Introduction

Patients with refractory temporal lobe epilepsy (TLE) are at higher risk for mental and cognitive impairments than healthy controls (Hermann et al., 1987, 1997; Trimble, 1988). Typically patients with right-sided TLE are frequently impaired in visuo-spatial retention tasks; patients with left-sided TLE may exhibit deficits of verbal memory. Because of these frequent and prominent memory deficits it is sometimes neglected that many TLE patients perform below healthy control subjects on a variety of neuropsychological tests including intelligence measures (Hermann et al., 1997). The probable reason is that the temporal epileptogenic zone is not only malfunctioning but also adversely influences remote cerebral structures resulting in additional cognitive deficits (Engel et al., 1991; Lüders and Awad, 1991). One of our recent studies confirmed that assumption (Jokeit et al., 1997).

We investigated 96 TLE patients by FDG-PET and neuropsychological assessment who had a corresponding unilateral temporal hypometabolism, left hemisphere speech dominance, full scale IQ of >70 and no extra-temporal lesion on MRIs. The regional glucose metabolism was determined in each patient in homologous regions including prefrontal cortex. A multivariate analysis of variance revealed that the observed prefrontal metabolic disturbances, that are remote from the temporal epileptogenic zone, were associated with impaired intellectual abilities. Patients who demonstrated prefrontal metabolic disturbances performed worse on verbal as well as performance IQ measures than patients without prefrontal metabolic disturbances. Although patients who demonstrated prefrontal metabolic disturbances had an earlier epilepsy onset, the revealed association with cognitive impairment was unrelated to the age at onset.