Infantile spasms are highly associated with developmental delay.19–21 In a population-based epidemiologic study of infantile spasms, Trevathan et al.20 found that 83% of children with a history of infantile spasms had mental retardation at 10 years of age. Profound mental retardation (IQ <20) was present in 56%. Of 79 children with profound mental retardation living in the Atlanta, Georgia, area, 10% had a history of infantile spasms.

Other authors have found infantile spasms to be a significant risk factor for subsequent mental retardation.19,21–23 In a case-control analysis of 106 patients with tuberous sclerosis, Jozwiak et al.23 found that the presence of infantile spasms was the only analyzed risk factor that showed a consistent and independent association with poor mental outcome.

The majority of infants who develop infantile spasms have developmental delay prior to the onset of the seizures, with both the developmental delay and spasms resulting from the same symptomatic etiology. However, infants who are previously normal are at substantial risk for developing delays with the onset of the spasms.24 Likewise, infants with developmental delay may regress after the onset of the spasms, with declines in attention, alertness, eye contact, and activity level. Unless this process can be reversed quickly, permanent impairment of the child is highly likely. Because early cessation of spasms is of prognostic significance, early diagnosis and treatment are essential.25–28

The degree of developmental regression that occurs in relation to the severity of the seizures is surprising. Although seizures may be frequent, spasms are relatively brief, lasting for only seconds, and rarely are associated with cyanosis or postictal sleep. As with Landau-Kleffner syndrome and continuous spikes and waves during slow sleep, the primary cause of cognitive impairment may be the interictal EEG activity. Hypsarrhythmia is a severely abnormal pattern, and this dysfunctional state probably prevents normal development. Developmental improvement in a child whose EEG remains so abnormal is unusual.

The reason that seizures during early development can be so detrimental remains undetermined. The combination of a severely chaotic EEG and spasms occurring at a critical developmental stage may be responsible. However, seizures occurring during early development display clinical and EEG features different from those seen in seizures in older children and adults, and so comparing the consequences of seizures in infants and adults is difficult. Infantile spasms and hypsarrhythmia never occur de novo in an adult, so it is unclear whether the developmental consequences of infantile spasms are unique to the patient’s age or to the specific clinical and EEG features of the syndrome.

I<