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Biomedical subjects

R A Harrad

Publications and source records attributed to R A Harrad.

At least 37 records · Page 2Linked to original sources
PubMed1997

Pulfrich's phenomenon in unilateral cataract.

AIMS: To determine whether unilateral cataract causes a pathological Pulfrich's phenomenon. METHODS: 29 subjects with unilateral cataract and contralateral pseudophakia were assessed on their ability to perceive the Pulfrich phenomenon. Using a computer generated pendulum image, and graded neutral density filters, a series of forced choice trials were performed in which the subject was required to describe the direction of any apparent pendulum rotation. A pathological Pulfrich effect was said to occur when apparent rotation was perceived in the presence of a zero strength neutral density filter. The size of any pathological Pulfrich effect which was present was quantified by neutralising the perceived pendulum rotation with neutral density filters of varying strength placed before the better seeing eye. RESULTS: 20 out of 29 subjects were able to perceive apparent pendulum rotation when uniocular filtering was performed. In the group (n = 12) which was tested both before and after cataract extraction with intraocular lens implantation, a statistically significant pathological Pulfrich effect was demonstrated preoperatively, compared with a group of normal control subjects. This effect was abolished after cataract extraction (p = 0.009). The median size of the effect was equivalent to a 0.25 log unit neutral density filter over the non-cataractous eye. The subjects who were unable to perceive the Pulfrich phenomenon at all had a significantly greater difference in the visual acuity of each eye (p = 0.045) and significantly worse stereoacuity than those who were able to perceive the effect (p = 0.002). CONCLUSIONS: Unilateral cataract can cause a pathological Pulfrich phenomenon. This finding may explain why some patients with unilateral cataract complain of visual symptoms that are not easily accounted for in terms of visual acuity, contrast sensitivity, or stereoacuity.

Adult↗
PubMed1996

Unilateral congenital ptosis with ipsilateral superior rectus muscle overaction.

PURPOSE: Congenital ptosis may be accompanied by weakness of the ipsilateral superior rectus muscle. We report the finding of a hypertropia of the ipsilateral eye in patients with isolated unilateral congenital ptosis that became manifest only in upgaze. METHODS: Seventy consecutive patients with congenital ptosis were recalled and 58 reexamined. Examination included assessment of visual acuities, palpebral apertures, levator muscle function, and an orthoptic examination. Particular attention was paid to the assessment of upgaze. RESULTS: Thirty-eight patients had an isolated unilateral congenital ptosis. Four patients had bilateral ptosis, six had upgaze deficits, and ten others had a variety of other syndromes associated with ptosis. Seventeen of the 38 patients with isolated unilateral congenital ptosis were found to have an ipsilateral hypertropia on upgaze. The size of the vertical deviation varied from 5 to 30 prism diopters and, in the more severe cases, produced a cosmetic problem that became more noticeable after successful ptosis surgery. In one of these patients, a superior rectus muscle posterior fixation suture was effective in reducing the hypertropia. CONCLUSION: Of a number of possible causes for the ipsilateral hypertropia in upgaze in patients with unilateral congenital ptosis that we observed either a misdirection syndrome within the superior division of the oculomotor nerve or an exaggerated Bell's reflex is the most likely.

Adolescent↗
PubMed1996

Magnetic resonance imaging in thyroid eye disease.

We examined 25 patients with thyroid eye disease, using both the STIR (Short Tau Inversion Recovery) sequence and cine MRI techniques. A number of characteristic features can be seen on the cine MRI. There is muscle enlargement with restriction of movement and, in the burnt-out phase of the disease, reduced elasticity of the muscles is manifest as their failure to stretch on eye movement. This is in contrast to the active phase of the disease, where although the muscles are enlarged, muscle stretching is clearly visible. The STIR sequence gives an assessment of muscle water content, and hence a high signal is seen in active disease. Combining these techniques is useful in assessing the level of disease activity in thyroid eye disease, and helps in planning further management.

Disease Progression↗
PubMed1996

Methodology for a randomised controlled trial of preschool vision screening. A new approach with the 'ALSPAC' project.

We present the methodology of a population-based Randomised Controlled Trial, comparing an intensive programme of primary preschool vision screening by orthoptists with the usual non-specialist screening. The aims of the trial are to compare the effectiveness and costs of intensive orthoptic screening with non-specialist measures. The orthoptic screening programme will be evaluated both as a composite package and in terms of the screening value of the individual tests at specific ages. This trial is nested within a large population-based longitudinal study. Additional demographic and developmental data on the children in the trial are therefore available. The results of the trial will be used to help clarify which methods of preschool ophthalmic population screening are best in terms of disease detection and cost efficiency.

