Hudson, in press). For 85 to 90 percent of cases, onset of spasms is within the first year of life. Incidence rates of infantile spasms range from 0.25 per 1,000 live births in Denmark and the United States to 0.4 per 1,000 live births in Finland (Leviton and Cowan, 1981).
Most investigators divide infantile spasms cases into two categories which are defined on the basis of the presence or absence of a presumed cause and the child's developmental status prior to the onset of spasms. What are commonly referred to as "symptomatic cases" are those in whom a presumed cause can be identified. Idiopathic cases are defined as infants with no identifiable causes for their spasms. This group is further subdivided by some into cryptogenic (those for whom there is no known cause of infantile spasms and whose development was essentially normal prior to the onset of spasms; ~10 percent of all cases) and doubtful (those for whom there is no known cause of infantile spasms but whose development prior to the onset of spasms may have been delayed).
Those cases considered to be idiopathic range between 30 and 50 percent (Cowan and Hudson, in press), although this proportion may be declining because of more sensitive diagnostic methods, such as neuroimaging techniques and positron tomography (Chugani et al., 1990). However, although approximately 70 to 90 percent of infantile spasms cases are reported to have abnormal computed tomography (CT) scans (Glaze and Zion, 1985; Pinsard and Saint-Jean, 1985), the significance of some CT diagnoses, for example, cortical atrophy, has been questioned (Ludwig, 1987). Thus, it is unclear that the proportion of infantile spasms cases considered to be idiopathic is really decreasing because of improved diagnosis of cerebral anomalies.
Among symptomatic cases, presumed causes are frequently grouped according to the timing of the suspected insult as occurring pre-, peri-, or postnatally. Prenatal factors are thought to account for 20 to 30 percent of cases. This category includes cerebral anomalies, chromosomal disorders, neurocutaneous syndromes such as tuberous sclerosis, inherited metabolic disorders, intrauterine infections, family history of seizures, and microcephaly (Bobele and Bodensteiner, 1990; Kurokawa et al., 1980; Ohtahara, 1984; Riikonen and Donner, 1979). Perinatal factors are thought to account for from 25 to 50 percent of infantile spasms cases. This category includes perinatal hypoxia, birth trauma, and metabolic disorders (Kurokawa et al., 1980; Pollack et al., 1979). Approximately 8 to 14 percent of infantile spasms are attributed to postnatal factors, including central nervous system (CNS) infections, trauma, immunizations, and intracranial hemorrhage (Bobele and Bodensteiner, 1990; Gibbs et al., 1954; Kurokawa et al., 1980; Lombroso, 1983a). Few of these factors have been subjected to systematic investigation, however, and the etiology of infantile spasms remains unknown for 30 to 50 percent of cases (Cowan and Hudson, in press).