Diagnostic conversion to bipolar disorder in unipolar depressed patients in Hong Kong: A 20-year follow-up study

Bipolar (BP) disorder, especially BP depression is common and yet remains enigmatic until the emergence of mania. The rates and risk factors of conversion from unipolar (UP) depression to BP disorder reported vary.

To study the long-term conversion rate from UP depression to BP disorder of an inpatient sample and identify the associated risk factors.

This is a retrospective longitudinal study conducted in 2017 based on review of medical records of patients admitted to a regional hospital in Hong Kong with diagnosis of major depressive disorder during the period from 1988 to 2000.

A total of 19.5% of subjects had diagnostic shift from UP depression to BP disorder at follow-up, with a mean conversion time of 10.8 years and about 1% shift annually in the first 10 years. Risk factors include family history of mental illness, young age at onset, repeated admissions, psychotic symptoms and use of electroconvulsive therapy. More unconverted UP subjects (9.0%) committed suicide than those converted to BP (3.5%).

The study is limited by its retrospective design.

Conversion from UP depression to BP disorder is dictated by its biological characteristics and clinical severity. Vigilance should be held in the first decade after onset when most conversion takes place.