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

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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.

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