Grabb-Oakes measurement

The Grabb-Oakes measurement (pB-C2 line) is a measure of the encroachment by the odontoid process into the upper spinal canal (basilar invagination) and thus, potential ventral brainstem compression. It is used to aid in the diagnosis of craniocervical instability.

Measurement
The pB-C2 line is the distance in milimeters from the dura to the line drawn from the basion to the posterior inferior edge of the C2 vertebra.

Clinical significance
The pB-C2 line was first proposed by Grabb, et al. as a means of quantifying ventral brainstem compression in pediatric and young adult patients with Chiari malformation. They found that in patients with pB-C2 lines less than 9mm, posterior fossa decompression surgery alone was sufficient to improve symptoms. However, following decompression surgery, patients with pB-C2 lines greater than 7mm (n=19) remained mildly symptomatic and those with measurements greater than 9mm (n=11), moderately symptomatic. In the latter group, treatment directed at improving the retroflexed odontoid was necessary. A second pediatrics study found that even among those with “normal” Grabb-Oakes measurements, higher measurements were correlated with a greater incidence of post-operative headache.

A consensus statement formed at the second International CSF Dynamics Symposium of the Chiari and Syringomyelia Foundation in 2013, proposed that a Grabb-Oakes measurement greater than 9mm, or in some cases, 8mm, could be seen as “potentially pathological.”

Epidemiology
As compared to the clivo-axial angle, there has been less work done to establish the average Grabb-Oakes measurement in the general population or in various patient cohorts.

Pediatrics
In Grabb et al.’s pediatric study, healthy control participants (n=12) had an average Grabb-Oakes measurement of 3.1mm, while the average for those with Chiari Malformations (n=40) was 7.0mm.

Adults
A study of the Grabb-Oakes measurement in 125 healthy adults found a mean of 6.5 ± 2.1mm, range of 0–11.2mm. The authors argue that normal ranges for adults are probably higher than those established in pediatric patients, and that the pediatric literature should not be used for establishing surgical values in adults. However, another study of 100 asymptomatic adults found a similar mean, 6.7 ± 1.0mm, range of 4.2–10.2mm. Only one person in this study had a Grabb-Oakes measurement greater than 9mm, the common pathological cutoff, suggesting a Grabb-Oakes measurement greater than 9mm is quite rare in the general population.