Standardized method for the measurement of Grabb's line and clival-canal angle

OBJECTIVE Grabb's line-the perpendicular distance from the basion-C2 line (pB-C2)-is a widely used radiographic measurement with significant clinical implications in patients with a complex Chiari malformation. Rigorous demonstration of the reproducibility of this measurement has not previously been reported. The authors report a standardized measurement technique with excellent inter- and intrarater reliability on T1-weighted sagittal MRI. METHODS The authors developed a standardized measurement technique that included specifications of midline slice selection, landmark and reference line definitions, and measurement technique on T1-weighted sagittal images. Twenty MR images were reviewed by 2 pediatric neurosurgeons, 1 pediatric radiologist, and 1 undergraduate student. Measurements were performed using the technique specified on 2 separate occasions. Intrarater and interrater reliabilities were calculated using the intraclass correlation coefficient. RESULTS A combined interrater reliability of 0.879 was achieved for the pB-C2, and 0.916 for the clival-canal angle, another measure of interest in patients with complex Chiari malformations. Intrarater reliability for these measurements exceeded 0.858 for all 4 reviewers. CONCLUSIONS Grabb's line-the pB-C2-can be measured with excellent reliability using a standardized measurement protocol. Individual clinicians and collaborative databases should consider using validated measurement techniques to guide clinical decision making in patients with craniocervical junction pathology.