W. Neil Charman

Visual performance with multifocal intraocular lenses: mesopic contrast sensitivity under distance and near conditions.

OBJECTIVE To evaluate distance and near visual performance under bright (photopic) and dim (mesopic) conditions in patients who had undergone uncomplicated cataract extraction with multifocal or monofocal intraocular lens (IOL) implantation. DESIGN Prospective, nonrandomized, masked, comparative, observational case series. PARTICIPANTS Thirty-two eyes of 32 patients after zonal-progressive multifocal IOL implantation (Allergan Medical Optics Array SA-40N) and 32 eyes of 32 age-matched patients after monofocal IOL implantation (Allergan Medical Optics SI-40NB). INTERVENTION All eyes underwent phacoemulsification and IOL implantation. MAIN OUTCOME MEASURES At 18 months after surgery, the monocular contrast sensitivity (CS) function was measured with sinusoidal grating charts at distance and near, at one photopic luminance level and 2 mesopic luminance levels (85, 5, and 2.5 candelas per square meter). RESULTS Under bright conditions, CS at distance in the multifocal group was not statistically different (P>0.01) from that in the monofocal group at any tested grating spatial frequency (1.5, 3, 6, 12, and 18 cycles per degree [cpd]). At low luminances, distance CS for the multifocal group was worse than that for the monofocal group at the highest test spatial frequencies (12 and 18 cpd; P<0.01). At near, photopic CS in the multifocal group was lower than at distance; patients with only a monofocal distance correction, however, could not detect the test gratings, even at the highest available contrast. With optimal near spectacle additions (i.e., using the distance correction of the multifocal IOL), there were no significant differences between the photopic near CS values for the multifocal and monofocal groups. When the luminance was decreased, near CS at all spatial frequencies was reduced in both groups. Contrast sensitivity in the near-corrected, multifocal group was significantly worse than in the near-corrected, monofocal group at high spatial frequencies (12 and 18 cpd). CONCLUSIONS This work supports the findings of earlier authors that the Array multifocal IOL, with its center-distance design, is distance biased. Distance CS is within normal limits under bright photopic conditions but shows deficits at higher spatial frequencies (more than approximately 12 cpd) under dim mesopic conditions. Near CS obtained with the multifocal IOL is below that which can be achieved by an appropriate monofocal near correction, for all spatial frequencies and illumination conditions.

The eye in focus: accommodation and presbyopia

Current understanding of the anatomy, function and performance of the accommodative system of the young, adult human eye is outlined. Most major current models of the accommodative mechanism are based on Helmholtzs original ideas but, despite of a growing volume of related research, uncertainty continues over the relative contributions made to the overall mechanism by different ocular structures. The changes with age are then discussed. Although the amplitude of accommodation decreases steadily from later childhood, the speed and accuracy of the system within the available amplitude are little impaired until the age of about 40, when the amplitude falls below that needed for normal near work. A review of the available evidence on age‐related change in the lens, capsule, ciliary body and other relevant ocular structures confirms that geometric and viscoelastic lenticular changes play major roles in the progressive loss of accommodation. Other factors may also contribute in an, as yet, unquantified way and a full understanding of the origins of presbyopic change remains elusive.

Choice of spatial frequency for contrast sensitivity evaluation after corneal refractive surgery.

PURPOSE To study the utility of measurements of contrast sensitivity at different spatial frequencies as an index of visual recovery following refractive surgery. METHODS Contrast sensitivity at 1.5, 3, 6, 12, and 18 c/deg was measured with the Stereo Optical FACT chart in 20 patients after photorefractive keratectomy (PRK) using the Nidek EC-5000 excimer laser system, and in 18 patients following laser in situ keratomileusis (LASIK). Contrast sensitivity was measured preoperatively and 1, 3, 6, and 12 months after surgery. RESULTS Results showed a statistically significant reduction (P<.01) in contrast sensitivity at all spatial frequencies in PRK patients during the first and third month, but contrast sensitivity recovered to preoperative values by 6 months after surgery (P>.1). In LASIK patients, decreased contrast sensitivity values 1 month after surgery were also obtained at all spatial frequencies. After 3 months, contrast sensitivity at 1.5 and 3 c/deg had recovered and did not differ significantly from preoperative values (P>.1), although contrast sensitivity at other frequencies remained reduced (P<.01). At 6 and 12 months, contrast sensitivity at all spatial frequencies did not differ from that obtained preoperatively (P>.1). CONCLUSIONS Contrast sensitivity measurements at 6 and 12 c/deg appear to be most useful in the assessment of patients who have undergone laser refractive surgery because defocus and optical aberrations primarily affect the higher spatial frequencies.

Peripheral refraction in orthokeratology patients.

