1296. Defining the Concept of the Condylar Load Index in Chiari 1 Malformation: A Useful Measure in Radiographic Phenotyping

Authors: Vijay Mysore Ravindra, MD; Evan Joyce, MD; Jonathan Scoville, MD; Christopher Wilkerson, MD; Robert Bollo, MD; Michael Karsy, MD, PhD; Huirong Zhu, PhD; Douglas Brockmeyer, MD (Salt Lake City, UT)

Introduction: The influence of the O-C1 joint position on cranio-cervical load-bearing structures is unknown. We hypothesize the O-C1 joint position in relation to the C2 inferior endplate – the condylar load index (CLI)– correlates with phenotype in Chiari Malformation (CM).  

Methods: A single-center prospective review of patients <18 years of age with CM from 2015-2017 was performed. Three CM phenotypes were identified: 1) Simple: tonsillar herniation 2) Intermediate: tonsillar and obex herniation, 3) Complex: tonsillar and obex herniation with a pBC2 > 9, CXA < 125 and/or craniocervical bony abnormality. Two blinded, independent reviewers measured the pBC2, CXA,  and CLI – defined as the position of a line dropped perpendicular from the O-C1 joint to the midpoint of the C2 inferior endplate (positive, neutral or negative). The sensitivity for CLI predicting the observed phenotype and kappa value for inter-observer reliability were calculated.

Results: 133 children were reviewed: 60 were simple, 44 intermediate and 29 complex. The finding of negative CLI showed significant differences with respect to phenotype (p<0.001). Between the three phenotypes, the mean CXA (simple 140 vs. intermediate 139 vs. complex 124, p<0.001), and pBC2 (simple 6.7 vs. intermediate 7.53 vs. complex 8.99, p=0.003) were significantly different. The sensitivity for negative CLI predicting a complex phenotype was 89% [0.718 – 0.977]. The κ=1 indicating a high level of agreement for CLI determination.

Conclusions: Among patients with CM, the CLI is a sensitive screening technique in identifying the abnormal craniocervical relationships of the complex phenotype. Further validation is necessary.