Mental retardation (MR) or severe and profound intellectual disability generally occurs with other severe neurologic or psychiatric impairments, especially in individuals with acquired encephalopathy. Speech defects, epilepsy, and cerebral palsy (CP) are among the most common associated comorbid conditions. However, there are a number of syndromes associated with MR, all of which have their own associated features. Individuals with MR also have a significantly higher risk of sudden death than those without MR. The combination of MR with epilepsy or CP doubles the mortality compared to individuals with CP alone.

Epilepsy, which has a prevalence of <1% in the general population, has a prevalence of 20% to 30% in children with MR, independent of race. Notably, a diagnosis of epilepsy in this population can be difficult, given that patients with epilepsy commonly exhibit behaviors that resemble epilepsy. For example, generalized tonic extension crisis may occur in individuals with severe spasticity, resembling tonic–clonic seizures. Absence seizures may be mistaken for episodes of unresponsiveness that are frequently seen in patients with MR. Additionally, psychiatric disorders potentially responsible for self-destructive behavior in MR patients often need to be evaluated by neurologists to rule out frontal or temporal lobe seizures.

The prognosis of epilepsy in individuals with MR ultimately depends on the etiology of the MR. Some studies have shown a remission of seizures by the second decade of life, but the rate of remission is less than half that of children with normal intelligence—30% versus 70%. Generalizations in the prognosis of children with epilepsy and MR cannot be made, however, because MR is not uniform (Tables 3.1 and 3.2).