Factors affecting image magnification in indirect ophthalmoscopy with Volk or similar lenses and a biomicroscope.

PURPOSE: To explore the factors affecting the linear magnification of the intermediate fundus image during indirect ophthalmoscopy with a slit-lamp biomicroscope. METHODS: A simple paraxial model, based on a 'reduced' eye and a 'thin' ophthalmoscopy lens, is used to develop equations showing the effects of the power and ametropia of the eye, and the equivalent power and position of the ophthalmoscopy lens on fundus magnification. Predicted magnifications are compared with practical results found in earlier published experimental studies, which used Volk ophthalmoscopy lenses in conjunction with physical model eyes with adjustable levels of axial ametropia. RESULTS: The model's magnification predictions, as a function of the eye's ametropia, are in good agreement with previous experimental measurements, provided that the equivalent powers of the Volk lenses are used rather than their labelled nominal powers. Magnification values typically change by approximately ±10% over the practical range of each parameter if other parameters are held constant. In particular, normal variations in the equivalent power of the adult emmetropic eye result in magnifications which range from about 90-120% of the nominal value given for an eye power of +60.00 D. It is demonstrated that the recommended working distances for different powers of Volk ophthalmoscopy lenses approximate optimal matching between the various pupils of the eye-Volk lens-slit-lamp biomicroscope system. CONCLUSIONS: All the parameters considered have marked effects on magnification. The magnification values quoted by manufacturers can be regarded as only approximations of those which may be found in practice. Better estimates of magnification can be obtained by inserting the appropriate parameter values into the equations derived in this paper, using, where appropriate, the equivalent power of the indirect ophthalmoscopy lens, rather than the lens' labelled, 'nominal' power.

Customised aberration-controlling corrections for keratoconic patients using contact lenses.

Technological advancements in the design of soft and scleral contact lenses have led to the development of customised, aberration-controlling corrections for patients with keratoconus. As the number of contact lens manufacturers producing wavefront-guided corrections continues to expand, clinical interest in this customisable technology is also increasing among both patients and practitioners. This review outlines key issues surrounding the measurement of ocular aberrations for patients with keratoconus, with a particular focus on the possible factors affecting the repeatability of Hartmann-Shack aberrometry measurements. This review also discusses and compares the relative successes of studies investigating the design and fitting of soft and scleral customised contact lenses for patients with keratoconus. A series of key limitations that should be considered before designing customised contact lens corrections is also described. Despite the challenges of producing and fitting customised lenses, improvements in visual performance and comfortable wearing times, as provided by these lenses, could help to reduce the rate of keratoplasty in keratoconic patients, thereby significantly reducing clinical issues related to corneal graft surgery. Furthermore, enhancements in optical correction, provided by customised lenses, could lead to increased independence, particularly among young adult keratoconic patients, therefore leading to improvements in quality of life.

Optical quality and visual performance with customised soft contact lenses for keratoconus.

PURPOSE: This study investigated how aberration-controlling, customised soft contact lenses corrected higher-order ocular aberrations and visual performance in keratoconic patients compared to other forms of refractive correction (spectacles and rigid gas-permeable lenses). METHODS: Twenty-two patients (16 rigid gas-permeable contact lens wearers and six spectacle wearers) were fitted with standard toric soft lenses and customised lenses (designed to correct 3rd-order coma aberrations). In the rigid gas-permeable lens-wearing patients, ocular aberrations were measured without lenses, with the patient's habitual lenses and with the study lenses (Hartmann-Shack aberrometry). In the spectacle-wearing patients, ocular aberrations were measured both with and without the study lenses. LogMAR visual acuity (high-contrast and low-contrast) was evaluated with the patient wearing their habitual correction (of either spectacles or rigid gas-permeable contact lenses) and with the study lenses. RESULTS: In the contact lens wearers, the habitual rigid gas-permeable lenses and customised lenses provided significant reductions in 3rd-order coma root-mean-square (RMS) error, 3rd-order RMS and higher-order RMS error (p ≤ 0.004). In the spectacle wearers, the standard toric lenses and customised lenses significantly reduced 3rd-order RMS and higher-order RMS errors (p ≤ 0.005). The spectacle wearers showed no significant differences in visual performance measured between their habitual spectacles and the study lenses. However, in the contact lens wearers, the habitual rigid gas-permeable lenses and standard toric lenses provided significantly better high-contrast acuities compared to the customised lenses (p ≤ 0.006). CONCLUSIONS: The customised lenses provided substantial reductions in ocular aberrations in these keratoconic patients; however, the poor visual performances achieved with these lenses are most likely to be due to small, on-eye lens decentrations.

