Is “routine” magnetic resonance imaging necessary in adolescent idiopathic scoliosis? A retrospective analysis in New Zealand
Article information
Abstract
Study Design
Retrospective case series.
Purpose
To determine the prevalence of neural axis abnormalities (NAA) in patients with adolescent idiopathic scoliosis (AIS) undergoing deformity corrective surgery and evaluate factors that may predict the presence of underlying NAA in these patients.
Overview of Literature
There is no clear consensus regarding the use of magnetic resonance imaging (MRI) to screen for potential NAA in patients with AIS. Various clinical and radiographic risk factors predicting underlying NAA have been suggested, but these remain controversial.
Methods
This study included 282 patients with presumed AIS who underwent preoperative MRI to exclude NAA between 2010 and 2020 in multiple centers. Spinopelvic parameters, including Cobb angle, thoracic kyphosis, lumbar lordosis, sacral slope, pelvic tilt, and pelvic incidence, were measured on preoperative and postoperative radiographs. Additional clinical data were gathered on curve characteristics, symptomatic back pain, and abnormal preoperative neurological examinations.
Results
The median age was 14 years (range, 11–18 years). The cohort consisted of 49 males (17%), 217 patients (77%) of European ethnicity, 30 (10.6%) Māori, and 7 (2.5%) Pacific Islanders. Twenty-one patients (7.4%) had NAA, of which five required neurosurgical intervention. Among the NAA group, four were diagnosed with Chiari malformations, seven with syringomyelia, and four with both. The presence of NAA did not affect curve reduction with surgery. No significant association was found between NAA and any investigated variable.
Conclusions
Routine preoperative MRI is justifiable, as 7.4% of the cohort had NAA, with five patients requiring neurosurgical intervention, thereby altering operative management.
Introduction
Adolescent idiopathic scoliosis (AIS) is a common form of scoliosis affecting the pediatric population. AIS is a diagnosis of exclusion, established after ruling out potential underlying etiological factors, including malignancy, inflammatory conditions, demyelination processes, and neurovascular or musculoskeletal congenital anomalies [1–4]. The association between scoliosis and lesions of the central nervous system is well documented. Thus, a proportion of patients diagnosed with AIS may have an underlying neural axis abnormality (NAA), such as syringomyelia, Arnold-Chiari malformations, tethered spinal cord, intraspinal tumors, or diastematomyelia [5,6].
New Zealand is a multicultural nation with a majority of the population identifying as European, while the largest ethnic minority is represented by Māori and Pacific Islanders (Polynesians) [7]. Ratahi et al. [7] reported a strong association between Polynesian ethnicity and having scoliosis secondary to syringomyelia. The authors also concluded that “true” idiopathic scoliosis is uncommon among the Polynesian population.
Identifying NAA through magnetic resonance imaging (MRI) before scoliosis corrective surgery is of critical significance in surgical planning. Several case reports have highlighted the risk of neurological complications in patients undergoing deformity correction surgery with undiagnosed or untreated NAA, particularly syringomyelia. Notably, patients with scoliosis secondary to underlying NAA may not exhibit abnormal neurological signs, making preoperative MRI screening essential [8–14]. However, the use of preoperative MRI to detect NAA remains a topic of debate, with some surgeons arguing that preoperative MRI screening and decompression prior to scoliosis corrective surgery is not clinically relevant in AIS patients who do not present with neurological signs or symptoms [1,8,15–17]. The increasing availability and accessibility of MRI in the developed world have reduced barriers to its use [18]. Historically, clinicians relied on clinical and radiographic findings to predict underlying NAA, but the widespread availability of MRI has changed this approach.
The primary aim of this study was to investigate the prevalence of NAA based on MRI results in patients undergoing surgery for AIS and to evaluate potential clinical and radiographic risk factors that may predict underlying NAA. We hypothesized that there are no reliable risk factors predicting the presence of NAA. The secondary aim of the study was to compare and contrast spinopelvic parameters between patients with and without NAA.
Materials and Methods
Study design
This multi-center retrospective study was conducted across two tertiary referral centers based in the Waikato and Auckland regions of New Zealand. All patients aged 11–18, reviewed from 2010 to 2020, with a diagnosis of AIS who underwent surgical correction, were included in the study. Patients with congenital, syndromic, pathological, or neuromuscular manifestations of scoliosis were excluded.
