FIG 1. 49-year-old woman with 5-week history of progressive diplopia.

A, Axial spin-echo T1-weighted noncontrast MR image (400/16/1 [TR/TE/excitations]; 256 x 192 matrix) shows a lesion with a short T1 signal projecting into the lower pons and upper medulla from an anterior extraaxial location (arrows).

B, Sagittal spin-echo T1-weighted image more clearly shows the extraaxial origin of the lesion (closed arrow). The brain stem is not significantly compressed or displaced, but the cervicomedullary junction and biventral lobules just project through the foramen magnum (open arrow).

C, Coronal T1-weighted images before (left) and after (right) contrast enhancement show no enhancing component to the lesion (arrows).

D, Axial fast spin-echo spin density–weighted (3700/19/1; echo train length, 3) (left) and T2-weighted (4200/92/1; echo train length, 8) (right) images. The cyst is of relatively high intensity on the spin density–weighted image (small open arrows) and of mixed intensity on the T2-weighted image, where a ventral short T2 component is seen (closed arrows) as well as a more dorsal component, which is approximately isointense with gray matter (large open arrows). It extends into the brain stem from an anterior and left-sided extraaxial location.

E, T1-weighted spin-echo image 1 year after surgery shows no evidence of recurrence of the lesion. The cervicomedullary junction has not changed its position, despite removal of the cyst.

F, Photomicrograph shows part of the cyst wall containing pseudostratified columnar epithelium (arrows) (hematoxylin-eosin, original magnification x150).

G, Photomicrograph of part of the cyst wall containing columnar ciliated (closed arrows) and nonciliated pseudostratified columnar epithelium (open arrows) (hematoxylin-eosin, original magnification x150).

H, Electron micrograph of the columnar ciliated epithelium shows the ultrastructural details of transversely cut microvilli demonstrating the "9 + 2" configuration (arrows). Note the prominent desmosomes between the adjacent cells (original magnification x18,000).