Pathological Status of the Mesial Temporal Lobe Predicts Memory Outcome from Left Anterior Temporal Lobectomy

Abstract

This investigation tested the hypothesis that the degree of impairment to memory function caused by an anterior temporal lobectomy (ATL) is inversely related to the pathological status of the resected hippocampus. Specifically, the greatest risk to postoperative memory function should be to patients with no or minimal hippocampal sclerosis, i.e., those with a functional hippocampus. Forty patients who underwent a partial resection of the left (n = 21) or right (n = 19) anterior temporal lobe were administered tests of immediate and delayed verbal and figural memory, both preoperatively and 6 months postoperatively. The degree of postoperative impairment in memory function was then investigated as a function of the degree of hippocampal sclerosis, as determined by a standardized procedure. For a left ATL, an absence or mild degree of hippocampal sclerosis was associated with significantly greater postoperative impairment of both verbal and figural memory, compared with patients with moderate or marked sclerosis. No statistically significant relationship was noted for patients who underwent a right ATL, but the findings were in the same direction for five of six memory measures. It may be possible to predict and avoid surgically induced impairment of memory function among patients who undergo left ATL through the use of preoperative hippocampal volumetric magnetic resonance imaging. Better clinical tests of right hippocampal function are needed to predict the outcome for patients who undergo a right ATL.

A Dissociation between Implicit and Explicit Verbal Memory in Left Temporal Lobe Epilepsy

“…The results from this study corroborate the previous literature on intentional (explicit) influences of memory in LTLE by showing that these patients clearly perform poorly on tasks that tap explicit memory (21)(22)(23)(24)(25)(52)(53)(54)(55). In contrast, performance on our measure of implicit memory, free of contamination from explicit memory effects, was intact and comparable to that of normal controls.…”

“…Patients with left temporal lobe epilepsy (TLE) show evidence of mesial temporal lobe hippocampal atrophy, as measured through magnetic resonance imaging (MRI) volumetry (16)(17)(18)(19)(20). Their deficits in explicit memory are well documented (21)(22)(23)(24)(25).…”

Naming Decline After Left Anterior Temporal Lobectomy Correlates with Pathological Status of Resected Hippocampus

“…The very significant differences in age at onset of epilepsy [(7 vs. 20 years) (26)] and the increased risk of early etiological insult among patients with the syndrome of MTLE appears to favor the possibility of reorganization of higher cortical function. Indeed, the minimal neuropsychological morbidity among patients with left MTLE in the context of the significant verbal memory decline associated with left ATL in patients without MTLE (27)(28)(29)(30)(31)(32) is consistent with such a thesis. The risk of postoperative language decline after left ATL is associated with later age at onset of left TLE, implying that the earlier age at onset associated with MTLE may involve reorganization of cortical language function (10).…”

“…This dichotomous grouping has been used in previous investigations that have demonstrated meaningful relationships with regard to memory status and clinical features in patients with TLE (24,26,27). Because grade 1 HS represents 4 0 % neuronal dropout, HS-essentially denotes absence of HS.…”

“…More recently, however, the relationship between the risk of pre-/postoperative memory change and the pathological status of the hippocampus has been clarified with greater precision; no or minimal HS is associated with greater risk of memory loss when ATL is performed on the languagedominant side (27,30,31). The implication is that absence of sclerosis indicates a functioning hippocampus, with its removal resulting in loss of function.…”