Amblyopia↗
PubMed1995

Uniocular fields of fixation in thyroid eye disease.

Immunosuppressive therapy is well established in the treatment of thyroid eye disease (TED). The best response has been observed in those with active (wet phase) disease of short duration. A prospective study was designed to observe the effects of orbital radiotherapy and oral immunosuppression on patients with TED, and to assess whether any pre-treatment parameters were predictive of the outcome. Significant improvements in uniocular fields of fixation (UFOF) and in the Mourits' disease activity scale were seen after treatment. The degree of improvement in UFOF was positively correlated with the level of initial disease activity. The use and technique of UFOF in assessing disease phase and activity are discussed.

Adult↗
PubMed1994

The STIR sequence MRI in the assessment of extraocular muscles in thyroid eye disease.

Nineteen patients with thyroid eye disease were examined with magnetic resonance imaging using the Short Tau Inversion Recovery (STIR) sequence and compared with normal controls. The Signal Intensity Ratio (SIR) of each of the four recti was obtained by comparison with the signal intensity of the adjacent temporalis muscle. The SIR was compared with disease activity assessed using Werner's grading system, a clinical inflammatory score, and the range of extraocular movements. Results show that a high SIR is associated with a high index of disease activity. This technique is likely to prove useful in assessing disease activity and planning the management of thyroid eye disease, particularly with respect to immunosuppression.

Adult↗
PubMed1994

Binocular rivalry disrupts stereopsis.

Does the shift from binocular rivalry to fusion or stereopsis take time? We measured stereoacuity after rivalry suppression of one half-image of a stereoacuity line target. After the observer signalled that the single stereo half-image had been suppressed, the other half-image was presented for a variable duration. Stereoacuity thresholds were elevated for 150-200 ms. A control experiment demonstrated that the threshold elevation was due to rivalry suppression per se, rather than masking effects associated with the rivalry-inducing target. Monocular Vernier thresholds, measured as the smallest identifiable abrupt shift in the upper line of an aligned Vernier target that had previously been suppressed by rivalry, were elevated for a much longer duration. This result shows that an appropriately matched stereo pair can break rivalry suppression more easily than can monocular changes in position. With the aid of a similar paradigm, we also measured the duration needed to detect a disparate feature in a random-dot stereogram after rivalry suppression of one half-image of the stereogram. Observers could correctly identify the location of the disparate feature (upper or lower visual field) when the other half-image was presented for a duration ranging from 150-650 ms. In the absence of the matching half-image, the first half-image was suppressed by the rival target for a far longer duration (a few seconds). These findings show that although stereopsis and fusion terminate rivalry, both are initially disrupted for a few hundred milliseconds by rivalry suppression.

Depth Perception↗
PubMed1993

Fusional suppression in normal and stereoanomalous observers.

Observers with normal stereopsis suppress some of the monocular information contained in each stereo half-image, a phenomenon we call fusional suppression. We measured vernier acuity for an ordinary vertical vernier test target, presented to one eye, that was paired stereoscopically with a vernier target with a large fixed offset, presented to the other eye. The size of the fixed offset, and hence, the disparity of the upper target line, was varied parametrically. Vernier thresholds for the test target rose as a function of disparity, reaching a maximum at 20 min of disparity and then decreasing gradually as the disparity exceeded the limits of foveal fusion (40-60 min arc). In the two normal observers, fusional suppression was symmetrical; neither eye had good access to monocular information. In the stereoanomalous observers, fusional suppression was not symmetrical. When they viewed the vernier test target with the stronger of their two eyes, their vernier thresholds were barely affected by the stereo half-image in the other eye, and so were better than those of the normal observers measured in the same condition. When the stereoanomalous observers viewed the test target with their weaker eye, their fusional suppression was similar in range and magnitude to the suppression found in normal observers. Amblyopic suppression in mild amblyopes may be a residual effect of normal fusional suppression, operating to suppress monocular signals in the weaker eye, without conferring the benefits of normal stereopsis and fusion.

Amblyopia↗
PubMed1993

Cine magnetic resonance imaging of eye movements.

Cine magnetic resonance imaging (MRI) is a technique in which multiple sequential static orbital MRI films are taken while the patient fixates a series of targets across the visual field. These are then sequenced to give a graphic animation to the eyes. The excellent soft tissue differentiation of MRI, combined with the dynamic imaging, allows rapid visualisation, and functional assessment of the extraocular muscles. Good assessment of contractility can be obtained, but the technique does not allow study of saccadic or pursuit eye movements. We have used this technique in 36 patients with a range of ocular motility disorders, including thyroid-related ophthalmopathy, blow-out fracture, post-operative lost or slipped muscle, and Duane's syndrome.

