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Binocular Vision Continuing education CET Binocular vision Part 5 – Binocular sensory status and miscellaneous tests In the latest addition to our occasional series on the assessment and management of binocular vision in practice, Priya Dabasia looks at sensory status and its measurement. Module C16058, one general CET point for optometrists and dispensing opticians he preceding accounts in ● Confusion – the superimposition this mini series of binocular of two dissimilar images in higher vision (BV) testing have processing, experienced predominantly detailed procedures for on observing complex scenes such as ‘a the cover test (CT), ocular room’. The same patient is more likely motility and heterophoria to report diplopia on viewing a small, Tcompensation. The final two articles bright target such as a penlight aim to outline the assessment of ● Retinal rivalry – the observation binocular sensory status, stereopsis and of alternating percepts or a combined convergence. ‘mosaic’ so that images from each eye Having two frontally positioned eyes are never seen simultaneously. separated by approximately 65mm enhances many aspects of our visual Anomalous retinal correspondence performance – a wide panorama, (ARC) is considered a more efficient higher acuity, and three-dimensional sensory adaptation to heterotropia as perception to a distance of 200 metres, suppression occurs in localised zones provided both eyes are fully functional Figure 1 to the fovea of one eye corresponds to a rather than spanning the binocular and coordinated together. Anomalies Worth 4-Dot point temporal to the fovea in the other field. It facilitates a weaker form of of binocular function have often been test eye. In reality, BSV can still be achieved BSV, relieving diplopia while enabling described as ‘the hidden learning with misaligned visual axes provided a good level of depth perception of up disability’ as they impair academic the disparity occurs within the limits of to 100’’. ARC is believed to develop performance in young children, ‘Panum’s fusional area’; a horizontally from an abnormal enlargement of often resulting in a permanent visual oval group of retinal elements that foveal suppression in the deviated eye impairment when left untreated. increase in size with eccentricity from to encompass the image of the fixation Early detection, using appropriate tests the fovea. When our visual system is object. An alteration in correspondence and exercising proper management is subjected to undue stress such as in takes place so that the fovea of the fixing of paramount importance, but often decompensated heterophoria, the axes eye is now paired with a new parafoveal seen as a challenging aspect of daily may deviate outside Panum’s area, locus of the deviated eye. This change practice. resulting in one of the following: in common visual direction continues Binocular single vision (BSV) ● Diplopia – the perception of two across the retina so that a small area occurs when the image from each eye different, non-fusible images of the nasal to the fovea of the fixing eye now contributes to a single, common percept. object of interest as they stimulate the correlates with elements temporal to the It is considered in three grades: fovea of the fixing eye and parafoveal new extrafoveal point of the deviated ● Sensory – the ability to perceive elements of the deviated eye. Double eye. As a prerequisite to any sensory an image formed with each eye vision is generally experienced in later adaptation, the heterotropia must simultaneously life where the stability of BSV prevents present early, typically under the age of ● Motor – the facility to maintain suppression of one image six years when the binocular system is sensory fusion through a range ● Suppression – sensory adaptation amenable to change. It should also be vergence that develops in early onset conditions small in angle, unilateral and constant ● Stereopsis – the perception of depth up to the age of eight years to avoid in presentation, resulting in a higher based on binocular disparity. undesirable symptoms such as diplopia, prevalence of ARC in esotropia. by inhibiting the image of one eye To gain a more in-depth understanding, in favour of the fellow eye during Tests for sensory status it is necessary to appreciate the concept binocular viewing. In heterotropia, The most recognised tests for sensory of retinal correspondence, a function it tends to occur in larger deviations status are Worth 4-Dot, Bagolini of higher processing that takes place misaligned by greater than 40Δ. At the and the Modified OXO tests. Details at the visual cortex. Normal retinal onset, two suppression zones develop of procedures, how to interpret the correspondence is the most favourable at the fovea and fixation point