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Travis R. Ladner, Michael C. Dewan, Matthew A. Day, Chevis N. Shannon, Luke Tomycz, Noel Tulipan and John C. Wellons III

OBJECT

Osseous anomalies of the craniocervical junction are hypothesized to precipitate the hindbrain herniation observed in Chiari I malformation (CM-I). Previous work by Tubbs et al. showed that posterior angulation of the odontoid process is more prevalent in children with CM-I than in healthy controls. The present study is an external validation of that report. The goals of our study were 3-fold: 1) to externally validate the results of Tubbs et al. in a different patient population; 2) to compare how morphometric parameters vary with age, sex, and symptomatology; and 3) to develop a correlative model for tonsillar ectopia in CM-I based on these measurements.

METHODS

The authors performed a retrospective review of 119 patients who underwent posterior fossa decompression with duraplasty at the Monroe Carell Jr. Children’s Hospital at Vanderbilt University; 78 of these patients had imaging available for review. Demographic and clinical variables were collected. A neuroradiologist retrospectively evaluated preoperative MRI examinations in these 78 patients and recorded the following measurements: McRae line length; obex displacement length; odontoid process parameters (height, angle of retroflexion, and angle of retroversion); perpendicular distance to the basion-C2 line (pB–C2 line); length of cerebellar tonsillar ectopia; caudal extent of the cerebellar tonsils; and presence, location, and size of syringomyelia. Odontoid retroflexion grade was classified as Grade 0, > 90°; Grade I,85°–89°; Grade II, 80°–84°; and Grade III, < 80°. Age groups were defined as 0–6 years, 7–12 years, and 13–17 years at the time of surgery. Univariate and multivariate linear regression analyses, Kruskal-Wallis 1-way ANOVA, and Fisher’s exact test were performed to assess the relationship between age, sex, and symptomatology with these craniometric variables.

RESULTS

The prevalence of posterior odontoid angulation was 81%, which is almost identical to that in the previous report (84%). With increasing age, the odontoid height (p < 0.001) and pB–C2 length (p < 0.001) increased, while the odontoid process became more posteriorly inclined (p = 0.010). The pB–C2 line was significantly longer in girls (p = 0.006). These measurements did not significantly correlate with symptomatology. Length of tonsillar ectopia in pediatric CM-I correlated with an enlarged foramen magnum (p = 0.023), increasing obex displacement (p = 0.020), and increasing odontoid retroflexion (p < 0.001).

CONCLUSIONS

Anomalous bony development of the craniocervical junction is a consistent feature of CM-I in children. The authors found that the population at their center was characterized by posterior angulation of the odontoid process in 81% of cases, similar to findings by Tubbs et al. (84%). The odontoid process appeared to lengthen and become more posteriorly inclined with age. Increased tonsillar ectopia was associated with more posterior odontoid angulation, a widened foramen magnum, and an inferiorly displaced obex.

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Travis R. Ladner, Michael C. Dewan, Matthew A. Day, Chevis N. Shannon, Luke Tomycz, Noel Tulipan and John C. Wellons III

OBJECT

The clinical significance of radiological measurements of the craniocervical junction in pediatric Chiari I malformation (CM-I) is yet to be fully established across the field. The authors examined their institutional experience with the pB–C2 line (drawn perpendicular to a line drawn between the basion and the posterior aspect of the C-2 vertebral body, at the most posterior extent of the odontoid process at the dural interface). The pB–C2 line is a measure of ventral canal encroachment, and its relationship with symptomatology and syringomyelia in pediatric CM-I was assessed.

METHODS

The authors performed a retrospective review of 119 patients at the Monroe Carell Jr. Children's Hospital at Vanderbilt University who underwent posterior fossa decompression with duraplasty, 78 of whom had imaging for review. A neuroradiologist retrospectively evaluated preoperative and postoperative MRI examinations performed in these 78 patients, measuring the pB–C2 line length and documenting syringomyelia. The pB–C2 line length was divided into Grade 0 (< 3 mm) and Grade I (≥ 3 mm). Statistical analysis was performed using the t-test for continuous variables and Fisher's exact test analysis for categorical variables. Multivariate logistic and linear regression analyses were performed to assess the relationship between pB–C2 line grade and clinical variables found significant on univariate analysis, controlling for age and sex.

RESULTS

The mean patient age was 8.5 years, and the mean follow-up duration was 2.4 years. The mean pB–C2 line length was 3.5 mm (SD 2 mm), ranging from 0 to 10 mm. Overall, 65.4% of patients had a Grade I pB–C2 line. Patients with Grade I pB–C2 lines were 51% more likely to have a syrinx than those with Grade 0 pB–C2 lines (RR 1.513 [95% CI 1.024–2.90], p = 0.021) and, when present, had greater syrinx reduction (3.6 mm vs 0.2 mm, p = 0.002). Although there was no preoperative difference in headache incidence, postoperatively patients with Grade I pB–C2 lines were 69% more likely to have headache reduction than those with Grade 0 pB–C2 lines (RR 1.686 [95% CI 1.035–2.747], p = 0.009). After controlling for age and sex, pB–C2 line grade remained an independent correlate of headache improvement and syrinx reduction.

CONCLUSIONS

Ventral canal encroachment may explain the symptomatology of select patients with CM-I. The clinical findings presented suggest that patients with Grade I pB–C lines2, with increased ventral canal obstruction, may experience a higher likelihood of syrinx reduction and headache resolution from decompressive surgery with duraplasty than those with Grade 0 pB–C2 lines.