PITTSBURGH — Subthreshold bipolar disorder is highly prevalent and disabling, according to a nationwide survey of more than 9,000 Americans sponsored by the National Institute of Mental Health.
About 2% of the U.S. adult population has subthreshold bipolar disorder, on top of the 2% who have symptoms that meet DSM-IV criteria for bipolar I and II disorder, Ronald C. Kessler, Ph.D., said at the Sixth International Conference on Bipolar Disorder. These are “staggeringly large numbers,” he said.
The survey results also documented that people who reported having subthreshold bipolar disorder, defined as having episodes with two or more symptoms of bipolar disorder for 4 or more days at a time, had an average of 43 days a year when they were totally unable to work or to perform other normal, daily activities. This level of impairment was very similar to what was reported by people who met the definition for classic bipolar I or II disorder.
“We need to figure out how we can deliver effective treatment to people” with subthreshold bipolar disorder, said Dr. Kessler, a professor in the department of health care policy at Harvard Medical School in Boston. “The subthreshold people sure have something that's much more impairing than most things we call illness,” including asthma, arthritis, and diabetes.
The National Comorbidity Survey-Replication, conducted from February 2001 to April 2003, consisted of face-to-face interviews with 9,282 randomly selected U.S. residents. Each participant was seen by a professional interviewer who followed a script that was designed to collect the data needed to diagnose the presence of any mental illness as well as its impact.
People with symptoms of subthreshold bipolar disorder had clinical characteristics that were very similar to those diagnosed with bipolar I or II disorder. The average age of onset for subthreshold disease was 22.5 years; the average number of lifetime episodes was 9.4; people reported having episodes over a period of an average of 9.2 years, and the 12-month persistence of subthreshold disorder was about 60%.
Severe impairment in at least one life activity was reported by 64% of people who met the criteria for subthreshold bipolar disorder, compared with 70% of those who met criteria for bipolar I and 68% who met criteria for bipolar II. The average reported number of days of impairment per year was 44 for people with bipolar I and 59 for those with bipolar II.
Among those with subthreshold bipolar disorder, 88% also met the diagnostic criteria for another mental illness, compared with 97% of those with bipolar I and 86% of those with bipolar II.
The prevalence of clinically significant, subthreshold bipolar disorder might potentially be even greater, because the definition was arbitrarily set as people who have episodes that last for a minimum of 4 days. It's likely that many more people have episodes that last for 1 day, Dr. Kessler said at the conference, sponsored by the University of Pittsburgh.
Further analysis of data collected in the survey showed that people who met any of the three criteria for bipolar disease reported an average of 50 days a year when they were unable to work effectively, either because they were absent from work (absenteeism) or because of problems they had at work (“presenteeism”).
In contrast, results from the survey showed that people who were diagnosed with major depressive disorder had an average of 32 days a year of absenteeism and presenteeism.
When extrapolated on a national scale, these numbers suggest that all forms of bipolar disorder produce about $26 billion in lost worker productivity in the United States each year, while major depression leads to about $44 billion in lost productivity annually, Dr. Kessler said.