Pachygyria with cortical thickness greater than 10 mm and a perisylvian predominant severity gradient. The severity gradient is determined based on the gyral width, with gyri typically wider than 5mm over the more severely affected regions. Perisylvian predominant gradient indicates pachygyria more severe other the occipital lobes but also includes a rare perisylvian-predominant pachygyria and a temporal predominant pachygyria. [from HPO]