Purpose. The purpose of this study is to measure refraction across the horizontal central visual field in orthokeratology patients before and during treatment. Methods. Refractions were measured out to 34° eccentricity in both temporal and nasal visual fields using a free-space autorefractor (Shin-Nippon SRW5000) for the right eyes of four consecutively presenting myopic adult patients. Measurements were made before orthokeratology treatment and during the course of treatment (usually 1 week and 2 weeks into treatment). Refractions were converted into mean sphere (M), 90° to 180° astigmatism (J180), and 45° to 135° astigmatism (J45) components. Results. Before treatment, subjects had either a relatively constant mean sphere refraction across the field or a relative hypermetropia in the periphery as compared with the central refraction. As a result of treatment, myopia decreased but at reduced rate out into the periphery. Most patients had little change in mean sphere at 30° to 34°. In all patients, the refraction pattern altered little after the first week. Conclusion. Orthokeratology can correct myopia over the central ± 10° of the visual field but produces only minor changes at field angles larger than 30°. If converting relative peripheral hypermetropia to relative peripheral myopia is a good way of limiting the axial elongation that leads to myopia, orthokeratology is an excellent option for achieving this.

Peripheral refraction and the development of refractive error: a review.

It has been suggested that emmetropic and low‐hyperopic eyes in which the refractive error in the periphery of the visual field is relatively hyperopic with respect to the axial refraction may be at greater risk of developing myopia than eyes with similar refractions but relatively myopic peripheral refractive errors. The animal and human evidence to support this hypothesis is reviewed. The most persuasive studies are those in which emmetropization has been shown to occur in infant rhesus monkeys with ablated foveas but intact peripheral fields, and the demonstration that, in similar animals, lens‐induced relative peripheral hyperopia produces central axial myopia. Evidence for emmetropization in animals with severed optic nerves suggests that emmetropization is primarily controlled at the retinal level but that the higher levels of the visual system play a significant role in refining the process: there appear to be no directly equivalent human studies. Since any contribution of the higher centres to the control of refractive development must depend upon the sensitivity to defocus, the results of human studies of the changes in depth‐of‐focus across the field and of the contribution of the retinal periphery to the accommodation response are discussed. Although peripheral resolution is relatively insensitive to focus, this is not the case for detection. Moreover accommodation occurs to peripheral stimuli out to a field angle of at least 10 deg, and the presence of a peripheral stimulus can influence the accommodation to a central target. Although the basic hypothesis that a relatively hyperopic peripheral refractive error can drive the development of human myopia remains unproven, the available data support the possibility of an interaction between the states of focus on axis and in the periphery.

Changes in posterior corneal curvature after photorefractive keratectomy

Purpose: To determine whether myopic ablation by excimer laser photorefractive keratectomy (PRK) affects only the anterior curvature of the cornea or whether changes also occur in the posterior corneal curvature. Setting: Department of Optometry and Neuroscience, UMIST, and Optimax Laser Eye Clinic, Manchester, United Kingdom. Methods: Sixteen patients who presented for correction of myopia in 1 eye by excimer laser PRK were followed for 3 months. Only newly presenting patients were recruited, and the untreated eyes were used as controls. The patients were examined at the initial visit (0 week) and 6 and 12 weeks post‐PRK. Measurements included Orbscan topography and pachymetry, autokeratometry, and ultrasound pachymetry. Results: The mean patient age of the 8 men and 8 women was 29.6 years ± 8.6 (SD) (range 20 to 47 years). The attempted mean spherical equivalent correction was between −1.73 and −6.43 diopters. Anterior corneal curvature and corneal thickness in the treated eyes changed systematically in relation to the amount of ablation. Posterior corneal curvature steepened in relation to the dioptric power treated. There were systematic differences between the pachymetry values obtained with the Orbscan and the ultrasound pachymeter. Conclusions: The results suggest that after myopic PRK, the thinner, ablated cornea may bulge forward slightly to steepen both anterior and posterior curvatures. This may account for the regression toward myopia that is typically found in the first few days posttreatment. The forward bulging is similar to the corneal relaxation effects observed after radial keratotomy.

Age-Related changes in ocular aberrations with accommodation

This study investigates the changes in aberrations with monocular accommodation as a function of age. Second-order and higher order wavefront aberrations and pupil size were measured as a function of accommodation demand over the range of 0-4 D in the right eyes of 47 normal subjects with ages between 17 and 56 years. Higher order ocular Zernike aberrations were analyzed for the natural pupil size in terms of their equivalent defocus and were also determined for fixed pupil diameters of 4.5 mm in the unaccommodated eyes and 2.5 mm in the accommodating eyes. With relaxed accommodation (0 D accommodation stimulus), the major change with age was in the value of C4(0), which increased in positive value over the age range studied, although the total higher order RMS wavefront aberration did not increase. When the data were analyzed for natural pupils, spherical aberration was again found to change systematically in the positive direction with age. The equivalent defocus of total higher order RMS error for natural pupils showed no significant correlation with age (p > .05). With active accommodation, spherical aberration was found to decrease and become negative as the accommodative response increased in the younger subjects (<40 years). Near-zero spherical aberration was found at accommodation levels of about 0.50 D in the youngest subjects (<20 years) and at around 2-3 D in subjects between 20 and 39 years. In the older subjects (>40 years), the spherical aberration showed only small changes, some of which were positive, within the limited amplitude of accommodation available. Other higher order aberrations and the RMS of higher order aberrations did not appear to change systematically with accommodation, except in the oldest subjects. The change with age in the relationship between aberration and accommodation is interpreted in terms of the changing gradients of refractive index and surface curvatures of the crystalline lens.

