Trends in Diagnosis Rates for Autism and ADHD at Hospital Discharge in the Context of Other Psychiatric Diagnoses

This study included all hospital discharges from calendar years 1989 to 2000 in the HCUP NIS for individuals from birth to age 21 years. Individuals could be included in the sample more than once if they had multiple discharges.

Psychiatric disorders were coded by using the International Classification of Diseases, 9th Edition (ICD-9) (27). Disorders were associated with a hospital discharge if they were coded in any position for that discharge. Comorbid conditions were double-counted for all analyses. Autism spectrum disorders were coded for any discharge with a 299.0 diagnostic code, ADHD for any discharge with codes 314.00 to 314.99, major affective disorders for any discharge with codes 296.00 to 296.99, and substance use disorder for any discharge with codes 303.00 to 305.99.

For each set of disorders, three rates were calculated. First, the rate of hospital discharges associated with each disorder was calculated for each calendar year as a function of the total number of hospital discharges for that year. Second, average rates were calculated across all years of the study period by age. Finally, the sample was divided into four periods: 1989 to 1991, 1992 to 1994, 1995 to 1997, and 1998 to 2000. Differences in trends were examined for each period.

A MEDLINE search using the term "attention deficit disorder" returned 185 articles in 1989 and 541 in 2000. A search that used the term "autistic disorder" returned 216 articles in 1989 and 359 in 2000. Increased awareness was not limited to the medical literature. A NEXIS search of all U.S. news outlets found 25 articles on ADHD and autism in 1988 and 838 articles in 2000. Popular awareness may have consequently increased, fueling parental concern and, in turn, changing physicians' behavior (31). Concurrently, treatments for developmental disorders have become more widely accepted and used. More children with a diagnosis of ADHD (32) as well as younger children with this diagnosis (33) are treated with stimulants. Likewise, children with autism are increasingly treated with stimulants and neuroleptics (34). The increasing popularity of these treatment options may increase the likelihood that providers will assign a related diagnosis.

Another possible reason for changing ascertainment is the expansion of Supplemental Security Income (SSI) and the Individuals With Disabilities Education Act (IDEA), which may have provided incentive for physicians to diagnose ADHD and autism. The number of children insured through SSI increased by more than 400 percent between 1988 and 1997 (35), in large part because of the Social Security Administration's broadened definition of disability to include mild mental retardation and mental illness.

Concurrently, the number of children served by IDEA grew from 4.5 to 6.3 million between 1988 and 2000 (36). The categories of both autism and "other health impairment," which includes ADHD, grew by more than 1,000 percent during this time. In 1990, IDEA was expanded to include early intervention services for children aged three to five years (36). School systems were required to implement new systems for screening and diagnosis, which may have increased the number of children receiving a diagnosis.

Byrd and colleagues (18) suggest that changes in diagnostic practices cannot explain the rise in diagnosis of autism. Several etiologic pathways for this increase have been posited. One hypothesis is an increase in exposure to environmental toxins, such as mercury (3,37,38). A second is an increase in viral loads associated with measles, mumps, and rubella (MMR) vaccines. The association between increasing rates of both MMR vaccination and autism has been noted (17), but evidence of causality is limited (4,39). Recent studies indicated no association between MMR vaccination and autism (40,41,42), and limited data are available on the risk of neurodevelopmental disorders associated with thimerosal exposure (41). A second possibility relates to the increasing survival rate of low-birth-weight and otherwise compromised babies, who are at increased risk of developmental disorders (43). From 1989 to 1997, rates of mortality among preterm infants decreased by more than 30 percent (44). However, it is unlikely that the increase in the number of low-birth-weight babies alone could account for a large change in rates of developmental disorders.

A number of studies have shown that children with developmental disorders use more health care services than other children (45,46). Although hospitalization rates for the general population remained constant from 1983 to 1991, rates of hospitalizations for persons with developmental disabilities increased by 56 percent (47). This change could not be accounted for by an increase in the number of persons with developmental disabilities and occurred at the same time as a reduction in hospital admissions for pediatric injury (48,49) and stable rates of admission for asthma (50) and influenza (51).

Gallaher and colleagues (52) found that having a developmental delay was independently associated with hospitalization, even after other health conditions had been controlled for. They also found that children with developmental delays were twice as likely as other children to have other chronic health conditions. Children with both developmental delay and another chronic health condition were much more likely to be hospitalized than other children, and, unlike the effect of having a chronic health condition only, this difference persisted over the first five years of life. Increased probability of hospitalization among children with developmental disabilities may partially account for trends observed in this study.

Some limitations of this study should be mentioned. As with any administrative data source, the HCUP is subject to coding errors (53). If these errors were differentially biased by disorder, or error rates changed over time, it would bias comparisons within or among diagnoses. Second, it is possible that although individuals with psychiatric disorders presented at hospitals in the data set, psychiatric diagnoses were not assigned. It may be that when presenting problems are more directly related to the mental disorder—for example, intoxication and substance use or self-injury and depression—the disorder is more likely to be coded. This dynamic might lead to underreporting of rates of diagnosis of ADHD and autism relative to other disorders. A related limitation is that for approximately three-quarters of individuals with a diagnosis of an affective disorder, ADHD, or an autism spectrum disorder, the diagnosis was secondary to some other morbidity. Primary diagnoses were not examined in these cases and may have been differentially associated with the assigned secondary diagnosis. Third, because this study used only inpatient hospital data from specific types of hospitals, the results may not be generalizable to other health care settings. Fourth, the hospitalization data do not include long-term psychiatric and drug treatment facilities, so individuals with alcohol and drug dependence are probably underrepresented. The number of such specialty hospital facilities have also been declining in recent years, and few children are admitted to these types of facilities, so it is difficult to ascertain exactly what bias the exclusion of the facilities may have on our results.