Memory Function Related to Hippocampal Imaging Findings

Abstract

Bilateral Hippocampal Atrophy: Consequences to Verbal Memory Following Temporal Lobectomy

Martin RC, Sawrie SM, Knowlton RC, Bilir E, Gilliam FG, Faught E, Morawetz RB, Kuzniecky R

Neurology 2001;57:597–604

Background

Bilateral hippocampal damage is a risk factor for memory decline after anterior temporal lobectomy (ATL).

Objective

To investigate verbal memory outcome in patients with temporal lobe epilepsy (TLE) with either unilateral or bilateral hippocampal atrophy as measured by MRI.

Methods

The authors selected 60 patients with TLE who had undergone ATL (left = 31, right = 29). They determined normalized MRI hippocampal volumes by cursor tracing 1.5-mm slices from three-dimensional MRI acquisition. Hippocampal volumes were defined as atrophic if the volumes were below 2 SD for control subjects. Bilateral hippocampal atrophy was present in 10 patients with left TLE and 11 patients with right TLE. The authors assessed acquisition, retrieval, and recognition components of verbal memory both before and after ATL.

Results

Groups did not differ across age, education, intelligence, age at seizure onset, or seizure duration. Seizure-free rates after ATL were 70% or higher for all groups. Before surgery, patients with left TLE displayed worse verbal acquisition performance compared with patients with right TLE. Patients with left TLE with bilateral hippocampal volume loss displayed the lowest performance across all three memory components. After surgery, both groups of patients with left TLE exhibited worse verbal memory outcome compared with patients with right TLE. Bilateral hippocampal atrophy did not worsen outcome in the patients with right TLE. A higher proportion of patients with left TLE with bilateral hippocampal atrophy experienced memory decline compared with the other TLE groups.

Conclusion

Bilateral hippocampal atrophy in the presence of left TLE is associated with worse verbal memory before and after ATL compared with patients with unilateral hippocampal volume loss or right TLE with bilateral hippocampal volume loss.

Commentary

Surgical treatment for partial epilepsy has been shown to be an effective and safe operative procedure for selected patients with medically refractory seizure disorders. Individuals with a successful operative outcome may experience a significant reduction in seizure tendency and an improvement in quality of life. The most common surgically remediable epileptic syndrome is temporal lobe epilepsy (TLE). The ictal onset zone in patients with TLE often involves the “detonator structures” of the mesial temporal lobe, that is, the amygdala or hippocampus. The hallmark surgical pathology associated with TLE is mesial temporal sclerosis characterized by hippocampal cell loss and gliosis. An anterior temporal lobectomy (ATL) is the most frequent operative method used in the management of TLE. This procedure includes an amygdalohippocampectomy and a lateral temporal neocortical resection. An important adverse effect of an ATL is an impairment in cognitive function. A significant decrement in verbal memory post-operatively is most likely to occur in patients with left TLE. Quantitative magnetic resonance imaging (MRI)-based hippocampal formation (HF) volumetry is of prognostic importance in determining the seizure and neuropsychological outcomes of ATL. Individuals with left HF atrophy and a normal right HF volume are less likely to experience a decline in verbal memory following a left ATL than those with symmetrical HF volumes or right HF atrophy. The structural MRI complements the intracarotid sodium amobarbital study in the preoperative evaluation of the patient's neurocognitive risk associated with epilepsy surgery.

Martin et al. evaluated the effect of bilateral HF atrophy on verbal memory in patients undergoing an ATL. The investigators identified 60 patients with TLE (left = 31, right = 29) who had a comprehensive presurgical evaluation prior to a TLE that comprised quantitative HF measurements, neuropsychological studies, and long-term EEG monitoring. Unilateral HF atrophy occurred in 39 patients (left = 21, right = 18). Twenty-one patients had bilateral HF atrophy (left = 10, right = 11). The patients were separated into four groups based on the neuroimaging studies: left TLE with left HF atrophy, left TLE with bilateral HF atrophy, right TLE with right HF atrophy, and right TLE with bilateral HF atrophy. Preoperatively, patients with left TLE and bilateral HF atrophy had the “lowest performance” on verbal memory testing. Postoperatively, both groups of patients with left TLE exhibited a significant reduction in verbal memory performance. Patients with left TLE and bilateral HF atrophy appeared to have the least favorable outcome. The results of the MRI study did not affect the neuropsychological outcome in patients with right TLE. The authors concluded that patients with left TLE and bilateral HF atrophy should be “carefully consulted regarding the risk for memory change after surgery.”

This study provides further evidence indicating the pivotal role of the left HF in determining verbal memory in patients with left TLE who proceed with epilepsy surgery. Previous studies have shown that patients with symmetrical HF volume measurements (with or without atrophy) and left TLE are at risk of a decline in verbal memory following surgery. Here we see that patients with left TLE and bilateral HF atrophy are at the highest risk for experiencing a postoperative significant reduction in verbal memory. Other clinical factors that may affect memory after an ATL include preoperative cognitive performance and surgical outcome. A presurgical neuroimaging evaluation may be useful in counseling and guiding the patient with left TLE. Ultimately, the decision regarding surgical treatment in patients with left TLE needs to be individualized. The potential risk of a clinically significant verbal memory decline must be balanced with the putative beneficial effects of successful surgery.