Complex sentence: People whose PTE does not respond to medication may undergo surgery to remove the epileptogenic focus, the part of the brain that is causing the seizures, but this may be more difficult than it is for epilepsy due to other causes.
Split into simpler sentences: People whose PTE does not respond to medication may undergo surgery to remove the epileptogenic focus, the part of the brain that is causing the seizures. However surgery may be more difficult than it is for epilepsy due to other causes.