Exploring the optimum step size for defocus curves.

PURPOSE: To evaluate the effect of reducing the number of visual acuity measurements made in a defocus curve on the quality of data quantified. SETTING: Midland Eye, Solihull, United Kingdom. DESIGN: Evaluation of a technique. METHODS: Defocus curves were constructed by measuring visual acuity on a distance logMAR letter chart, randomizing the test letters between lens presentations. The lens powers evaluated ranged between +1.50 diopters (D) and -5.00 D in 0.50 D steps, which were also presented in a randomized order. Defocus curves were measured binocularly with the Tecnis diffractive, Rezoom refractive, Lentis rotationally asymmetric segmented (+3.00 D addition [add]), and Finevision trifocal multifocal intraocular lenses (IOLs) implanted bilaterally, and also for the diffractive IOL and refractive or rotationally asymmetric segmented (+3.00 D and +1.50 D adds) multifocal IOLs implanted contralaterally. Relative and absolute range of clear-focus metrics and area metrics were calculated for curves fitted using 0.50 D, 1.00 D, and 1.50 D steps and a near add-specific profile (ie, distance, half the near add, and the full near-add powers). RESULTS: A significant difference in simulated results was found in at least 1 of the relative or absolute range of clear-focus or area metrics for each of the multifocal designs examined when the defocus-curve step size was increased (P<.05). CONCLUSION: Faster methods of capturing defocus curves from multifocal IOL designs appear to distort the metric results and are therefore not valid. FINANCIAL DISCLOSURE: No author has a financial or proprietary interest in any material or method mentioned.

Optical quality for keratoconic eyes with conventional RGP lens and simulated, customised contact lens corrections: a comparison.

PURPOSE: To compare monochromatic aberrations of keratoconic eyes when uncorrected, corrected with spherically-powered RGP (rigid gas-permeable) contact lenses and corrected using simulations of customised soft contact lenses for different magnitudes of rotation (up to 15°) and translation (up to 1mm) from their ideal position. METHODS: The ocular aberrations of examples of mild, moderate and severe keratoconic eyes were measured when uncorrected and when wearing their habitual RGP lenses. Residual aberrations and point-spread functions of each eye were simulated using an ideal, customised soft contact lens (designed to neutralise higher-order aberrations, HOA) were calculated as a function of the angle of rotation of the lens from its ideal orientation, and its horizontal and vertical translation. RESULTS: In agreement with the results of other authors, the RGP lenses markedly reduced both lower-order aberrations and HOA for all three patients. When compared with the RGP lens corrections, the customised lens simulations only provided optical improvements if their movements were constrained within limits which appear to be difficult to achieve with current technologies. CONCLUSIONS: At the present time, customised contact lens corrections appear likely to offer, at best, only minor optical improvements over RGP lenses for patients with keratoconus. If made in soft materials, however, these lenses may be preferred by patients in term of comfort.

Changes in refraction, ocular aberrations, and corneal structure after suspending rigid gas-permeable contact lens wear in keratoconus.

PURPOSE: This study reports on changes in visual acuity, ocular higher-order aberrations, and refraction after suspending rigid gas-permeable lens wear for 1 week in 15 patients with moderate to severe keratoconus. Alterations in the anterior surface, central corneal powers and axes, and central corneal thickness were also explored. METHODS: Scheimpflug photography and Hartmann-Shack aberrometry were performed at 2 visits, 7 days apart, after the patients had removed their habitual contact lenses. Subjective refraction and both high- and low-contrast logarithm of the minimum angle of resolution visual acuities were also recorded at both visits. RESULTS: Reductions in both high-contrast visual acuity (P = 0.001) and low-contrast visual acuity (P = 0.002), along with an increase in third-order root mean square aberrations (P = 0.008), occurred after rigid gas-permeable lens wear was suspended in these patients with keratoconus. However, no significant changes in subjective refraction were found over the 1-week period (P ≥ 0.10). Significant correlations were observed between third-order coma root mean square aberrations and the measured high-contrast (r(p) ≥ 0.59; P ≤ 0.02) and low-contrast visual acuities (r(P) ≥ 0.61; P ≤ 0.015). In addition to increases in the anterior surface central corneal powers (P ≤ 0.02), a reduction in central corneal thickness also was found between the 2 visits (P = 0.00016). CONCLUSIONS: Changes in the optical and structural parameters of the keratoconic cornea occur after suspending rigid gas-permeable contact lens wear. This information may be of interest to practitioners concerned with prescribing aberration-controlling soft contact lenses for such patients.

