Convergence accommodation to convergence CA/C ratio: convergence versus divergence.

AIM: To determine whether the convergence accommodation to convergence (CA/C) ratio during divergence with base-in (BI) prisms is of a similar or different magnitude to that measured during convergence with base-out (BO) prisms. METHODS: Eighteen participants with normal binocular single vision were recruited. The participants viewed a pseudo-Gaussian target, which consisted of a light emitting diode (LED) behind a diffusing screen at 40 cm. After 5 minutes of dark adaptation, the refractive status of the eye was measured without any prism using a Shin-Nippon SRW-5000 autorefractor. The participant held the selected prism (5Δ or 10Δ BO or BI, counterbalanced) in front of their right eye and obtained a single, fused image of the target while refractive measures were taken with each. A 30-second rest period was given between measurements. The mean age of the participants was 20.6±3.22 years. RESULTS: The mean CA/C ratios for the 5Δ BO, 10Δ BO, 5Δ BI, and 10Δ BI were 0.108 (±0.074) D/Δ, 0.110 (±0.056) D/Δ, 0.100 (±0.090) D/Δ, and 0.089 (±0.055) D/Δ, respectively. A 2-factor repeated measures ANOVA found that the CA/C ratio did not significantly change with differing levels of prism-induced convergence and divergence (p=0.649). CONCLUSIONS: Change in accommodation induced by manipulating vergence is similar whether convergence or divergence are induced. The CA/C ratio did not show any change with differing levels of prism-induced convergence and divergence.

Non-linearity of the response accommodative convergence to accommodation ratio.

PURPOSE: Previous studies have reported variation in stimulus accommodative convergence to accommodation (AC/A) ratio across differing accommodative stimuli. Response AC/A ratio was assessed across 4 accommodative demands to determine if these differences could be due to accommodative inaccuracies to stimuli. METHODS: Twenty-three student participants aged 18 to 26 years (mean age 20.3 ± 1.7 years) successfully completed all testing conditions. The modified Thorington technique was used at 4 m to measure heterophoria. The Shin Nippon SRW 5000 infrared autorefractor was used to determine accommodative change to -1.50, -3.00, -4.50, and -6.00D lens stimuli. RESULTS: Significant differences were found in response AC/A ratio between different minus lens stimulated accommodative demands (p < 0.001). Mean AC/A ratio values were 3.11 ± 1.29 with the -1.50D lens stimulus; 4.03 ± 2.11 with -3.00D; 4.14 ± 1.40 with -4.50D; and 4.48 ± 1.56 with -6.00D. No differences in linearity were noted between myopes and non-myopic participants, but myopes tended to have higher response AC/A ratios than non-myopes, mean 4.88 ± 1.89 for myopes vs 3.61 ± 1.47 for non-myopes (p = 0.045). CONCLUSIONS: Response AC/A ratio did not display linearity across 4 minus lens accommodative stimuli, but tended to increase with accommodative demand. Significant variability in response AC/A ratio was found, both within individuals to different accommodative demands, and between individuals across the data set.

Depth of suppression in anisometropic amblyopia (with or without microtropia).

BACKGROUND AND PURPOSE: There are conflicting reports concerning the relationship between depth of suppression and level of amblyopia in strabismics. Little attention has been given to anisometropes. This study examines the density of suppression in anisometropic amblyopes, with or without microtropia, and investigates whether there is a relationship with level of amblyopia. METHODS: Patients with anisometropia (defined as a difference of 1D or 0.5 D cyl), binocular single vision and a difference in corrected visual acuity of at least 0.1 logMAR between eyes were recalled. The degree of amblyopia was expressed as the interocular difference using the Bailey-Lovie logMAR chart. Stereoacuity (Titmus test), binocular alignment and fixation were recorded. The depth of suppression was measured using the neutral density filter bar together with the Worth four dot test at 4.5m (subtending an angle of 0.5 degrees). Best spherical equivalent (BSE) was calculated to represent anisometropia. RESULTS: Thirteen participants aged 8.3 years to 12.1 years (mean 9.7 years) completed the study. No significant correlation was present (r=0.10, p=0.74) between the depth of suppression and degree of amblyopia. However, there was a correlation between depth of suppression and level of stereoacuity (r=0.59, p=0.03). Six participants had microtropia and showed stronger suppression (p=0.03) and worse stereoacuity (p=0.001) than the pure anisometropes. CONCLUSIONS: No evidence was found of a relationship between density of suppression and amblyopia in this cohort of anisometropic amblyopes.

