No evidence of hippocampal sclerosis in healthy subjects on magnetic resonance imaging

Purpose: The presence of hippocampal sclerosis on MRI has a great impact for the treatment of patients with temporal lobe epilepsy. It influences the clinical evaluation and counselling and is considered as a key criterion for the decision to recommend epilepsy surgery in patients with temporal lobe epilepsy (TLE). However, neuropathological studies describe evidence of hippocampal sclerosis (HS) in up to 10% of non-epileptic patients, challenging the impact of MRI findings in patients with TLE. Therefore, we evaluated the prevalence of hippocampal sclerosis on MRI in the general population, using a specific protocol, sensitive for the detection of hippocampal sclerosis

Methods: 100 healthy subjects and 10 patients with TLE due HS were investigated in a prospective study using 1.5T MRI with a specific protocol for the detection of hippocampal pathology (coronal FLAIR and coronal T2 TSE perpendicular to the long axis of the hippocampus and a T1 weighted 3D SPGR sequence). The MRI films were read independently by two experienced and blinded reviewers.

Results: HS was diagnosed in none of the healthy subjects (95% CI=0%-3.6%) proving that the prevalence is significantly less than 0.1. Inter-rater agreement was perfect for presence of HS. Thirty-three subjects had an unilateral enlarged temporal horn, one of the secondary criteria for HS and the inter-rater agreement was poor for this point. Incidental pathologic findings occurred in 2% of the subjects, including an astrocytoma of low malignancy in the left frontal lobe and an aneurysm of the posterior communicating artery.

Discussion: HS is not an incidental finding on the MRI of healthy subjects, supporting its impact for the evaluation of patients with temporal lobe epilepsy. An unilateral enlarged temporal horn that occurred in one third of the healthy subjects however should not be considered as a pathologic finding or even as a single marker for HS.