group. In the future, the comparative validity of DXA and BIA should be verified. Finally, the prevalence of sarcopenia may be underestimated in this study due to the extremely small sample size. Utilizing a larger sample may have revealed a statistically significant difference between patients and the control. In future research, verification should be carried out by increasing the number of people to be measured. However, since DHS is a rare disease, our findings valuable basic research data.