Obex/Nucleus gracilis Position: Its Role as a Marker for the Cervicomedullary Junction

Abstract

Traditionally, the decussation for the pyramids has been the designated marker for the cervicomedullary junction. With Chiari malformations abnormal caudal migration of posterior fossa structures occurs, resulting invariably in a low position of the caudal poles of the cerebellar tonsils (i.e. tonsillar heterotopia) and a low position of the cervicomedullary junction. Since there is considerable variability in normal caudal tonsillar position and because the pyramidal decussateons are difficult to image, diagnostic confusion may exist when the cerebellar tonsils reside in an intermediately low position. This study utilized anatomic specimens to establish the normal morphologic relationship between the position of the obex/nucleus gracilis (O/NG) and the pyramidal decussations; the former consistently lies 5–6 mm rostral to the latter. Having established that anatomic relationship, the O/NG position was then used to assess the cervicomedullary junction position in a radioanatomic survey of normal brain MR examinations. This analysis established a mean O/NG position 10–12 mm above the plane of the foramen magnum. The O/NG position was then evaluated in patients with low tonsillar position. This analysis revealed, as expected, that patients with Chiari II malformation exhibited tonsillar and O/NG positions well below the plane of the foramen magnum. It also distinguished between two additional groups of patients with less florid MR findings. These include patients who exhibit minimally low cerebellar tonsils but essentially normal O/NG position; these patients should be considered normal expressing only minor variation of tonsillar position. The remaining patients exhibit evidence of both intermediately low tonsillar and low O/NG positions, suggesting an underlying anomaly of both tonsil and brain-stem development. It is this latter group of patients that might be considered Chiari I malformations.

Asymmetry of Tonsillar Ectopia in Chiari I Malformation

“…One patient with a 40° progressive levoscoliosis had no syrinx, and the progression of the curvature was halted following posterior cranial fossa decompression. Interestingly, Quisling et al [9]reported a mean distance of 1.7 and 1.5 mm inferior to the plane of the foramen magnum for the inferior pole of the left and right cerebellar tonsils, respectively, in normal patients aged 1–25 years (mean 12.7 years). Curiously, in our study, two patients had a ‘unilateral’ Chiari I malformation, with the inferior pole of the left cerebellar tonsil being located at the plane of the foramen magnum in both cases.…”

Cranial roots of the accessory nerve exist in the majority of adult humans

“…Measurements were made using a digital caliper (Mitutoyo, Tokyo, Japan). The obex was used as the external landmark for the junction of the brain stem and spinal cord (Quisling et al, ). Only branches found superior to the obex were defined as cranial roots of the spinal accessory nerve.…”

A points-based algorithm for prognosticating clinical outcome of Chiari malformation Type I with syringomyelia: results from a predictive model analysis of 82 surgically managed adult patients

“…16 The 8-mm mean distance of the obex from the FM in our patient cohort was less than the 10-to 12-mm distance observed in healthy populations. 24,29 This suggests that, along with the classic tonsillar descent in CMI, the cervicomedullary junction also tends to be caudally displaced toward the FM.…”