Corneal refractive surgery is a safe mode of vision correction, but a variety of complications have been described in the literature.1 Many of these complications do not compromise the integrity of the cornea, but can cause serious visual distortions, some of which cannot be re-treated with surgery or compensated with spectacles. Complaints of poor night vision are among the most common complaints described by patients.2 Halos are among the most commonly reported symptoms, particularly after laserassisted in situ keratomileusis (LASIK) surgery,3 and a certain increase inthe halo disturbance index can be present even when the procedure is considered entirely successful.4,5 However, other types of visual distortion can be present after corneal refractive surgery, including glare, starburst, hazy vision, monocular diplopia and polyopia, simultaneous vision and defocus.6–8 Such problems are primarily due to light scatter through tissues with compromised transparency, or different refraction through operated, non-operated and transition zones. With current refractive surgical procedures, the optical aetiology is the most commonly accepted source of visual distortion and several aberration coefficients are more highly correlated with these symptoms, particularly spherical aberration.5
Multiple factors seem to predispose to problems of vision after refractive surgery, including attempted refractive correction, age and post-refractive surgical refraction.9,10 Surprisingly, pupil size and pre-surgical corneal curvature are not considered significant predictors of post-surgical visual complaints under scotopic conditions.11
Even today, after years of debate, the disagreement between the patient’s self-reported symptoms and the objective visual assessment as measured by the surgeon under photopic conditions causes frustrations for both patient and surgeon. The former cannot vocally express what she/he actually feels, and the latter cannot obtain objective measures of such complaints. Therefore, it is urgent that we find an objective measuring method to characterise the subjective symptoms of night-vision distortion. One attempt to do so is the method described by Lackner et al.8 using a commercial computer program. Using this instrument, the authors evaluated the size of the halo in a longitudinal fashion from pre-surgery to six months post-surgery under mesopic conditions. This method consists of the subjective assessment of the patient by moving a stimulus from the periphery of clear vision up to what he/she judges as the limit of the halo, and has shown good repeatability on measuring halo size in patients with multifocal intraocular lens implantation given a value of the halo diameter. The main drawback is that if a defined halo is not seen or the patient does not understand what a halo really is, the patient could have difficulties in identifying the limit of clear and handicapped fields of vision. Moreover, in many cases the luminous distortions are not rotationally symmetrical, which can cause even more confusion to the patient when asked to delimitate the margin of the distortion. The fact that the stimulus is moved from the periphery to the centre after the patient has seen the stimulus could cause some problems with fixation to the central point; however, this has not been quoted as a problem by the authors. Finally, this method does not allow the differentiation of the area of the visual field where the patient is ‘blind’ to visual stimuli because the patient is asked to delimitate the margin of the halo, but he/she could see the stimulus even inside this limit.