EEG charateristics in patients with CT or MRI localized intra-axial brain tumors at a tertiary hospital in Uganda.

Abstract

Introduction: EEG findings in brain tumors vary. The various types of electroencephalographic alterations, when they occur, may include delta slowing, spikes and spike-wave elements & “typical interictal epileptiform discharges” among others. These observations promise a useful insight into the pathophysiology of epilepsy. Objectives: we described EEG findings in 31patient with intra-axial brain tumors with the primary aim of delineating the neuroanatomical structures involved in modulating the observed EEG findings. Methods: This was a cross-sectional study in 31 patients with intra-axial brain tumors. Size and intra-axial positions of the brain tumours was determined and confirmed using CT or MRI scans then surface EEG was done and its characteristics described and documented by a blinded reader in the corresponding cases. Results: For both infratentorial and supratentorial loci we find that the tumor size is not linearly related to the EEG characteristics. The clear cut EEG findings were three (i.e. Normal, polymorphic slowing and interictal epileptiform spikes and sharp waves). The total number of normal EEG was 11 (35.5%) in patients with tumors of both infratentorial and supratentorial locations. Polymorphic slowing was 12 in total (38.7%) mainly in patients with tumors located in the superficial cerebral white matter and at the cerebral white-grey matter boundary. Total interictal epileptiform discharges were 9, of which 4 were overlapping with polymorphic slowing. The interictal epileptiform spikes were mainly in tumors of the cerebral grey matter. Significance: The lack of linearity of the EEG characteristics to the tumor size and the distribution of normal EEGs throughout the brain in our study indicate that certain neuroanatomical pathways must first be compromised before any EEG change can be realized. Reduction of the cholinergic tone in neuromodulation has in previous studies been associated with delta and polymorphic EEG. We now propose and implicate the destruction of cholinergic fibres in the production of polymorphic EEG changes which we observed in the current series. The observation of interictal spikes (some sleep potentiated) on polymorphic slow background EEG activities of 4 patients with white matter tumors implicate polymorphic slowing as a precursor to interictal epileptiform spikes and focal cortical hyper- excitability in cases of white matter & subcortical tumors. We therefore propose that the aforementioned destruction of cholinergic fibres set the stage for this focal cortical hyper excitability by abetting an abnormal form of frequency dependent synaptic plasticity that has previously been described as augmenting response. In cases of purely grey matter tumors where interictal epileptiform discharges were observed without polymorphic background slowing of the EEG the mode of epileptogenesis suggested above therefore may not apply.