Relationship of Cine Phase-Contrast MRI to Outcome after Decompression for Chiari I Malformation.

Abstract

BACKGROUND Many patients with symptomatic Chiari I malformation experience symptom recurrence after surgical decompression. Identification of predictors of outcome is needed to better select patients most likely to benefit from surgical intervention. We examined whether CSF flow dynamics assessed by cine phase-contrast MRI could independently predict response to posterior fossa decompression for Chiari I malformation. METHODS Pre and postoperative CSF flow dynamics were assessed by cine phase contrast magnetic resonance imaging in 130 consecutive patients receiving posterior fossa decompression for Chiari I malformation between 1997 and 2003. CSF flow was classified as “abnormal CSF flow” if biphasic flow was either absent or decreased through the aqueduct, fourth ventricle and its outlets, the foramen magnum, or ventral or dorsal to the cervical spinal cord. If no evidence of decreased flow was noted CSF flow was classified as “normal.” The association between pre-operative CSF flow dynamics, all recorded variables, and long-term outcome was assessed utilizing multivariate proportional hazards regression analysis. RESULTS All patients had tonsil herniation > 5mm below the foramen magnum (average 11 ± 5mm). Abnormal hindbrain CSF flow was observed in 81% of patients (43% complete obstruction, 38% reduced flow). Normal CSF flow was observed in 19% of patients. In multivariate analysis, patients with normal preoperative hindbrain CSF flow were 4.8-fold more likely to experience symptom recurrence following surgery (RR, 4.85; 95%CI, 1.88–12.5, P < 0.001) regardless of degree of tonsillar ectopia or presence of syringomyelia. Isolated frontal headache (RR, 4.16; 95%CI, 1.7–9.8, P < 0.05) and scoliosis (RR, 9.2; 95% CI, 1.7–10.5, P < 0.001) were also independent risk factors for symptom recurrence. CONCLUSIONS Normal preoperative hindbrain CSF flow was an independent risk factor for treatment failure after decompression for Chiari I malformation regardless of the degree of tonsilar ectopia. Cine phase-contrast MRI may be a valuable tool in identifying patients less likely to respond to surgical decompression for Chiari I malformation.