Night vision disturbance (2005)

“Gespenster sind fur solche Leute nur Die sie sehn wollen.” (Ghosts only come to those who look for them.) Holeti The term night vision disturbance (NVD) describes a decrease in the quality of vision secondary to glare disability, with decreased contrast sensitivity and consequential image degradation. The terminology surrounding NVD can be confusing. Glare is light that appears bright and intense. Dazzle is to be blinded partially and temporarily by sudden excessive light or light intensified by scatter. Temporary visual disability at night is a consequence of the above. A ghost is a faint trace or possibility of something. Ghosting in the visual sense is to be troubled by a faint, partial, monocular double image. Finally, starburst images refer to radial or regular radiating scatter of light from a point source. These visual phenomena characterize the complaints of a small but often vociferous cohort of patients who have had corneal laser refractive surgery. In their survey of dissatisfied refractive surgery patients, Jabbur and coauthors1 discovered that among the most common subjective complaints were blurred distance vision (59.0%), glare, and NVD (43.5%). Night vision disturbance may occasionally affect untreated eyes because of intrinsic micropathology or as a consequence of contact lens wear.2 The relationship between NVD and corneal laser surgery (CLS) was recognized quite early in the evolution of the process,3–6 when disparity between the centrally treated corneal zone and the wider pupil, which obtains under dim-light conditions, was reported. This was a consequence of the limited treatment zone that could be effected by early excimer laser technology. Before the era of CLS, corneal refractive surgery was effected through corneal incisions; radial keratotomy for myopia was the most prevalent. Driving at night was the main side effect that patients had to tolerate as a consequence of radial scars and a small untreated central (optical) zone.7 Patients did tolerate these problems in return for the general visual satisfaction they obtained from unaided vision. Surprisingly, not all patients complained of NVD, and some who did initially were not troubled later. This suggests that physical symptoms could be suppressed or filtered out of a patient's awareness through supratentorial mechanisms. As patients' awareness and demands became heightened by the dramatic increase in the incidence of corneal refractive surgery engendered by the “LASIK revolution,” aided and abetted by modern communications, NVD became more of an issue.8–10 The source of post-CLS dissatisfaction has been fueled in part by web sites dedicated to bringing unhappy patients together in a movement not possible before the advent of web-based communication. Accordingly, informed consent for patients undergoing CLS dwelt significantly on the possibilities of the side effects of treatment, notwithstanding the possible existence of pretreatment symptoms of a similar nature. From the professional aspect, debate ensued to determine the nature and cause of NVD with the hope of prevention, always preferable to cure, especially as the answers to NVD have not been forthcoming. The debate coincided with the clinical availability of aberrometers, which faciliated understanding of more subtle optics of the eye before and after treatment. Among these investigations, there was a return to the original explanation that a disparity between the treated zone of the cornea and the pupil could account for many of the problems reported.8–10 Thus, clinical pupillometry became an issue, not least because pupil measurement is much more complex than placing a ruler in front of the eye. Pupils are dynamic and are influenced by many variables.11 Anisocoria is almost universal when pupil studies are performed by bilateral simultaneous methodology. Pupil measurement must heed the exact luminance at which the pupils are measured and because pupillary unrest is the norm, multiple readings are required to assess a mean diameter under given conditions as well as a peak value. Only then can the treated zone of the cornea be related to the pupil size. A large pupil size measured preoperatively correlates with an increased frequency of subjectively experienced post-LASIK visual disturbances during scotopic conditions.12,13 If NVD is then believed to be related to a pupil–treated zone disparity, pharmacologic pupil size reduction may prove the point. Attention to the corneal treated zone14 showed that WYTINWYG (what you treat is not what you get, at least in functional terms). The effective optical zone is a facet of the treated diameter, the degree of correction, the transitional zone; ie, the function of the laser platform, the initial keratometry, and the biomechanical response. Thus, a disparity between pupil size in dim light and the effective or functional optical zone is the key rather than the treated zone–pupil relationship. In this issue, Nepomuceno and coauthors (pages 379–384) return to this theme from their original publication,15 reminding us that small optical functional zones and large pupils create an increase in spherical aberration. They consider that “glare is induced by rays of light that enter the pupil through the cornea which is outside the area of ablation.” For sure, such rays of light will degrade the image (loss of contrast, blur halo) but may not contribute to glare as defined above. The authors demonstrate that the area treated does not equal the central part of the cornea that has a high level of optical quality and minimal spherical aberration, which defines the functional or effective optical zone. Further, the degree of induced spherical aberration with large treatment zones relates to the degree of correction applied. Sandhu and coauthors (pages 446–450) draw attention to a post-LASIK visual complaint that would be expected to relate to the surgery as the patient complained of a shadow affecting the vision in the left eye. Coincidence in medicine is unusual but should never be discounted, for this patient had a pituitary tumor whose symptomatology was masquerading as a post-LASIK problem. If a patient is deemed unsuitable for CLS because he or she has large “scotopic” pupils or even large “photopic” pupils and a “thin” cornea, are they suitable for a phakic implant solution or will the risk for NVD apply? This issue is addressed by Dick and coauthors (pages 302–307). Anecdotal experience indicates that posterior chamber phakic intraocular lenses reduce pupil size under all conditions compared with preoperatively. This is confirmed in Dick and coauthors' study. An additional advantage is that quality of vision is generally better with a lenticular solution for ametopia than a corneal solution.16