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PubMed1999

Emmetropization in the rhesus monkey (Macaca mulatta): birth to young adulthood.

PURPOSE: To provide baseline measurements on the postnatal changes in refractive error, corneal curvature, and axial elongation of the eyes of normal monkeys. Little is known about the course of normal eye growth from birth to adolescence, particularly how refractive parameters co-vary during development. In animal models of ametropia, usually one eye is manipulated and the fellow eye serves as a control. However, given individual differences, and without baseline data, it is impossible to determine whether either eye develops normally. METHODS: Measurements were obtained on 237 rhesus monkeys, whose ages ranged from birth to 5 years. Examinations included cycloplegic refraction by retinoscopy, keratometry measurements, and A-scan ultrasound measurements of axial length. The time course of development was evaluated using a growth curve analysis appropriate for a mixture of cross-sectional and longitudinal data. RESULTS: At birth, all three parameters were normally distributed and only weakly correlated. Monkeys had +7 D (SD=2.3 D) of hyperopia, corneal power of 58 D (SD=1 D), and axial length of 13.2 mm (SD=0.4 mm). Refractive error ranged from +0.5 D to +14.5 D, with a mean difference between the two eyes of 0.5 D. Corneal curvature ranged from 61 D to 54 D, with a mean difference between the two eyes of 0.8 D. Axial length ranged from 12.0 mm to 14.2 mm, with a mean difference between the two eyes of 0.1 mm. Although the degree of hyperopia achieved asymptote, of + 2 D, shortly after 1 year of age, corneal curvature and axial length did not achieve asymptote until nearly 5 years of age. By this time, refractive error had declined by 5 D, corneal curvature had declined by 7 D, and axial length had increased by 6 mm. CONCLUSIONS: The magnitude of the individual differences that can occur in a small sample of experimental subjects is large enough to necessitate reference to age norms derived from a large population. Our results provide a baseline for studies of normal and abnormal eye growth and ametropia in primates. Our results also led to the confirmation of a set of "rules" that have been offered as an explanation of how these three parameters interact during emmetropization.

Aging↗
PubMed2003

Emmetropisation responses when visual information is presented at only one or two near target planes in chick.

PURPOSE: When visual information is confined to one object plane and zero or hyperopic defocus is present, emmetropisation is directed towards this plane, when myopic defocus is present, emmetropisation processes fail. We investigated the effect of introducing information at a second nearer plane on emmetropisation responses under these conditions. METHODS: The visual environment was controlled using lenses (+30 D, +40 D and +50 D) and cones with a Maltese cross target (MX) at one or two distances. Four different target configurations were used: 1. a single target was located at 3.3 cm, 2. a vertical hemi-field target was added at 2.5 cm, 3. a transparent target was added at 2.5 cm or 4. a single target was located at 2.5 cm. An additional cone length of 4.0 cm and nearer target distance of 3.3 cm was used with the +50 D lens. The imaging devices were applied monocularly to eight-day-old chicks and worn for four days. At the end of the treatment, refractive errors and eye growth were measured. Potential regional differences in growth were also assessed. RESULTS: The configuration of the target and the interaction between target configuration and lens power had significant refractive and axial growth effects. With a single target plane, myopic and hyperopic defocus resulted in myopia. When defocus was experienced at two planes, refractive errors shifted towards the plane with the lower defocus and emmetropisation responses, although still not normal, were more consistent. CONCLUSIONS: When visual information is provided at two distances, the target with the lesser incident defocus has the greater influence on the resultant refractive error. Emmetropisation responses are more accurate when information is presented at many distances.

Accommodation, Ocular↗
PubMed1994

Treatment of low, moderate, and high myopia with the 193-nm excimer laser.

PATIENTS AND METHODS: Photorefractive keratectomy using the VISX 2015 193-nm excimer laser was performed on 134 consecutive eyes of 97 myopic patients by the two authors. Preoperative refractive errors (spherical equivalent) ranged from -1.63 to -14.25 diopters (D) (mean, -5.76 +/- 2.40 D). Follow-up of 6 months was available on 110 eyes. At six months, the average residual refractive error was +0.16 +/- 1.13 D (range -2.88 to +3.38). Correction within 1 D of that attempted was obtained in 77 eyes (70%). Uncorrected visual acuity of 20/40 or better was achieved in 89 eyes (81%), and 20/25 or better in 54 eyes (49%). RESULTS: At one year, follow up was available on 57 eyes. The average residual refractive error was -0.22 +/- 0.87 D (range -3.00 to +2.00 D). Correction within 1 D was achieved in 47 eyes (82%). Visual acuity was 20/40 or better uncorrected in 50 eyes (88%), and 20/25 or better in 35 eyes (61%). One patient lost three lines of best corrected visual acuity and 4 patients lost 2 lines of best corrected visual acuity from corneal haze or irregular astigmatism, while all other patients returned to best corrected visual acuity within one line of their preoperative best corrected visual acuity. CONCLUSION: Photorefractive keratectomy with the 193-nm excimer laser appears to be a useful treatment modality for the reduction of low to moderate myopia.

