Clinical variables and language outcome after hemispherectomy: a review of 20 cases
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Research in the field of outcomes of syntactic abilities in patients after cerebral hemispherectomy (i.e. complete disconnection and/or removal of one cerebral hemisphere due to drug-resistant seizures) has not resulted in conclusive findings. A few studies that have been carried out have not reported uniform findings, and it remains unclear what effect on language the removal of left (language-dominant) or right hemispheres is. Important limitations of all existing studies are small numbers of patients (except for one study by Liégeois 2008b) and the use of tests that are not sufficiently grounded in a current linguistic theory. Our first task was to unify and critically assess previous literature. In total, fifteen studies have been reviewed in terms of syntactic tests used and clinical variables such as aetiology reported. Eight studies (total number of patients, n = 14) qualified and their findings are summarized in Figures 1 – 3. A review of literature suggests that aetiology may be an important factor to take into account in addition to side removed similar to hypotheses that have been put forward by (Curtiss et al. 2001, Liégeois 2008b). To further investigate this hypothesis we designed a study that partially overcomes limitations of previous reports. We investigated the effect of aetiology x side interactions in a relatively large population of hemispherectomized individuals (n = 20) with well-defined aetiologies limited to two major groups based on the timing of primary insult. We further employed three tests of syntactic ability (ranging from a focus on core syntactic elements to more general grammatical development). Our fourth test addressed lexical comprehension (receptive vocabulary). It was included to examine whether effect of cerebral hemispherectomy on core hard-wired aspects of language are similar in a domain of language that is more influenced by general cognitive abilities (such as IQ). As a result of the methodological improvements made in this study, we were able to show that aetiology x side do predict outcomes when taken into account together. On all our tests the group with either right or left hemisphere remained perform similarly, but only if their primary insult was prenatal. This suggests that a newborn’s brain possesses some degree of equipotentiality when it comes to language acquisition. However, even a very short (one year) period of normal development eliminates this equipotentiality. On all tests participants whose lesion occurred after birth performed similarly to adult stroke patients in that the isolated left hemisphere outperformed its right counterpart. This difference in the pattern of language development following hemispherectomy was not limited to syntactic tests only. The same trend was present in the test of receptive vocabulary suggesting that the hemispherectomy effects are not limited to strictly computational aspects of language (such as syntax).