Clinical significance of changes in pB-C2 distance in patients with Chiari Type I malformations following posterior fossa decompression: a single-institution experience

Abstract

OBJECT The coexistence of Chiari malformation Type I (CM-I) and ventral brainstem compression (VBSC) has been well documented, but the change in VBSC after posterior fossa decompression (PFD) has undergone little investigation. In this study the authors evaluated VBSC in patients with CM-I and determined the change in VBSC after PFD, correlating changes in VBSC with clinical status and the need for further intervention. METHODS Patients who underwent PFD for CM-I by the senior author from November 2005 to January 2013 with complete radiological records were included in the analysis. The following data were obtained: objective measure of VBSC (pB-C2 distance); relationship of odontoid to Chamberlain's, McGregor's, McRae's, and Wackenheim's lines; clival length; foramen magnum diameter; and basal angle. Statistical analyses were performed using paired t-tests and a mixed-effects ANOVA model. RESULTS Thirty-one patients were included in the analysis. The mean age of the cohort was 10.0 years. There was a small but statistically significant increase in pB-C2 postoperatively (0.5 mm, p < 0.0001, mixed-effects ANOVA). Eleven patients had postoperative pB-C2 values greater than 9 mm. The mean distance from the odontoid tip to Wackenheim's line did not change after PFD, signifying postoperative occipitocervical stability. No patients underwent transoral odontoidectomy or occipitocervical fusion. No patients experienced clinical deterioration after PFD. CONCLUSIONS The increase in pB-C2 in patients undergoing PFD may occur as a result of releasing the posterior vector on the ventral dura, allowing it to relax posteriorly. This increase appears to be well-tolerated, and a postoperative pB-C2 measurement of more than 9 mm in light of stable craniocervical metrics and a nonworsened clinical examination does not warrant further intervention.

Keywords: CM = Chiari malformation; Chiari malformation; PFD = posterior fossa decompression; VBSC = ventral brainstem compression; brainstem compression; posterior fossa decompression.

Figure 1

Subjective evaluation of ventral brainstem compression. (A) No compression. (B) Mild compression. (C) Moderate compression.

Figure 2

pB-C2 measurement. A line is drawn between the basion and the inferior-posterior aspect of C2 (B-C2 line). pB-C2 is the perpendicular distance between B-C2 and the ventral cervicomedullary dura.

Figure 3

Other lines and relationships used in the study. McRae’s, Chamberlain’s, McGregor’s, and Wackenheim’s lines are represented with dotted lines in A–D, respectively. (A) Odontoid-McRae distance (C2-McR) and tonsillar descent (TD) are shown. (B) Odontoid-Chamberlain distance (C2-Cham), basal angle (BA) and clival length (CL) are shown. (C) Odontoid-McGregor distance (C2-McG) and ventral kink angle (VKA) are shown. (D) Odontoid-Wackenheim distance (C2-Wack) is shown.

Figure 4

Graphical representation of the change in pB-C2 values after surgery.