Effect of induced fixation disparity on binocular visual acuity.

Fixation disparities were artificially created for distance vision by prisms, and the monocular and binocular visual acuities were measured. The normal approximate 10% improvement in binocular visual acuity compared to monocular visual acuity, deteriorated in proportion to the amount of fixation disparity created by the prisms. This was true in both eso- and exo-disparity, although not to the same extent.

Distance and near readings of associated heterophoria taken on 500 patients.

Distance and near readings of associated heterophoria were taken, using standard Mallett units, on 500 patients passing consecutively through an optometric practice and any symptoms, connected with distance and near vision, were noted. Complete sets of results were obtained on 383 people. Distance readings, either in size or direction, did not appear related to symptoms and such readings did not appear helpful in patient management. For near vision a fair degree of association existed between readings of associated phoria and symptoms and it was possible in the under 40 years and 40-59 years age groups to express clinically useful rules regarding the relationship. Over 60 years of age such a relationship was complicated by the fact that high degrees of exophoria readings existed in both symptomatic and asymptomatic patients and it is suggested that this may be related to the quality of binocular vision.

Binocular vision without visual stress.

The use of changes in fixation disparity, the associated phoria, and the dissociated phoria in assessing the degree of visual stress requires that we know the effect of vision under close working conditions (40 cm) on these parameters. The authors have previously reported that reading under the stress of abnormally low illumination, or at too close a working distance (20 cm), increases fixation disparity and associated phoria and is associated with symptoms and visual discomfort. The current study shows that reading at photopic levels of illumination and at a working distance of 40 cm does not change the magnitudes of fixation disparity and associated phoria. Reading under these conditions is not accompanied by an onset of symptoms or visual discomfort. This result indicates that changes in fixation disparity and associated phoria are a product of visual stress and are not produced by reading itself.

Binocular vision, age and symptoms.

Changes with age in heterophoria, associated heterophoria, fixation disparity and stereopsis for near vision were investigated in a sample of 187 subjects (age range from 10 to 65 years) divided into six age groups. With increasing age, heterophoria, associated heterophoria and fixation disparity increased in the exo-direction. No change in stereopsis was found. A comparison with visual symptoms for near vision indicated a relationship between symptoms and the magnitude of fixation disparity and associated heterophoria for all age groups, but no relationship was found between the presence of symptoms and heterophoria measurement. No relationship between the presence of symptoms and the central slope of the forced vergence fixation disparity curve was found. These findings suggested that neither the measurement of heterophoria nor the forced vergence disparity curve are appropriate tests in the assessment of binocular vision anomalies. However, fixation disparity and associated heterophoria are related to binocular problems for near vision in all age groups.

Criteria for decompensation in binocular vision.

Two groups of patients, one under 40 years of age and the other 40 years and over, were divided into subgroups according to whether they had symptoms associated with near vision. A method developed in 'signal detection theory' was used to determine whether a value could be found for heterophoria, or associated heterophoria, which would predict which patients had symptoms due to decompensated heterophoria in normal routine investigation. Receiver operator characteristic curves (ROC curves) were constructed to look for these cut-off values between patients with, and without, symptoms. No value for dissociated heterophoria was found that would discriminate between the symptomatic and asymptomatic patients. It is concluded that heterophoria measurement is not useful as a routine procedure. In the case of associated heterophoria, for the under 40 years age group, patients with a value of 1 prism dioptre or more are more likely to have symptoms than not, and one-third of patients with close work problems have a value of 2 prism dioptres or more. It was rare to find any asymptomatic patient with a value as high as this. In the 40 years and over age group those with an associated heterophoria of 2 prism dioptres or more are more likely to have symptoms than not. One-third of patients in the latter age group, with symptoms, have a value of 3 prism dioptres or more whilst it is rare to find an asymptomatic patient with a value as large as this.

The subjective assessment of cataract.

The medical treatment of cataract now appears to be a distinct possibility. A number of anti-cataract formulations are being clinically tested, and more clinical trials are being planned. To obtain a true assessment of a drug's efficacy, a battery of tests are needed which can accurately assess cataract progress. A clinical trial of the proposed anti-cataract drug Bendalina is being conducted. The six subjective methods used to assess cataract progression in the trial are presented. These are refractive error, LogMAR visual acuity, contrast sensitivity, glare sensitivity, retinal visual acuity and displacement threshold hyperacuity. The reasons for using each technique and their method of measurement is explained.

Comparison of associated heterophoria measurements using the Mallett test for near vision and the Sheedy Disparometer.

The Mallett test for near vision and the Sheedy Disparometer are compared in terms of their ability to measure associated heterophoria. Twenty-two asymptomatic subjects were used. With the Disparometer, ten of these subjects, familiar with making prism vergence measurements, exhibited stable forced vergence fixation disparity curves, and the associated heterophoria values using both instruments were not significantly different. However, the twelve subjects not familiar with making such measurements generated fluctuating, unstable fixation disparity curves, and the associated heterophoria values were significantly different. Repeated trials with the randomly selected naïve subjects led to flatter, more stable fixation disparity curves and lower values for the x- and y-intercepts when using the Disparometer. The Mallett test, however, produced identical values for associated heterophoria in each successive trial. Therefore, the validity of measurements using the Disparometer in a clinical setting is questioned.

