Cervical myelopathy caused by hypoplasia of the atlas: two case reports and review of the literature.

OBJECTIVE AND IMPORTANCE: Congenital anomalies of the posterior arch of the atlas (C1) are uncommon. They range from partial clefts to total agenesis of the posterior arch. Developmental cervical canal stenosis is a congenital anomaly that may cause cervical myelopathy. Myelopathy caused by cervical stenosis at the level of the atlas has been reported in only three cases. We present two cases of nontraumatic cervical myelopathy caused by spinal stenosis at the level of the atlas associated with a hypoplastic but complete posterior arch of C1. CLINICAL
PRESENTATION: Two elderly Chinese men developed cervical myelopathy gradually during months to years, without preceding trauma. Imaging revealed a hypoplastic but complete posterior C1 arch associated with changes of spondylosis in both patients, producing severe spinal stenosis and spinal cord compression. Posterior decompression was achieved in both by the removal of the posterior arch of C1 with its surrounding thickened posterior ligaments. Symptoms and clinical findings improved in the two patients during the follow-up period.
CONCLUSION: The anomaly presented in our two cases differs from the established classification of congenital abnormalities of the posterior arch of the atlas, suggesting a different embryological defect. The hypoplastic posterior C1 arch created a congenitally narrowed spinal canal in our patients, rendering the spinal cord more susceptible to compression related to degenerative changes of the spine. Surgical removal of the shortened posterior C1 arch and surrounding degenerative ligaments is an effective treatment for symptomatic patients with this condition.