This results in the formation of an abnormally small posterior fossa. Subsequent development of the cerebellum and brainstem within the abnormally small posterior fossa leads to upward herniation and downward herniation of the cerebellar vermis and brainstem through an enlarged foramen magnum into the upper cervical spine. This disorder is usually associated with myelomeningocele. Downward dislocation of the cervicomedullary junction, pons, 4th ventricle and medulla. Cerebellar tonsils located are located at or below the foramen magnum.
Findings are due to brain stem and lower cranial nerve dysfunction.
Onset in adulthood is rare. Most patients present in early childhood or in the neonatal period. If patient has swallowing difficulty, stridor or apneic spells occur, urgent posterior fossa decompression is recommended.