INTRODUCTION: The 'Hikikomori' syndrome (HS) consists of prolonged and severe 
social withdrawal. It has been studied first in Japan and recently has 
increasingly drawn the attention of researchers and clinicians all over the 
world. It is unclear whether it exists in other cultural contexts than Asia. The 
existing systematic reviews did not provide a quantitative synthesis on its 
prevalence. In addition, a summary of the co-occurring rates of psychiatric 
disorders is lacking. To provide a more comprehensive understanding of the 
clinical picture, it seems important to investigate which psychiatric disorders 
listed in the classification systems are most frequently associated with this 
psychological condition affecting young people. This paper describes a 
systematic review and meta-analysis protocol summarising worldwide prevalence of 
the HS in general population and clinical samples with psychiatric disorders. 
The review will also assess the co-occurrence between HS and each psychiatric 
disorder defined by any version of the Diagnostic and Statistical Manual of 
Mental Disorders (DSM) or International Classification of Diseases (ICD) in any 
clinical samples with psychiatric disorders.
METHODS AND ANALYSIS: A systematic review will be conducted according to 
Preferred Reporting Items for Systematic Reviews and Meta-analyses (PRISMA) 
guidelines. Studies will be included if they use youth aged 12-35 years, 
recruited from general population or population with psychiatric disorders, if 
they use international criteria to diagnose HS. No restriction about design or 
language will be applied. The search will be conducted during the first week of 
November 2019 by two independent reviewers through the databases Scopus, PubMed, 
PsycINFO, Web of Science, by examining study references, by looking for 
conference proceedings/dissertations/theses, by contacting study corresponding 
authors. Random-effect meta-analysis will be performed by computing effect sizes 
as logit event rates. Study quality will be assessed through the 
Newcastle-Ottawa Scale.
ETHICS AND DISSEMINATION: The current review does not require ethics approval. 
The results will be disseminated through conference presentations and 
publications in peer-reviewed journals.
PROSPERO REGISTRATION NUMBER: CRD 42018098747.