Worldwide, as many as 150 million people or roughly 2% of the global population are classified as homeless, and as many as 1.6 billion people (20% of the world population) lack adequate housing.

Homeless people are more likely than the general population to have mental disorders. Evidence suggests that the vast majority of homelesss people suffer from mental, neurological, and substance disorders including schizophrenia, depression, bipolar disorder (BD), substance use, and personality disorders.

Specifically to BD, numerous studies have reported that the prevalence estimates of BD might be substantially high among homeless people when compared to the general population. The reported prevalence estimates of BD among the homeless people range from 2.41% to 42.42% depending on the study. Several factors contribute to this wide range of prevalence rates, including: (1) the use of different instruments to assess BD; (2) the presence of comorbidities and other mental, substance use and physical health problems; and (3) the difference in sociodemographics and other characteristics of the participants.

Epidemiological evidence suggests that the presence of BD among the homeless increases the likelihood that the person stays homeless for longer and increases the risks of mortality from suicide, drug use, and general medical conditions. It is also associated with an elevated risk of the vulnerability of the homeless person, such as criminality and violent victimizations.

Why did we do this study?

Although homeless people have been disproportionately affected by BD, the global prevalence estimate of BD among homeless people is unknown.  We therefore systematically reviewed and meta-analyzed the existing epidemiological data on the prevalence of BD worldwide. The findings of the study may assist the concerned bodies in the planning and implementation of better screening and management strategies to address the burden and impacts of BD among homeless people.

We systematically searched Embase, PubMed, and Scopus to identify pertinent studies that reported the prevalence of BD among homeless people in March 2019.  A meta-analysis using a random effect model was conducted to pool the prevalence estimated from individual studies.

What did we find?

Of 3236 studies identified, 10 studies with 4300 homeless individuals were included in the final analysis. The estimated global prevalence of BD among homeless people was 11.4%. This result is remarkably higher (11.35-fold higher) than the reported prevalence of BD among the general population. The prevalence of BD among the homeless was 10.0% in Europe and it was 13.2% in other countries.

The estimated global prevalence of BD among homeless people was 11.4% which is remarkably higher than the reported prevalence of BD among the general population.

What do our findings imply?

In this study, we found that the prevalence of BD among homeless people was considerably high as compared to the reported prevalence in the general population. This result implies that greater attention and action are required to identify and treat BD among homeless people. In addition, the findings suggest the need for further studies evaluating the possible contributing factors for this remarkably high prevalence estimate of the disorder as well as better prevention and treatment strategies for this population group.