MRI findings of olivary degeneration after surgery for posterior fossa tumours in children: incidence, time course and correlation with tumour grading.

PURPOSE: Olivary degeneration is due to many posterior cranial fossa (PCF) lesions affecting the dentato-rubro-olivary pathway, also known as Guillain-Mollaret triangle. Triangle damage results in hyperexcitation and consequently in hypertrophy of the inferior olivary nucleus (ION). The aim of our study was to evaluate the incidence of magnetic resonance (MR) imaging changes in the ION after surgery in a large cohort of paediatric patients and to determine their correlation with tumour grade.
MATERIALS AND METHODS: We retrospectively evaluated 58 patients treated surgically for PCF tumours who underwent MR imaging between 2007 and 2014, 1 week to 5 years after surgery. Histopathology revealed 29 medulloblastomas (WHO IV), 6 ependymomas (WHO II), 2 anaplastic ependymomas (WHO III) and 21 pilocytic astrocytomas (WHO I). ION MR imaging changes were correlated with surgery-to-MR interval and with tumour grading.
RESULTS: ION MR imaging changes were observed in 19/64 (33 %), and all consisted of T2 signal alterations, 15 bilateral and four unilateral, with dentate nucleus damage in all cases. Olivary enlargement was observed in few cases only (7/19). ION T2 hyperintensity was always present between 1 and 6 months after surgery with a trend to decrease, becoming faint after 1 year. The Fisher test demonstrated a significant (p = 0.005) correlation between ION MR imaging changes and high tumour grade.
CONCLUSIONS: Our results demonstrate that olivary degeneration, with or without hypertrophy, is a relatively frequent consequence of posterior fossa surgery, particularly in children treated for high-grade tumours. Knowledge of this condition can prevent misdiagnoses and unnecessary investigations.