Quantitative CT Evaluation of Posterior Fossa Volume in Chiari Type II Malformation

INTRODUCTION: To quantitatively evaluate total posterior fossa volume and single-slice volume at the level of the internal acoustic canal on brain CT in patients with Chiari type II malformation and in controls.
METHODS: In this retrospective single-center study, consecutive patients who underwent brain CT between 2014 and 2018 were identified from the institutional archive using the term “Chiari type II.” Images of all identified cases were re-evaluated. Controls underwent brain CT during the same period and had no abnormality affecting posterior fossa anatomy. Total posterior fossa volume and single-slice internal acoustic canal-level volume were measured on axial noncontrast CT images by using a freehand volume-of-interest tool. Group comparisons, correlation analyses, age-restricted sensitivity analysis, receiver operating characteristic analysis, and age- and sex-adjusted multivariable models were performed.
RESULTS: Eighty participants were included (30 controls and 50 patients with Chiari type II malformation). The Chiari type II group was younger than the control group (median age, 5.5 months [interquartile range {IQR}, 1.0–12.0 months] vs 36.0 months [IQR, 24.0–48.0 months]; P < 0.001), whereas sex distribution did not differ (P = 1.000). Total posterior fossa volume was significantly lower in the Chiari type II group (34.78 cm³ [IQR, 26.59–58.80] vs 165.80 cm³ [IQR, 159.38–181.52]; P < 0.001), as was single-slice internal acoustic canal-level volume (IAC-level single-slice volume) (6.27 cm³ [IQR, 4.17–7.87] vs 9.79 cm³ [IQR, 8.23–11.63]; P < 0.001). Total posterior fossa volume correlated positively with age (ρ = 0.796, P < 0.001), remained significantly lower in the 1–5-year subgroup (P < 0.001), and showed an AUC of 1.000 (95% CI, 1.000–1.000). The corresponding AUC for single-slice internal acoustic canal volume was 0.823 (95% CI, 0.725–0.911). In age- and sex-adjusted models, Chiari type II malformation remained independently associated with lower total posterior fossa volume (adjusted ratio, 0.34; 95% CI, 0.27–0.43; P < 0.001) and lower IAC-level single-slice volume (adjusted ratio, 0.78; 95% CI, 0.61–0.99; P = 0.045).
DISCUSSION AND CONCLUSION: Total posterior fossa volume is markedly reduced in Chiari type II malformation and provides stronger group discrimination than single-slice internal acoustic canal-level volume on brain CT.

Keywords: Chiari type II, posterior fossa, volume, computed tomography