While both usually occur, sometimes a child has a problem only with the structure of the brain and at other times, a child has a problem only with the function of the brain. All the problems with the brain can range from minor to very serious. Others have an abnormal corpus callosum.
Some people also have other parts of the brain which are abnormally formed.
Problems with the formation of the brain can be quite varied often involving the hemispheres and the ventricles. In some people with ONH, these brain problems are minor and do not cause abnormal brain function. In others, brain function is affected, ranging from mild to severe. Many of these problems can be helped with therapy.