Grouped data do not tell the whole story: individual analysis of cognitive change after temporal lobectomy

Studies of group measures of central tendency describe the cognitive sequelae of anterior temporal lobectomy; however, information regarding clinically significant change is scarce. This study examined the frequency of clinically significant variability (i.e., a change score > or = 1 standard deviation from preoperative to postoperative performance) in 50 temporal lobectomy patients (19 left-sided; 31 right-sided). A range of cognitive domains was examined. Most (68%) indices showed no change. Of those that changed, an approximately equivalent number were losses and gains. Such results do not support restricting surgical treatment of epilepsy to avoid major cognitive decline. In right-handers, the observance of material-specific memory decline was variable. Only 36% of right-handers with left-temporal lobectomy showed only postsurgical loss in verbal memory, while 50% showed both losses and gains across several verbal memory measures. Almost 60% of right-temporal lobectomy patients exhibited some verbal memory loss, but only one-third showed nonverbal memory loss. We conclude that group studies identifying material-specific memory loss overestimate the consistency and pervasiveness of the negative effects of temporal lobectomy.