Soft tissue swelling after anterior surgery for degenerative cervical spinal disease occurs in almost all patients, and this is identified by increases in the width of prevertebral soft tissues on plain radiographs. Many studies have attempted to determine whether cervical spine areas have been damaged by measuring the widths of prevertebral soft tissues in trauma patients. Weir1)
advised that the normal range of PSTS at C3 was 2.6-4.8 mm, Templeton et al.4)
defined a PSTS of > 10 mm in the retropharyngeal space as abnormal, Miles et al.5)
defined a PSTS size of larger than half of the diameter of the vertebral body in the retropharyngeal space as abnormal, and Pope and Riddervold defined a PSTS of > 7 mm at C2-3 as abnormal.6)
However, few studies have reported the abnormal range of PSTS observed after surgery for degenerative cervical spinal diseases. Sanfilippo et al.10)
advised that most swellings disappear within 6 weeks and that large abnormal swellings should be monitored or treated, but they did not mention the normal range or a treatment method. The general progress of swelling peaks at day 2 or 3 after surgery and thereafter decreases.11-13)
Sanfilippo et al.10)
advised that although soft tissue swelling was observed even at two weeks after surgery, it normalized at 6 weeks. Similarly, in our study, degrees of swelling progressed and peaked on day 2 at the anterior 3rd. cervical spine at an average of 12.1 mm. When quantifying swelling, the diameters of prevertebral soft tissues measured in the middle of the area between the lower end plate of the C3 and C4 reflect the most sensitive swelling.13)
It was noted that the lower vertebral levels experienced little change postoperatively when compared with the upper cervical spine. This is most likely because of the more constrained anatomy of the lower cervical spine. The potential retropharyngeal space appears to be much greater in the pharynx and hypopharynx than in the more distal trachea/esophageal portion of the neck. Therefore, greater consistency would be found between the preoperative and postoperative measurements. Furthermore, in our study, differences in results were also compared based on PSTS anterior to the C3 on day 2 when changes in PSTS were at their largest. In this study, we identified that the multiple level fusion and upper level fusion showed significant correlations with degrees of swelling. In comparing single level and two level fusions, Suk and colleagues11,12)
reported that a difference in postoperative degrees of swelling was not evident, but advised the possibility that results might differ when more than two levels are involved. In addition, they advised that when surgery is performed at an area proximal to the 5th cervical spine, soft tissue swelling is severe in the C2 and C3 areas, whereas Andrew and Sidhu13)
advised that although swelling increased greatly in the upper level fusion, it was not related to the location. We found that if PSTS reflects the degree of damage to soft tissues occurring during surgery, its degree would be higher for multiple segment surgery. Therefore, degrees of damage to soft tissues occurring during surgery in degenerative cervical spinal disease may also be indirectly identified using plain radiographs as with acute cervical spine trauma patients. Although airway obstruction rarely occurs after cervical spine surgery, it is potentially fatal, and has been reported with swelling and hematoma.14-16)
In our study, re-intubation after surgery was not observed in airway obstruction, dyspnea that required treatment was observed in 13% of the 135 study subjects, and its incidence was higher in those who showed more severe PSTS. However, the time when dyspnea mainly occurred (between day 0 and day 2) and the time when soft tissue swelling was most severe (days 2 or 3) were not the same, and thus, the degrees of soft tissue swelling cannot be safely used to predict the development of dyspnea.
Dysphagia is a complication that occurs relatively frequently after anterior cervical spine surgery. Although no clear mechanism has been suggested, swelling by traction, hematoma, damage to the pharyngeal nerve plexus and to the hypoglossal nerves during surgery in the upper area have been suggested as risk factors.17-20)
In addition, damaged soft tissues may adhere to the larynx or esophagus during the healing process and induce dysphagia.21,22)
Bazaz et al.8)
reported that after anterior cervical spine surgery, swelling occurred more frequently in females and those patients that underwent multiple segment surgery. No significant differences were shown between revision surgery and primary surgery, and between the locations of surgery. The present study shows that soft tissue swelling was not significantly different in males and females, but that it increased markedly when multiple segments and upper level surgery cases were included. Furthermore, dysphagia frequently occurred in patients that experienced marked soft tissue swelling.
Dysphonia after surgery in the anterior cervical spine occurs less frequently than dysphagia, and is known to be caused by traction damage to the laryngeal nerves.17,19)
However, laryngeal swelling could also be a cause, and both are more likely to occur when damage to soft tissue during surgery is more severe. In our study, dysphagia and dysphonia persisting for at least three months after surgery occurred in 12% and 4% of our study subjects, respectively, and these frequencies were significantly related to PSTS.
The limitations of this study are that corrections were not made for other factors, such as, the degrees of traction during surgery, which might affect postoperative soft tissue swelling or the occurrence of complications. Furthermore, patterns of complications relative to PSTS severity were not quantitatively analyzed. Nevertheless, for the first time, the present study describes the links between PSTS and the complications associated with surgery, and thus, sheds light on the clinical relevance of PSTS.
Increments in PSTS after anterior cervical fusion for degenerative spinal disorders are greater and incidences of complications are higher in patients that undergo multi-level or high-level fusion. Thus, the measurement of PSTS using consecutive cervical lateral radiographs after anterior cervical surgery is a clinically meaningful procedure.