Platybasia is defined by a nasion-tuberculum-basion angle greater than 143° (122) and was mostly found in groups 2 (14.3%) and 4 (30.3%).
Basilar invagination is an upward displacement of the odontoid dens into the foramen magnum. Radiologic definition of basilar invagination is the crossing of the McRae line – connecting the basion to the opisthion – by the tip of the odontoid process (123). It has been associated with 12-35% of CM-1 but the relationship between those two deformities has not been totally understood (124). In our cohort, we measured the Klaus index (91) to evaluate basilar invagination, which is defined by an index below 30 mm. Using the same cutoff value to define basilar invagination in a pediatric population, whatever the patient’s age was, limits the interpretation of the prevalence of basilar invagination throughout the different subgroups – it overestimates it in younger patients. We found a 40.3% rate of basilar invagination in our cohort according to this definition which is superior to the data from the literature in adult series of CM-1 patients. This is not surprising and explained at least partially by the use of same definition as in adult population. Patients with basilar invagination were predominant in groups 2 and 4, with respectively 97% and 81.4%. Those two groups were also the only ones with absolute mean values of the Klaus index inferior to 30 mm.
Ventral brainstem compression by the odontoid process may happen even if there is no basilar invagination according to the traditional definition. Hence, the use of the pB-C2 length (distance between C2 tip and the line drawn between the basion and the posteroinferior aspect of the C2 body) may be of interest to evaluate this anterior compression. High pB-C2 has been
36 associated with eye motion abnormalities and upper cervical osseous anomalies, and some authors suggest that anterior decompression should be considered if pB-C2 is greater than 9 mm (89). In our cohort, only 17% of the patients had a pB-C2 greater than 9 mm and were mostly found in group 4 (36%) and 5 (40%). No patients with pB-C2 higher than 9 mm were present in group 2.
Some studies identified basilar invagination, type 1.5 Chiari malformation and clivo-axial angle < 125° – but not pB-C2 ≥ 9mm – as preoperative risk factors for occipitocervical fusion (125). In our cohort, we lacked sufficient paraclinical data to assess the stability of the craniovertebral junction. The unfavorable outcomes after posterior fossa decompression without stabilization in groups 2 and 4 compared to other groups may suggest a persistent instability phenomenon of the craniovertebral junction and/or of the atlantoaxial joint.
Surprisingly, basilar invagination and ventral compression of the brain stem were almost not correlated (correlation coefficient of 0,02). In group 5, patients often presented with a ventral compression but without an upward displacement of the dens through the foramen magnum, whereas in group 2, ventral compression was infrequent and upward displacement of the dens was frequent. Another feature characterizing patients from group 2 is the significantly higher position of the obex compared to the whole cohort. We hypothesize that this may reveal an upward displacement of not only the dens but also the brainstem itself in group 2 (Figure 7). Patients from this group also had the most variability in C1C2 angle throughout the postoperative course.
Our understanding of these phenomena is that patients from this group suffer from a craniovertebral junction instability on the sagittal plan.
37 Figure 7 – Hypothesis on the mechanisms
underlying the radiological features in group 2 – Craniovertebral instability leads to an upward displacement of both the dens and the brainstem that may explain the high position of the obex and the variation in C1C2 angle.
In group 5, ventral brainstem compression was generally not associated to an upward displacement of the dens. Tonsillar herniation was significantly higher in this group compared to the cohort (p<0.0001). Outcomes were favorable after duroplasty and tonsillectomy. Fourth ventricle stenting was performed only in 2.6% from this group and showed rather unfavorable outcomes. Our interpretation of these results is that anterior compression of the brainstem by the dens is responsible for the crowding of the foramen magnum and tonsillar herniation, therefore explaining the efficacy of surgical strategies that make more space in the posterior fossa, i.e.
duraplasty and tonsillectomy. The good evolution without osteosynthesis in this group is in favor of the absence of instability at the level of the craniovertebral junction in this group.
38 Interestingly, four of the six patients that had a C0C1 fusion in the cohort were found in this group. Although no statistical analysis can be performed on such a small sample, this may be suggestive of a bony anomaly of Pang’s central pivot (9), which could be one of the factors at the origin of the ventral compression of the brainstem. Patients in this group were older than in the other groups, presented mostly with headaches and sometimes with central sleep apneas. We think that the mechanisms underlying tonsillar herniation in these patients are close to those in adult patients with CM-1. Some authors have advocated a transoral odontoidectomy in CM-1 patients with irreducible deformities of the craniovertebral junction compressing the brainstem (126,127). We believe that an anterior decompression should only be associated in the most severe CM-1 cases with an important anterior compression of the brainstem and associated symptoms, i.e. brainstem dysfunction. However, no patient in our cohort underwent a transoral decompression, hence we lack data to analyze the efficacy of such strategies that will have to be assessed in future prospective studies.