Adult↗
PubMed1994

[The accuracy of optical power calculations of intraocular lenses in cataract surgery].

The author selected 12 groups of patients (858 patients altogether). The operation was carried out by the same surgeon and the same type of IOL was implanted in each group. An individual A-constant was calculated for each patient. Using individualized A-constant for each group a postoperative refractive error was calculated for each patient. Following formulas were used for the calculation: SRK, SRK II, Holladay formula (A-constant), Holladay formula (surgeon factor) and SRK/T. No substantial difference between the results of formulas SRK, SRK II, Holladay formula (surgeon factor) and SRK/T was found. The postoperative refractive error +/- 1.0 D was found in the interval 65.6% - 67.8% for all these formulas. Holladay formula (A-constant) yielded the postoperative refractive error +/- 1.0 D in 62.2% of cases.

Cataract Extraction↗
PubMed2000

Intraocular Lens Calculation for Cataract Treated with Photorefractive Keratectomy Using Ray Tracing Method.

Purpose: Conventional methods (such as the SRK-II formula) do not accurately calculate the power of the intraocular lens (IOL) after refractive surgery. Therefore, we compared a new formula including a ray tracing method to the conventional method for foldable IOL lens implantation.Method: Foldable IOLs (MA 60 BM) were implanted in 26 patients (32 eyes) using the phakoemulsification technique. The power of the IOL was measured preoperatively using the SRK-II formula in all cases. From the results of postoperative refractive errors of these cases, the power of IOL calculated by the ray tracing method was compared to the SRK-II formula. Cataract patients first treated with photorefractive keratectomy (PRK) received IOL implants using our ray tracing method and their postoperative refraction was measured.Results: The average postoperative refractive error was 1.32 D in SRK-II formula, 0.95 D in the ray tracing method with Ray 1 used and 0.89 D with Ray 2 used. Postoperative refraction of both eyes first treated with PRK was -1.00 D.Conclusion: The average postoperative refractive error was reduced in the ray tracing method using Olsen's predicted ACD (Ray 2) compared to SRK-II formula. This new tracing method appears to be useful for determination of IOL power and it may be applied for IOL calculation for cataract surgery after refractive surgery.

Journal Article↗
PubMed1975

Genetic analysis of indices of corneal power and corneal astigmatism in human populations with varying incidences of strabismus.

Heritability estimates for corneal power were found to be high and similar for two populations which differed in their incidence of esotropia. This similarity suggest (1) that genetic differences for corneal power do not contribute to the difference in heritability for spherical refractive error reported for these populations and (2) that this character is not a critical variable contributing to the pathophysiology of esotropia. Heritability estimates for corneal astigmatism were, in most cases, rather low. The pattern of population and sex differences among heritability estimates was consistent with those previously reported for cylindrical refractive error. These population differences in heritability suggest that they contribute to population differences found for cylindrical refractive error.

Adult↗
PubMed2003

[Results of scanning and flying spot technologies in photorefractive keratectomy (PRK) for hypermetropia].

