Background
Globally, as many as 100 million people were categorized as homeless, and over 1.6 billion individuals lacked adequate housing [
1]. A wide range of multiple and interrelated factors contributes to homelessness such as domestic violence, social isolation, unemployment, addictions, mental illness, sexual assault, poverty, a critical shortage of affordable housing, financial difficulties, family breakdown, and adverse childhood experiences [
2‐
6].
As compared to the general population, homeless people suffer from a greater prevalence of psychiatric disorders including schizophrenia and other psychotic disorders [
7,
8]. The reported magnitude of mental disorders among homeless people ranged from 25 to 50% across the studies [
9‐
11]. This prevalence rate rises to about 92% among those who are street homeless [
7]. The presence of mental disorders among homeless people is linked with a higher risk of disability and mortality from suicide and general medical [
12‐
14], as well as alcohol and drug-related causes [
15].
Epidemiologic evidence from several studies has shown that schizophrenia and other psychotic disorders are among the most common psychiatric disorders among homeless people [
7,
8,
16] and they are associated with a greater risk of comorbid physical disease, substance use, and disability as well as mortality from different causes [
12‐
14,
16‐
18]. The reported prevalence estimates of schizophrenia among the homeless people range from 2.29 to 60% depending on the studies [
7,
8,
16‐
22]. The impacts of psychotic disorders among homeless people could be severe and extensive, it negatively impacts the homeless person suffering from it, the family, relatives and other population [
12,
15,
23,
24].
A better understanding of the epidemiology of schizophrenia and other psychotic disorders among the homeless people could assist the development of better policies and intervention strategies as well as best intervention mechanisms to address the impacts associated with these disorders. Although there are previous systematic reviews on overall mental disorders and psychosis among homeless people, to our knowledge, this is the first systematic review and meta-analysis aimed to provide the pooled prevalence estimates of schizophrenia and other psychotic disorders among the homeless. we hypothesized that the prevalence of schizophrenia and other psychotic disorders are notably high among homeless people. Therefore, the purpose of this study was to evaluate the results of available epidemiologic studies in order to provide a high level of evidence on the global prevalence of schizophrenia and other psychotic disorders among homeless and forward implications of the findings for the future clinical practice as well as research.
Discussion
To the best of our knowledge, this is so far the first study to systematically search, assess, and synthesize the estimated poled prevalence of schizophrenia and other psychotic disorders among homeless people. We conducted a systematic review and meta-analysis over 31 studies on the prevalence of schizophrenia and other psychotic disorders among homeless people. Our analysis resulted in a remarkably higher prevalence of overall psychosis (21.21%), schizophrenia (10.29%), schizophreniform disorder (2.48%), schizoaffective disorder (3.53%), as well as psychotic disorders not otherwise specified (9%) among homeless people The findings of our meta-analysis highlighted that the prevalence of schizophrenia and other psychotic disorders were higher among homeless people as compared to the reported prevalence in the general population. These findings underline schizophrenia and other psychotic disorders as a significant and serious global public health issue among homeless people.
The present systematic review and meta-analysis revealed that the existing epidemiologic data on the prevalence of schizophrenia and other psychotic among homeless people demonstrated a considerable difference based on the country (the origin of the study), the tools used to quantify schizophrenia and other psychotic disorders, and the reported quality of the studies. The majority of the included articles were performed in developed countries (87.1%; n = 27) and only a small number of studies were conducted in developing countries (12.90%; n = 4). To quantify the magnitude of schizophrenia and other psychotic among the homeless, some of the articles used screening tools and some used diagnostic instruments.
The results of our study underlined that the prevalence estimates of overall psychosis (21.21%) were remarkably higher than the previous systematic review reports from western countries among the homeless (12.7%) [
65]. The possible reasons for the observed difference includes: first, the previous review considered specific categories of psychotic disorders including schizophrenia, schizoaffective and other distinct categories of psychotic disorders as psychosis which obviously underestimates the estimated prevalence whereas in the current meta-analysis, studies that reported the estimate of overall psychosis only were included under psychosis analysis and for articles that reported the specific categories of psychotic disorder we conducted separate analysis; second, the present review included studies from developed as well as developing countries whereas the previous study was conducted only in western countries and in our subgroup analysis we found that the magnitude of psychosis in developed countries was lower than that of developing countries. Therefore, the possible reason for the higher magnitude of psychosis in the previous study may be due to the inclusion of new studies including studies from developing countries.
