Beyond the CA1 subfield: Local hippocampal shape changes in MRI-negative temporal lobe epilepsy.

OBJECTIVE

Hippocampal atrophy in temporal lobe epilepsy (TLE) can indicate mesial temporal sclerosis and predict surgical success. Yet many patients with TLE do not have significant atrophy (magnetic resonance imaging (MRI) negative), which presents a diagnostic challenge. We used a new variant of high-dimensional large-deformation mapping to assess whether patients with apparently normal hippocampi have local shape changes that mirror those of patients with significant hippocampal atrophy.

METHODS

Forty-seven patients with unilateral TLE and 32 controls underwent structural brain MRI. High-dimensional large-deformation mapping provided hippocampal surface and volume estimates for each participant, dividing patients into low versus high hippocampal atrophy groups. A vertex-level generalized linear model compared local shape changes between groups.

RESULTS

Patients with low-atrophy TLE (MRI negative) had significant local hippocampal shape changes compared to controls, similar to those in the contralateral hippocampus of high-atrophy patients. These changes primarily involved the subicular and hilar/dentate regions, instead of the classically affected CA1 region. Disease duration instead co-varied with lateral hippocampal atrophy, co-localizing with the CA1 subfield.

SIGNIFICANCE

These findings show that patients with "MRI-negative" TLE have regions of hippocampal atrophy that cluster medially, sparing the lateral regions (CA1) involved in high-atrophy patients. This suggests an overall effect of temporal lobe seizures manifesting as bilateral medial hippocampal atrophy, and a more selective effect of hippocampal seizures leading to disease-proportional CA1 atrophy, potentially reflecting epileptogenesis.