Side of surgery
Our sample allowed us to directly compare two children with very early epilepsy onset and lesions of the occipital-temporoparietal region, one in the left hemisphere (Patient 5) and one in the right (Patient 1). Both children were impaired as to reading, but showed a double dissociation in two aspects. First, although Patient 5 was significantly impaired on irregular word reading, Patient 1 showed normal performance. The lexicosemantic path necessary for irregular word reading is thought to involve the anterior part of the inferior frontal gyrus and of the fusiform gyrus (Mechelli et al., 2005), which had been resected in Patient 5 but not in Patient 1. Lesioned in the left hemisphere, Patient 5 was greatly impaired in lexical knowledge. In contrast, Patient 1 was able to directly read with the left hemisphere, where lexical knowledge is well developed. Second, although Patient 1 showed significant pseudo-word reading errors, Patient 5 had normal performance. Words are first processed within each hemisphere in the V4 area and then transferred through the corpus callosum to the left hemisphere, in the VWFA, around the temporooccipital sulcus (Cohen et al., 2000). Pseudo-word reading may further involve the dorsal premotor cortex (Mechelli et al., 2005). Our results suggest that pseudo-word reading may in fact involve other networks, including the right hemisphere. Both the right and left hemisphere fusiform gyri process newly presented words (Dehaene et al., 2001), and the posterior fusiform area has been shown to correlate with the frontal region involved in pseudo-word reading (dorsal premotor cortex) (Mechelli et al., 2005). We posit a right hemisphere contribution in the early detection of pseudo-words that could mimic serial reading processes known to activate biparietal regions (Vinckier et al., 2006; Cohen et al., 2008).
In summary, our sample was too small to quantify cognitive outcome with respect to dysplastic tissue, localization, resection size, time of surgery, seizure freedom, and pharmacotherapy reduction. However, the visuospatial impairment profile of our sample can be related to the neuroanatomic framework. We observed that the one child (Patient 2) showing the highest nonverbal IQ score was also the one with the smallest dysplastic tissue as visualized on MRI, had the least amount of resection, and was operated on at the youngest age. Indeed, this patient showed a very good outcome considering the classic long-term prognosis of infantile spasms (Jambaqué et al., 1993). Furthermore, the child showing the worst overall results (Patient 4), including deficits in oral–verbal attention, also showed the most extended dysplasia, involving the left temporal lobe. The other four patients may also show residual nonepileptogenic dysplastic tissue not obvious on MRI. In these cases, the dysplastic tissue may or may not be cognitively functional (Janszky et al., 2000). Patient 1, the only child with a right occipitoparietal resection, was the most impaired as concerns visual attention, visuospatial orientation, simultagnosia, and visuospatial construction abilities. In contrast, Patient 5, with a very similar resection of the left occipitoparietal region, showed the least visual attention and visuospatial working memory difficulties, but was the most impaired on reading. Finally, the only patient in whom the resection involved the left occipital-temporoparietal region (Patient 3), including the superior parietal lobe, was the only one to show numerical cognition impairments and had relatively preserved reading abilities.
From our small series, we can only convey that a number of variables including dysplasia size, resection size, and location are important in predicting verbal and nonverbal cognitive outcomes. However, given that verbal IQ improvements occurred after surgery, we can ascertain a beneficial role for seizure freedom and antiepileptic drug reduction in verbal brain processing. The mechanisms responsible for the visuospatial outcomes remain unclear. Whereas performance IQ did not significantly change after the surgery, the level of competence of each child is still striking when one considers the lesion sizes in some of the children. Possible recovery factors in visuospatial tasks may include distinct cognitive strategies, brain reorganization, and normalized brain activity resulting from seizure freedom and reduced medication.
One limitation of this study includes the fact that the more extensive neuropsychological evaluation was conducted only postoperatively. This limitation comes from the young age of the patients included in this study. For future work, more comprehensive preoperative assessment could help in better characterizing their improvements or worsening after surgery.
In conclusion, early surgical intervention in children with posterior intractable epilepsies offers the possibility of optimizing cognitive outcome and quality of life. We believe that, in future, more attention should be paid to neuropsychological assessment of these patients to better understand their visual perceptive deficiencies and provide better clinical and psychological management.