The pooled prevalence estimates of schizophrenia in the general population have been found to be (0.4%) [
66], and the pooled prevalence estimates in the current study exceed this estimate more than 25 fold (10.29%). The current systematic review and meta-analysis also resulted in a remarkably greater prevalence of schizophreniform disorder (2.48%), schizoaffective disorder (3.53%), and any psychosis (21.21%) among the homeless. These results are higher than the reported prevalence in the general population [
67]. There are a range of explanations for the observed remarkable difference in the magnitude of schizophrenia between homeless people and general populations: First, the presence of a considerably higher prevalence rates of serious physical health problems such as tuberculosis, HIV/AIDS and other medical conditions possibly could contribute to the greater prevalence of psychiatric disorders including schizophrenia among the homeless people [
23,
68‐
72]; second, evidence shows that homeless people are more likely to experience traumatic events including physical and sexual abuse, which has been linked with greater risks of mental health problems including psychotic disorders in those who are exposed [
73‐
76]; thirdly, the presence of considerably higher magnitude of mental, neurologic and substance use disorders among homeless people including anxiety, depressive, bipolar, and other psychiatric and substance use disorder among homeless people as compared with other populations are the other possible reasons for the observed high prevalence of schizophrenia among the homeless [
7,
8,
40,
77].
This review revealed the highest prevalence of schizophrenia and other psychotic disorders was higher in developing countries than in developed countries. There are many interpretations of the observed variation in the magnitude of schizophrenia and other psychotic disorders among homeless people among the countries. (1), the possible methodologic variations such as the tools utilized to quantify schizophrenia and other psychotic disorders among the included studies is the possible reason for the observed considerable difference. For instance, in our subgroup analysis according to the tools used we found we found a remarkably high prevalence of psychosis as measured by screening instrument (32.19%) than diagnostic instruments (17.42%); (2) the possible variations in in the magnitude of the potential contributing factors that increase the risk of schizophrenia and other psychotic disorders among the study participants across the countries including potential life treating traumatic events and disasters and serious physical health problems are the other possible explanations for the observed variation in the magnitude of schizophrenia and other psychotic disorders across the nations; (3) the variations in the severity of the disease and comorbid other psychiatric and substance-related disorders among homeless people across the countries are the other potential reasons for the observed difference. In fact, the observed the difference in magnitude of schizophrenia psychosis among the developed and developing countries was not statically significant. Therefore, the difference is a chance of finding rather than a real difference. Additionally, the lower number of studies in developing countries as compared with developed countries must be considered. Finally, the inclusion of studies conducted on-street homeless in developing countries than developed might overestimated the prevalence.
Strength and limitations
The present systematic review and meta-analysis had numerous strengths: First, the study utilized predefined procedure for searching selection of the articles, data extraction as well as analysis; second, the extradition of information from the included articles and quality evaluation were performed by two independent; third, we conducted a subgroup and sensitivity analysis based on the country of the study, the tools used, and the quality of the studies. Finally, we also conducted a meta-regression analysis to quantify the impacts of important covariates such as year of study, the status of the country and instruments used to quantify psychotic disorders. The meta-regression provided no evidence of a significant effect by all of the above covariates.
However, this review also had some limitation: first, most of the included studies were conducted in developed countries and only 4 studies were conducted in developing countries; second, we included small number of articles a subgroup and sensitivity analysis which could possibly reduce the power of the estimate; third, we observed a considerable heterogeneity across the studies.
The implication of the findings for future research and clinical practice
This study has found some implication for the future clinical practice as well as research; Firstly, we found a higher magnitude of schizophrenia and other psychotic disorders among homeless people as compared to the reported magnitude in the general population, which needs further strong studies to explore the possible reasons for the higher magnitude. Secondly, we found a few numbers of studies that estimated the prevalence of schizophrenia and other psychotic disorders in developed countries. So, further studies are required to understand the prevalence of schizophrenia and other psychotic disorders in developing countries and compare them with the prevalence estimate from developed countries. Thirdly, attention need to give by the concerned bodies to address schizophrenia and other psychotic disorders among homeless people including better ways of prevention and treatment strategies possibly integrating the mental health service of the homeless with the existing medical services, which suggests utilizing holistic approaches to address the health needs of the homeless people such as physical, mental, and substance use problems as well as suicide.
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