Morphological changes in amblyopic eyes in choriocapillaris and Sattler's layer in comparison to healthy eyes, and in retinal nerve fiber layer in comparison to fellow eyes through quantification of mean reflectivity: A pilot study

Purpose

Establishing the reliability of a new method to check the mean retinal and choroidal reflectivity and using it to find retinal and choroid changes in amblyopia.

Methods

Design: Retrospective case-control. Population: 28 subjects of which 10 were healthy controls (20 eyes): 8 with refractive errors, 1 with strabismus, and 1 with both. 18 patients with unilateral amblyopia included: 7 anisometropic, 6 isoametropic, 1 strabismic, and 4 combined. Mean participants’ age: 13.77 years ± 10.28. Observation procedures: SD-OCT and ImageJ. Main outcome measure: mean reflectivity of retinal and choroid layers. Amblyopic, fellow, and healthy eyes were compared.

Results

The method of measuring reflectivity is good to excellent reliability for all regions of interest except the fourth. The mean reflectivity of the choriocapillaris and Sattler’s layer in amblyopic eyes were significantly lower than in healthy eyes (p = 0.003 and p = 0.008 respectively). The RNFL reflectivity was lower than that of fellow eyes (p = 0.025). Post-hoc pairwise comparisons showed statistically significant differences between amblyopic and healthy eyes for choriocapillaris (p = 0.018) and Sattler’s (p = 0.035), and between amblyopic and fellow eyes for RNFL (p = 0.039).

Conclusion

A decrease in reflectivity of the choriocapillaris and Sattler’s in amblyopic compared to healthy eyes, and a decrease in reflectivity of the RNFL in the amblyopic compared to fellow eyes, indicate that the pathophysiology is partly peripheral and might be bilateral.

Effect of optical correction on subfoveal choroidal thickness in children with anisohypermetropic amblyopia

The purpose of this study was to determine the effect of optical correction on the best-corrected visual acuity (BCVA) and subfoveal choroidal thickness (CT) in the eyes of children with anisohypermetropic amblyopia. Twenty-four anisohypermetropic amblyopic eyes and their fellow eyes of 24 patients and twenty-three eyes of 23 age-matched control children were studied. After one year of optical correction, the BCVA in the anisohypermetropic amblyopic eyes was significantly improved. Before the treatment, the mean subfoveal CT in the amblyopic eyes was 351.9 ± 59.4 μm which was significantly thicker than that of control eyes at 302.4 ± 63.2 μm. After the treatment, the amount of change in the subfoveal CT in the amblyopic and fellow eyes was greater than that in the control eyes. The amblyopic and fellow eyes with thicker choroids had a greater thinning of the choroid whereas eyes with thinner choroids had a greater thickening of the choroid. We conclude that wearing corrective lenses improves the visual acuity, and induces changes of the subfoveal CT in eyes with anisohypermetropic amblyopia.

Association between Ocular Sensory Dominance and Refractive Error Asymmetry

Purpose

To investigate the association between ocular sensory dominance and interocular refractive error difference (IRED).

Methods

A total of 219 subjects were recruited. The refractive errors were determined by objective refraction with a fixation target located 6 meters away. 176 subjects were myopic, with 83 being anisometropic (IRED ≥ 0.75 D). 43 subjects were hyperopic, with 22 being anisometropic. Sensory dominance was measured with a continuous flashing technique with the tested eye viewing a Gabor increasing in contrast and the fellow eye viewing a Mondrian noise decreasing in contrast. The log ratio of Mondrian to Gabor’s contrasts was recorded when a subject just detected the tilting direction of the Gabor during each trial. T-test was used to compare the 50 values collected from each eye, and the t-value was used as a subject’s ocular dominance index (ODI) to quantify the degree of ocular dominance. A subject with ODI ≥ 2 (p < 0.05) had clear dominance and the eye with larger mean ratio was the dominant one. Otherwise, a subject had an unclear dominance.

Results

The anisometropic subjects had stronger ocular dominance in comparison to non-anisometropic subjects (rank-sum test, p < 0.01 for both myopic and hyperopic subjects). In anisometropic subjects with clear dominance, the amplitude of the anisometropia was correlated with ODI values (R = 0.42, p < 0.01 in myopic anisometropic subjects; R = 0.62, p < 0.01 in hyperopic anisometropic subjects). Moreover, the dominant eyes were more myopic in myopic anisometropic subjects (sign-test, p < 0.05) and less hyperopic in hyperopic anisometropic subjects (sign-test, p < 0.05).

Conclusion

The degree of ocular sensory dominance is associated with interocular refractive error difference.