Visual performance and optical quality with soft lenses in keratoconus patients.

PURPOSE: To assess visual performance and ocular aberrations in keratoconic patients using toric soft contact lenses (SCL), rigid-gas-permeable (RGP) contact lenses and spectacle lens correction. METHODS: Twenty-two keratoconus patients (16 RGP lens wearers and six spectacle wearers) were fitted with toric SCL. Ocular aberrations were measured with and without the patient's habitual RGP lenses and with the SCL in place. In the spectacle wearers, aberrations were measured with and without the SCL. Visual performance (high- and low-contrast visual acuity) was evaluated with the patient's habitual correction and with the SCL. RESULTS: In the RGP lens wearers both the habitual lenses and the toric SCL significantly reduced coma, trefoil, 3rd-order, 4th-order cylinder and higher-order root-mean-square (RMS) aberrations (p ≤ 0.015). In the spectacle wearers the toric SCL significantly reduced coma, 3rd-order and higher-order RMS aberrations (p ≤ 0.01). The patients' habitual RGP lenses gave better low-contrast acuity (p ≤ 0.006) compared to the toric SCL; however, no significant difference was found between lens types for high-contrast acuity (p = 0.10). In the spectacle wearers no significant differences in visual performance measurements were found between the patients' spectacles and the toric SCL (p ≥ 0.06). CONCLUSION: The results show that RGP lenses provided superior visual performances and greater reduction of 3rd-order aberrations compared to toric SCL in this group of keratoconic patients. In the spectacle-wearing group, visual performance with the toric SCL was found to be comparable to that measured with spectacles. Nevertheless, with the exception of spherical aberration, the toric SCL were successful in significantly reducing uncorrected higher-order aberrations.

Forward light scatter and contrast sensitivity in keratoconic patients.

PURPOSE: The aim of this study was to obtain intraocular forward light scatter (FLS) data in keratoconus patients and to explore possible relationships with contrast sensitivity. METHOD: Intraocular straylight was assessed in the right eye of 10 visually-normal subjects (31±5 years) and 5 keratoconus patients (34±9 years) using the van den Berg straylight meter. Contrast sensitivity was also measured in both groups, with and without a glare source provided by the Brightness Acuity Tester (BAT), using a Pelli-Robson chart. Inclusion criteria for the keratoconic patients were chosen to avoid natural causes of increased scatter and to limit the influence of factors that could affect the straylight measurements. RESULTS: Intraocular light scatter was significantly greater in the keratoconic patients than in normal subjects for each of the three eccentricities tested (Mann-Whitney U test (2-tailed): Z≤-2.2, p≤0.027). Baseline Pelli-Robson contrast sensitivity was also found to be lower in the keratoconus group than in the normal subjects (Mann-Whitney U test: Z=-2.2, p=0.023). Under glare conditions contrast loss was significantly greater in the keratoconic patients than in the normal subjects (Mann-Whitney U test: Z=-2.5, p=0.013). High levels of negative correlation were found between the magnitudes of intraocular straylight and baseline contrast sensitivity in the keratoconus group (Spearman's correlation coefficient (RS)≤-0.87, p≤0.027). CONCLUSION: The high values of straylight measured in the keratoconic patients cannot be explained by age or scarring and could be related to the structural degradation of the cornea. Further investigation of intraocular scatter in keratoconus is warranted to improve our current understanding of the visual deficits associated with this pathology.

Repeatability of ocular aberration measurements in patients with keratoconus.

