Twenty-three studies (2,012 patients) were included in the review.
IQ (four studies): Epilepsy surgery was associated with an average 11% loss and 16% gain in IQ for combined left and right surgical groups.
Memory (12 studies): In studies that reported results by side of surgery (eight studies), in left operated patients, the average rate of verbal memory decline was 44% (95% CI 34 to 55) versus 20% (95% CI 14 to 28) in right sided patients. Gains in verbal memory (seven studies) were 7% (95% CI 3 to 16) in left sided surgery and 14% (95% CI 7 to 27) in right sided surgery. For visual memory (six studies), average loss was 21% (95% CI 13 to 31) for left sided surgery and 23% (95% CI 18 to 29) for right sided surgery and average gain was 15% (95% CI 10 to 21) in left sided surgery and 10% (95% CI 7 to 13) in right sided surgery. In one study involving children where side of surgery was not reported, gains in verbal and visual memory were 10% and 25% of children, respectively and risks of loss were 5%.
Language (five studies): Average risk of decline in naming (four studies) was 39% (95% CI 32 to 47) for left sided surgery. Gain (one study) for right sided surgery was 4%. A loss of 4% in receptive comprehension of language was reported in one study of left sided surgery.
Executive functioning and attention outcomes (four studies): Left and right sided surgery were associated with a loss of 1% and 0% respectively (one study) and a gain of 9% and 4% (one study) in mental flexibility. Left and right sided surgery were associated with an average loss of 10% (95% CI 4 to 23; 3 studies) and 21% (two studies) respectively and an average gain of 27% (95% CI 10 to 55; 2 studies) and 16% respectively (two studies) in word fluency. In left and right sided surgery, two studies reported an average loss of 6% and 2% respectively and an average gain of 10% and 15% respectively in attention.
Overall subjective change in multiple abilities (three studies): Regardless of side of surgery, average overall loss was 9% and average overall gain was 18%.
Substantial heterogeneity was identified in all pooled estimates of verbal memory, average loss in left sided surgery for visual memory and most measures of word fluency.