Dysphagia contributes to higher self-reported disability and lower physical health status. The most probable explanation for postoperative dysphagia is that it is a multifactorial phenomenon, explained by esophageal retraction, direct cervical plate stimulating the esophagus, prevertebral swelling, among others. Persistent and severe dysphagia may lead to various degrees of discomfort, and increase the risk of various complications such as such as difficulty in eating or drinking, bronchospasm, aspiration pneumonia, dehydration, asphyxia and malnutrition. However, the pathophysiology and risk factors of postoperative dysphagia are not fully understood.
The incidence of dysphagia after anterior cervical spine surgery has been reported as ranging from 1–80%. Baron et al believed that the incidence of transient dysphagia after anterior cervical fusion was as high as 80% and that the symptoms of most patients were relieved after treatment.13 These greatly varying incidence rates may be related to factors such as Surgical approach, sample size, and differences in case selection and evaluation methods, particularly regarding the criteria used to define postoperative dysphagia.14,15 Theoretically, if an anterior cervical titanium plate is placed directly into the esophagus, the titanium plate may affect the incidence of postoperative dysphagia. Any mechanical irritation or impingement of the esophagus may cause symptoms of dysphagia. No-notch implants were considered to be associated with anterior plate + cage. Our result showed that the incidence of dysphagias of patients used Zero-P system was lower than that of the previous reported.
Many scholars have developed classification systems to define and classify postoperative dysphagia, but the inconsistent usage and lack of consensus limit its research progress. X-ray fluoroscopic examination of swallowing function has long been regarded as the gold standard for assessing swallowing difficulty. The Bazaz grading system is widely used to assess the incidence of dysphagia after cervical spine surgery. But it has many shortcomings. This scale is based on qualitative information collected by an investigator to assess the patient’s subjective sensation of difficulty when swallowing liquids and solids, possible sensory disruptions causing postoperative dysphagia may be challenging to explain and may not reflect accurate clinical outcomes. In addition, the correlation between the indicators of dysphania and subjective performance is not clear. Anderson believed that the self-assessment questionnaire for patients' main symptoms might be a relatively reliable method for evaluating clinical dysphagia after anterior cervical surgery.16 SWAL QOL score was recommended to be used to assess the degree of dysphagia. Postoperative dysphagia was evaluated using the Bazaz scoring system in our result, and it was found that dysphagia was a common early complication after anterior cervical surgery, and the incidence and severity of dysphagia gradually decreased over time. After 1 year of follow-up, almost all patients had resolved their dysphagia, and only a few still had mild dysphagia. Bazaz's prospective study of 249 patients who underwent anterior cervical surgery found that the incidence of dysphagia was 50.2%, 32.2%, 17.8%, and 12.5% at 1, 2, 6, and 12 months after surgery, respectively. At 6 months after surgery, only 4.8% of patients had moderate or severe dysphagia, which was basically consistent with the results of this study.
Recently, several studies have compared the clinical outcomes spacer and cervical sagittal balance in ACDF for treating cervical disorders.17–19 And the result of some studies indicated the Cobb angle, T1 slope and some other parameters are closely related to clinical outcomes.20–22 To the best of our knowledge, few reports have described the effect of the difference between postoperative and preoperative sagittal balance on postoperative dysphagia after ACDF with Zero-P. Consequently, one objective of this retrospective analysis was to summarize and identify the effect of the cervical sagittal balance and other possible related factors on dysphagia. And provides for future spinal surgeries with evidence on how to reduce the incidence of dysphagia after ACDF with Zero-P.
Miyata believed that posterior cervical surgery, such as posterior occipito-cervical fusion, if the surgery changes the physiological curvature, it will result in mechanical strictures of oropharynx, which may lead to postoperative swallowing disorders.23 This provides an idea for us to study the swallowing disorder caused by the overall curvature change after cervical spine surgery. Our study indicated that dO-C2 angle and dC2–7 angle were significantly related with postoperative dysphagia.
Khaki believed that anterior cervical surgery could affect the throat stage in the four stages of swallowing process, thus leading to postoperative dysphagia.24 Based on the previous studies, we concluded that the posterior pharyngeal wall protruded forward due to the large angle of dC2-7 angle after surgery, which reduced the throat volume and affected the squeezing effect of pharynx during eating, thus leading to dysphagia. However, there is still a lack of direct imaging evidence to confirm this view, and the hypothesis proposed by us can only explain to a certain extent the mechanism of the dC2-7 angle and postoperative dysphagia. In the anterior cervical decompression, the intervertebral space should be extended as far as possible with the use of a retractor to restore the normal curvature of the cervical spine and reduce postoperative cervical degeneration. The results of this study suggest that excessive expansion of cervical spine space may cause excessive changes in C2-7 Angle, which may lead to forward protrusion of the posterior pharyngeal wall, resulting in postoperative dysphagia. Therefore, the recovery of anatomical force line should not be pursued only, but also the change of cervical curvature should be controlled to reduce the occurrence of postoperative dysphagia in ACDF. This suggests that intraoperative control of C2-C7 angle within a reasonable range may reduce the probability of postoperative dysphagia symptoms.
Operative time was another factor associated with dysphagia in our study. In a prospective study of 38 patients undergoing single-segment and double-segment ACDF surgery, found that extended duration of surgery was the only variable associated with the severity of dysphagia 12 weeks after surgery.25 We hypothesized that prolonged traction of the trachea and esophagus would inevitably lead to more severe soft tissue swelling. Therefore, in complex cervical spine surgery, where the operation is expected to take a long time, surgery by a senior cervical surgeon may reduce the incidence of postoperative dysphagia more than surgery by a junior physician or resident.
There are some limitations in the current study. First, the sample size of the retrospective study was small though it was adequate to evaluate the variables. A further prospective study should be designed using a standardized scoring system and postoperative examination. Second, the follow-up period was relatively short, with a mean of 18.5 months. Therefore, the long-term clinical effect should be evaluated. Third, not all potential risk factors, such as the thickness and length of the plate, were not considered in the statistical analysis. In addition, the mechanisms by which the T1 slope affects the development of dysphagia after ACDF with the Zero-P implant System are not completely clear. Therefore, multicenter and randomized controlled studies are needed to verify our conclusions in the future.