Significance of Modified Clivoaxial Angles in the Treatment of Adult Chiari Malformation Type I

Objective: To investigate the influence of modified clivoaxial angle (MCAA) on the surgical planning and outcome in adult patients with Chiari malformation type I.

Methods: Clinical data of 268 adult patients undergoing posterior fossa decompression without (PFD) or with duraplasty (PFDD) or with resection of tonsils (RT) for Chiari malformation type I over 10 years were reviewed retrospectively, with outcome evaluated by Chicago Chiari Outcome Scale (CCOS) score. By means of receiver operating characteristic curve, the threshold of MCAA was calculated to stratify the patients.

Results: MCAA was positively correlated with CCOS score. In the MCAA ≤ 127° group (n = 57) with severe ventral cervicomedullary compression (VCMC), CCOS score of PFD, PFDD, and RT was 11.00, 11.06, and 12.42 (P < 0.05), respectively. In the 127° < MCAA ≤ 138° group (n = 87) with moderate VCMC, CCOS score of PFD, PFDD, and RT was 11.71, 12.72, and 13.00 (P < 0.01), respectively. In the MCAA > 138° group (n = 124) with mild or no VCMC, CCOS score of PFD, PFDD, and RT was 13.19, 13.90, and 13.67 (P < 0.05), respectively. The mean MCAA increased by 4.4° postoperatively (P < 0.05), which was positively associated with syringomyelia shrinkage.

Conclusions: MCAA may play a role in guiding the surgical treatment and predicting the prognosis in adult CMI patients. The larger the MCAA, the less invasive surgery is preferred with higher CCOS. PFDD should be the first choice in patients with mild or no VCMC (MCAA >138°) and RT in patients with moderate VCMC (127° < MCAA ≤ 138°). For patients with severe VCMC (MCAA ≤ 127°), RT could be considered as the primary surgery with awareness of the possible insufficiency of posterior decompression alone.