Purpose: Amygdala enlargement (AE) has been reported in drug resistant lesional and non-lesional temporal lobe epilepsy (TLE). Its contribution to development of intractability of epilepsy is at best uncertain. Our aim was to study the natural course of AE in a heterogenous group of TLE patients with follow-up imaging and clinical outcomes. Methods: A prospective observational study in patients with TLE with imaging features of AE recruited from epilepsy clinics between 1994 and 2018. Demographic data, details of epilepsy syndrome, outcomes and follow up neuroimaging were extracted. Results: Forty-two patients were recruited including 19 males (45 %). Mean age at onset of epilepsy was 30.6 years and mean duration of epilepsy was 19.9 years. On MRI, 33 patients had isolated unilateral AE and eleven had AE with hippocampal enlargement (HE). Twenty (48 %) underwent temporal resections with most common histopathology being amygdalar gliosis (40 %). Engel Class IA outcome at last follow up (mean, 10 years) was 60 %. Thirty-four patients had neuroimaging follow up of at least 1 year (mean, 5 years). AE resolved in 6, persisted in 25, evolved into bilateral HS in 1, bilateral mesial temporal atrophy in 1 and ipsilateral mesial temporal atrophy in 1. Resolution of AE was associated with better seizure free outcomes (p = 0.013). Conclusions: TLE with AE is associated with favourable prognosis yet not benign. Over 50 % were drug resistant and surgical outcomes were similar to mTLE. Resolution of AE on follow up neuroimaging was associated with better seizure free outcomes.

Introduction

Mesial temporal lobe epilepsy (mTLE) is major contributor to the group of drug resistant epilepsies and is one of the leading indications for epilepsy surgery, the most common pathology being hippocampal sclerosis (HS) [1]. There is a significant group of patients with mTLE that do not have any lesions on MRI, called “non-lesional” or “MRInegative [2]. These patients pose a challenge for treatment in terms of difficult presurgical work-up, frequently warranting intracranial EEG recordings. There is evidence that non-lesional patients fare worse in surgical outcomes in general as compared to lesional ones [3]. Interestingly, there is accumulating evidence that MRI-negative epilepsies are not necessarily non-lesional [4]. The amygdala, which is part of the limbic system, is known to be part of the epileptogenic network of patients with mTLE [5]. Intracranial EEG recordings have revealed interictal epileptiform discharges (IEDs) arising from the amygdala [6]. Structural abnormalities of the amygdala may be difficult to detect by conventional MR techniques [7], and thus may contribute to the so called non-lesional temporal lobe epilepsy (TLE-NL). Imaging studies of TLE-NL patients have identified amygdala enlargement (AE) as an increase in grey matter and amygdalar volume in 12 % of patients [8]. The same authors reported similar proportion (14 %) of AE in patients with mTLE and HS [9]. While the role of the amygdala is functionally defined and evidence of epileptogenicity in the amygdala exists, the significance of AE remains undefined.