Cortical dysplasia occurs when the top layer of the brain does not form properly. It is one of the most common causes of epilepsy. The most common type of cortical dysplasia is focal cortical dysplasia (FCD). There are three types of FCD: 

Type I − is hard to see on a brain scan. Often the patients do not start having seizures until they are adults. This type usually involves the temporal lobe of the brain. 

Type II is a more severe form of cortical dysplasia. It is seen more often in children. This type normally involves both the temporal and frontal lobe of the brain. 

Type III − includes one of the above described dysplasias. It also involves damage in another part of the brain.   This type may also be due to some form of brain injury early in life.