Table 1

Main characteristics of included studies

Study	Country	Ethnicity	Study design	Sample size	Mean age	Definition of asthma	Definition of DED	Adjustment for confounders
Alshamrani et al,37 2017	Saudi Arabia	Arabian	Cross-sectional	Asthma: 139 Control: 1719	39.3	Questionnaire	Six-item questionnaire	Age, gender and smoking
Chia et al30 2003	Australia	Australian	Cross-sectional	Asthma: 135 Control: 1039	60.8	Medical history of confirmed diagnosis	Interviewer-administered questionnaire	Age and gender
Huang et al29 2018	Taiwan	Asian	Cohort	Asthma: 41 229 Control: 164 916	49.56	ICD-9-CM 493	ICD-9-CM 375.15	Age and gender
Kim et al52 2016	Korea	Asian	Cross-sectional	Asthma: 556 Control: 16 494	50.88	Medical history	Interviewer-administered questionnaire	Age, gender, residential area, education level, occupation and history of eye surgery
Vehof et al36 2014	UK	Caucasian	Cross-sectional	Asthma: 608 Control: 3216	57.1	Medical history	ICD-9-CM 375.15	Age and gender
Wang et al35 2012	Taiwan	Asian	Cross-sectional	Asthma: 2548 Control: 45 480	52.4	Elixhauser comorbidity index	ICD-9-CM 375.15	Age, gender, urbanisation levels and monthly incomes

DED, dry eye diseas; ICD-9-CM, The International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM) is based on the World Health Organization’s Ninth Revision, International Classification of Diseases (ICD-9).