Diffusion abnormalities on intraoperative magnetic resonance imaging as an early predictor for the risk of posterior fossa syndrome

Abstract

Background: Posterior fossa syndrome (PFS) is an important complication of posterior fossa surgery in children. The pathophysiology of this condition remains unclear, but there is evidence implicating surgical injury of the proximal efferent cerebellar pathway (pECP) and the cerebellar vermis to PFS. We aimed to evaluate if diffusion abnormalities involving these structures on the final intraoperative MRI can predict the development of PFS.

Methods: Diffusion-weighted imaging from 31 posterior fossa resections were anonymized and evaluated for abnormalities involving the dentate nucleus, superior cerebellar peduncle, and the mesencephalic tegmentum forming the pECP, vermis, and middle cerebellar peduncle. The case notes were independently evaluated for evidence of PFS.

Results: The diffusion imaging in 28 cases was of optimal quality for evaluation. Diffusion abnormalities were identified in 10 cases, 7 of which involved the pECP. Retrospective evaluation revealed evidence of PFS in 6 cases. There was a significant association between abnormalities involving pECP structures (P = .001) and development of PFS. Bilateral involvement of pECP (P = .006) was a highly specific risk factor for predicting the development of PFS. Diffusion abnormality of the inferior vermis was significantly associated with PFS (P = .001) but may not represent a risk factor in isolation.

Conclusion: This study demonstrates the feasibility of identifying children at risk for developing PFS at the earliest stage post tumor resection and thus adds to the growing evidence base on its pathophysiology.

Keywords: cerebellar mutism syndrome; diffusion-weighted imaging; efferent cerebellar pathway; intraoperative MRI; posterior fossa syndrome.

Fig. 1.

T2-weighted images of the posterior fossa at the level of the upper (A) and lower (B) pons showing the regions of interest being evaluated.

Fig. 2.

Diffusion-weighted b800 images demonstrating absence of distortion (A) and significant distortion (B). There is partial distortion (C) affecting the right DN (arrow) due to a focus of air shown on the T2 sequence (D).

Fig. 3.

Cases illustrating various patterns of diffusion abnormalities. (A) Bilateral diffusion restriction of the MT in a 3-year-old girl who developed PFS. (B) IoMR in a 15-year-old girl who developed PFS, showing bilateral hyperintensity of the MT on the b800. The corresponding region on the ADC map appeared isointense, but a subsequent scan 6 days later showed hypointensity on ADC in keeping with restricted diffusion. (C) Diffusion restriction involving the left SCP (large arrows) and the vermis (small arrows) noted in an 11-year-old boy who did not develop PFS.

Fig. 4.

Preoperative imaging in a 15-year-old boy with recurrent pilocytic astrocytoma reveals an enhancing tumor infiltrating the DN, SCP, and MT on the axial T2 (A and B) and T1-postcontrast sequence (C). T2-weighted image (D) on the final intraoperative scan shows partial resection of the tumor with minimal hemorrhage along the surgical margins. The b1000 images (E & F) show no diffusion abnormality along the pECP and vermis. The patient did not develop PFS.