Mark A. Bullimore

What is low vision? A re-evaluation of definitions

PURPOSEnTo re-evaluate definitions of low vision, visual impairment, and disability.nnnMETHODSnWe review current definitions of legal blindness and low vision and how these definitions are variably based on disability or impairment. We argue for a definite distinction being made between criteria for visual impairment and visual disability, low vision being defined as the presence of a visual impairment that results in a disability. Visual impairment is defined according to population norms and a statistical cut-off is used. Visual disability is defined by consideration of the level of visual measures which result in measurable or reportable disability. We consider the evidence that contrast sensitivity should be a criterion for visual disability in addition to visual acuity and visual field.nnnCONCLUSIONSnAccording to the current information, we define visual impairment as best monocular or binocular visual acuity <(worse than) 6/7.5, total horizontal visual field <146 degrees (Goldmann III-4e) or <109 degrees (III-3e), and contrast sensitivity <1.5 (PelliRobson); we define visual disability as best monocular or binocular visual acuity <6/12 or contrast sensitivity <1.05.

CHARACTERISTICS OF CORNEAL ECTASIA AFTER LASIK FOR MYOPIA

Purpose: There are numerous reports of corneal ectasia after laser in situ keratomileusis (LASIK) for myopia without a consistent definition of this condition or a definitive etiology. We conducted a retrospective analysis of published case reports to describe common characteristics of this postoperative event and compared them with findings from a group of successful LASIK patients. Methods: A MEDLINE search for “LASIK” and “ectasia” yielded 21 relevant articles published before May 2003 (n = 86 eyes, 59 patients). A comparison group (n = 103 eyes, 63 patients) was selected from a clinic-based sample of successful LASIK patients with 12 months of follow-up after treatment. Descriptive statistics are reported as median and interquartile range. Comparisons were performed using the Wilcoxon rank sum, Wilcoxon signed rank, and chi-square tests. Results: Time to diagnosis of ectasia after LASIK was 13 months (6 to 20 months). Residual myopia in the ectasia group was −3.69 D (−6.00 to −2.13 D) and was significantly greater than the comparison group, −0.38 D (−0.75 to 0.00 D), P < 0.001. After surgery, eyes with ectasia had increased corneal toricity 2.87 D (2.00 to 4.9 D) with increased oblique astigmatism 1.3 D (0.23 to 2.89 D) relative to eyes in the comparison group 0.00 D (0.00 to 0.08 D), and a loss of 2 lines (−0.5 to −6 lines) of best spectacle-corrected visual acuity (all P < 0.001). Thirty-five percent of reported cases resulted in subsequent corneal transplantation. Conclusions: Preoperative characteristics of corneal ectasia include worse visual acuity, less corneal thickness, greater residual myopia, and greater corneal toricity than nonectatic eyes. Treatment factors associated with corneal ectasia after LASIK are greater stromal ablation and less residual stromal bed thickness. Postoperative characteristics of corneal ectasia are myopic refractive error with increased astigmatism, worse spectacle-corrected visual acuity, increased corneal toricity with topographic abnormality, and progressive corneal thinning.

Lens thickness with age and accommodation by optical coherence tomography.

Purpose:u2002 To utilize time‐domain optical coherence tomography (OCT) to measure changes in the crystalline lens with age and accommodation.

An Evaluation of the Mars Letter Contrast Sensitivity Test

Purpose. The Mars Letter Contrast Sensitivity Test (initially known as the Lighthouse Letter Contrast Sensitivity Test) is similar in design to the Pelli-Robson Test but may offer several advantages. This study evaluates the repeatability of the Mars test and its agreement with the Pelli-Robson test in normal and low-vision subjects. Methods. Fifty-four subjects were tested (age 22–86 years), including 20 normally sighted young adults, 17 normally sighted older adults, and 17 adults with low vision (20/16 to 20/250). Subjects were tested with both contrast sensitivity tests and with the ETDRS visual acuity chart. After a short break, subjects were retested with an alternate form of each contrast sensitivity test. The chart forms used (two Pelli-Robson and three Mars) and the order of testing were varied systematically. Testing was monocular with habitual correction and, for subjects over 40 years of age, included appropriate near add. Letter-by-letter scoring was used for both tests. Repeatability and agreement were assessed by determining the 95% limits of agreement (LoA): ± 1.96 standard deviations of the differences between administrations or tests. Results. The Mars test showed excellent agreement with the Pelli-Robson test, with 95% LoA of ± 0.21 log units for all subjects. The Mars test was similarly repeatable (95% LoA = ±0.20 log units) to the Pelli-Robson test (95% LoA = ±0.20 log units) among all subjects. Conclusion. The new Mars Letter Contrast Sensitivity Test shows excellent agreement with the Pelli-Robson test and has similar repeatability. There are subtle differences in the actual contrast levels on different forms of the Mars test, and adjusting for these differences leads to superior repeatability of the Mars test. Thus, the Mars test may be a useful alternative to the Pelli-Robson test offering several advantages, including smaller size, improved durability, and ease of use.

