Due to CM-Ⅰ often associated with other CVJ malformations, there are inevitably some confounding factors that make the evaluation criteria of CM-I inconsistent. [25] Moreover, children and adults share different clinical courses, not only for the natural history but also the postoperative prognosis. Our study focused on the preoperative clinical and radiologic parameters in simple CM-I patients with syringomyelia to find out the relationship between the clinical status of the patients and the morphometrical characteristics of the syrinx with surgical outcome.
Differences between pediatric and adult patients with CM-Ⅰ and syringomyelia
It is considered that children are still in the stage of growth and development. In a subset of pediatric patients with Chiari malformation without surgical treatment, the syringomyelia could even resolve spontaneously at follow-up.[26] According to comparisons of the preoperative clinical and radiologic characteristics in our continuous series of patients between adults and children, it illustrated that the main difference lies in the duration of preoperative symptoms. What’s more, children often found CM-Ⅰ with syringomyelia on further workup due to scoliosis. In adults, syringomyelia is mostly diagnosed when weakness or paresthesia of limbs occurs. All patients in the two groups completed the operation successfully, and there was no perioperative death or serious fatal complications.
Improvements in syringomyelia-related symptoms, however, do not always correspond to reductions in the size of the syrinx,[27, 28] so we analyzed both syrinx resolution and CCOS score respectively to comprehensively assess the surgical outcomes. The prognosis of both groups was generally favorable, no significant differences existed in the syrinx resolution and clinical outcomes between the two groups. Through further comparisons of radiologic parameters, it was found that syrinx in moniliform type was more common in children, which may be related to the relatively shorter natural history of CM-I in the pediatric group. There were no statistically significant differences in other baseline factors. It is unknown whether the more frequent occurrence of the moniliform syrinx in children is due to the high compensation of syringomyelia in pediatric CM-I. If so, it is necessary to further include phenotypic factors of syringomyelia in adults to analyze its association with prognosis through multivariate analysis.
Univariate and multivariate logistic regression models in adult patients
By reviewing the natural history of CM-I in children and adults, combined with the comparison between the two groups of cases in this series (children vs adults), it is suggested to further explore the relevant factors affecting the prognosis in adult patients, and whether it is related to the specificity of typical moniliform type.
Firstly, through univariate analysis in adults, we concluded that preoperative syndrome duration (P = .000), observation time (P = .000), the length of the syrinx (P = .002), and with/without moniliform type (P = .008) were related factors affecting the prognosis. Next, the multivariate analysis illustrated that the duration of preoperative symptoms, observation time before surgery, and with/without moniliform type were independent prognostic factors. Moreover, the effectiveness of syrinx resolution in patients with moniliform type was significantly higher than that with non-moniliform type.
Appropriate time of intervention for syringomyelia was important.[19, 29] The shorter the preoperative symptom duration and the shorter the observation time for decompression surgery, the better the prognosis. Timely and effective decompression surgery could achieve a better outcome, which was reflected in a higher CCOS score and syrinx resolution rate. The AOC curve also illustrated the duration of preoperative symptoms to guide prognosis with high accuracy. Therefore, surgical decompression should be taken as soon as possible once the indication for surgery is determined, which is beneficial to improve the prognosis. According to the natural history of the occurrence and development of syringomyelia, timely surgical intervention for the moniliform syringomyelia may allow patients to obtain better surgical efficacy, which would be discussed further in the following text.
There was a paucity of detailed studies on the relationship between prognosis after decompression surgery and syringomyelia morphology, which contained syrinx length, width, configuration, and deviation.[30–32] Syrinx configuration has rarely been regarded as predicting factors in previous studies. Our results confirmed that most factors above were not involved in predicting surgical outcomes. The syrinx length was only a related factor, rather than an independent factor affecting the prognosis. However, we interestingly found that the CCOS score improvement in syrinx with moniliform type was relatively better than that with non-moniliform type.
