Chiari identified herniated cerebellar tonsils on postmortem more than a century ago [1]. With the development of high-resolution magnetic resonance imaging (MRI) in the last decade, such tonsillar ectopias (Chiari Malformation, CM) have become more common and acknowledged as an important cause of syringomyelia (SM) [2]. Both these pathologies (CM and SM) have been independently recognized as risk factors for scoliosis [3,4]. While scoliosis has been reported in 25% to 85% of patients with Chiari Malformation type I (CM-I) and SM; the incidence of CM-I and SM in scoliosis patients ranges between 4% and 58% [3–5].

The exact pathogeneses underlying the association between these conditions are still poorly understood [6]. The malfunction of the central nervous system, damage to anterior horn cells, and changes in the dynamics of cerebrospinal fluid flow as a result of tonsillar herniation are all thought to play a role in the formation or propagation of syrinx, and subsequent neurogenic changes in paraspinal musculature possibly lead to the spinal deformity [7,8]. Although deformity patterns associated with SM and CM are different from the deformity characteristics observed in adolescent idiopathic scoliosis patients, the typical curve morphology is not well-documented hitherto [9]. The role of posterior fossa decompression (PFD) on the progression of the syrinx, deformity magnitude, and outcome in patients presenting with a combination of CM-I, SM, and scoliosis are still largely unclear [10,11]. As a result, the current study was designed to evaluate our experience with patients who underwent PFD after presenting with CM-I, SM, and scoliosis. The overall pattern of scoliosis (and its correlation with syrinx morphology and CM-I), its progression, morphological patterns of the syrinx, the effect of PFD on syrinx size and curve magnitude, corrective scoliotic surgeries or other non-surgical interventions undergone; and thus, the overall outcome in such scenarios were all examined.

The study included 42 patients in total. The patients’ average age was 14.2±5.8 years, and there were 20 female patients (six of whom were premenarchal) (Table 1). Sixteen patients (38.1%) had an abnormal superficial abdominal reflex (SAR; absent on the right, left, and bilaterally in three, eight, and five patients, respectively). The rest of the neurological examinations were all normal in all of the patients.

It is well-acknowledged that a subset of children with “presumed idiopathic scoliosis” may have an underlying abnormality of the neural axis [13]. The most common such anomalies include ACM-I and SM, which are primarily diagnosed on MRI [1,13]. Although factors like age, sex, and certain clinical findings (e.g., stigmata of spinal dysraphism, abnormal SAR, etc.) may indicate the possibility of underlying neuraxial abnormalities, a few radiographic guidelines have also been put forth to help clinicians in suspecting such neural axis disorders in scoliotic children with normal clinical examination [4]. The identification of “atypical curves” assumes a major significance in this context, as it has been well-demonstrated that such curve patterns have a higher frequency of association with abnormal neural axis [1,13,14].

ACM-I is a congenital hindbrain disorder that results from abnormal development of para-axial mesoderm [15]. It is defined as a caudal migration of cerebellar tonsils ≥5 mm below the foramen magnum [16]. ACM can result in the development of spinal syrinx, and syrinx can lead to scoliosis as a result of asymmetrical insult to the spinal cord secondary to an expanding cyst [14]. ACM has been associated with scoliosis even in the absence of syrinx [13]. This phenomenon has been explained by the truncal imbalance due to asymmetrical compression of the cervicomedullary junction by asymmetrically herniated cerebellar tonsils [16,17]. Researchers have also reported scoliotic curves in patients with idiopathic SM, in the absence of ACM [14]. The current study was designed to evaluate our experience with patients who underwent PFD after being diagnosed with ACM-I, SM, or scoliosis.

In our series, one-third of the patients (14 patients) were 16 years or older, which is the typical (and relatively late) pattern of presentation in developing countries. SAR was abnormal in about 38% of the people. The side of abnormal SAR, on the other hand, did not correspond to the side of tonsillar descent or syrinx. Fujimori et al. [18] had previously reported a strong association between abnormal SAR and SM. They concluded that abnormal SAR had sensitivity, specificity, positive predictive value, and negative predictive value for SM in a patient with presumed idiopathic scoliosis of 89%, 95%, 80%, and 98%, respectively. None of the patients in our series had any signs of major sensorimotor, lower cranial nerve, or evidence of medullary dysfunction.

