Objectives Tumefactive demyelinating lesions (TDLs) occur spontaneously during the course of MS or as the first manifestation of demyelination and may be precipitated by drug withdrawal, vaccination, viral infection or fingolimod. The objective of this study was to identify the frequency, clinical context and radiological features of TDLs in a hospital based cohort.

Methods This is a retrospective analysis of 515 MS patients from the MS clinic at Royal North Shore Hospital. Lesions were regarded as tumefactive when larger than 2 cm in diameter. Clinical, laboratory and radiological data were collected. The lesions were characterised according to the radiological features and topography and correlated with the clinical presentation.

Results Forty-five (8%) patients had TDLs. More than half of TDLs exhibited ‘ring-like’ enhancement on MRI (58%). A ‘Balo-like’ pattern occurred in 24%. In 71% (32) of patients TDLs developed without a precipitant during the course of MS, and in 27% of patients (12) TDLs appeared in the context of a precipitating factor (prior vaccination, viral infection, treatment with fingolimod or cessation of medication due to pregnancy). In 20 (44%) patients the lesions were exclusive to the WM. The most frequent location of TDLs were the frontal lobes (56%), followed by parietal (53%) and temporal lobes (46%). In six patients TDLs in the cerebellum or brainstem were identified. Patients with multifocal, non-precipitated TDL’s exhibited the highest EDSS (mean 2.44).

Conclusions Tumefactive MS is predominantly a white matter disease and lesions are frequently in the frontal lobes and exhibit ring-like enhancement. Curiously, TDLs occur most frequently during the course of otherwise prototypic MS and in this scenario may cause significant disability.