Abstracts

Rationale: It is frequently assumed that language and verbal memory functions are lateralized within the same hemisphere. Patients with early left hemisphere seizure onset are known to have a higher probability of at least some reorganization of language to the right hemisphere (about 33% in our sample of 86 patients). Reorganization of memory has also been reported, specifically when there is damage to the mesial temporal structures. However, little is known about the dissociation of these functions when reorganization occurs. This report describes left temporal lobe patients in which there is apparent reorganization of memory function to the right hemisphere despite persistent left hemisphere language dominance. Methods: A retrospective record review identified 6 left temporal lobe epilepsy surgery candidates who failed the Wada memory test with the right injection and passed with the left injection. All cases demonstrated left hemisphere language dominance and completed presurgical neuropsychological testing. The Wada memory test consisted of a maximum of 16 items (8 pictures, 4 words, and 4 designs) but only items presented prior to first motor return were scored for each injection. The neuropsychological test battery included the Wechsler Memory Scale-IV subtests of Logical Memory (LM) and Verbal Paired Associates (VPA), Verbal Selective Reminding Test (vSRT), Nonverbal Selective Reminding Test (nvSRT), and Boston Naming Test (BNT). Postoperative memory outcome was evaluated using the same measures in patients who underwent left mesial temporal resections. Results: Patients were all right handed with no family history of left handedness. All 6 cases were referred for epilepsy surgery, 5 of whom had left mesial temporal sclerosis on MRI. Mean age of seizure onset was 9.17 years (range = 1 to 30). Wechsler Adult Intelligence Scale-IV Full Scale IQs ranged from 86 to 116 (x = 97). Mean presurgical cognitive test scores were within normal limits in each domain, including verbal memory (scaled scores: LM-I = 10.50, LM-II = 8.33, VPA-I = 9.33, VPA-II = 9.67; % correct: vSRT Best Recall = 91.60, vSRT Delayed Recall = 49.20), nonverbal memory (standard score: nvSRT Long-Term Storage = 95.60; % correct: nvSRT Delayed Recall = 65.00), and naming (BNT raw score = 49.67). All 6 patients failed Wada memory tests in the left hemisphere following right injection (x = 24% correct) but passed memory tests in the right hemisphere (left injection; x = 77% correct). Postsurgical neuropsychological test scores were available for 3 patients, all of whom underwent left temporal lobectomy including the mesial temporal structures. In 2 of 3 cases, all cognitive scores remained stable or slightly improved following surgery. The third patient demonstrated decline on only one memory measure. (Mean scaled scores: LM-I = 10.00, LM-II = 9.00, VPA-I = 9.00, VPA-II = 9.33; % correct: vSRT Best Recall = 77.67, vSRT Delayed Recall = 55.67; standard score: nvSRT Long-Term Storage = 96.00; % correct: nvSRT Delayed Recall = 79.17; raw score: BNT = 48.67). Conclusions: These findings are consistent with reorganization of memory function to the right hemisphere as evidenced by intact baseline verbal memory scores and the absence of significant postoperative decline. All patients maintained unequivocal left hemisphere language dominance. These cases provide an alternative interpretation of intact verbal memory at baseline and have implications for preoperative counseling, as many patients facing left temporal resection are considered at risk for postoperative verbal memory decline even when the risk of postoperative amnesia has been ruled out. This study also highlights the continuing value of the Wada test, especially to help clarify the memory capacity of each hemisphere. Funding: N/A