Preoperative MRI is a routine practice followed by the senior authors. This practice has been informed by previous research by Ratahi et al. [7], and further influenced by the increasing accessibility of MRI, the gradual reduction in MRI costs, and minimal adverse effects, notably, the absence of radiation exposure. Additionally, in the absence of intraoperative computed tomography (CT)-based navigation, which is often the case, anatomic details such as pedicle dysplasia are often gleaned from the MRI.
Ethics approval
Ethical approval was obtained from each participating hospital prior to study commencement, in accordance with local guidelines. The requirement for informed consent from individual patients was omitted because of the retrospective design of this study.
Outcomes and variables
Data pertaining to patient demographics (sex, age, and ethnicity), scoliotic curve characteristics (Cobb angle magnitude, the direction of apex, and the Lenke classification), patient history, examination findings, MRI findings, and description of the scoliosis management were retrieved from electronic clinical records. Additionally, preoperative and postoperative lumbar lordosis (LL) and thoracic kyphosis (TK) were measured along with spinopelvic parameters (sacral slope [SS], pelvic tilt [PT], and pelvic incidence [PI]). These radiographic measurements were collected by the primary author. Erect lateral and anteroposterior scoliosis radiographs obtained preoperatively and postoperatively were used for these measurements. The Picture Archiving and Communication System imaging system was used to view and measure the parameters.
LL was measured as the sagittal Cobb angle between the superior endplates of L1 and the sacral end plate. TK was measured from the superior endplate of T5 and the inferior endplate of T12. PI was measured as the angle between a line from the hip axis to the midpoint of the sacral endplate and a line perpendicular to the sacral endplate. PT was measured as the angle between the line connecting the hip axis to the midpoint of the sacral endplate and the vertical. SS was measured as the angle between the line along the sacral endplate and the horizontal [19]. The study by Vrtovec et al. [20] served as a guide for making these measurements.
All patients in this study underwent preoperative MRI, as determined by the treating spinal surgeon, to rule out potential NAA.
Statistical analysis
Independent-sample t-test and chi-square test were used to identify statistically significant differences between NAA and non-NAA cohorts with respect to spinopelvic parameters, LL, TK, curve characteristics, clinical findings, and patient demographics. p-values <0.05 were deemed indicative of statistical significance.
Results
The study included 282 patients with a median age of 14 years (standard deviation=1.8; range, 11–18 years). The cohort was predominantly female (233 patients; 82.6%) with a smaller proportion of males (49 patients; 17.4%). In terms of ethnicity, 217 patients (77%) identified as New Zealand European, 30 patients (10.6%) were Māori, 26 patients (8%) were of Asian origin, and seven patients (2.5%) were Pacific Islanders. Two patients did not report their ethnicity.
The primary scoliotic curves affected the thoracic spine in 57.8% of patients, the thoracolumbar spine in 41.1% of patients, and the lumbar spine in one patient. The mean Cobb angle was 55.8°±11.7° (range, 28°–95°). A left-sided curve pattern was observed in 60 patients (21.3%). Five patients (1.8%) had a true atypical curve pattern in the form of a left thoracic curve. Double-major curve patterns were present in 35 patients (12.4%). The distribution of patients according to the Lenke classification is illustrated in Fig. 1.
Distribution of patients according to Lenke classification. NAA, neural axis abnormality.
The majority of the patients underwent posterior spinal fusion, with three patients receiving anterior spinal tethering and two undergoing anterior instrumented fusion. No perioperative neurological complications occurred in any of the patients. Nine patients had abnormal preoperative neurological findings, including absent superficial abdominal reflexes, hyper-reflexia, areflexia, positive Babinski sign, and ataxia. In the NAA group, only one patient had an abnormal preoperative neurological finding, specifically absent superficial abdominal reflex. Back pain was reported by 66 patients (23.4%).