The particularity of syringomyelia with moniliform type
Why does syrinx with moniliform type show better prognostic improvement? Does the formation of moniliform syrinx indicate some kind of protective mechanism, or bring about some inspiration in the clinical diagnosis and treatment of the syringomyelia-related disease? The clinical prognosis of syringomyelia in moniliform type is relatively better, simultaneously with higher effectiveness of syrinx resolution shown by the survival curve in both pediatric and adult group. (Fig. 2) On the one hand, this may be related to the relatively short duration of its natural history, on the other hand, such syrinx may have strong compensatory effects for decompression surgery. During the postoperative follow-up, we found that the moniliform syringomyelia tended to move towards a smaller S/C, but the intrinsic separation of the syringomyelia appeared to persist in the short period after surgery. (Figs. 3 and 4) In terms of imaging characteristics, its continuous separation located inside the syrinx may be the source of the highly compensatory pathophysiological mechanism of such syringomyelia.[33]
From the point of the association among cerebellar tonsillar herniation, CSF circulation obstruction, and syringomyelia, we agreed that syringomyelia generally underwent a dynamic process: pre-syringomyelia stage, progressive stage, stable stage.[34] We further analyzed the differences of various factors at baseline between moniliform type and non-moniliform type syringomyelia and tried to trace the particularity from the origin and development of the moniliform syringomyelia. Such configuration tends to have a shorter duration of preoperative symptoms, and ventral SAS ≥ 2 mm are more concentrated in this type, suggesting that a moniliform type may represent the morphological feature in the early course of the disease. Because the cerebellar tonsillar herniation obstructs the circulation of CSF at the foramen magnum (especially in dorsal SAS), ventral SAS ≥ 2 mm was more common in the moniliform type group, which could be derived from some “buffer space” in the ventral SAS in the early stage.
What's more, straightened cervical physio-curve appears to be more common in patients with moniliform syrinx than non-moniliform syrinx. Some scholars have previously reported that compared with normal people, patients with syringomyelia will have a certain loss in cervical lordosis, but they have not mentioned the intrinsic relationship between the specific syringomyelia type and the corresponding changes in cervical sagittal alignment.[35] We speculated that the compensatory decrease in cervical lordosis during syringomyelia formation may act as a compensatory physiological response to get better CSF circulation in the SAS. This mechanism may be more pronounced in the moniliform syrinx. With the syrinx resolution after surgery, the lordosis may have a certain tendency to recover (Fig. 4), but that still needs to be confirmed by more RCTs.
Clinical significance of this study
The results in this study did not show that other factors such as CSI, TH grade, CVJ measurements, the presence or absence of scoliosis, and cervical physio-curve were directly related to clinical prognosis, nor did it show a difference in these factors between the moniliform and non-moniliform syringomyelia. Although they may be relevant factors that need to be comprehensively considered before surgery, they had no obvious predictive significance for guiding prognosis. However, CM-Ⅰ often accompanies other CVJ malformations, and it is difficult to identify the prognosis through only one single index. It must be analyzed on a case-by-case basis combined with other comprehensive factors.
This paper proposes a special morphological feature of syringomyelia, the moniliform type syringomyelia, associated with the surgical outcome from the perspective of clinical symptoms, imaging features (phenotypes of syringomyelia and biomechanical structures of the CVJ and cervical spine), and multivariate prognostic analysis. Based on the previous studies,[24] we give a new definition for this type of syringomyelia, that is moniliform syrinx with continuous obvious separation on MRI, which presents as a wide ventral SAS at FM, and tends to have a relatively shorter natural history and more common straightened cervical curve, and most importantly it may suggest a better prognosis. In the future, more clinical research is needed to dynamically observe the change of syringomyelia from the perspective of the pathophysiological mechanism of the occurrence and development of syringomyelia, finally to make it clear the structure and function of syringomyelia separation and the biological characteristics of moniliform syringomyelia.
Strengths and limitations
Moniliform type was elicited based on clinical and radiographic comparisons between different age groups. It was confirmed that such type may have an impact on the prognosis in adults, which was assessed using both syrinx resolution and CCOS. Finally, the particularity of moniliform syringomyelia was analyzed.
However, it was undeniable that there were some shortcomings in our study. The survival curve was analyzed based on the event of the effective resolution of the syrinx by postoperative MRI, so a short time interval was selected as much as possible, but there was still interval error, for example, the occurrence of the event happened to fall within the empty period between the checkpoints. Besides, there was a lack of longer follow-up after the effective resolution of the syringomyelia, so more research and evaluations are needed in the future to achieve the maximal ablation effect of the syringomyelia.