[Macular thickness in unilateral amblyopia as measured by optical coherence tomography: a meta analysis]

OBJECTIVE

To offer a clue for the etiology of amblyopia by systematically comparing the macular thickness measured by optical coherence tomography (OCT) between the amblyopic eye and the fellow normal eye in patients with unilateral amblyopia.

METHODS

Databases including Wanfang, PubMed and others were searched, and data were extracted after quality assessment, then the Meta analysis was performed by the RevMan 5.02 software.

RESULTS

Four studies using the time-domain OCT (TD-OCT) were enrolled in the analysis and all those were anisometropic amblyopic patients. The only one study showed that the foveal minimum thickness is thicker in the amblyopic eyes than the fellow ones (P = 0.045). The meta analysis indicated the amblyopic eyes were 6.82 µm (P < 0.05, 95% CI: 2.81-10.83) thicker in the fovea (1 mm diameter region ). Five trials using the spectral-domain OCT (SD-OCT) were included in the analysis. It showed no significant difference in the foveal minimum thickness between amblyopic and fellow eyes. Also, no significant difference were detected in either anisometropic or strabismic subgroup (P = 0.50). Amblyopic eyes were 7.38 µm thicker in the fovea (1 mm diameter region) (P < 0.05, 95% CI: 3.13 -11.63) and significant difference in the anisometropic subgroup while no significant difference in the strabismic subgroup were detected.

CONCLUSIONS

The macular fovea is thicker in the amblyopic eyes than the fellow ones by the use of OCT, so it is effective to measure the macular thickness of amblyopia in searching evidence for the etiology of amblyopia.

Effects of local myopic defocus on refractive development in monkeys

PURPOSE

Visual signals that produce myopia are mediated by local, regionally selective mechanisms. However, little is known about spatial integration for signals that slow eye growth. The purpose of this study was to determine whether the effects of myopic defocus are integrated in a local manner in primates.

METHODS

Beginning at 24 ± 2 days of age, seven rhesus monkeys were reared with monocular spectacles that produced 3 diopters (D) of relative myopic defocus in the nasal visual field of the treated eye but allowed unrestricted vision in the temporal field (NF monkeys). Seven monkeys were reared with monocular +3 D lenses that produced relative myopic defocus across the entire field of view (FF monkeys). Comparison data from previous studies were available for 11 control monkeys, 8 monkeys that experienced 3 D of hyperopic defocus in the nasal field, and 6 monkeys exposed to 3 D of hyperopic defocus across the entire field. Refractive development, corneal power, and axial dimensions were assessed at 2- to 4-week intervals using retinoscopy, keratometry, and ultrasonography, respectively. Eye shape was assessed using magnetic resonance imaging.

RESULTS

In response to full-field myopic defocus, the FF monkeys developed compensating hyperopic anisometropia, the degree of which was relatively constant across the horizontal meridian. In contrast, the NF monkeys exhibited compensating hyperopic changes in refractive error that were greatest in the nasal visual field. The changes in the pattern of peripheral refractions in the NF monkeys reflected interocular differences in vitreous chamber shape.

CONCLUSIONS

As with form deprivation and hyperopic defocus, the effects of myopic defocus are mediated by mechanisms that integrate visual signals in a local, regionally selective manner in primates. These results are in agreement with the hypothesis that peripheral vision can influence eye shape and potentially central refractive error in a manner that is independent of central visual experience.

The ocular biometric and corneal topographic characteristics of high-anisometropic adults in Taiwan

BACKGROUND

To investigate the difference of ocular biometric and corneal topographic characteristics between the two eyes in high anisometropes with difference of 4 D or more in spherical component.

METHODS

Fifty-one young anisometropic men were collected. Detailed ocular examinations, including cycloplegic autorefraction, best-corrected visual acuity, intraocular pressure, A-scan, and Orbscan topography were done and recorded. The comparisons between two eyes were performed and the correlations between different ocular parameters were evaluated.

RESULTS

The mean axial length in the more myopic/less hyperopic eye was longer than that in the less myopic/more hyperopic eye [difference 1.8 mm, 95% confidence interval (CI) 1.6-2.0 mm, p < 0.001]. The mean thinnest corneal thickness in the more myopic/less hyperopic eye was an average of 4.0 μm thicker than that in the other eye (95% CI 1.2-6.8 μm, p = 0.007). The mean anterior chamber depth in the more myopic/less hyperopic eye was an average of 0.05 mm (95% CI 0.02-0.07 mm, p < 0.001) more than that in the other eye. The curvature and size of cornea were not significantly different.

