Seizure disorder is characterized by recurrent episodes of abnormal electrical impulses in the brain. Some individuals appear to have a lower-than-normal threshold for seizure activity. Seizure activity may occur in anyone, given the right conditions. Head injury, meningitis, brain tumors, and metabolic disorders (electrolyte imbalance, fever, acidosis) may predispose an individual to having seizures. Initiation of seizure activity may occur in a particular brain area (the epileptogenic focus). Nearby and distant neurons may then be recruited into the seizure. When sufficient neurons are involved, the seizure becomes clinically evident as involuntary movement or unusual sensations. Partial seizures involve a part of the brain; generalized seizures involve the entire brain at the onset. Partial seizures are further classified as simple, in which consciousness is retained, and complex, in which consciousness is impaired. Seizures may begin as partial and then generalize to affect the entire brain. Generalized seizures include absence, myoclonic, atonic, and tonic-clonic types.