Bipolar disorder in children is a controversial topic. It has traditionally been thought to be very rare but the rate of diagnosis has increased dramatically over the last decade. There is a lot of heated debate as to whether this increase reflects past under-diagnosis or present over-diagnosis. The last large-scale study found a rate of 0.1% of mood disturbances in children under 13 but an official diagnosis requires lengthy periods of disturbance. If shorter periods of altered mood are included then the number of children affected by the condition could be much higher. One study found that 25% of children who had the shorter period of mood disturbance went on to develop bipolar disorder within the next two years. Researchers led by Argyris Stringaris from the Institute of Psychiatry, King's College London studied 5,326 8-19 year-olds from the general population. They found that while only 0.1% of the sample met the criteria for bipolar disorder, 1.1%-1.5% (depending on whether the parents or children respectively were asked about symptoms) had shorter episodes of mood disturbance. These mood disturbances were associated with social impairments over and above those caused by other mental-health problems but the researchers questioned whether, although they were superficially similar to bipolar disorder, they were actually part of the same condition with the same origin and potential treatments.
Stringaris, Argyris ... [et al] - Youth meeting symptom and impairment criteria for mania-like episodes lasting less than four days: an epidemiological enquiry Journal of Child Psychology and Psychiatry January 2010, 51(1), 31-38