CONCLUSION

The anterior chamber depth is deeper, axial length is longer, and thinnest corneal thickness is thicker in the more myopic/less hyperopic eye of high-anisometropic patients. Anisometropic eyes provide the chance to understand the biometric changes of eyeball with different refractive statuses in the same person. Such information is helpful for us to calculate the intraocular lenses power in cataract surgery and to do the surgical planning for corneal refractive surgery in eyes of different refractive power.

[Comparison of deficits in visual cortex between anisometropic and strabismic amblyopia by fMRI retinotopic mapping]

OBJECTIVE

To study the neural mechanism of visual cortical deficits between anisometropic and strabismic amblyopia comparatively by BOLD-fMRI retinotopic mapping.

METHODS

Ten anisometropic amblyopes, 10 strabismic amblyopes and 9 normal subjects underwent fMRI with retinotopic mapping and luminous spots stimuli (spatial frequency: 6 cpd, contrast: 0.5). 1.5T MRI system was used to obtain functional images of visual cortex. Responses in primary and secondary visual cortex were compared among the dominant (normal subject group), anisometropic and strabismic amblyopic eyes by one-way ANOVA, successively analyzed by paired-samples t test between amblyopic eyes and fellow fixing eyes (anisometropic and strabismic amblyopia group respectively). Their fMRI deficits of amblyopes were analyzed regressively in two amblyopia groups respectively.

RESULTS

The result of one-way ANOVA showed significantly a lower activation (average T value) in V1, V2, V3, Vp and V7 visual areas (P < 0.05, P values 0.018, 0.007, 0.002, 0.000, 0.025 respectively) between anisometropic amblyopia and normal group. This was in accordance with the result of paired-samples t test between amblyopic eyes and fellow fixing eyes in anisometropic amblyopia group (P < 0.05, P values 0.035, 0.007, 0.020, 0.009, 0.023 respectively). Statistical difference was found in V1, V2 and Vp areas between strabismic amblyopia and normal group (P < 0.05, P values 0.010, 0.007 & 0.003 respectively). The paired-samples t test in strabismic amblyopia group showed statistical difference only in V2, Vp areas (P < 0.05, P values 0.026 and 0. 009 respectively. ). So the two results were discordant. Between the two amblyopic groups, there was no statistical difference (P > 0.05) except in V7 area (P < 0.05, P value = 0.048). There was no causal relation between the primary visual cortical deficits and the secondary cortex in amblyopia (P > 0.05).

CONCLUSION

Anisometropic amblyopia and strabismic amblyopia both have functional deficits in the primary and secondary visual cortex. The neural mechanism of secondary visual cortical deficits may be more complex than decreased cortex activation induced by the deficit of primary cortex. In the primary cortex, strabismic amblyopia and anisometropic amblyopia have neuronal deficits and/or abnormal interaction. In addition, strabismic amblyopia may also have suppressive influences of the fixing eyes upon the amblyopic eyes. Anisometropic amblyopia has the neural undersampling at a high spatial frequency in the secondary visual cortex as compared to amblyopic amblyopia.

Corneal Hysteresis, Resistance Factor, Topography, and Pachymetry After Corneal Lamellar Flap

PURPOSE

To measure prospectively the early changes in corneal hysteresis, topography, and pachymetry after the creation of a stromal flap cut without laser photoablation.

METHODS

A 37-year-old man was referred for a bioptic procedure to correct for compound myopic astigmatism in the left eye. A 159-microm-thick 8x8.5-mm superior hinged flap was created with a mechanical microkeratome in the left cornea. Changes in the corneal hysteresis, corneal resistance factor, Goldmann correlated intraocular pressure (lOP), corneal compensated IOP, anterior and posterior topography, and optical and ultrasound pachymetry were monitored prospectively before and at 1 hour, 1 day, 5 days, and 25 days after flap creation. The right eye served as a control.

RESULTS

In the left eye, corneal hysteresis and corneal resistance factor decreased immediately after the flap cut and remained lower than preoperatively at 1 hour, 1 day, 5 days, and 25 days. Corneal compensated IOP varied significantly less than Goldmann correlated IOP in both eyes. Central flattening of the horizontal meridians was observed on the difference topography maps. The values of the left eye posterior best fit sphere increased after the flap cut. Increased central corneal thickness occurred immediately after the flap cut and decreased over time without returning to its preoperative value.

CONCLUSIONS

The creation of a stromal flap can modify the biomechanical properties of the cornea, including a reduction in corneal hysteresis. The topographic changes were consistent with previously reported cases of flap cut in normal corneas.

Prevalence and associations of anisometropia and aniso-astigmatism in a population based sample of 6 year old children

AIM

To study the distribution of anisometropia and aniso-astigmatism in young Australian children, together with clinical and ocular biometry relations.

