Why do we still not know whether refractive error causes headaches? Towards a framework for evidence based practice

In this paper, we systematically review the available experimental and clinical evidence concerning the causation of headache by refractive error. Despite the apparent belief of both medical and optometric professionals that provision of an appropriate correction may alleviate various types of headache, there is little if any robust evidence in support of this position. We identify four serious methodological and theoretical difficulties with studies to date, which currently render it impossible to assess the relationship between refractive error and headache. The provision by the International Headache Society of the diagnostic category "headache associated with refractive error" is called into question. Five research questions are posited in the form of a framework for the development of evidence-based practice in optometry and the treatment of headache.