Pathology and pathogenesis of chronic myelopathy in atlanto-axial dislocation, with operative or postoperative haematomyelia or other cord complications.

Pathological features are described, and clinico-pathological correlations are made, in 11 patients presenting the unusual combination of chronic compressive myelopathy of the upper cervical cord due to congenital atlanto-axial dislocation, and sudden haematomyelia and respiratory arrest developing at open surgery for release of pressure on the medullospinal tissues, or as a postoperative catstrophe. Necropsy and histological examination confirmed that the neuropathological substrate of intermittent or progressive quadriplegia and sensory changes (mainly deep sensory loss) was degeneration of the lateral and posterior columns. Loss of anterior horn cells of the lower cervical and upper dorsal cord led to weakness of hand and arm muscles. Acute paracentral haemorrhages occurring at the C1 to C2 or medullospinal level, in the territory of distribution of the sulcal branches of the spinal artery (due to sudden release of pressure at operation) were responsible for postoperative paralysis and sensory loss below C2 segment. In terms of the bony anomaly (dissected at necropsy), the cases fell into 2 sibgroups: a) The majority with the odontoid attached to the body of the axis, but short and ill developed, at times accompanied by other bony anomalies and a poorly formed transverse ligament. This tended to produce a fixed type of dislocation with a higher level (medullospinal) and more severe compression of the CNS. b) Fewer cases with a well formed but detached odontoid, producing a less severe myelopathy at a lower level (C1-C2), and generally more readily reducible on extension.