AIM OF THE STUDY: To compare the results of scanning and flying spot laser beam technologies of photorefractive keratectomy (PRK) in eyes with hypermetropic refractive error. PATIENTS AND METHODS: In Group I (n = 800) eyes were treated with scanning technology (Aesculap-Meditec MEL 60), in Subgroup I/1 (n = 482) those eyes, which had a preoperative refractive error between +1.0 and +3.5 D; in Subgroup I/2 (n = 318) the eyes between +3.75 and +6.5 D. In Group II (n = 200) eyes treated with flying spot technology (Aesculap-Meditec MEL 70 G-Scan) were evaluated; in Subgroup II/1 (n = 106) eyes between +1.0 and +3.5 D; in subgroup II/2 (n = 94) eyes between +3.75 and +7.5 D. Follow-up time was 12 months. RESULTS: The preoperative correction need decreased in Group I/1 from +2.88 +/- 1.34 D to +1.26 +/- 1.24 D; in Group I/2 from 64 +/- 2.96 D to +2.46 +/- 1.84 D; in Group II/1 from +2.94 +/- 1.42 D to +0.42 +/- 0.14 D and in Group II/2 from 48 +/- 2.62 D to +0.86 +/- 0.6 D 12 months after PRK. Postoperative uncorrected visual acuity (UCVA) was 1.0 or better in 75.7% within the eyes of Group I/1; it was 22.3% in Group I/2; 80% in Group II/1 and 64.8% in Group II/2. The percentage of the eyes within +/- 1.0 D of targeted refraction was: In Group I/1 86.1%, in Group I/2 45.3%, in Group II/1 92.4% and in Group II/2 78.7%. The best spectacle-corrected visual acuity (BSCVA) decreased by 2 or more Snellen lines among the eyes of Group I/1 in 12%; in Group I/2 in 21%; in Group II/1 in 2.8% and in Group II/2 in 9.6%. In Group I/1 2%, in Group II/1 3.8% of the treated eyes gained 2 or more lines of BSCVA. Among the eyes treated with the scanning model (Group I/2) a central bump-like opacity was observed in 4 eyes (1.2%); among the eyes treated with the flying spot model no similar complication occurred. The postoperative increase of intraocular pressure was observed in 7.5% in Group I/1; in 6.8% in Group I/2; in 7.0% in Group II/1; and in 6.4% in Group II/2. CONCLUSIONS: Flying spot technology was superior to scanning method in each treatment group, difference was greatest in eyes treated with a preoperative refractive error higher than +3.75 D. The upper limit of hypermetropic treatments has increased to +6.0 D in case of flying spot treatments from the previous +3.5 D upper limit of scanning technology.

Humans↗
PubMed1990

Pitfalls in the conception, manipulation, and measurement of visual accommodation.

Reports of applied research in visual accommodation frequently contain points of confusion, suggesting that there may be some widespread misunderstanding regarding the nature of accommodation, the complexities involved in its measurement, and the multiple meanings inherent in its terminology. The present paper reviews several of these problem areas. Included are discussions of the effect of refractive error on accommodation assessment; the dangers involved in assuming equivalence among physiological, refractive power, and conjugate distance changes; and multiple meanings in the conception of resting accommodation. A variety of manipulation and measurement problems also are reviewed, including the specification of accommodative target distance, the use of lenses, and the correction of refractive error. The review concludes with recommendations regarding conceptions and procedures useful in avoiding the difficulties described.

Accommodation, Ocular↗
PubMed2002

Clear lens extraction with intraocular lens implantation during retinal detachment repair in patients with Acquired Immune Deficiency Syndrome (AIDS) [correction of autoimmune deficiency syndrome] and cytomegalovirus retinitis.

OBJECTIVE: To assess the outcomes of clear lens extraction with intraocular lens (IOL) implantation during repair of retinal detachment by vitrectomy with silicone oil tamponade in patients with acquired immunodeficiency syndrome (AIDS) and cytomegalovirus (CMV) retinitis. DESIGN: Retrospective, noncomparative case series. PARTICIPANTS: Twelve eyes of 10 patients with AIDS, CMV retinitis, and retinal detachment. INTERVENTION: All patients underwent phacoemulsification with posterior chamber IOL placement at the time of vitrectomy with silicone oil tamponade for repair of retinal detachment. A targeted postoperative refractive error of -5.00 diopters (D) to -3.00 D was chosen in an attempt to counteract the hyperopic effect of silicone oil. MAIN OUTCOME MEASURES: The following factors were evaluated: postoperative visual acuity, refractive error, and intraoperative and postoperative complications. RESULTS: Median follow-up was 7 months (range, 1-46 months). For patients without macular necrosis, median best-corrected preoperative visual acuity was 20/75 (range, 20/20-20/800), and median best postoperative visual acuity was 20/50 (range, 20/20-20/400). Median final visual acuity was 20/140 (range, 20/25 to count fingers at 1 foot). The median postoperative refractive error (spherical equivalent) was -1.00 D (range, -4.00 D to +7.88 D). Reoperation was required in 3 of 12 eyes for recurrent macular detachment (1 with silicone oil underfill; 2 with proliferative vitreoretinopathy). The macula was attached in all eyes at last follow-up. Reattachment of the peripheral retina was achieved in 10 of 12 eyes. There were no anterior segment complications. CONCLUSIONS: Clear lens extraction with IOL placement during repair of retinal detachment with silicone oil tamponade does not seem to increase complications and may improve long-term visual rehabilitation, improve retinitis management by allowing better posterior segment visualization throughout the postoperative course, and decrease overall cost and morbidity associated with cataract extraction as a second procedure.

