Neuropsychological Investigation of Temporal Lobe Epilepsy

Formal neuropsychological testing of patients with neurological disorders has been accepted for some 40 years or more as contributing valuable information to their diagnosis and treatment. The emphasis of neuropsychological assessments has shifted somewhat over the yearsand their role now includes the provision of a functional description of the patient's cognitive strengths and weaknesses, the identification of rehabilitation goals and the establishment of baseline measures against which to evaluate the effects of pharmacological, neurosurgical or radiological measures. The neuropsychological approach of particular relevance to the assessment of temporal lobe epilepsy (TLE) patients is the delineation of cognitive deficits associated with a particular brain lesion. Consequently, neuropsychological assessments ofTLE patients have generally focused on the assessment of memory, in view of the body of knowledge which has accumulated concerning the role of temporal lobe structures (in particular temporal neocortex, hippocampus and amygdala) in learning and memoryv". Such knowledge has accumulated largely from the study of memory in patients who have undergone temporal lobectomy (TL) for intractable epilepsy. The earliest suggestion that the temporal lobes were critically involved in human memory dates from a report by Bekhterev' who described a patient who had exhibited a severe memory impairment and who on autopsy was shown to have bilateral softening in the region of the uncus, hippocampus and adjoining medial temporal cortex. In the 1950s, the importance of the hippocampus and amygdala in human memory was clearly demonstrated in patients with bilateral damage. Scoville & Milner's patient" HM is the most extensively studied, having now been followed up for more than 30 years. In 1953 a bilateral medial temporal lobe resection, for intractable epilepsy, produced a severe anterograde memory impairment in this man, which has persisted with little improvement to this day. He is impaired in any memory test where there is a delay between the presentation of material and its recall, especially if given an interpolated distractor task. New motor skills can be acquired, and other instances of procedural learning have been documented's". At least one other patient has been described whose severe memory deficits were attributed to bilateral hippocampal damage'. The evidence concerning memory impairment in patients with unilateral temporal lobe lesions has also come, for the greater part, from neurosurgicallytreated patients. Attempts have been made to relate memory deficits to the extent of hippocampal and neocortical removal. -Iones-Gotman" compiled a table of tasks eliciting material-specific memory deficits after unilateral TL, classifying results according to whether small or large hippocampal excisions were performed. Three patterns of results emerge after TL. Firstly there are tasks on which the deficit is not exacerbated by encroachment on hippocampal tissue. In a second subgroup, there are tests sensitive to both large and small hippocampal removals, with the large excisions producing greater deficits than the smaller ones. In the third subgroup, deficits are only found with large hippocampal removals. Jones-Gotman's summary of data reflects the well accepted pattern of material-specific memory deficits seen after left and right TLs with in general, verbal learning and memory affected by a left TL and nonverbal learning and memory by a right TL9-17. Clearly, when conducting a neuropsychological assessment on non-operated patients however, it is uncertain whether the effect of hippocampal or more widespread pathology is being measured. Thus to some extent the assessment leads to the description of the functional abilities of patients, in the context of what else is known on the basis of history, EEGs, scans, medication, etc. It is well accepted that a combination of neuropsychological tests is more effective than any single test in detecting organic dysfunction, and the use of many tests for the patients may be especially important as such patients may only show subtle, if any, overt indications of cognitive impairment. Although reports may exist of single, particularly sensitive tests with the patients! ", it is still preferable to administer a selection of tests in order to demonstrate that a circumscribed area of dysfunction exists. As indicated by Rausch19, interpretation of neuropsychological assessment results in a complex, multistage process. Absolute test scores do not reveal the complete picture, although raw or derived scores must indeed be related to age and IQ normative data, so that an initial impression can be gained of which scores are impaired. The pattern of scores obtained on a variety of tests must then be examined to see whether a subgroup of scores suggest impairments in specific functions. This is in order to be able to identify the key impairment. For example, a low score on a test of auditory verbal memory may be explained by deficits in verbal memory, auditory comprehension attention or learning, and the use of multiple tests may reveal the underlying deficit. The qualitative nature of the patient's responses must also be considered. For example patients with either left temporal or left frontal lobe impairment may perform poorly on a verbal learning task, although the pattern of impairment will differ. Correct interpretation of results therefore depends on clinical experience of Paper read to Section of Psychiatry, 13 November 1990