Fractal Biomarker Of Activity In Patients With Bipolar Disorder

Background The output of many healthy physiological systems displays fractal fluctuations with self-similar temporal structures. Altered fractal patterns are associated with pathological conditions. There is evidence that patients with bipolar disorder have altered daily behaviors. Methods To test whether fractal patterns in motor activity are altered in patients with bipolar disorder, we analyzed 2-week actigraphy data collected from 106 patients with bipolar disorder type I in a euthymic state, 73 unaffected siblings of patients, and 76 controls. To examine the link between fractal patterns and symptoms, we analyzed 180-day actigraphy and mood symptom data that were simultaneously collected from 14 patients. Results Compared to controls, patients showed excessive regularity in motor activity fluctuations at small time scales (<1.5 h) as quantified by a larger scaling exponent (alpha(1) > 1), indicating a more rigid motor control system. alpha(1) values of siblings were between those of patients and controls. Further examinations revealed that the group differences in alpha(1) were only significant in females. Sex also affected the group differences in fractal patterns at larger time scales (>2 h) as quantified by scaling exponent alpha(2). Specifically, female patients and siblings had a smaller alpha(2) compared to female controls, indicating more random activity fluctuations; while male patients had a larger alpha(2) compared to male controls. Interestingly, a higher weekly depression score was associated with a lower alpha(1) in the subsequent week. Conclusions Our results show sex- and scale-dependent alterations in fractal activity regulation in patients with bipolar disorder. The mechanisms underlying the alterations are yet to be determined.