METHOD

The Sydney Myopia Study examined 1765 predominantly 6 year old children from 34 randomly selected Sydney schools during 2003-4. Keratometry, cycloplegic autorefraction, and questionnaire data were collected.

RESULTS

Spherical equivalent (SE) anisometropia (> or =1 dioptre) prevalence was 1.6% (95% confidence interval (CI) 1.1% to 2.4%). Aniso-astigmatism (>or =1D) prevalence was 1.0% (CI: 0.6% to 1.6%). Both conditions were significantly more prevalent among moderately hyperopic (SE > or =2.0D) than mildly hyperopic (SE 0.5-1.9D) children. Myopic children (SE < or =-0.5D) had higher anisometropia prevalence. Neither condition varied by age, sex, or ethnicity. In multivariate analyses, anisometropia was significantly associated with amblyopia, odds ratio (OR) 29, (CI: 8.7 to 99), exotropia (OR 7.7, CI: 1.2 to 50), and neonatal intensive care unit (NICU) admission (OR 3.6, CI: 1.1 to 12.6). Aniso-astigmatism was significantly associated with amblyopia (OR 8.2, CI: 1.4 to 47), maternal age >35 years (OR 4.0, CI: 1.3 to 11.9), and NICU admission (OR 4.6, CI: 1.2 to 17.2). Anisometropia resulted from relatively large interocular differences in axial length (p<0.0001) and anterior chamber depth (p = 0.0009). Aniso-astigmatism resulted from differences in corneal astigmatism (p<0.0001).

CONCLUSION

In this predominantly 6 year old population, anisometropia and aniso-astigmatism were uncommon, had important birth and biometry associations, and were strongly related to amblyopia and strabismus.

[Refractive surgery--possibilities to maximize postoperative visual acuity]

Laser refractive surgery has an ascendant evolution on the context of the progress in the medical field. The functional result can be negatively influenced by residual errors, irregular astigmatism or anisometropia, situations in which rigid gas permeable (RGP) contact lenses are a successful alternative in optimizing visual acuity. The paper presents the particularities and the protocol of RGP fitting, outlining the contribution of the corneal topography in the ocular pre and post-operative evaluation.

Voxel-based analysis of MRI detects abnormal visual cortex in children and adults with amblyopia

Amblyopia, sometimes called "lazy eye," is a relatively common developmental visual disorder well characterized behaviorally; however, the neural substrates associated with amblyopia in humans remain unclear. We hypothesized that abnormalities in the cerebral cortex of subjects with amblyopia exist, possibly as a result of experience-dependent neuronal plasticity. Anatomic magnetic resonance imaging (MRI) and psychophysical vision testing was carried out on 74 subjects divided into two age ranges, 7-12 years and 18-35 years, and three diagnoses, strabismic amblyopia, anisometropic amblyopia, and normal vision. We report a behavioral impairment in contrast sensitivity for subjects with amblyopia, consistent with previous reports. When the high-resolution MRI brain images were analyzed quantitatively with optimized voxel-based morphometry, results indicated that adults and children with amblyopia have decreased gray matter volume in visual cortical regions, including the calcarine sulcus, known to contain primary visual cortex. This finding was confirmed with a separate region-of-interest analysis. For the children with amblyopia, additional gray matter reductions in parietal-occipital areas and ventral temporal cortex were detected, consistent with recent reports that amblyopia can result in spatial location and object processing deficits. These data are the first to provide possible neuroanatomic bases for the loss of binocularity and visual sensitivity in children and adults with amblyopia.

Thicknesses of macular retinal layer and peripapillary retinal nerve fiber layer in patients with hyperopic anisometropic amblyopia

This prospective study was performed to measure the macular and the peripapillary retinal nerve fiber layer (RNFL) thicknesses using optical coherence tomography (OCT) in patients with anisometropic amblyopia. Thirty-one patients with hyperopic anisometropic amblyopia were included. The macular retinal thickness and the peripapillary RNFL thickness were measured using OCT. The mean refractive error was +3.71 diopters (D) and +1.00 D, the mean macular retinal thickness was 252.5 microm and 249.7 microm, and the mean RNFL thickness was 115.2 microm and 109.6 microm, inthe amblyopic eye and the normal eye, respectively. OCT assessment of RNFL thickness revealed a significantly thicker RNFL in hyperopic anisometropic amblyopia (P=0.019), but no statistically significant difference was found in macular retinal thickness (P>0.05). In conclusion, the amblyopic process may involve the peripapillary RNFL, but not the macula. However, further evaluation is needed.

