Symptomatic Adult Chiari I Malformation: Posterior Fossa Morphometric Analysis and Clinical Outcomes

Introduction: The data on correlation of posterior fossa morphometric analysis (PFMA) in adult Chiari I Malformation (CIM) with the extent of tonsillar herniation (EOTH), syrinx formation, and clinical outcomes are lacking. The objective of this study is to elucidate the same and, additionally, assess the role of EOTH and syrinx formation on clinical outcomes.

Methods: Symptomatic adult patients (≥21 years old) who underwent surgery for CIM from 2001 to 2010 were retrospectively analyzed (n = 23). Adults with acquired CIM or having EOTH < 5 mm were excluded from the study. Comparative posterior fossa morphometric analysis (PFMA) was done with normal adult controls (n = 23). The patients were further divided into four groups; Group 1(syrinx), Group 2(no syrinx), Group 3 (EOTH < 15 mm), and Group 4 (EOTH ≥15 mm) for intergroup analysis. Good outcome was defined as improvement in preoperative KPS score at follow-up (mean, 10 months).

Results: The mean age of the patients was 38 years. In the comparative PFMA, length of the supraocciput was shorter in CIM patients than control (35 mm vs. 41 mm, P = 0.0004), whereas the anteroposterior diameter (APD) of foramen magnum was larger in CIM patients than controls (41 mm vs. 37.5 mm, P = 0.02). In the intergroup analysis, no variable, including EOTH, was significantly different between Groups 1 and 2. The angle between the supraocciput and the cerebellar tentorium was significant in the patients who underwent posterior fossa decompression with duraplasty. Postoperative CSF leaks were seen in 13%. Good outcomes were noted in 55% of patients, although no patient showed deterioration in KPS score. EOTH and presence of syrinx were not associated with clinical outcomes.

Conclusion: Significant difference in length of the supraocciput and APD of foramen magnum in our study suggests altered bone development in adult CIM patients. The cohort of patients having tonsillar herniation ≥15 mm had higher angle between the supraocciput and cerebellar tentorium. EOTH and presence of syrinx were not associated clinical outcomes.