Radiologic Evaluation of Basilar Invagination Without Obvious Atlantoaxial Instability (Group B Basilar Invagination): Analysis Based on a Study of 75 Patients.

BACKGROUND: We evaluated the radiologic features of 75 patients with group B basilar invagination who exhibited no evidence of atlantoaxial instability based on the conventional parameter of an abnormal increase in the atlantodental interval. We specifically studied the variability and possible significance of the presence of cerebrospinal fluid (CSF) within and outside the confines of neural tissues.

MATERIALS AND METHODS: During the period January 2008-May 2015, we encountered 75 cases with group B basilar invagination. These patients were divided into 4 groups depending on cervical spinal imaging that showed the presence of syringomyelia (group B1), increased CSF volume in the extramedullary space or external syrinx (group B2), the presence of both syringomyelia and external syrinx (group B3), and no abnormality of CSF cavitation in the spinal canal (group B4).

RESULTS: Our cohort comprised 39 group B1 cases, 10 group B2 cases, 20 group B3 cases, and 6 group B4 cases. The neck size and posterior fossa height were simultaneously reduced, by 15.89% and 15%, respectively, but the length of the neural structures remained within the normal range. Excessive amounts of CSF were present within or outside the confines of neural structures, including the spinal cord, brainstem, and cerebellum.

CONCLUSIONS: In cases of basilar invagination, various musculoskeletal and neural alterations seem to have a common functional role in protecting the craniocervical cord and delaying or stalling neurologic dysfunction.