Eye Movements↗
PubMed1993

Visual disturbance of the uncovered eye in patients wearing an eye patch.

We questioned 104 consecutive eye casualty patients about any visual disturbance experienced while wearing an eye patch as part of their treatment. Many (31%) reported momentary loss of vision in the uncovered eye. Some experienced this while driving. It was more common (p < 0.0008) if the 'dominant eye' had been patched (47%) rather than the 'non-dominant eye' (16%). In a separate experiment, a number of co-operative subjects were provided with a translucent-coloured occluder to be worn over each eye in turn. They reported a recurring coloured interference in the uncovered eye sparing the monocular crescent. We propose that the loss of vision in the unpatched eye is due to binocular rivalry suppression by the patched eye. This visual disturbance may be a cause of anxiety or risk for the patient. Patched patients should be warned this may occur, and advised to avoid driving or operating machinery.

Automobile Driving↗
PubMed1992

Binocular integration of contrast information in amblyopia.

We asked whether suppression in amblyopia could be accounted for by dichoptic masking as described in normals, operating in the presence of a contrast threshold difference between the two eyes. A dichoptic masking paradigm was employed to investigate binocular interaction in a mixed group of amblyopic subjects. Normal dichoptic masking was not seen after threshold differences between the two eyes were accounted for in the majority of subjects studied. We found that the binocular dysfunction did not merely follow as a consequence of the known monocular loss and that it depended upon the aetiology of the amblyopia and the spatial frequency of the stimulus.

Adolescent↗
PubMed1992

Complicated extracapsular cataract surgery in pseudoexfoliation syndrome: a case report.

Crystalline lens dislocation and zonular dialysis during intraocular surgery are recognised features of the pseudoexfoliation syndrome (PES). A case is reported in which zonular dialysis occurred in both eyes during extracapsular cataract extraction. In particular surgery was complicated by difficulty in performing anterior capsulotomy. Careful preoperative assessment and peroperative technique may help to reduce the risk of surgical complication in PES.

Aged↗
PubMed1988

Amblyopia and strabismus in congenital ptosis.

Seventeen per cent of 216 cases of simple congenital ptosis developed amblyopia and 19% had a squint. Of those patients with amblyopia, 14% had amblyopia attributable to stimulus deprivation, 21% had anisometropic amblyopia and 51% had strabismic amblyopia. Early refraction, orthoptic assessment and treatment and, where the pupillary axis is occluded, surgery to prevent stimulus deprivation amblyopia are recommended.

Amblyopia↗
PubMed1985

The effect of hypoglycaemia on visual function: a clinical and electrophysiological study.

Disturbance of vision commonly accompanies hypoglycaemia. This study was designed to investigate the nature of the visual disturbance, the blood glucose threshold at which the disturbance occurred and the physiological basis. Measurements were made of the corrected visual acuity, colour vision (100 Hue test), visual evoked potentials (VEP), electroencephalography (EEG) frequency analysis and psychometry (digit recall) during stepwise induction of controlled hypoglycaemia produced by an intravenous insulin infusion. Six male volunteers and five insulin-dependent diabetic subjects were studied. During hypoglycaemia corrected visual acuity was unchanged. Colour vision was significantly impaired. Baseline VEP were normal in both groups but significantly prolonged during hypoglycaemia (mean increment 10.8 ms) and increased by greater than 5 ms in nine out of 11 subjects. Quantitative EEG analysis demonstrated slowing with a power density spectral shift from fast alpha to slow alpha, theta and delta which correlated with VEP latency and amplitude changes. The findings have clinical implications. A deterioration in colour vision is likely to impair the ability to read reagent strips by eye. VEP measurements in diabetic patients are likely to be misleading if hypoglycaemia is present; EEG changes are a sensitive index of cortical dysfunction during hypoglycaemia and provide a theoretical basis for developing a portable device to detect early hypoglycaemia.

Adult↗
PubMed1985

Argon laser iridotomy.

Argon laser iridotomy was successfully performed on 47 out of 52 eyes of 35 patients by the continuous wave argon laser. The technique of argon laser iridotomy is described in detail. A number of complications were observed. The reasons for these are discussed, and measures for the avoidance of complications are suggested.

Adult↗
PubMed1982

Failure of postural manoeuvres to prevent lumbar puncture headache.

Diagnostic lumbar puncture was performed on 76 neurological inpatients. They were randomly allocated to one of four bed rest positions for four hours following the procedure (supine and horizontal, prone and horizontal, supine with head-down tilt and prone with head-down tilt) after which they were allowed to get up. There was no substantial or significant difference in the incidence of headache between the four groups. Expectation of headache did not appear to be an important factor in its development.

Adolescent↗