Mesopic contrast sensitivity function after excimer laser photorefractive keratectomy.

PURPOSE To evaluate contrast sensitivity under mesopic conditions in patients who had undergone uncomplicated excimer laser photorefractive keratectomy (PRK) for myopia. METHODS Monocular contrast sensitivity function was measured with the Stereo Optical F.A.C.T. chart in 26 patients who had received PRK using the Nidek EC-5000 excimer laser system. Mean preoperative refractive error was -6.23 +/- 1.69 D (range, -4.00 to -8.25 D); postoperatively, mean refractive error was -0.36 +/- 0.58 D (range, -0.75 to +0.50 D). Contrast sensitivity function was measured 6 months after surgery using four different chart luminances: 85, 5.0, 2.5, and 0.1 cd/m2, the first being a photopic level and the rest mesopic. A control group of eight emmetropic subjects was also studied to allow comparison of results for statistical purposes. RESULTS Logarithmic values of contrast sensitivity at each spatial frequency were used for statistical analysis and normalized values were used for graphical representation. The results showed a statistically significant reduction (P < .01) in contrast sensitivity for the PRK patients in comparison with the control group under mesopic conditions for each spatial frequency tested (1.5, 3, 6, 12, and 18 c/deg), although no significant contrast sensitivity differences were observed between PRK and control groups at the photopic (85 cd/m2) level (P > .01 for all frequencies). CONCLUSION Photorefractive keratectomy can induce significant reductions in contrast sensitivity under mesopic conditions, even though the photopic contrast sensitivity function is normal.

Power Profiles of Multifocal Contact Lenses and Their Interpretation

Purpose Many contact lens (CL) manufacturers produce simultaneous-image lenses in which power varies either smoothly or discontinuously with zonal radius. We present in vitro measurements of some recent CLs and discuss how power profiles might be approximated in terms of nominal distance corrections, near additions, and on-eye visual performance. Methods Fully hydrated soft, simultaneous-image CLs from four manufacturers (Air Optix AQUA, Alcon; PureVision multifocal, Bausch & Lomb; Acuvue OASYS for Presbyopia, Vistakon; Biofinity multifocal- “D” design, Cooper Vision) were measured with a Phase focus Lens Profiler (Phase Focus Ltd., Sheffield, UK) in a wet cell and powers were corrected to powers in air. All lenses had zero labeled power for distance. Results Sagittal power profiles revealed that the “low” add PureVision and Air Optix lenses exhibit smooth (parabolic) profiles, corresponding to negative spherical aberration. The “mid” and “high” add PureVision and Air Optix lenses have bi-aspheric designs, leading to different rates of power change for the central and peripheral portions. All OASYS lenses display a series of concentric zones, separated by abrupt discontinuities; individual profiles can be constrained between two parabolically decreasing curves, each giving a valid description of the power changes over alternate annular zones. Biofinity lenses have constant power over the central circular region of radius 1.5 mm, followed by an annular zone where the power increases approximately linearly, the gradient increasing with the add power, and finally an outer zone showing a slow, linear increase in power with a gradient being almost independent of the add power. Conclusions The variation in power across the simultaneous-image lenses produces enhanced depth of focus. The through-focus nature of the image, which influences the “best focus” (distance correction) and the reading addition, will vary with several factors, including lens centration, the wearer’s pupil diameter, and ocular aberrations, particularly spherical aberration; visual performance with some designs may show greater sensitivity to these factors.

Intraocular pressure after excimer laser myopic refractive surgery

The aim of this study was to determine whether intraocular pressure (IOP), as measured by Goldmann applanation or non‐contact tonometry, shows systematic changes in patients who have undergone photorefractive keratectomy (PRK) or laser in situ keratomileusis (LASIK). IOP was measured by central Goldmann and non‐contact tonometry in 54 patients pre and post‐PRK, and in 43 patients pre‐ and post‐LASIK. An interval of 12 months was allowed after surgery. Patients were selected to have one of four specific initial values of refractive error (−2.5, −5.0, −7.5 and −10.0 D). Fellow unoperated eyes were used as controls. A paired Students t‐test and a one‐way ANOVA test were used for statistical analysis. After PRK and LASIK, a statistically significant decrease (p<0.01) was observed in the IOP of the treated eyes (but not for control eyes; p>0.01). Although the magnitude of the change increased with the attempted refractive correction, this trend was not statistically significant ( >p>0.01). No statistically significant differences were found between the results obtained following the two types of surgery, although the recorded fall in IOP was smaller following LASIK( >p>0.01). The IOP measured after PRK and LASIK for myopia may be reduced because of reduced corneal thickness and curvature and, possibly, tissue softening after natural healing. The presence or absence of Bowmans membrane does not appear to be important in this context. The reduction in measured IOP following refractive surgery, by about 0.5 mmHg/D of myopic correction, needs to be remembered when possible abnormality of IOP in such patients is being considered.