Quantification of age-related and per diopter accommodative changes of the lens and ciliary muscle in the emmetropic human eye.

PURPOSEnTo calculate age-related and per diopter (D) accommodative changes in crystalline lens and ciliary muscle dimensions in vivo in a single cohort of emmetropic human adults ages 30 to 50 years.nnnMETHODSnThe right eyes of 26 emmetropic adults were examined using ultrasonography, phakometry, anterior segment optical coherence tomography, and high resolution magnetic resonance imaging. Accommodation was measured both subjectively and objectively.nnnRESULTSnIn agreement with previous research, older age was linearly correlated with a thicker lens, steeper anterior lens curvature, shallower anterior chamber, and lower lens equivalent refractive index (all P < 0.01). Age was not related to ciliary muscle ring diameter (CMRD) or lens equatorial diameter (LED). With accommodation, lens thickness increased (+0.064 mm/D, P < 0.001), LED decreased (-0.075 mm/D, P < 0.001), CMRD decreased (-0.105 mm/D, P < 0.001), and the ciliary muscle thickened anteriorly (+0.013 to +0.026 mm/D, P < 0.001) and thinned posteriorly (-0.011 to -0.015, P < 0.01). The changes per diopter of accommodation in LED, CMRD, and ciliary muscle thickness were not related to subject age.nnnCONCLUSIONSnThe per diopter ciliary muscle contraction is age independent, even as total accommodative amplitude declines. Quantifying normal biometric dimensions of the accommodative structures and changes with age and accommodative effort will further the development of new IOLs designed to harness ciliary muscle forces.

Estimation of pupil size by digital photography

Purpose: To evaluate a digital photography method of pupil size estimation over a broad range of illumination conditions and to compare this method with common clinical techniques. Setting: College of Optometry, Ohio State University, Columbus, Ohio, USA. Methods: Two examiners measured the pupil diameter in 45 right eyes at 3 illumination levels: <0.63 lux (dark), 5 lux (dim), and 1000 lux (bright). Estimation by infrared video recording, the reference standard, was compared with measurements by digital photography, ruler, semicircular templates, and the Colvard pupillometer. Masked graders measured pupil size from infrared video recordings and digital photographs. Results: The repeatability of the measurement method determined by the mean intraclass correlation coefficients was highest for video recording across conditions (0.86–0.97), followed by digital photography (0.76–0.94), Colvard pupillometry (0.63–0.82), ruler (0.71–0.85), and templates (0.70–0.83). An analysis of variance showed a significant difference in pupil size by method (P<.001). All methods except digital photography estimated smaller pupil sizes under dark and dim illumination than infrared video measurements (all P<.01). Under bright illumination, the ruler measurements were significantly smaller (–0.15 mm) and the Colvard pupillometer measurements were greater (+0.30 mm) than the reference (P<.01). The 95% limits of agreement (LoA) between examiners were smallest for video measurements at all light levels. The remaining measures ranked from best to worst by 95% LoA were digital photography, Colvard pupillometry, ruler, and templates. Conclusions: Estimation of pupil size by digital photography was more repeatable and accurate than estimates by common clinical techniques over a wide range of illumination. Although not as quick as other methods, digital photography is relatively inexpensive, permits lasting documentation, and allows independent grading suitable for clinical research purposes.

Visual acuity and the accuracy of the accommodative response

Previous investigators have observed that some subjects show large amounts of accommodative lag. We hypothesized that less accurate accommodation might be associated with poorer visual acuity and/or smaller pupil sizes. Sixty subjects (30 emmetropes and 30 myopes) aged 20–30u2003years, participated. All had best‐corrected visual acuity of 6/6 or better [mean=−0.10u2003±u20030.07u2003logarithm of the minimum angle of resolution (logMAR)]. Subjects monocularly viewed reduced Bailey–Lovie charts through a +6.50u2003D Badal lens on a Canon R1 auto‐refractor. Visual acuity, accommodative response and pupil diameter were measured for 0, 2 and 4u2003D accommodative stimuli. For accommodation measurements (N=10) subjects were instructed to fixate the smallest letters that they could read. The mean accommodative response was +0.22u2003±u20030.28, +1.83u2003±u20030.23 and +3.71u2003±u20030.27u2003D for the 0, 2 and 4u2003D stimuli, respectively. The mean visual acuity was −0.06u2003±u20030.10, −0.11u2003±u20030.07 and −0.11u2003±u20030.07 logMAR for the 0, 2 and 4u2003D stimuli, respectively. Visual acuity for the 0u2003D stimulus was significantly poorer than for other conditions (pu2003<u20030.001) and associated with increased accommodative lead (pu2003<u20030.01). There was also an association between visual acuity and accommodative response (or lag) for the 4u2003D stimulus (p=0.002). The emmetropes showed significantly better visual acuity than the myopes (p=0.004). No significant difference was observed in the accommodative response between emmetropes and myopes. Pupil diameter was not associated with the accuracy of the accommodative response (pu2003>u20030.17). Increased accommodative lead (0u2003D stimulus) and accommodative lag (4u2003D stimulus) are associated with decreased visual acuity. Smaller pupil diameters are not associated with increased accommodative lag.