Ocular dominance column width and contrast sensitivity in monkeys reared with strabismus or anisometropia

PURPOSE

To study the relationship between the width of ocular dominance columns in primary visual cortex and spatial contrast sensitivity functions in monkeys with strabismus or anisometropia during infancy.

METHODS

Adult monkeys having had monocular visual abnormalities induced in infancy were tested behaviorally for spatial contrast sensitivity and then subjected to functional enucleation of one eye to reveal the ocular dominance columns (ODCs) of the primary visual cortex by cytochrome oxidase (CO) staining. The relative widths of the left and right eyes' ODCs were measured and related to the contrast sensitivity functions.

RESULTS

The relative widths of the ODCs having input from eyes with strabismic or anisometropic amblyopia were reduced in proportion to the age of onset and the duration of the early visual abnormality. The relative losses in contrast sensitivity were in ordinal agreement with the losses in relative width of the ODCs.

CONCLUSIONS

Amblyopia induced by the early monocular abnormalities of strabismus or anisometropia is proportional to the loss in cortical afference as reflected in the reduction in width of the respective ODCs in the primary visual cortex.

[Higher-order aberrations in myopic and astigmatism eyes]

OBJECTIVE

To analyze the distribution of the higher order aberrations in the population of Chinese with myopia and astigmatism and determine whether the pupil size, degree of myopia and astigmatism affects the higher order aberrations.

METHODS

Using a Tscherning method, 147 myopic and astigmatism eyes were evaluated before LASIK. The root mean square (RMS) values of higher order aberrations for each subject with pupil diameters of 4.5, 5.0, 5.5, 6.0, 6.5 and 7.0 mm were calculated. The subjects were divided into following groups: high myopia (> -6.00 D) and low myopia (<or= -6.00 D); astigmatism group I (>or= 0.75 D) and astigmatism group II (< 0.75 D).

RESULTS

The root mean square (RMS) values was decreased from 3rd order to sixth order aberrations in the same eye. The individual difference was from 6.1-fold to 36.6-fold in same type of aberration. All types of aberrations, in particular spherical and coma aberration, was increased significantly with increasing pupil size. Secondary spherical aberration, spherical and secondary coma aberration were affected by myopia, but coma aberration; astigmatism only affected the coma aberration, but spherical aberration.

CONCLUSIONS

The aberrations were increased with the increasing of refractive error such as myopia and astigmatism. Spherical aberration is more common in high myopia, Coma aberrations is related with astigmatism.

Photorefractive keratectomy for anisometropic amblyopia in children

PURPOSE

To assess the safety and efficacy of photorefractive keratectomy (PRK) in children with anisometropic amblyopia and to define the characteristics of children who may be candidates for PRK.

METHODS

This thesis comprises four parts: (1) a retrospective analysis of risk factors predictive of amblyopia treatment failure in 104 children, (2) a prospective study of pachymetry in 198 eyes of 108 children, (3) development and implementation of a protocol to perform PRK under general anesthesia, and (4) a prospective interventional case-comparison study of PRK in 11 noncompliant children with anisometropic amblyopia to evaluate safety and long-term outcomes. Compliant and noncompliant children with anisometropic amblyopia were analyzed as controls.

RESULTS

Factors associated with conventional anisometropic amblyopia treatment failure were poor compliance (P = .004), age 6 years or older (P = .01), astigmatism > or = 1.5 diopters (P = .0002), and initial visual acuity of 20/200 or worse (P = .02). Central and paracentral pachymetry measurements were similar to published adult values. The general anesthesia protocol was efficient, and the laser functioned properly in all cases. All children did well with no anesthesia-related or treatment-related complications. Two years following PRK, the mean reduction in refractive error was 9.7 +/- 2.6 diopters for myopes (P = .0001) and 3.4 +/- 1.3 diopters for hyperopes (P = .001). The cycloplegic refractive error in 9 of 11 treated eyes was within 3 diopters of that in the fellow eye. Uncorrected visual acuity in the amblyopic eye improved by > or = 2 lines in seven of nine children; best-corrected visual acuity improved by > or = 2 lines in six of nine children. Stereopsis improved in five of nine children. The mean visual acuity of the PRK patients at last follow-up was significantly better than that of noncompliant controls (P = .003). The safety and efficacy indices for PRK in this study were 1.24 and 1.12, respectively.

CONCLUSIONS

Photorefractive keratectomy can be safely performed in children with anisometropic amblyopia. Visual acuity and stereopsis improved in most eyes, even in older children. Photorefractive keratectomy may have an important role in the management of anisometropic amblyopia in noncompliant children.