Neural axis abnormalities were noted in 21 patients (7.4%), consisting of seven cases of syringomyelia, four cases of Arnold-Chiari malformations, and four cases of concurrent syringomyelia and Arnold-Chiari malformations. Five patients (1.8%) required foramen magnum decompression before undergoing scoliosis corrective surgery. Additionally, six patients had minor NAA findings, including two cases of mega cisterna magna and individual cases of a cystic lesion of the pineal gland, gliosis of the left cerebellar hemisphere, infratentorial arachnoid cyst, and a white matter lesion of the frontal lobe with unknown etiology. These findings are summarized in Table 1.
List of patients with neural axis abnormality identified in the study and summary of outcomes
Table 2 presents a comparison between patients with and without NAA. The analysis revealed no significant differences in the frequency of atypical curve patterns between the two groups, with a notable absence of atypical patterns in the NAA group.
Comparison of demographics, clinical findings and curve characteristics between NAA and Non-NAA cohorts
Table 3 summarizes the spinopelvic measurements for the entire cohort and according to the presence or absence of NAA. Significant differences were observed between the two groups in terms of the amount of TK gained postoperatively, with the NAA group experiencing a greater increase (5.2° vs. 2.6°, p=0.002), and in postoperative PT, with the NAA group having a smaller change (11.5° vs. 15.3°, p=0.03).
Summary of age, curve size, thoracic kyphosis and spinopelvic parameters within the study population and the comparison between the NAA and Non-NAA cohorts
An infographic summarizing the key findings of the study is presented in Fig. 2.
A summary of key findings within the study population. MRI, magnetic resonance imaging; AIS, adolescent idiopathic scoliosis; NAA, neural axis abnormality.
Discussion
The prevalence of NAA detected on MRI in patients awaiting scoliosis corrective surgery ranges from 2.1% to 14% [21]. Several potential risk factors, including male sex, atypical curve patterns, curve severity, thoracic hyperkyphosis, abnormal neurological examination, and back pain, may suggest an underlying NAA in patients with presumed AIS [3,15]. However, there is no clear consensus regarding the need for a preoperative MRI scan to detect NAA before proceeding with corrective surgery [1,2,4,7,15,16,21].
The 2003 New Zealand census reported that 17.7% of the population is represented by Māori, while 8.9% is represented by Pacific Islanders [22]. Thus, the topic of this study is very relevant to the New Zealand population, given the increased prevalence of scoliosis secondary to NAA within the Māori and Pacific Islander populations [7].
The aim of our study was to identify the prevalence of NAA in patients with AIS and to review risk factors that may predict the presence of NAA, thereby justifying the use of preoperative MRI as a screening tool for patients with AIS awaiting corrective surgery. All patients in this study had undergone routine preoperative MRI, as it is readily accessible in hospitals in New Zealand and it has minimal side effects. Historically, MRIs have been costly and difficult to obtain, but in the modern era, they are readily accessible in developed nations [18].
No statistically significant association was found between the presence of NAA on MRI and any of the assessed risk factors in this study. Therefore, no reliable clinical markers were identified that could predict potential NAA in patients with AIS. Additionally, no significant difference was noted between NAA and Non-NAA cohorts in terms of spinopelvic parameters, including LL and TK. Furthermore, no difference was identified in the degree of correction achieved postoperatively between the NAA and Non-NAA cohorts, suggesting that the presence of an underlying NAA did not compromise deformity correction.
Minor differences were noted in spinopelvic parameters between the two cohorts, with the NAA cohort experiencing a greater gain in TK following surgery. One could hypothesize that the presence of NAA may lead to a more cautious correction due to concerns about neurologic compromise, but this was not the case. This is also evident in the greater improvement in Cobb angle in the NAA cohort, albeit not statistically significant. Overall, the presence of NAA did not appear to limit the achievement of significant deformity correction. The only other notable difference was that PT was 3.8° higher in the non-NAA cohort following surgery, which may be related to the relatively lower improvement in TK and reflect natural compensation in alignment. Interestingly, PT did not decrease after posterior spinal fusion for AIS in this study cohort, as is often observed. However, spinopelvic parameters may evolve over time, well after fusion, as the spine adapts to the deformity correction [23,24].
