Introduction
Parkinson’s disease (PD) is a neurodegenerative disease that mainly affects middle-aged and elderly people and has increasing incidence rates seen in many countries, including Japan [
1]. PD has characteristic motor features, such as bradykinesia, resting tremor, rigidity, and abnormal posture [
2]. Although the typical abnormal posture in PD is a stooped appearance with flexion of the hips and knees and rounding of the shoulders, some patients may exhibit more severe abnormal posture, such as kyphoscoliosis, camptocormia, Pisa syndrome, or dropped head syndrome (DHS) [
2,
3]. Camptocormia is a postural disorder in the sagittal plane characterized by marked flexion of the thoracolumbar spine [
2]. DHS is known as anterocollis, with the forward neck flexion exaggerated compared to truncal postural changes [
2]. DHS is considered a common feature in multiple system atrophy; however, in patients with PD, DHS is relatively rare, with a reported incidence (in PD) of approximately 6% [
2]. To our knowledge, only a few reports have described DHS in PD, and details of its pathology remain to be elucidated. This study aimed to clarify the characteristics of DHS based on sagittal spinopelvic alignment in patients with PD and DHS.
Discussion
In the present study, 50% (4/8) of the patients developed DHS within three years of PD onset, including the three who developed DHS before the diagnosis of PD. In a study of 15 patients with PD and DHS, Kashihara et al. [
14] reported the development of DHS within three years of PD onset in seven patients, including one who developed DHS before the onset of parkinsonism. In the Movement Disorder Society Clinical Diagnostic Criteria for PD, “disproportionate anterocollis (dystonic) within the first 10 years” is considered to be a red flag [
15]. The current study indicated that DHS could appear in the early stage of PD, suggesting that DHS should be considered in the diagnosis of PD. It was also suggested that the differential diagnosis of neurodegenerative diseases, such as PD, is crucial for the management of patients with DHS.
To our knowledge, this study is the first to report on the characteristics of spinopelvic alignment in patients with PD/DHS. In a Japanese cohort survey on sagittal spinal alignment in the general elderly population [
9], the SVA values of healthy individuals in their 60 and 70s were 9 ± 38 and 22 ± 30 mm in men, and 5 ± 30 mm and 30 ± 36 mm in women, respectively. Furthermore, the respective cervical SVA values in the same age groups were 28 ± 8 and 29 ± 12 mm in men and 16 ± 8 and 17 ± 11 mm in women, respectively [
9]. In the present study, the cervical SVA values in patients with PD/DHS tended to be greater when compared to the data on the general elderly population. However, the SVA values differed among patients and exhibited no specific trend. A recent study reported that patients with cervical kyphotic deformity exhibit two different types of thoracolumbar compensatory mechanisms according to the C7 plumb line: the head-balanced and trunk-balanced types [
12]. In the former type, the center of gravity of the head-plumb line is located in the pelvis due to a posterior shift in the C7 plumb line (negative SVA). This type is characterized by a small T1 slope, straightened thoracolumbar junction, and lumbar hyper-lordosis [
12]. In contrast, in the trunk-balanced type, the center of gravity of the head-plumb line is located anteriorly due to the absence of a posterior shift in the C7 plumb line (positive SVA) [
12]. This type is characterized by a large T1 slope and low lumbar lordosis. A retrospective observational study in Japan reported that the global sagittal alignment parameter including SVA might have notable impacts on the surgical outcomes of DHS [
16]. In our study, four patients with PD/DHS showed a posterior shift of the C7 plumb line, and the remaining four patients showed an anterior shift of the C7 plumb line. These results suggest that DHS in PD also exhibits different subtypes with different thoracolumbar compensatory mechanisms according to the underlying pathology.
In a study of patients with DHS without PD or other neurological diseases, Murata et al. [
7] classified DHS into the diffuse kyphosis- (cervical lordosis > -10˚) and cervical kyphosis-types (cervical lordosis ≤ -10˚), based on the degree of cervical lordosis. They demonstrated that the T1 slope and T4–T12 thoracic kyphosis were significantly greater in the diffuse kyphosis-type than in the cervical kyphosis-type. In our study, all patients with DHS exhibited cervical kyphosis with severity of ≤ -10˚. However, based on the visual assessment of spinopelvic alignment as well as the assessment of T1 slope and thoracic kyphosis, the severity of kyphosis from the cervicothoracic junction to the thoracic spine also differed among patients with PD/DHS. In patients with a larger T1 slope and greater thoracic kyphosis, anterocollis tended to be more severe.
The pathogenesis of DHS in PD is poorly understood. Fujimoto [
17] identified dystonia of the flexor neck muscles and weakness of the extensor neck muscles as the pathophysiological mechanisms of DHS in PD, highlighting the involvement of isolated neck extensor myopathy in the weakness of the extensor neck muscles. Furthermore, Lava et al. [
18] reported a case of levodopa-responsive parkinsonism accompanied by anterocollis due to focal neck extensor myopathy. In contrast, Kashihara et al. [
14] reported in a study of 15 patients with PD/DHS that although they had no neck muscle weakness, many patients exhibited predominant neck muscle rigidity and characteristic marked contraction of the neck extensor muscles in the upright position. Our study also showed that all the PD/DHS patients had stiffness in the neck extensor muscle.
Various cases of DHS associated with antiparkinsonian medications, particularly DAs, have been reported, and our study also included one case strongly suggestive of such an association [
2]. With regard to DAs, several studies have reported differences in the sensitivity of muscles to L-dopa and other DAs; however, how DA affects neck muscles and triggers DHS remains unknown [
17].
There are no consistent guidelines for the treatment of DHS associated with PD. In some cases, improvement occurs after adjustment of antiparkinsonian drugs (including dose reduction and discontinuation of dopamine agonists); therefore, dose adjustment should be attempted as a first-line management strategy. In our study, drug adjustment led to improvement in two out of eight patients. The efficacy of botulinum toxin therapy for DHS is inconsistent. There are fewer reports on the efficacy of surgery for DHS in PD, and the long-term prognosis is unclear. In our study, two patients underwent surgery and showed postoperative improvement of DHS. Future large-scale longitudinal studies are needed to determine the optimal treatment of DHS in PD.
Our study has certain limitations. First, because it was a single-center case series study, the sample size was relatively small, similar to previous reports [
14]. We suggested the possible diversity of DHS in PD based on an analysis of the characteristics of sagittal spinopelvic alignment; however, we could not perform detailed analysis of the pathology and could not make a firm conclusion. Second, detailed examinations, such as electromyography and muscle biopsy, were not conducted on our patients, which precluded detailed analysis of the pathogenesis and etiology of DHS. To address these issues, longitudinal studies of a large sample size using electromyography or muscle biopsy need to be performed in the future.
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