Chiari malformation type I (CMI) is diagnosed as herniation of the cerebellar tonsils by at least 5 mm below the foramen magnum. However, the degree of tonsillar herniation is a poor predictor of the need for decompression surgery. Exploration for an alternative morphological predictor for surgical intervention could provide greater insight into the development of an appropriate treatment plan for these patients. To investigate this issue, the authors calculated the soft tissue density within the foramen magnum as a measure of impaction of the cerebellar tonsils. Soft tissue density within the foramen magnum and degree of tonsillar herniation were then assessed for their correlation with the need for surgical intervention.
The authors conducted a retrospective, longitudinal chart review of pediatric patients with CMI. Those who had undergone surgical intervention were considered symptomatic and those who had been treated conservatively, as asymptomatic. Soft tissue density was found by dividing the soft tissue occupancy of the foramen magnum (brainstem and cerebellar tonsils) by the total area of the bony foramen magnum. The predictive value of these two measurements for the need of surgery was determined.
Of the 465 patients seen for CMI at the authors’ institution between July 1, 2011, and May 31, 2017, 80 underwent surgical intervention and 385 were asymptomatic. The average tissue density was significantly greater in the surgical group than in the asymptomatic group (83.3% and 78.6%, respectively, p < 0.0001). The average tonsillar descent for surgical patients was 10.8 mm compared to 9.8 mm for asymptomatic patients (p = 0.140). The point-biserial correlation coefficient was assessed, and soft tissue density was found to positively correlate with the need for surgical intervention (rpb = 0.199, p = 0.0001), whereas tonsillar herniation did not correlate with the need for surgery (rpb = 0.083, p = 0.115). Additionally, the degree of tonsillar herniation did not correlate with soft tissue density (r = 0.09), indicating that soft tissue density is an independent morphological parameter.
The study findings suggest that the need for surgical intervention in CMI patients is positively correlated with increasing soft tissue density within the foramen magnum, whereas the degree of tonsillar herniation did not show a correlation with the need for surgical intervention. Additionally, soft tissue density is a factor independent of the degree of tonsillar herniation. Further investigation of tissue density within the foramen magnum is needed in the hope of discovering a clinically applicable parameter that would indicate a need for surgical intervention in patients with CMI.