Spondylolisthesis is less commonly found in the cervical spine compared to the lumbar spine, but it is more prevalent than originally thought with reported rates ranging from 4 to 20%.11,12 Degenerative cervical spondylolisthesis is one potential cause of myelopathy in the elderly population. One retrospective study found that 55 of 80 (69%) patients with cervical spondylotic myelopathy had evidence of degenerative spondylolisthesis with displacement greater than 2mm.13 A separate retrospective study observed the progression of degenerative cervical spondylolisthesis where neck pain was the initial symptom in all patients presenting with degeneration of the facet joints. At more advanced staging, wherein degeneration was observed in both the facet joints and vertebral bodies, the predominant presenting complaint was radiculomyelopathy or myelopathy. In the most severe spine deformities, severe myelopathy was the leading symptom.14 Park et al. found that the natural history of cervical listhesis appeared to be stable in patients who did not develop myelopathy during an extensive eight-year follow up period in their study.15 Therefore, at an early stage of listhesis with less slippage (1-2mm), patients will likely remain relatively asymptomatic with management focused on conservative management. These findings highlight the importance of monitoring the natural progression of cervical spondylolisthesis in high-risk patients to prevent further development of symptoms.
This is the first study to compare the longus colli CSA in patients with degenerative cervical spondylolisthesis. Our study suggests that decreased longus colli CSA is independently associated with a higher grade and degree of preoperative cervical spondylolisthesis. Smaller CSA of the longus colli may be attributed to muscle disuse as a result of axial and radicular pain causing the patient to limit activation of the muscles.16,17 It has been theorized that smaller CSA of the cervical multifidus in females with chronic neck pain may be a result of disuse of the muscle secondary to pain.6 One MRI study analyzing progressive degeneration of the cervical multifidus found a significant association of neck muscle fatty infiltrate and persistent neck disability, resulting in the inhibition of normal muscular activity and function.18 In addition, avoidance behavior, which may ultimately lead to disuse based atrophy, is exhibited in patients with axial neck pain, further complicating this self-propagating cycle of pain and instability.19 In a 2004 prospective study, custom electrodes were used to record electromyographic activity of deep cervical flexors of the cervical spine and found lower amplitude of deep cervical flexor EMG activity in a patients with neck pain when compared to a control group.16 Although not significant, their study also demonstrated greater sternocleidomastoid and anterior scalene muscle EMG activity in patients with neck pain, creating a theorized relationship between weakening of deep flexors and resultant compensatory hypertrophy of the superficial flexors. The role of targeted deep and superficial cervical flexor strengthening, in addition to targeted electrical muscle stimulation, as conservative management for this cohort, needs further study.20,21
The implications of our study have yet to be fully elucidated from both a conservative and operative treatment standpoint. The idea that disuse muscular atrophy of the cervical spine stabilizers and its association with instability represents both a challenge and opportunity for physicians. Recognizing that axial neck pain begets atrophy and ultimately instability allows for more targeted strengthening with physical therapy. A study by Tamai et al. observed this relationship using kinematic MRIs for patients with symptomatic neck pain or radiculopathy and found that muscle degeneration was correlated with disk degeneration and degenerative spondylolisthesis, suggesting that physical training should be employed to maintain or improve cervical alignment.22 Although this may be one opportunity to help patients avoid surgery by preventing further progression of symptoms, prospective studies are needed to clarify the efficacy of deep cervical flexor strengthening and its benefits in reducing symptoms in the setting of cervical spondylolisthesis.
This retrospective study is not without its limitations. First and foremost, the retrospective nature of our study subjects our findings to the same inherent biases associated with all retrospective studies, which include, but are not limited to, differences in patient characteristics and demographics between study groups. Nonetheless, these variables were identified and controlled for in our regression analysis to minimize their impact on our study’s conclusions. Another limitation of this study is the fact that we assessed longus colli CSA at the midpoint of the C5 vertebral body to standardize measurement across patients regardless of fusion level. There is undoubtedly anatomic variation that this standardized measurement may fail to appreciate; however, degenerative cervical spondylolisthesis has been shown to occur most often at the C4-C5 level.23 Lastly, our study population only consisted of patients with Grade 1 and 2 spondylolistheses who were indicated for surgery which therefore limits the generalizability of our findings. Further studies observing both operative and nonoperative patients are needed to provide additional insight into the effect of longus colli CSA on the clinical presentation and progression of cervical spondylolisthesis.