Atypical scoliotic curve patterns were found in about 83% of our patients. Atypical patterns included left-sided convexity and apical hyperkyphosis. A double major curve pattern appeared in 23.8% of cases. Zhu et al. [1] observed that the thoracic spine in patients with scoliosis tended to be convex on the dominant side of an asymmetrically-displaced tonsil as well as the deviated side of an eccentrically-located syrinx. In their series, 66.7% had asymmetric tonsillar descent and 74.4% had an eccentrically-located syrinx. In our study, we could observe that the convexity of scoliosis was more commonly located on the same side as an eccentrically-located syrinx [1,4,14]. We were unable to find a statistically significant relationship between the side of tonsillar descent and the side of scoliosis. In our study, 83.3% of patients had asymmetrically descended cerebellar tonsils, and 50% had an eccentric syrinx. These findings support the role of spinal cord dysfunction in the pathogenesis of SM-associated scoliosis. Sun et al. [15] also reported that the position of cerebellar tonsils in a cohort of 206 children with “presumed idiopathic scoliosis” did not significantly differ with different curve patterns or severity, although there was a higher incidence of tonsillar ectopia in children with atypical curves or double thoracic curves.

Fujimori et al. [18], in their study, had observed that patients with SM requiring surgery had a mean preoperative Cobb angle of 78° (range, 56°–105°) and mean curve flexibility of 58% (range, 50%–78%). They concluded that SM presented with more severe and rigid curves, in comparison with idiopathic deformities. Of the patients, 33% had a severe deformity of magnitude >70° and 76.2% had significant apical vertebral rotation (Moe’s grade ≥2). Zhu et al. [12] observed that sagittal spinopelvic parameters (including TK, LL, PT, and SS) in patients with ACM and scoliosis were significantly different from those with idiopathic scoliosis. While the PT was significantly lower in these patients, TK was relatively higher. We could also observe a higher prevalence of apical thoracic or TL hyperkyphosis (31%) in our patients.

Previous studies have reported good results following bracing in patients with scoliosis secondary to ACM-I, although the outcome has not been as successful in those with associated SM [19]. Sha et al. [19,20] reported that bracing after PFD was effective in preventing curve progression in 62% to 64% of patients with ACM. This result was significantly better than those who were only observed (where the curve stabilized in only 30% of patients). After PFD, all patients who were eligible for bracing (Risser 0, 1, 2; curve magnitude between 25° and 50°; interim progression of the curve by at least 5°) were advised to wear an additional brace.

Yeom et al. [8] observed that early diagnosis and decompression of syrinx in scoliosis before 10 years of age was crucial in decreasing the curve size and limiting scoliosis progression. A previous study by Brockmeyer et al. [21] also revealed an excellent outcome after PFD in ACM-I patients younger than 10 years, where the surgery could halt scoliosis progression in 91% of situations. We found no significant difference in curve stabilization after PFD in younger children (as compared to children >10 years) in our study. This observation could, however, be because our cohort had a smaller number of younger children. Although our observations suggest that PFD can potentially stabilize deformities of less than 40°, its role in stabilizing deformities of larger magnitudes is unclear and needs to be investigated further. Many previous studies have made the same observation [1,4,8,10,14].

In all patients who present with a combination of ACM-I, SM, and scoliosis at our institution, we prefer to perform PFD first. Additional shunt procedures are performed in patients whose syrinx size is persistently large or whose neurological status is deteriorating. There was no neurological deterioration in this series of patients after PFD. One patient, however, had a persistently large syrinx that necessitated an SSAS procedure three months after PFD. All deformity correction surgeries are performed no sooner than 6 months after PFD as a standard procedure at our institution. We had no neurological complications with this approach in the current series.

We could observe a significant reduction in the size of the syrinx (as revealed by a significantly reduced S/C ratio) after PFD. Even based on the evidence in the literature, a significant proportion of patients with ACM-I have a substantial reduction in syrinx size with PFD alone [11]. Zhu et al. [6] recommended that neurosurgical procedures are necessary for patients presenting with syringomyelia when the S/C ratio is greater than 0.7 (in large syrinx). In this series, 23 patients underwent definitive deformity correction surgery. While the literature has suggested a good outcome in these patients even with selective thoracic fusion [9], only one patient was amenable to a selective fusion procedure. All patients had a satisfactory outcome with the achievement of adequate fusion at the end of 2-year follow-up.

We did not include a control group of people with idiopathic scoliosis in our study to compare curve characteristics and morphology. In our study, we observed all of the inherent flaws of a retrospective trial. Alternative management strategies such as definitive fusion without prior PFD, single-staged PFD and deformity correction, or regular shunt procedures for the syrinx were not compared. The follow-up MRI was obtained at 6 months after PFD, despite the fact that only patients with at least 2 years of clinical follow-up were included in this retrospective study. Nonetheless, we were able to see excellent results from our management team. This is one of the largest studies heretofore published, which not only discuss the characteristics of deformities in these rare scenarios but also elaborates upon the outcome following PFD and deformity correction.

A majority of patients with ACM-I and SM present with atypical curve patterns. In our cohort, the side of syrinx deviation correlated significantly with the convexity of scoliosis. PFD, followed by surgical deformity correction is a safe approach in such scenarios with low neurological risk. PFD reduces the size of the syrinx by a significant amount. PFD may not play a significant role in the stabilization of scoliosis in curves with magnitudes >40°.