Stiles-Crawford effect of the first kind (SCE-I) in post-photorefractive keratectomy and anisometropic subjects

The Stiles-Crawford effect of the first kind (SCE-I) was measured on both post-photorefractive keratectomy (PRK) and anisometropic subjects at six different locations of retina. The eye that underwent the PRK procedure showed results that were typical of myopic eyes, although the vision in that eye has been maintained at 6/5 (or 20/15) since the operation. The anisometropic subject had one eye that was emmetropic and the other eye was myopic with a refractive error of -3.00 DS. The emmetropic eye showed the normal well-centered SCE-I functions across the retina, whereas the myopic eye of the same subject showed the nasal tilting of receptors in the nasal retina, which has been the typical finding among myopic subjects.

The effect of axial length on ocular blood flow assessment in anisometropes

BACKGROUND

Pulsatile ocular blood flow (POBF) assessment measures the choroidal circulation and therefore provides data with diagnostic value in certain ocular diseases, such as glaucoma. The technique assumes a constant pressure-volume relationship. The current study investigated the effect of axial length on POBF from subjects with axial anisometropia. Ocular blood supply in the ophthalmic artery was also determined using colour Doppler ultrasonography.

METHODS

Thirty-one normal, anisometropic subjects were recruited, whose ages ranged from 20 to 34 years. They had axial anisometropia (expressed by spherical equivalent) of at least 2 D. After Goldmann tonometry, the POBF of each eye was measured in a supine posture by one examiner, followed by a measurement of the blood flow velocity in the ophthalmic artery using colour Doppler ultrasonography in the same posture by another examiner. There was a 10 min rest between the two techniques. All the measurements were made at around the same time to eliminate any effect from diurnal variation.

RESULTS

The mean anisometropia (expressed by spherical equivalent) was 3.89 +/- 1.96 D and the mean inter-ocular axial length difference was 1.49 +/- 1.00 mm. The anisometropia and axial length were significantly different between the two eyes (paired t-tests: p < 0.001). However, the intra-ocular pressure was similar between the two eyes (paired t-test: p = 0.41). The POBF was significantly lower in the eye with the longer axial length (459.3 microL min-1) than the fellow eye (590.8 microL min-1), paired t-test: p < 0.001. The pulse amplitude was also significantly lower in the eye with the longer axial length (1.61 mmHg) than the fellow eye (1.89 mmHg), paired t-test: p < 0.001. However, the blood flow velocity from colour Doppler ultrasonography did not demonstrate any significant difference between the two eyes (paired t-test: p > 0.05).

CONCLUSIONS

The POBF and pulse amplitude were found to be reduced in the eye with the longer axial length but colour Doppler ultrasonography did not show any significant difference. This suggests that a new pressure-volume relation should be considered in deriving POBF. Practitioners should measure the axial length in POBF assessment.

Thickness of the retinal nerve fiber layer in patients with anisometropic and strabismic amblyopia

PURPOSE

To compare the retinal nerve fiber layer [RNFL] thickness of the amblyopic eye with that of the normal eye in patients with anisometropic and strabismic amblyopia.

METHODS

Scanning laser polarimetry was performed with the GDx Nerve Fiber Analyzer (NFA), a scanning laser ophthalmoscope that measures the thickness of the peripapillary RNFL quantitatively and objectively, in vivo, based on the birefringence of the RNFL. Eighteen anisometropic, two strabismic and four combined amblyopic (both anisometropia and strabismus) patients were enrolled in the study. There were 9 male and 15 female patients, age range 7-66 years. Using Student's t-test, 14 GDx parameters in the amblyopic eyes and sound eyes were compared.

RESULTS

The GDx parameters showed no significant differences between the two groups [p > 0.5]. The superior average and inferior average values were 79.50 microm and 80.41 microm, respectively, in the amblyopic group, and 80.75 microm and 82.75 microm, respectively, in the healthy eyes. The average thickness did not differ between amblyopic and sound eyes (65.50 microm and 66.45 microm, respectively).

CONCLUSION

Assessment of RNFL thickness by means of scanning laser polarimetry revealed no difference between the two eyes in patients with unilateral amblyopia.

Binocularity and spatial frequency dependence of calcarine activation in two types of amblyopia

OBJECTIVE AND BACKGROUND

Strabismus and anisometropia early in life frequently causes monocular amblyopia. Activation of the visual cortex is compared between the two types of amblyopia to elucidate differences in the pathogenetic mechanism of the disease.

METHODS

Using an EPI gradient echo sequence in 1.5T MRI, calcarine activation by monocular viewing of checkerboard patterns with reversal was examined in terms of binocularity of the activation and dependence on the spatial frequency of the stimuli.