The response AC/A ratio: differences between inducing and relaxing accommodation at different distances of fixation.

PURPOSE: To determine if there is a difference between the response AC/A ratios when measured using the gradient method at near and distance fixation with plus and minus lenses respectively in young adults with normal binocular single vision. METHODS: A repeated measures design was used. The accommodative response of the right eye was measured objectively using the Shin-Nippon SRW-5000 autorefractor (Grand Seiko Company, Fukuyama, Japan) (open view) at 33 cm with and without plus lenses (2DS or 3DS) and at 3.8 m with and without minus lenses (2DS or 3DS) dependent on the participants' ability to obtain subjectively "clear" vision. The angle of deviation was measured using the alternate prism cover test at 33 cm and 3.8m fixing with the right eye with the participant sat at the autorefractor. LogMAR 0.0 (6/6) was used for fixation. Response AC/A ratios were calculated. RESULTS: Twenty-five participants were examined; mean and standard deviation of their ages were 21.2 ± 4.04 years. The mean and standard deviation of the near response AC/A ratios was 4.73 ± 2.34/1 and at distance was 3.05 ± 1.71/1. Pearson's Product Moment Correlation Coefficient showed no correlation between the 2 sets of data. Paired t-test showed that there was a statistically significant difference between the near and distance response AC/A ratios (t = 3.30, p = 0.003). The difference was found to be greater in participants who were non-orthoptic students. CONCLUSION: The response AC/A ratio was found to be slightly higher at 33 cm with plus lenses than at 3.8 m with minus lenses. No reason was identified for this difference but adaptation and perceptual effects could be further explored.

The effect of testing distance on the bielschowsky head tilt test.

AIMS: The Bielschowsky Head Tilt Test (BHTT) is a commonly used test in Orthoptic practice and is used mainly to differentiate between a long standing Superior Oblique and Superior Rectus palsy. No consistent test distance is recommended in the literature and therefore the aim of this study was to determine whether test distance had an effect upon the measurements obtained. METHOD: Thirteen participants were recruited. Participants had either longstanding or recent onset unilateral Superior Oblique palsy. Prism Cover Test measurements of the vertical angle of deviation were taken whilst the participant tilted their head to either side whilst fixating on a target at 33 cm, 3 m and 6 m. RESULTS: The test distance does have a significant effect on the change in vertical angle measured with head tilted to either side (x2+7.747,DoF 2, p=0.021). When the median values are considered it appears that the significant effect occurs due to a smaller change in angle between head tilt to the affected side when fixing at 3 metres. This was confirmed using the Wilcoxon signed rank test (33cmv3m p=0.039, 3m v 6m p=0.013 and 33cm v 6m p=0.67). CONCLUSIONS: The testing distance at which the BHTT is performed appears to have an effect upon the measurements obtained. The clinical importance of the difference in the change of angle with head tilt to either side is debatable due to the fact that the difference in the median measurement value between the three test distances is a maximum of 3 prism dioptres.

Comparison of the visual effects of Fresnel prisms in normal and amblyopic eyes.

PURPOSE: To assess the effects of Fresnel prisms on visual acuity and peak contrast sensitivity in the amblyopic and sound eyes of participants with amblyopia and to determine whether these functions were affected by Fresnel prisms to a different degree than those of controls. METHODS: The LogMAR visual acuity and peak contrast sensitivity of 10 unilateral amblyopic participants (mean age, 22.6 years) and 9 controls (mean age, 26.2 years) were tested with Fresnel prisms of powers 5(Delta), 10(Delta), 15(Delta), 20(Delta), and 25(Delta) and without a Fresnel prism. RESULTS: A statistically significant reduction in visual acuity with increasing prism power was found for all 3 groups, with the visual acuity of the amblyopic eyes being the least affected by the prisms. No statistically significant differences were found between the control and the sound eyes. No statistically significant differences in the effects of the prisms on peak contrast sensitivity could be detected between the 3 groups. Fresnel prisms were found to have a smaller effect on those amblyopic eyes with a poorer baseline visual acuity, indicating that these eyes may tolerate strong prisms without substantially impairing their visual acuity. CONCLUSIONS: Fresnel prisms have a lesser effect on the visual acuity of amblyopic eyes than on controls. In contrast, results for peak contrast sensitivity were very similar for each of the groups tested, and no significant differences were evident between the amblyopic, sound, and control eyes.