PURPOSE: To explore the repeatability of lower-order and higher-order ocular aberrations measured in patients with keratoconus. METHODS: The IRX-3 (Imagine Eyes, Paris, France) aberrometer was used to record lower-order and higher-order aberrations in 31 eyes of 31 patients with keratoconus. Four monocular measurements were taken consecutively for each patient. The aberrometry data were analysed up to the 5th Zernike order for a 4-mm pupil diameter. The data were evaluated using repeated-measures anova and Friedman analyses. Repeatability was analysed using within-subject standard deviation (S(W) ) and the repeatability limit (r) calculated as 1.96 ×√2 × S(w). RESULTS: Of the 11 aberration terms evaluated, the repeatability of Z (2,0) (mean = 1.36 µm; S(W) = 0.09 µm; r = 0.26 µm); Z (2,±2) RMS (mean = 1.05 µm; S(W) = 0.09 µm; r = 0.24 µm) and Z (4,0) aberrations (mean = 0.34 µm; S(W) = 0.09 µm; r = 0.24 µm) showed the highest variability. In contrast, Z (3,±1) RMS aberrations (mean = 0.85 µm; S(W) = 0.06 µm; r = 0.16 µm) and Z (4,±2) RMS aberrations (mean = 0.40 µm; S(W) = 0.07 µm; r = 0.18 µm) showed comparatively better repeatability. CONCLUSIONS: The lower-order and higher-order aberrations measured in this group of keratoconic patients showed higher levels of variability compared to previous investigations of visually-normal subjects. These results may be of interest to eyecare practitioners involved in the design and fitting of aberration-controlling contact lenses for patients with keratoconus.

Corneal changes after suspending contact lens wear in early pellucid marginal corneal degeneration and moderate keratoconus.

OBJECTIVES: This case study reports on how refraction, visual acuity, and ocular higher-order aberrations changed after rigid gas-permeable (RGP) lens wear was suspended by a patient with early pellucid marginal corneal degeneration (PMD) and by a patient with moderate keratoconus. Alterations in central corneal power and axes, central corneal thickness, and corneal topography were also explored. METHODS: Ocular aberrations and Scheimpflug photography were measured at 2 visits, 7 days apart, after the patients had removed their contact lenses. Subjective refraction and logarithm of the minimum angle of resolution visual acuities were also recorded at both visits. RESULTS: In contrast to the keratoconic patient, the patient with early PMD showed changes in subjective refraction (approximately -1.75 diopter cylinder of astigmatism), front surface central powers (∼1.3 diopter [D] horizontally and 2 D vertically), and anterior surface topography (∼1.2 D) between visits. Both patients showed an increase in total higher-order root mean square (HORMS) aberrations after suspending lens wear (∼0.40 µm for the moderately keratoconic patient and 0.22 µm for the early PMD patient). CONCLUSIONS: Changes in the optical and structural parameters of the cornea after suspending lens wear are likely to be dependent on a multitude of factors, such as the lens fitting and the biomechanical properties of the cornea. The findings of this report may be useful to practitioners when refracting or refitting existing RGP lens wearers with ectasia or to those involved in prescribing aberration-controlling contact lenses for conditions such as keratoconus and PMD.

Pellucid corneal marginal degeneration: A review.

Pellucid marginal corneal degeneration (PMD) is a rare ectatic disorder which typically affects the inferior peripheral cornea in a crescentic fashion. The condition is most commonly found in males and usually appears between the 2nd and 5th decades of life affecting all ethnicities. The prevalence and aetiology of this disorder remain unknown. Ocular signs and symptoms of patients with PMD differ depending on the severity of the condition. Unless corneal topography is evaluated, early forms of PMD may often be undetected however, in the later stages PMD can often be misdiagnosed as keratoconus. Visual signs and symptoms include longstanding reduced visual acuity or increasing against-the-rule irregular astigmatism leading to a slow reduction in visual acuity. In rare cases, patients may present with a sudden loss of vision and excruciating ocular pain due to corneal hydrops or spontaneous perforation. The vast majority of PMD patients are managed using spectacles and contact lenses. Several surgical procedures have been used in an attempt to improve visual acuity when spectacles and contact lenses do not provide adequate vision correction. Since patients with PMD make poor candidates for laser vision correction, an awareness of the topographical and slit-lamp features of PMD will be useful to clinicians screening for signs of corneal abnormality before corneal refractive surgery. This review describes the clinical features of PMD, its differential diagnosis and various management strategies presently available.

A comparison between subjective refraction and aberrometry-derived refraction in keratoconus patients and control subjects.