AIDS-Related Opportunistic Infections↗
PubMed2004

Laser in situ keratomileusis for high hyperopia in awake, autofixating pediatric and adolescent patients with fully or partially accommodative esotropia.

PURPOSE: To establish the safety and efficacy of laser in situ keratomileusis (LASIK) in pediatric and adolescent patients with bilateral visual acuity of 20/30 or better and accommodative or partially accommodative esotropia. SETTING: Department of Ophthalmology and Visual Science, University of Texas-Houston Medical School, Houston, Texas, USA. METHODS: The study comprised 30 eyes of 15 consecutive patients with accommodative or partially accommodative esotropia who met eligibility requirements and had bilateral LASIK using the Alcon Summit Autonomous LADARVision excimer laser to correct a refractive error after January 2001. All patients were awake and autofixating during the procedure. RESULTS: The mean age of the patients was 13.9 years (range 9.1 to 18.8 years) and the mean refractive error, +5.35 diopters (D) (range +3.75 to +8.50 D) with anisometropia of 2.0 D or less. The mean follow-up was 15.7 months (range 9.5 to 22.5 months). No intraoperative complications were encountered. The percentage of undercorrection [100% -[(treatment achieved/treatment attempted) x 100%]] [mean 34% +/- 17% (SD), coefficient of variation (SD/mean) 0.50, range 5% to 58%] was higher than expected. Seven patients (47%) required enhancement due to undercorrection of hyperopia with diplopia (6 patients) or astigmatism with decreased visual acuity (1 patient). In this small series, no patient lost best corrected visual acuity or stereo acuity. CONCLUSION: Laser in situ keratomileusis can safely and effectively reduce refractive error in this group of patients; however, patient selection is extremely critical and enhancement was required in almost half the patients.

Accommodation, Ocular↗
PubMed1989

Visual acuity outcome in isometropic hyperopia.

Refractive amblyopia may occur as a unilateral or bilateral condition. Although bilateral refractive amblyopia may account for 1 to 2% of all refractive amblyopia, there is little consistent information in the literature regarding isoametropic amblyopia resulting from bilateral hyperopia. Hence, this retrospective study investigated the prevalence of reduced aided acuity in patients aged 10 years and younger (mean age 3.97 years) with 5 D or more of isometropic hyperopia and considered the following factors that may influence visual acuity: (1) age at first correction; (2) magnitude of hyperopia; and (3) duration of refractive correction of the hyperopia. The results indicate that the majority of patients (87%) have aided acuity poorer than 6/6 at initial correction of refractive error. However, if the full hyperopic correction was worn for 1 year or longer, only 43% of these patients demonstrated acuity poorer than 6/6 and none showed acuity poorer than 6/12. The magnitude of the hyperopia appeared to have the greatest influence on the visual acuity outcome both at initial correction of refractive error and 1 year or longer after correction. Duration of correction also influenced the visual acuity outcome, but to a lesser extent than the magnitude of refractive error. In contrast, the age of first correction showed little correlation with visual acuity either at the time of first refractive correction or after a minimum of 1 year of correction.

Age Factors↗
PubMed2002

Induced optical aberrations following formation of a laser in situ keratomileusis flap.

PURPOSE: To determine how refractive error, visual acuity, and high-order aberrations (3rd- and 4th-order) are affected by the formation of a lamellar corneal flap during laser in situ keratomileusis (LASIK). SETTING: University refractive surgery center. METHODS: The effect of lamellar corneal flap formation was analyzed in 15 myopic eyes (mean preoperative refraction -4.72 diopters [D] [range -1.25 to -7.25 D]). The flap was created using a 2-step procedure: (1). a nasally hinged lamellar corneal flap was created; (2). the flap was lifted and stromal ablation performed 2 months after the flap was made. A Hartmann-Shack aberrometer was used to measure the aberrations. RESULTS: There was no significant change in the refractive error (spherical equivalent pre-flap -4.72 +/- 1.99 D and post-flap -4.62 +/- 1.99 D [P =.28]) or visual acuity (pre-flap uncorrected visual acuity [UCVA] 0.07 and best corrected visual acuity [BCVA] 0.96; post-flap UCVA 0.08 and BCVA 0.95 [P =.16 and P =.33, respectively]). A statistically significant increase in total higher-order wavefront aberrations was observed following flap formation (root mean square pre-flap 0.344 +/- 0.125 and post-flap 0.440 +/- 0.221 [P =.04]). CONCLUSION: Flap formation during LASIK can modify the eye's existing natural higher-order aberrations (especially spherical and coma-like aberrations along the axis of the flap's hinge), while visual acuity and refractive error remain unaffected.

