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.
Keywords: CD = cord diameter; CM-I = Chiari I malformation; Chiari I malformation; MSD = maximum syrinx diameter; PFDD = posterior fossa decompression with duraplasty; ROC = receiver operating characteristic; craniocervical junction; posterior fossa decompression; syringomyelia.