NEW YORK (Reuters Health) – The increasing prevalence of autism cases in California seen since the 1990s is in large part real, not simply the result of changes in diagnostic criteria, new research suggests.

This study is the first to assess whether the autism trends in California might be explained by changes in age at diagnosis or by inclusion of milder cases, Dr. Irva Hertz-Picciotto and Dr. Lora Delwiche, from the University of California Davis, note.

Using data from the California Department of Development Services, the researchers found that childhood (5 years or younger) autism rates rose steadily from 0.8 per 10,000 person-years in 1990 to 11.2 in 2006. The cumulative incidence per 10,000 births climbed from 6.2 in 1990 to 42.5 in 2001.

The proportion of cases that were diagnosed by 5 years of age rose only slightly from 54% to 61% for 1990 to 1996 births, the report in the January issue of Epidemiology indicates.

A change in the age at diagnosis could explain 12% of the increase in autism rates, while inclusion of milder cases could explain 56%.

“With evidence of a leveling off, the possibility of a true increase in (autism) incidence deserves serious consideration,” the authors emphasize.

“It’s time to start looking for the environmental culprits responsible for the remarkable increase in the rate of autism in California,” Dr. Hertz-Picciotto said in a statement.

Reference:
Epidemiology 2009;20:84-90.