Astigmatism↗
PubMed1979

Clinical evaluation of the DBR A-scan unit.

One hundred and eight eyes of 106 patients were evaluated preoperatively with the DBR-300 A-scan ultrasonic instrument. Intraocular lens power was then calculated using the formula supplied with the instrument, and the accuracy of the instrument was evaluated. It was found that 61.1% of the eyes were within +/- 1.00 D of the calculated refractive error and 81.5% of the eyes were within +/- 1.50 D of the calculated refractive error when using a standard 3.5-mm anterior chamber depth. Various factors were then analyzed to determine which factors contributed to the inaccuracy of the instrument. No single factor could be found to account for the slight tendency toward myopia, although several were found to contribute. No single factor was found to contribute to large errors.

Adult↗
PubMed1987

Vision for coloured pictures at different acuities: the Sonksen picture guide to visual function.

The vision 70 normal-sighted children was tested for both Snellen letters and life-size colour pictures of everyday objects. All the children wore spectacles for the tests: 60 were fitted with lenses which would induce a refractive error, the remainder had glassless frames only. Compared with acuity measures based on the Snellen letters, the pictures needed to be brought much closer than might be expected before the children with induced minor visual errors could recognise them. This highlights the importance of detecting and correcting refractive errors in preschool children who learn from such material, and suggests that the pictures may be a useful tool for detecting children with such errors. 18 pictures, comprising the Sonksen Picture Guide to Visual Function, have now been selected for this purpose.

Child↗
PubMed2001

Refraction and Anterior Chamber Depth Change After Vitrectomy for Pseudophakia.

Purpose: To evaluate the association between the vitreous and the refractive error in pseudophakia.Methods: Vitrectomy was performed in 67 eyes of 61 patients who underwent cataract surgery. Vitrectomy was needed for epiretinal membrane in 30 eyes, macular edema in 22 eyes, macular hole in 3 eyes, lamelar macular hole in 2 eyes, macular hole in 3 eyes, lamelar macular hole in 2 eyes, vitreous opacity in 6 eyes, and vitreous hemorrhage in 4 eyes. Refraction was measured before the operation, and 1 month, 3 months, 6 months, and 12 months after operation. Anterior chamber depth was measured in 10 eyes before the operation, and 1 month, 3 months, and 6 months after operation. We evaluated the refractive error after vitrectomy in 49 eyes that had predicted refraction.Results: The mean value of refractive change was -0.3 D 1 month postoperatively, and then gradually became positive. The variation of refraction (6-month postoperative refraction minus preoperative refraction) was negatively correlated with preoperative refraction (P =.0052, R(2) = 0.146). If preoperative refraction was more myopic than -1.5 D, then refraction became positive. If preoperative refraction was not more myopic than -1.5 D, then refraction became negative.Conclusion: About 15% of postoperative refractive error may be associated with the vitreous, but further investigation is required.

Journal Article↗
PubMed1990

Dopamine and myopia.

Dopamine may play a role in form-deprivation myopia in chickens. Anomalies of the dopaminergic system are believed to be responsible for manifestations of symptoms in Tourette Syndrome (TS), a neuropsychiatric disorder. Visual anomalies have been identified in TS. In this paper, refractive error in people suffering from TS is compared with available population studies. Refractive errors in TS are found to be similar to those in the normal population.

Adult↗
PubMed1997

Ocular components measured by keratometry, phakometry, and ultrasonography in emmetropic and myopic optometry students.

Ocular components were measured by keratometry, phakometry, and ultrasonography on 176 young adults. Refractive error was measured by retinoscopy. Mean vitreous depth was greater and the cornea was steeper in myopes than in emmetropes. There were no significant differences between myopes and emmetropes in mean anterior chamber depth, crystalline lens thickness, anterior crystalline lens radius, posterior crystalline lens radius, or crystalline lens power. Coefficients of correlation were calculated for each of the ocular components with refractive error for the entire group of 176 subjects. The components that showed statistically significant correlations with refractive error were vitreous depth and anterior corneal radius. Eyes with greater vitreous depths tended to have flatter anterior corneal surfaces. The slope of the principal axis relating these two variables was significantly different in emmetropes and myopes. Eyes with greater vitreous depths tended to have lesser crystalline lens power. Vitreous depth showed a statistically significant correlation with posterior lens radius, but not with anterior lens radius.

Adolescent↗