PURPOSE: This study compares the differences in the magnitude of the subjective refraction and three aberrometry-derived refractions along with visual acuity achieved with these refractions in a group of keratoconic patients and age-matched normal subjects. MATERIALS AND METHODS: Subjective refraction and Hartmann-Shack aberrometry was performed on six keratoconus patients and 12 normal subjects. In addition, the logMAR visual acuity achieved using the subjective and aberrometry auto-refraction data were measured in the six keratoconic subjects. RESULTS: The subjective and aberrometry-derived spherical equivalent refraction data were significantly different in the keratoconus group (p = 0.015) but not in the normal group (p = 0.10). In the keratoconic patients, subjective refraction data gave better logMAR acuity than the aberrometry-derived auto-refraction data. The magnitudes of vertical coma and higher-order RMS (root mean square) error showed significant correlations with the subjective refraction logMAR visual acuities. Significant correlations were found between the magnitudes of manifest vertical coma and higher-order RMS error and the difference in the M (the mean equivalent sphere) power vector terms between the subjective and aberrometry-derived auto-refraction data in the keratoconic group. CONCLUSIONS: The subjective and aberrometry-derived spherical equivalent refraction data were significantly different in the keratoconus group. The larger the magnitude of the higher-order aberrations in keratoconic eyes, the poorer the subjective refraction logMAR acuity and the larger the difference between the subjective and aberrometry-derived M power vector terms. Further investigation into deriving objective refraction data from aberrometry measurements is warranted in keratoconus.

Visual acuity and ocular aberrations with different rigid gas permeable lens fittings in keratoconus.

OBJECTIVES: The aim of this case report is to investigate the effect of rigid gas permeable (RGP) lens fitting on visual acuity and ocular higher-order aberrations by systematically applying RGP lenses with different back optic zone radii (BOZR) to a keratoconic eye. METHODS: Eight Menicon-EX RGP lenses (Menicon Co., Nagoya, Japan), which ranged from a BOZR of 7.40 to 8.10 mm in 0.10-mm steps, were fitted to a moderate keratoconic eye. All the trial lenses fitted had a total diameter of 9.60 mm, centre thickness of 0.15 mm, and a back vertex power of -3.00 diopter sphere. Measurements of the higher-order aberrations were made with the RGP lenses in place using a Shack-Hartmann aberrometer (IRX-3; Imagine Eyes, Paris, France). Visual acuities with the lenses were recorded using a high-contrast Early Treatment Diabetic Retinopathy Study letter chart at a distance of 4 m. RESULTS: As the lens BOZR value changed from 7.40 to 8.10 mm, the vertical coma component was found to reduce from +0.34 to +0.21 microm; third-order root mean square error also reduced from +0.34 to +0.23 microm and total higher-order root mean square aberrations decreased from +0.35 to +0.23 microm. More over, changes in the lens BOZR from 7.40 to 8.10 mm led to a substantial change in best-corrected LogMAR acuity from +0.58 to -0.30 log units. CONCLUSIONS: The results showed that a flatter RGP lens fit lead to a reduction in higher-order aberrations and improvement in visual acuity in this moderate keratoconic patient. Further investigations into the changes in aberrations with different RGP lens fittings and BOZR values in keratoconic eyes will help to improve our current understanding of how contact lens fitting impacts on optical performance.

Dynamics of ocular aberrations in keratoconus.

BACKGROUND: The aim was to investigate the fluctuations in monochromatic ocular aberrations with accommodation and tear-film changes in moderate keratoconic eyes. METHODS: We measured the changes in ocular higher-order aberrations in 10 moderate keratoconic and 10 visually normal eyes to accommodative stimuli ranging from zero to 5.00 DS using a Hartmann-Shack aberrometer. In addition, the changes in ocular higher-order aberrations were measured for up to 15 seconds after a blink in eight keratoconic and eight visually normal eyes. RESULTS: These results show that ocular spherical (p = 0.68) and coma-like (p = 0.71) aberrations did not change significantly with accommodation from zero to 5.00 DS in keratoconic eyes. In contrast to normal eyes, the ocular higher-order RMS error tended to decrease in magnitude after a blink in keratoconic eyes. Vertical coma became less negative with time after a blink in the keratoconic group, therefore, reducing the manifest ocular higher order RMS error by counteracting the negative vertical coma of the cornea. CONCLUSIONS: Compared to the manifest monochromatic higher-order aberrations, any dynamic fluctuations in ocular aberrations with accommodation and tear film changes are relatively small in moderate keratoconic eyes. This implies that the correction of monochromatic higher-order aberrations in keratoconus using customised soft contact lenses will not be significantly hindered by such dynamic aberrational changes.