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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).