Re-examining the crowding hypothesis in pediatric epilepsy

In adults with left-sided epilepsy, reorganized language may "crowd out" right-hemisphere visual-spatial skills, with relative sparing of language (i.e., the crowding hypothesis). However, this effect has not consistently been demonstrated in pediatric epilepsy studies. The objective of this study was to investigate the crowding hypothesis using a heterogeneous sample of children with intractable epilepsy and typical (left) language dominance or atypical (right or bilateral) language dominance. We examined the relative contributions of seizure onset (before or after age 5), handedness (right versus left), seizure localization (temporal versus extratemporal), as well as language dominance on verbal versus visual cognitive skills.

We retrospectively analyzed neuropsychology assessment results from a sample of 91 children who completed presurgical evaluation at the Hospital for Sick Children in Toronto, Canada (34 with typical language, 57 with atypical language, mean age = 12 years). We considered a selection of verbal skills (naming, vocabulary knowledge, verbal abstract reasoning) and visual skills (visual-motor integration, block construction, visual abstract reasoning).

Consistent with several previous adult studies supporting the crowding hypothesis, univariate analyses showed that the typical and atypical language groups were comparable on the measures of vocabulary knowledge and abstract verbal reasoning whereas the atypical language group produced lower scores across visual measures. Multivariate analyses (taking into account language dominance and associated factors) showed that language dominance was the strongest predictor of performance on two of three visual measures whereas language dominance was not a significant predictor of performance on most verbal measures. Unexpectedly, both sets of analyses indicated that the atypical language group had poorer naming abilities than the typical language group.

Our data provide some evidence of right-hemisphere functional crowding effects in a heterogeneous sample of children with intractable left-sided epilepsy. Specifically, those with atypical versus typical language dominance showed poorer visual-motor integration and visual-motor problem-solving skills, with comparable scores on certain verbal measures. It is critical that potential crowding effects be considered when interpreting the neuropsychological profiles of children being evaluated for epilepsy surgery.