Normalization of hindbrain morphology after decompression of Chiari malformation Type I

Abstract

Object: Chiari malformation Type I (CM-I) is characterized by hindbrain deformity. We investigated the effects of craniocervical decompression surgery on the anatomical features of hindbrain deformity with a prospective MRI study of patients with CM-I.

Methods: A prospective longitudinal study was conducted in 48 patients with CM-I (39 with syringomyelia) treated with craniocervical decompression. Clinical examinations and cervical MRI were performed before surgery and 1 week, 3-6 months, and annually after surgery. Hindbrain deformity was defined by tonsillar ectopia, pointed cerebellar tonsils, and/or cervicomedullary protuberance. The length of the clivus, basiocciput (sphenooccipital synchondrosis to basion), supraocciput (internal occipital protuberance to opisthion), and anteroposterior (AP) width of CSF pathways at the foramen magnum were measured and compared with those from 18 healthy volunteers (control group).

Results: Before surgery, the patients' posterior fossa bones were short and their CSF pathways were narrow. All patients had tonsillar ectopia (mean [± SD] 12.3 ± 5.1 mm; normal 0.3 ± 1.0). The majority of patients had pointed tonsils and more than two-thirds exhibited a cervicomedullary protuberance. Clivus and basiocciput lengths were significantly shorter than the values obtained in the control group. However, the supraocciput length did not differ significantly from control measurements. The mean bulbopontine sulcus distance superior to the basion was 9.5 ± 2.6 mm (vs 13.6 ± 2.8 mm in controls; p < 0.0001). The AP widths of the CSF pathways at the level of the foramen magnum were significantly narrowed. After surgery, CSF pathways significantly expanded both ventrally and dorsally. By 3-6 months after surgery, pointed tonsils became round, cervicomedullary protuberance disappeared, and tonsillar ectopia diminished by 51% (to 6.0 ± 3.3 mm; p < 0.0001).

Conclusions: The cerebellar tonsils and brainstem assumed a normal appearance within 6 months after craniocervical decompression. These findings support the concept that the CM-I is not a congenital malformation of the neural elements but rather an acquired malformation that arises from pulsatile impaction of the cerebellar tonsils into the foramen magnum. Clinical trial registration no.: NCT00001327.

Fig. 1

Diagram of the measurements taken in the midsagittal plane. Linear measurements (mm) included: cerebellar tonsillar ectopia (T, maximum extension of the cerebellar tonsils caudal to the foramen magnum); bulbopontine sulcus height (bps, distance from basion to pontomedullary junction); supraocciput length (so, internal occipital protuberance to opisthion); clivus length (cl, apex of dorsum sellae to basion); basiocciput length (bo, sphenooccipital synchondrosis to basion); AP width of the ventral subarachnoid space (v, inset), and AP width of the dorsal subarachnoid space (d, inset). The Boogaard angle (B, inner angle formed by the clivus, basion, and opisthion) was also measured.

Fig. 2

A and B Midsagittal T1-weighted MR images of the posterior fossa and cervical spine obtained before (A) and 6 months after (B) craniocervical decompression. Abnormally shaped (pointed) tonsils, dorsal cervicomedullary protuberance (arrow), and obliterated CSF spaces at the foramen magnum are evident preoperatively (A). Following surgery (B), the tonsils have assumed a normal shape, the cervicomedullary protuberance has disappeared, and CSF spaces including the foramen of Magendie have enlarged. C and D: The upper portions of the images in A and B, respectively, showing the Boogaard angle (inner angle between intersecting white lines in C and D) that was established before surgery (C) and was used to determine the level of the foramen magnum after surgery (D). In D, black dashes were added to indicate the position of the inner table of the supraocciput before surgery. Tonsillar ectopia was much reduced after surgery (D, black arrows).

Fig. 3

Midsagittal T1-weighted MR images of the posterior fossa and upper cervical spine obtained in a patient with CM-I before (left) and 6 months after (right) craniocervical decompression. The preoperative image demonstrates pointed, ectopic cerebellar tonsils (arrow), impression of the anterior medulla against the tip of the odontoid process, absence of CSF spaces at the foramen magnum, and a cervical syrinx (arrowhead). Postoperatively, the tonsils have ascended and become rounded, anterior compression has resolved, CSF spaces have expanded, and the syrinx has completely resolved. The CSF spaces in the superior half of the posterior fossa are unchanged.