of the results and suggestions of recording the condition in which the fovea of one deviated eye, eventually joining to findings are detailed in turn. eye corresponds with the fovea of span a larger area if left untreated. the other, sharing a common visual Suppression that swaps between the ● Worth 4-Dot test – a dissociative direction during binocular viewing. eyes such as in alternating heterotropia test comprised of four circular lights in This correlation continues for every can also be described as ‘complete’ or a diamond formation viewed against a locus of the retina so that a point nasal ‘total lack of retinal correspondence’ black-grey surround (Figure 1). The test 14 | Optician | 11.03.11 opticianonline.net CET Continuing education Patient response Observation Diagnosis/recording ‘Four lights’ NRC or ARC • One red • Two green • Bottom light – yellow mixture, red or green ‘Five lights’ ‘Five lights’ Exotropia with Esotropia with • Two red lights on • Three green on heteronymous ‘crossed’ homonymous the left the left diplopia ‘uncrossed’ diplopia • Three green lights on • Two red lights on the right the right ‘Five lights’ L/R hypertropia with vertical diplopia • Two red lights above • Three green lights below ‘Three green lights’ ‘Two red lights’ Right suppression Left suppression ‘Lights changing between 2 and 3’ Alternating suppression Figure 2 A summary of patient responses and interpretation of the Worth 4-Dot test it perceives a vertical streak, while the fellow eye observes the spotlight. For can be wall-mounted, arranged over the demonstration purposes, two Bagolini glass plate of a flashlight for near testing lenses have been used to illustrate the or displayed on a computerised chart results in Figure 4. This arrangement such as Test Chart 2000. Red-green is also vital for investigating alternating goggles are used to separate the images heterotropia. of each eye – the right eye observing c) Direct the patient to fixate on a through the red filter perceives the top spotlight held 33cm from the eyes red circle, while the left eye views the d) Ask the patient to report the number two central green circles. The bottom of lines and spots seen, their relative white target provides a fusional stimulus positions and whether any gaps are for binocular viewing as it is common perceived along the streaks to both eyes, perceived either as a yellow e) Repeat for 3m and 6m as required mixture, red/green depending on ocular using a Grade 4 lens at far distance dominance or an alternation of the two to produce a brighter streak for colours in retinal rivalry. easier viewing, reducing the room a) Place the R/G goggles over any near Figure 3 Bagolini lens illumination accordingly. spectacles, taking care that the target is not seen without these filters detecting central suppression and is ● Modified OXO test – a near Mallet b) Turn the room lights off more conducive to natural viewing unit test comprised of two green strips c) Hold the flashlight 40cm from the through minimal dissociation as it is positioned above and below the ‘X’ of an patient’s eyes just below the horizontal performed under ambient illumination. OXO panel (Figure 5). It is essentially midline to mimic the reading position. It is also a perfect illustration of the cost a larger version of the disparity test, Check it is correctly oriented with the efficiency of BV tests – a lens can be as smaller markers are more likely to red and white circles at the top and ‘created’ by smearing the surface of a fall into a central suppression zone, bottom respectively low power trial lens with a finger using rendering it unsuitable for detecting d) Ask the patient to report how many Vaseline or any suitable alternative to ARC and global suppression. dots they see, confirming the colours mimic the Bagolini striations. a) Ask the patient to hold the unit at and relative positions of each. Possible a) Clean a Grade 2 Bagolini lens with their normal reading distance and angle outcomes are illustrated in Figure 2 a microfibre cloth to avoid more than of viewing e) Repeat the test for distance fixation one image being perceived b) Direct the patient to fixate on the with any required spectacle correction. b) Position one lens in front of each Modified OXO target eye with the striations at 45° and c) Position the cross-polarising ● Bagolini test – a lens made up of 135°, either mounted in a lorgnette or visor before the eyes, over any near fine parallel striations that distort placed in a trial frame with refractive correction a spotlight target into a line image correction as required. The test can also d) Ask the patient to report the number perpendicular to their axis (Figure 3). be conducted with the lens oriented of strips perceived and their relative It supersedes
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