RESULTS

First, the proportion of voxels activated by both normal and amblyopic eye monocular stimulations is lower in the strabismic group than in the anisometropic group. Second, the activation by higher-spatial-frequency stimuli is reduced in the anisometropic group, but not in the strabismic group.

CONCLUSIONS

These findings from the human visual cortex are consistent with the view proposed based on animal research that the loss of binocular interaction and the undersampling of high-spatial-frequency components of visual stimuli are each one of the underlying changes in strabismic and anisometropic amblyopia, respectively.

Photorefractive Keratectomy for Visual Rehabilitation of Anisometropia Induced by Retinal Detachment Surgery

PURPOSE

To evaluate the efficacy of unilateral photorefractive keratectomy to correct anisometropia induced by retinal detachment surgery.

METHODS

Photorefractive keratectomy was performed in 10 eyes of 10 patients with anisometropia induced by previous retinal detachment surgery. The Aesculap Meditec MEL 60 excimer laser was used.

RESULTS

Preoperative mean spherical equivalent refraction was -5.20 D. Mean postoperative spherical equivalent refraction was -0.25 D after a mean follow-up of 12.9 months. Mean preoperative spherical equivalent refraction difference between two eyes of 4.87 D was decreased to a mean 0.60 D postoperatively (t-test, P < .0001). All patients were free of anisometropic symptoms after laser surgery.

CONCLUSION

Unilateral photorefractive keratectomy seems to be an effective method to correct anisometropia induced by conventional retinal detachment surgery, especially for patients with spectacle and contact lens intolerance.

PRK-induced anisometropia in the rabbit as a model of myopia

BACKGROUND

Current animal models of myopia, such as the chick and the tree shrew, have characteristics that limit their applicability to human myopia and/or their use among researchers. The purpose of this study was to establish a rabbit model of myopia based on photorefractive keratectomy (PRK)-induced anisometropia.

METHODS

A group of five pigmented rabbits was treated with a monocular -5 D PRK at 5 weeks of age. At 10 weeks of age, two of the eyes were retreated with a second -5 D PRK procedure to compensate for partial regression of the refractive effect. A second group of six pigmented rabbits was treated with a monocular -6 D PRK at 10 weeks of age. Longitudinal measurements of corneal curvature, refraction, and axial length were performed until the rabbits were 13 and 21 weeks of age in groups 1 and 2, respectively. The rabbits in each group were from the same litter.

RESULTS

Keratometry and retinoscopy measurements confirmed the refractive effect of the PRK procedures. At the final measurement point in group 1, the PRK-treated eyes were significantly longer than the untreated eyes (16.01 +/- 0.45 mm vs 15.45 +/- 0.56 mm). In group 2, the PRK-treated eyes were significantly longer by 0.19 mm and 0.20 mm at ages 19 and 21 weeks, respectively.

CONCLUSIONS

PRK-induced anisometropia is an effective technique to induce hyperopic error compensation in the rabbit as a model of myopic development. The technique is effective if the PRK procedure is performed at either 5 or 10 weeks of age. However, after PRK at 5 weeks of age, partial retreatment may be necessary due to regression of the PRK effect.

Neuronal correlates of amblyopia in the visual cortex of macaque monkeys with experimental strabismus and anisometropia

Amblyopia is a developmental disorder of pattern vision. After surgical creation of esotropic strabismus in the first weeks of life or after wearing -10 diopter contact lenses in one eye to simulate anisometropia during the first months of life, macaques often develop amblyopia. We studied the response properties of visual cortex neurons in six amblyopic macaques; three monkeys were anisometropic, and three were strabismic. In all monkeys, cortical binocularity was reduced. In anisometropes, the amblyopic eye influenced a relatively small proportion of cortical neurons; in strabismics, the influence of the two eyes was more nearly equal. The severity of amblyopia was related to the relative strength of the input of the amblyopic eye to the cortex only for the more seriously affected amblyopes. Measurements of the spatial frequency tuning and contrast sensitivity of cortical neurons showed few differences between the eyes for the three less severe amblyopes (two strabismic and one anisometropic). In the three more severely affected animals (one strabismic and two anisometropic), the optimal spatial frequency and spatial resolution of cortical neurons driven by the amblyopic eye were substantially and significantly lower than for neurons driven by the nonamblyopic eye. There were no reliable differences in neuronal contrast sensitivity between the eyes. A sample of neurons recorded from cortex representing the peripheral visual field showed no interocular differences, suggesting that the effects of amblyopia were more pronounced in portions of the cortex subserving foveal vision. Qualitatively, abnormalities in both the eye dominance and spatial properties of visual cortex neurons were related on a case-by-case basis to the depth of amblyopia. Quantitative analysis suggests, however, that these abnormalities alone do not explain the full range of visual deficits in amblyopia. Studies of extrastriate cortical areas may uncover further abnormalities that explain these deficits.

