Figure 1: CT axial view shows marked narrowing of the foramen magnum, deviating the medullo-spinal junction to the left.
Figure 2: CT axial view with a smaller left posterior fossa.
Figure 3: CT sagittal view shows absent apex of dens, assimilation of atlas into basi-occiput, platybasia, narrowing of the foramen magnum, fusion of atlas and axis spinous processes, and remnant of atlas fused to anterosuperior aspect of odontoid process.
Figure 4: CT coronal view showing the incorporation of the atlas and its condyle into the occiput, particularly on the right.
Figure 5: Sagittal MRI shows stenosis of the foramen magnum and distortion of the medullo-spinal junction.
Figure 6: Coronal T1 MRI scan shows a kinked medulla on the left, a smaller left posterior fossa, and the head tilted to the right.
This 56-year-old patient had chronic pain without an injury. He was neurologically intact, apart from moderate restriction of neck movements. He declined an operation.
There are initially five occipital somites, with the most rostral fading away to leave only four. The boundary between the head and neck corresponds to that between the remaining fourth and fifth somite. The four occipital somites fuse to form the clivus; the caudal part of the occipital four somite and rostral cervical somite combine to form the transitional “pro-atlas.” Its rostral part joins the three occipital somites as the anterior foramen magnum and the apex of the dens. Its lateral part forms the occipital condyles. The caudal half of somite five and the cranial half of somite six form the first cervical somite, giving rise to the base of the odontoid. The second cervical somite (from caudal six and cranial seven) form the body of the axis. The current instance is a result of many anomalies in this process.
Suresh Ramnath, MD, FRCSC; firstname.lastname@example.org
Paul Park, MD; email@example.com
Megan E. B. Foldenauer, PhD, CMI; firstname.lastname@example.org
Department of Neurosurgery, University of Michigan