Effect of reading in low illumination on fixation disparity.

Fixation disparity has been shown to be produced by prism stress, and it has therefore been proposed that its presence constitutes a sign that binocular vision is under stress (decompensated heterophoria). Fixation disparity can be present in the absence of any other indication of stress, and therefore may have a physiological basis. This study investigates the effect on fixation disparity and visual discomfort of placing the visual system under a different type of stress, that of requiring the subject to read under conditions of inadequate illumination. In 50 young adults, it was found that fixation disparity increased under inadequate illumination and that this was accompanied by symptoms in the form of visual discomfort. After a period of rest from any near-vision task, the fixation disparity decreased and the symptoms were reduced.

Fixation disparity in binocular stress.

Fixation disparity has been taken as a sign of stress on binocular vision because it is established that prism stress creates fixation disparity. This paper looks at the effect on fixation disparity of the stress caused by requiring subjects to read in inadequate illumination. It is found that the reduction in illumination does not in itself immediately change the magnitude of the fixation disparity. There is, however, an increase in the mean slope of the central part of the fixation disparity curve which suggests that when the effect of reduced illumination is added to prism stress, fixation disparity is increased. The stress created by asking subjects to read in reduced illumination for half an hour resulted in the mean associated heterophoria being increased, and over half the subjects reported symptoms of stress. It is concluded that fixation disparity is changed by this type of visual stress in some subjects, and in near vision is increased to a more marked degree of exo-disparity. Most of this increase occurs in the first ten minutes.

Lateral short-term prism adaptation in clinical evaluation.

Data were obtained from a normal clinical population regarding heterophoria, associated phoria and short-term prism adaptation in an attempt to differentiate between those subjects with symptoms and those without. Correlation between heterophoria, associated phoria, prism adaptation and symptoms was low. A short prism-adaptation test was found to be of little value in diagnosing decompensated heterophoria in the general population. Subjects diagnosed as having decompensated heterophoria did not have an abnormal short-term prism-adaptation system. The clinical implications of these findings are discussed in relation to the diagnosis of decompensated heterophoria and the prescribing of prismatic corrections.

Convergence insufficiency in a rural population.

Convergence insufficiency represents a significant problem in optometric practice. The percentage of cases in a rural area was compared with that in a previous sample from an urban area, with respect to the general incidence, and the effects of age and taking of medicines as an indicator of health. The percentage of convergence insufficiency in the rural area was about half that in the urban area, and the percentage taking medicines was about two-thirds. The incidence of convergence insufficiency increased with age in both populations. When both samples were taken together, the incidence of convergence insufficiency was three times greater in those taking medicines.

Convergence insufficiency in patients taking medicines.

The incidence of convergence insufficiency in patients taking medicines is compared with its incidence in patients taking no therapeutic drugs, in a sample of 648 taken from a normal optometric practice. It is found that the incidence was significantly higher in those patients on medicines considering the group as a whole. It is also found to be higher in older patients, and that most of the older patients were taking medicines whilst the younger patients were not. However, the increase in the incidence of convergence insufficiency in the older patients taking medicines was disproportionately high. It is concluded that age is not the only factor in the increased incidence, and that the taking of medicines as a possible indicator of poor general health, is also an important factor in the cause of convergence difficulties.

Physiological exophoria.

The evidence for the existence, nature and degree of physiological exophoria was examined with respect to a sample of 663 optometric patients. It was found that the mean value for heterophoria is the same for distance and near vision up to the age of the early twenties. After this age, the mean value for distance heterophoria remains unchanged throughout life whilst there is a change in the value for near vision towards exophoria, giving 5 delta physiological exophoria. No differences between males and females were found. Physiological exophoria is shown to increase regularly with age and no sudden increase was found at the age when presbyopic additions were first worn. It is concluded that presbyopic additions are not the cause of the physiological exophoria.

Success rate in the treatment of amblyopia by conventional methods.

Conventional methods of treatment were given to 50 young amblyopes, with the result that 26% showed no improvement and 50% improved by at least two Snellen lines. A 0.67 correlation coefficient was found between acuity before and after treatment. Whilst there was some differences in approach to convergent squint amblyopes and to non-squinting amblyopes, the degree of success in the two subdivisions was similar.

Orthoptic treatment in teenage patients.

A survey of 100 patients who received orthoptic treatment in the Bradford University Clinic is examined to see the level of success with the different binocular anomalies, indicating what factors were common in successful cases, and the type of treatment given. A successful outcome or worthwhile improvement was achieved in enough cases to conclude that orthoptics in teenage patients is often appropriate, even in strabismus, if the cases are selected carefully. There was no indication that the actual age made any difference once the patients was between 11 and 19 years, but the starting acuity was significant.