Factors affecting corneal endothelial morphology.

Purpose: To study the effects of age, race, years of contact lens wear, and degree of myopia on endothelial cell density, the coefficient of variation, and the percentage of hexagonal cells. Methods: A total of 149 subjects with a broad variation in age (19 to 71 years), refractive error (39.6% had myopia over -5.00 D), and contact lens history (13.4% had >20 years of wear) were examined. Most subjects were white, but one third were Asian. The Konan specular microscope was used to capture images from 1 eye of each subject and analyzed with the KSS-300 software. Multiple regression analysis was used to build models of endothelial cell density, polymegathism, and pleomorphism. Results: Age and race significantly affected endothelial cell density (ECD = 3274 - 12.7 × Age + [164 if Asian]). Years of contact lens wear, age, and their interaction significantly affected coefficient of variation (CoV = 24.3 + 0.72 × CLyears + 0.18 × Age - 0.011 × interaction). Years of contact lens wear, age, and refractive error all independently affected the percentage of hexagonal cells (% Hex = 70 - 0.29 × CLyears - 0.21 × Age + 0.4 × Rx). Conclusions: The three key aspects of corneal endothelial morphology are influenced by different factors. Any impact of contact lens wear on endothelial cell density may be caused by the confounding effect of age, and refractive error was associated with the percentage of hexagonal cells. Therefore, in studying the influence of drugs, devices, or disease, it is important to control for several subject-related factors.

Visual performance after photorefractive keratectomy with a 6-mm ablation zone

PURPOSEnTo prospectively examine the effect of photorefractive keratectomy with a 6-mm ablation zone on best-spectacle-corrected visual performance.nnnMETHODSnA prospective study was conducted of 164 eyes of 164 patients with an average (+/-SD) of -4.02 +/- 1.74 diopters (range, -0.63 to -8.38 diopters spherical equivalent). Best-spectacle-corrected high-contrast and low-contrast visual acuity (18% Weber contrast) was measured with both natural and dilated pupils. Patients were tested preoperatively and at 3, 6, and 12 months after photorefractive keratectomy. Photorefractive keratectomy was performed with an argon fluoride excimer laser. Fifty-five eyes of 55 patients also underwent astigmatic keratotomy.nnnRESULTSnTwelve months after photorefractive keratectomy, best-spectacle-corrected high-contrast visual acuity with natural pupils showed no significant change from preoperative values; mean (+/-SD) change was 0.004 +/- 0.10 logMAR (t = 0.45, P = .65). Best-spectacle-corrected low-contrast visual acuity with natural pupils was significantly reduced compared to baseline; mean (+/-SD) change was 0.04 +/- 0.13 logMAR (t = 3.3, P = .001). The low-contrast loss was larger (1.5 lines) with dilated pupils; mean (+/-SD) change was 0.13 +/- 0.15 logMAR (t = 9.31, P < .001). Greater losses in dilated low-contrast visual acuity were associated with concurrent astigmatic ketatotomy (t = 2.28, P = .025) and corneal haze of grade 1 or greater (t = 2.71, P = .005).nnnCONCLUSIONSnReductions in visual performance occur after photorefractive keratectomy with a 6-mm zone. These changes are greatest for low-contrast visual acuity with dilated pupils. Corneal haze and concurrent astigmatic keratotomy are associated with greater losses in low-contrast visual acuity. Best-spectacle-corrected low-contrast visual acuity is a sensitive measure for evaluating visual performance after refractive surgery.

Light-Scattering Study of the Normal Human Eye Lens: Elastic Properties and Age Dependence

The human ocular lens is a tissue capable of changing its shape to dynamically adjust the optical power of the eye, a function known as accommodation, which gradually declines with age. This capability is the response of the lens tissue to external forces, which, in turn, is modulated by the biomechanical characteristics of lens tissues. In order to investigate the contributions of lens sclerosis to loss of accommodation, we report on in vitro confocal Brillouin light scattering studies of human ocular lenses spanning over a 30-70 year age range. Using this nondestructive measurement method, we determined that the longitudinal bulk modulus (average ± SD) of the lens nucleus (2.79 ± 0.14 GPa) was consistently greater than the bulk modulus of the lens cortex (2.36 ± 0.09 GPa). Moreover, our results showed that these differences were not age dependent over the 40 year age range that we evaluated using healthy lens tissues. Our results are consistent with the hypothesis that an age-dependent change in the bulk modulus of lens tissues does not fully account for the natural decline of accommodation.