Stereoacuity is affected by induced phoria but returns toward baseline during vergence adaptation.

PURPOSE: In normal binocular single vision adaptation to an induced deviation occurs over a period of several minutes. This study investigates the effect on stereoacuity during vergence adaptation. METHODS: Stereoacuity, using the Frisby near stereotest, was measured in 20 participants aged 20.2 +/- 1.9 years with normal binocular single vision. Measurements were taken immediately on inducing a phoria with 12(Delta) base-out prisms (split), and after 3, 6, and 9 minutes of adaptation. A measure of stereoacuity was also taken with the same size prisms that were used concomitantly to control for the reduced visual acuity. RESULTS: Stereoacuity was found to decrease following introduction of the 12(Delta) base-out prismatic glasses (9.4 +/- 2.5 arcsec compared with 24.4 +/- 21.4 arcsec) and then increase over the 9 minute period of adaptation (ANOVA, p = 0.0002). Using post-hoc tests with Bonferroni correction, the decrease of stereoacuity on introduction of the prism was significant (p = 0.0039), and although an increase in stereoacuity appeared to occur after 3 and 6 minutes of wearing the 12(Delta) glasses (14.9 +/- 4.3 arcsec, 12.3 +/- 4.5 arcsec), this did not return to the baseline value until 9 minutes, when the stereoacuity had increased to 12.6 +/- 10.6 arcsec (p = 0.1982). CONCLUSIONS: In these participants, inducing a deviation with base-out prisms negatively affected near stereoacuity. However, as adaptation occurred, the level of stereoacuity was seen to increase back toward the baseline measurement.

Tilt and reading speed.

PURPOSE: To measure the effect of head tilt on reading rate and to determine whether a difference existed between the body or the text being tilted. METHODS: The Wilkins Rate of Reading Test was used to measure rate of reading in a population of normal readers. In the first part of the study, rate of reading with a head tilt of 15 degrees and 30 degrees was measured; in the second part of the study, rate of reading with a body tilt of 45 degrees and 90 degrees or a text tilt of 45 degrees and 90 degrees was measured. RESULTS: For the first experiment, 30 participants, with a mean age 20.3 years, showed no effect on reading speed with a head tilt of 15 degrees or 30 degrees. In the second experiment, 20 participants, with mean age of 19.75 years, showed significantly slowed rate of reading when text was tilted to 45 degrees and 90 degrees (by a mean of 19 and 36 words per minute, respectively), and when reading while lying horizontally (90 degrees) with text upright (mean reduction of 34 words per minute). With a body tilt of 45 degrees, the effect on reading rate (mean reduction of 15 words per minute) just failed to reach significance (p = 0.058). CONCLUSIONS: When text is tilted in relation to the body, by tilting either the text or the body, reading speed is affected. The effect of tilt on word recognition is discussed as a cause of the observed test results.

Is the Cardiff Acuity Test effective in detecting refractive errors in children?

PURPOSE: Refractive error is a common cause of reduced visual acuity in young children. This reduced vision should be detected as soon as possible to avoid development of squinting and educational disadvantage. The Bailey-Lovie Chart (BLC) is based on the logarithm of the minimal angle of resolution (logarithm of the minimum angle of resolution logMAR) and is widely accepted as the clinical standard for visual acuity testing. However, most young children are unable to perform this test because of its symbolic level. The Cardiff Acuity Test (CAT) overcomes these symbolic demands and is the test of choice for young children in most U.K. orthoptic departments. The purpose of this study is to determine how effective the CAT is in detecting reduced visual acuity caused by refractive error in young children. METHODS: Visual acuity of the right eye was tested without spectacles in 68 children (mean age, 74 +/- 14.6 months) with known bilateral symmetric refractive error (40 male) using the CAT and the BLC. Subjects were randomized to receive CAT or BLC first in a single assessment using a crossover design. Subjects scoring 0.2 logMAR or better were classified as having passed the test. RESULTS: The CAT correctly identified reduced vision caused by uncorrected refractive error in 25% (17) of the children compared with a detection rate of 97% (66) for the BLC using the specified pass criteria. Further analysis with an adjusted cut point for the CAT (0.0 logMAR), as used to identify abnormal vision in clinical practice, identified a detection rate of 56% (38 of the children). CONCLUSIONS: This study casts doubt on the current clinical practice used in orthoptic clinics by suggesting that assessment of visual acuity with the CAT alone will underdiagnose reduced acuity caused by refractive errors.

