Type I Chiari Malformation Without Concomitant Bony Instability: Assessment of Different Surgical Procedures and Outcomes in 73 Patients

Abstract

Objective: Posterior fossa decompression is the treatment of choice in type 1 Chiari malformation (CM-1) without bony instability. Although surgical fixation has been recommended by a few authors recently, comparative studies to evaluate these treatment strategies using objective outcome tools are lacking.

Methods: Seventy-three patients with pure CM-1 (posterior fossa bony decompression [PFBD], n = 21; posterior fossa bony and dural decompression [PFBDD], n = 40; and posterior fixation [PF], n = 12) underwent a postoperative outcome assessment using Chicago Chiari Outcome Score (CCOS). Logistic regression analysis detected predictors of an unfavorable outcome.

Results: Minimally symptomatic patients generally underwent a PFBD while most of the clinically severe patients underwent a PFBDD (p = 0.049). The mean CCOS score at discharge was highest in the PF (12.0 ± 1.41) and lowest in PFBDD group (10.98 ± 1.73, p = 0.087). Patients with minimal preoperative clinical disease severity (adjusted odds ratio [AOR], 4.58; 95% confidence interval [CI], 1.29-16.31) and PFBDD (AOR, 7.56; 95% CI, 1.70-33.68) represented risks for an unfavorable short-term postoperative outcome. Though long-term outcomes (CCOS) did not differ among the 3 groups (p = 0.615), PFBD group showed the best long-term improvements (mean follow-up CCOS, 13.71 ± 0.95), PFBDD group improved to a comparable degree despite a poorer short-term outcome while PF had the lowest scores. Late deteriorations (n = 3, 4.1%) occurred in the PFBDD group.

Conclusion: Minimally symptomatic patients and PFBDD predict a poor short-term postoperative outcome. PFBD appears to be a durable procedure while PFBDD group is marred by complications and late deteriorations. PF does not provide any better results than posterior fossa decompression alone in the long run.

Keywords: Chiari malformation; Functional outcome; Posterior fixation; Posterior fossa decompression; Predictors; Syrinx.

Fig. 1.

(A) A set of images of a patient with Chiari malformation type 1 with a normal bony anatomy. Sagittal section of the magnetic resonance imaging of cervical spine shows a cervical syrinx, tonsillar displacement below the foramen magnum reaching just above the posterior arch of atlas. On computed tomography evaluation, there are no abnormal bony fusions (B, C) and the odontoid tip is not extending more than 3 mm from the Chamberlain line (yellow) and lying below the McRae (red) and Wackenheim line (green).

Fig. 2.

(A) A set of images of another patient with Chiari malformation type 1 with an abnormal bony anatomy. Sagittal section of the magnetic resonance imaging of cervical spine shows a cervical syrinx, tonsillar displacement below the foramen magnum, and ventral encroachment of the medulla by the retroverted odontoid. (B–D) On computed tomography evaluation, there was assimilation of atlas, C2–3 fusion, platybasia with a retroverted odontoid. The odontoid tip is extending more than 3 mm from the Chamberlain line (yellow) but lying below the McRae (red) and Wackenheim line (green), suggesting a basilar impression or type B basilar invagination. In the panel B, the opisthion has been considered to be the point where the 2 cortices of the occipital squama join in view of assimilation of the atlas.