Association of HLA type with pseudoexfoliation of the lens capsule

Editor,—I read with interest the recent article by FitzSimon et al.1 The finding of association of particular HLA types in pseudoexfoliation assumes importance in view of reports of possible cross reaction between antigens of infectious agents and particular HLA types in disorders such as ankylosing spondylosis.2 Genetic predisposition in association with infectious disease in the aetiology of anterior uveitis is another example.3 Hence, the following points need attention in drawing any conclusions from the study.

In apprehension of bias towards null hypothesis, sampling bias has led to systematic differences between study populations and general populations, posing a threat to external validity. This could prevent generalisation in the population under study. Deliberate introduction of selection bias between cases and controls also resulted …