Visual side-effects from transdermal scopolamine (hyoscine).

Transdermal scopolamine may be used to reduce drooling in children with disabilities. Side-effects include dilated pupils and a reduction in the near point of accommodation (the closest point at which clear vision is possible). Two male children with epilepsy, one with spinal dysraphism (aged 7y 6mo) and one with cerebral palsy (aged 5y 8mo), who have undergone treatment for drooling with transdermal scopolamine are described. Near visual acuity was reduced, and both children showed dilated pupils with reduced or no response to light. These responses became normal on cessation of the scopolamine patch. As the effect of this drug may be cumulative, and many patients are unable to communicate difficulties, clinicians need to be aware of these possible side-effects.

Heroin and diplopia.

AIMS: To describe the eye misalignments that occur during heroin use and heroin detoxification and to give an overview of the management of persisting diplopia (double vision) which results from eye misalignment. METHODS: A literature review using Medline and the search terms strabismus, heroin and substance withdrawal syndrome is presented. General management of cases presenting to the ophthalmologist and orthoptist with acute acquired concomitant esotropia is described. FINDINGS: A tendency towards a divergence of the visual axes appears to be present in heroin users, although when present it may not always lead to diplopia. Following detoxification intermittent esotropia or constant esotropia (convergence of the visual axes) can occur; if intermittent the angle tends to be small and diplopia present when viewing distance objects. Occlusion of one eye to eliminate the second image could encourage the development of a constant deviation. The deviation is not caused by a cranial nerve palsy. Constant deviations of this type are classified as 'acute acquired concomitant esotropia'. Relief from the diplopia may be gained by prismatic correction, and the deviation may then resolve spontaneously. Botulinum toxin or surgical intervention may be necessary in cases that do not resolve. CONCLUSIONS: Heroin use may lead to intermittent or constant exotropia and withdrawal may result in intermittent or constant esotropia. Awareness of the mechanism causing this may avoid referral to other specialties (e.g. neurology) and awareness of treatment modalities could encourage patients to seek appropriate help for relief of symptoms.

Binocular sighting ocular dominance changes with different angles of horizontal gaze.

BACKGROUND AND PURPOSE: Ocular dominance testing is generally carried out in the primary position. A change of eye dominance when viewing in the contralateral field of horizontal gaze has recently been reported. The purpose of this study was to determine whether this occurs with other tests of eye dominance. METHODS: Ten right handed females (mean age 21.6 +/-0.8 years) with normal binocular single vision and right eye ocular dominance in primary position were tested for ocular dominance on three tests (pointing, hole-in-card, Miles ABC) performed fixing at positions at 10 degree intervals from 40 degrees left gaze to 40 degrees right gaze, with the head stationary. Four trials were undertaken in each position for each test. RESULTS: Not all participants showed a change in dominance in left gaze on all of the tests. However, a change did occur at a median of 30 degrees for the hole- in-card and Miles ABC tests and 20 degrees for the pointing test. Examining the position of change across the 3 tests, a borderline "statistically significant" difference occurred (p=0.055, Friedman test). A "statistically significant" difference was shown between the hole-in-card and pointing tests (p=0.041, Wilcoxon signed ranks test). For pointing versus Miles ABC, p=0.084; and hole-in-card versus Miles ABC, p=0.705. CONCLUSIONS: A change in eye dominance occurs when viewing in the contralateral field. Differences may exist in the angle at which this occurs due to the different conditions of the various tests for ocular dominance.

Vertical vergence adaptation does improve with practice.

PURPOSE: To investigate the effect of repeated testing on adaptation to a 2 Delta vertical prism in subjects with normal binocular single vision. METHODS: Subjects were required to have good visual acuity and normal binocular single vision. The residual deviation was measured after a 1-min period of adaptation to a 2 Delta vertical prism using a modified Maddox Rod technique. This measurement was repeated 10 times, with a 5-min rest period between each trial. RESULTS: For eight young adult subjects, the mean residual deviation at 1 min reduced with consecutive trials, and this effect was statistically significant (p < 0.0001). No significant correlation was present between the amplitude of vertical vergence and the increase in adaptation with repeated demand. CONCLUSIONS: A practice effect for vertical vergence adaptation appears to be present in young adult subjects with normal binocular single vision.