Ramirez et al. [25] suggested that lumbar pain can be a clinical marker for underlying spinal pathology in patients with AIS, but their findings included non-neural abnormalities such as sacral cysts, vertebral haemangiomas, and disc herniations, in addition to NAA. Lee et al. [8] concluded that males with AIS are at a higher risk of having underlying NAA. Wu et al. [26] recommended preoperative MRI for patients with AIS who present with a left-sided curve, are male, or have a severe curve. However, in this study, none of the patients with atypical curve patterns had underlying NAA, despite the prevalence of NAA falling within the range reported in the literature [21]. No statistically significant associations were found between NAA prevalence and the risk factors described in the literature. Additionally, there was no correlation between NAA prevalence and the six curve types of the Lenke classification (p=0.83). These findings are consistent with previous reports, such as Fruergaard et al., who found no significant differences between NAA and non-NAA cohorts with respect to curve type, direction, severity, progression, level of pain, or sex [1].
Although this study found no significant association between sex and the presence of NAA, 14.3% of male patients had an underlying NAA compared to 6% of female patients. This suggests that with a larger cohort, a statistically significant difference between the sexes may emerge, with a potentially higher prevalence of NAA among males with AIS.
Syringomyelia and other NAAs can increase the risk of neurological complications during surgical correction of scoliosis due to intraoperative spinal cord distraction and instrumentation [1]. This risk prompts the use of preoperative MRI scans. However, Wang et al. [27] suggest that in the absence of neurological symptoms, it may be safe to treat the syrinx nonoperatively and proceed with deformity correction. In the present study, five patients (1.8%) required neurosurgical intervention prior to scoliosis correction, necessitating a change in the initial operative plan due to preoperative MRI findings. Furthermore, since the advent of the MRI in the 1970s, its use has expanded due to greater accessibility [18]. Thus, a routine preoperative MRI for AIS will be obtainable with fewer barriers in the 21st century, especially in the developed world.
The preoperative MRI scan provides additional benefits beyond excluding NAA in patients with AIS. Sielatycki et al. [28] introduced a three-tier classification of thoracic spinal cord anatomy and its relation to the apical concave pedicle. Furthermore, intraoperative neuromonitoring (IONM) alerts can predict postoperative neurological deficits. Keshavarzi et al. [29] found that patients with a type 3 spinal cord shape, as described in Sielatycki et al. [28], with a large Cobb angle (>65°), have an increased risk of IONM alerts. This additional anatomical information from the MRI scan supports its routine use in preoperative planning for deformity correction, enabling surgeons to better anticipate and mitigate potential complications.
Some limitations of this study should be acknowledged. The retrospective design led to inaccuracies in certain clinical variables, such as back pain, which was inconsistently documented, making it challenging to find a potential association with underlying NAA. Additionally, inconsistent documentation of factors such as family history, body mass index, and prior bracing precluded analysis of their association with underlying NAA. Furthermore, it is uncertain whether neurological complications would have arisen if some NAA had gone undiagnosed.
The small sample size of the present study and the rarity of NAA limit the ability to draw definitive conclusions about the use of preoperative MRI in presumed AIS, as confirmed by post-hoc power analysis. Nevertheless, the study remains relevant as routine preoperative MRI can mitigate the potential catastrophic risk of neurological compromise due to undiagnosed NAA in patients undergoing scoliosis correction.
Conclusions
In this study, 21 patients (7.4%) were diagnosed with a NAA on MRI, with five requiring foramen magnum decompression prior to deformity correction. No correlation was identified between NAA and risk factors, indicating that MRI is necessary to exclude NAA in asymptomatic patients with presumed AIS. Given the widespread accessibility and minimal side effects of MRI, we recommend that all patients with presumed AIS planned for surgical deformity correction undergo a preoperative MRI scan.
Key Points
21 patients (7.4%) with adolescent idiopathic scoliosis (AIS) had a neural axis abnormality (NAA) on MRI, with five requiring neurosurgical intervention.
No correlation was found between NAA and commonly described clinical or radiographic risk factors.
We recommend preoperative magnetic resonance imaging for all patients with AIS awaiting surgical deformity correction.
Notes
Conflict of Interest
No potential conflict of interest relevant to this article was reported.
Author Contributions
Conceptualization: HC, JFB. Data curation: HF. Formal analysis: HF, JFB. Methodology: JFB. Investigation: HF, EL. Visualization: HF. Writing–original draft: HF. Writing–review & editing: HF, JFB. Supervision: AF, HD, HC, JFB. Final approval of the manuscript: all authors.