Screening for anisometropia in preschool children

PURPOSE

A preschool vision screening program was reviewed to evaluate eccentric photoscreening (EP), visual acuity, and stereopsis in identifying anisometropia.

METHODS

Patients referred by the screening were examined to assess efficacy of the three screening techniques in a population of preschool children. Testability and comparison of screening results to the classification of anisometropia (> or = 1 D) by retinoscopy obtained during a complete examination were evaluated.

RESULTS

Although EP identified 94.5% of the anisometropic children as abnormal, only 27.8% were classified as anisometropic by EP. Of the anisometropic children, 36.1% failed acuity, but only 19.4% failed based on a 2 line or greater interocular acuity difference. Stereopsis correctly identified only 7.3% of anisometropes as abnormal.

CONCLUSIONS

The sensitivity of EP in identifying anisometropic children as abnormal was superior to acuity and stereopsis, yet its ability to identify anisometropia specifically was poor. Anisometropia of low magnitude or that masked by the dead zone of the EP system was frequently classified as isometropic. Altering the EP referral criterion and/or taking photographs through adequate power plus lenses may improve the sensitivity for specifically identifying anisometropia. However, caution must be exercised when using EP to examine the prevalence of anisometropia in a population or if used to screen for only amblyogenic refractive errors (i.e., anisometropia), because many anisometropes will be missed, resulting in inaccurate prevalence data and significant underreferrals.

Distribution and localization of NMDA receptor subunit 1 in the visual cortex of strabismic and anisometropic amblyopic cats

ACTIVATION of NMDA (N-methyl-D-aspartate) receptors has recently been proposed as a prerequisite for the induction of experience-dependent modification of visual cortical neurones seen during early postnatal development. A new monoclonal antibody to the NMDA receptor subunit 1 (NMDA-R1) has been used to localize and compare the distribution of the receptors in the primary visual cortex of normal cats and those raised with either amblyopia induced by monocular optical blur or monocular esotropic strabismus. Although all three groups showed densest labelling in layers II-III, a comparison of immunopositive cells at any depth below the cortical surface showed a significantly lower frequency in strabismic and anisometropic cats than in normal cats, but a greater frequency in anisometropic cats that in strabismic animals. There appears to be no direct relationship between the expression of NMDA-R1 receptors and the level of excitability, binocularity or neuronal acuity known to exist in either of these two cat models of amblyopia, thus raising further questions as to the precise nature of the role of NMDA receptors in the processes of visual cortical plasticity.

Traumatic cycloplegia and myopic anisometropia

The pathogenesis of myopia and the mechanism of atropine in preventing myopic progression have long been widely discussed. Recent studies with animals have pointed to the possible role of the muscarinic receptor of the retina itself in regulating eye growth. This paper stresses that, for myopia in humans, the importance of accommodation still holds. Twenty-five recovered cases of previous traumatic hyphema, occurring under age 16, were collected. For four or more years, the patients were examined for ocular refraction, axial length, intraocular pressure, gonioscopy and accommodation time; the latter was measured with an accommodo-polyrecorder. Data from injured eyes and fellow eyes were compared and analyzed. The results indicated that, in the injured eyes the refractive status was invariably less myopic than in the fellow eye, and was not related to intraocular pressure. In injured eyes the accommodation time was highly correlated with the extent of angle recess. And the difference of anisometropia was also related to the extent of angle recess. These observations showed that there was a significant correlation between the degree of myopic anisometropia and the impairment of accommodation.

Computation of retinal contour in anisomyopia

It is well documented that myopia is associated with an increase in axial length of the posterior vitreous chamber. Whether equatorial or transverse dimensions are likewise affected in myopia is relevant to further understanding of the development of ametropia. We have utilised a computing method to determine retinal contour from real eye measurements of keratometry, A-scan ultrasonography and peripheral refraction as a means of assessing the transverse dimensions of the vitreous chamber. This technique has been applied to a 21-year-old female Caucasian anisomyope with a refractive error of R -1.50/-0.50 x 130 and L -4.00/-0.50 x 160. Anisomyopia offers a special opportunity for inter-eye comparison of different degrees of myopia. The repeatability of the technique was assessed by taking 10 separate sets of the aforementioned measurements and thus generating 10 retinal contours for each eye. We conclude that this method is repeatable and is capable of demonstrating differences between anisomyopic eyes although validation against in vivo measurements is required.