Simultaneous pattern visual evoked potential and pattern electroretinogram in strabismic and anisometropic amblyopia.

BACKGROUND: Amblyopia is a relatively common condition in which visual acuity through an eye is subnormal despite no overt pathology. Pattern visual evoked potential (PVEP) can detect any defect from optic nerve to occipital cortex and pattern electroretinogram (PERG) can detect retinal defects specially the ganglion layers. This study was performed to evaluate the cortical and retinal activity in strabismic and anisometropic amblyopia. METHODS: PVEP and PERG were recorded simultaneously in 40 amblyopes (20 strabismics and 20 anisometropics) and 20 normal control subjects. Normal subjects were age and sex matched with patients. RESULTS: The P(100) latency in PVEP was increased in both groups of patients but the P(100) amplitude was reduced only in anisometropic group. In PERG, the amplitude of P(50) was reduced in all patients with no significant change in latency. CONCLUSION: Beside reduced PVEP responses in strabismic and anisometropic amblyopia, the activity of retina reduced too. It is likely that retinal impulses can affect the development of visual system.

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.

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.

The effect on fixation disparity and associated heterophoria of reading at an abnormally close distance.

It is well established that prism stress on binocular vision produces fixation disparity. It is therefore suggested that fixation disparity is a sign of stress on binocular vision. The authors have previously looked at the stress caused by reading in reduced levels of illumination, and found that the average fixation disparity and associated heterophoria become more exophoric. In this paper the effect on fixation disparity and associated heterophoria of reading at an abnormally close distance is reported. It was found that in 50 young adults fixation disparity and associated heterophoria increased to a greater degree of exo-disparity. An analysis of the visual symptoms indicated that there was also an increase in the visual discomfort and other symptoms.

The clinical assessment of binocular vision before and after a working day.

This paper looks at the clinical investigative and measurement procedures used in assessing binocular vision at the beginning of a normal working day and compared with the same procedures carried out in the afternoon which included several hours of close work. In 84 young adults, it was found that dissociated heterophoria, associated heterophoria and fixation disparity increased. This was accompanied by visual symptoms. It is likely that any decompensation of heterophoria is worse at the end of a working day. The results of this investigation raise the question of the value of some clinical tests for binocular assessment carried out at the beginning of the day.