Binocular Vision Complaining of Some Form of Binocular Vision Anomaly

Continuing education CET t has been estimated that up to 10 per cent of primary care patients attend an eye examination Binocular vision complaining of some form of binocular vision anomaly. With the ever-increasing use Part 1 – The cover test Iof computers in the workplace and home, it is essential that some form Priya Dabasia begins a new series looking at the basic skills used in of basic binocular vision assessment is conducted on every patient seen in practice to assess binocular vision status. She begins with the cover practice, particularly young children test. Module C14710, one general CET point for OOs and DOs or those presenting with suspicious symptoms indicative of active pathology. Practitioners often consider (a) (b) binocular vision as a challenging part of general practice requiring sophisticated techniques and equipment only used during undergraduate training. In reality, the basic tests available are inexpensive, easily accessible and yield a significant amount of information in a short space of time. The average full-term newborn possesses all the basic elements required to achieve binocular single vision (BSV) in the form of two frontally located eyes that generate monocular images of similar size and clarity, and Figure 1 Visual axes in (a) heterotropia and (b) heterophoria (dotted line represents axis normal visual pathways to process without cover in position) and integrate this information. BSV, however, is not present at birth but and convergence. The selection of any detecting the presence of a deviation develops in a visually stimulated process supplementary tests such as stereopsis, it definitively distinguishes the two between three and five months of age. accommodative facility and tests for sub-types: This refinement of innate processes retinal correspondence are dependent ● Heterotropia (otherwise known as and cortical connections continues on the knowledge of previous ocular a manifest strabismus or ‘squint’) – in a process called synaptogenesis history, refraction and the outcome of where the visual axes do not coincide during a two-stage critical period for these preliminary investigations. These at the object of interest or at infinity development primarily up to two years tests will be considered in a series of for distance viewing (Figure 1a). It of age and to a lesser degree up to eight four articles discussing their indications can be unilateral, alternate between years at which 80 per cent of cortical for use, procedures and interpretation the eyes or only present with tiredness neurons are expected to respond to of results. or at a particular fixation distance. binocular stimulation. This article will consider the cover Heterotropia is most commonly Any anomaly during this test (CT) – one of the simplest objective primary in aetiology but can be developmental process that prevents tests conducted during the course described as secondary when it occurs the fovea receiving a clear image or of an eye examination in which the as a result of pathology causes the eye to suppress can result in examiner observes the behaviour of ● Heterophoria (otherwise known as amblyopia – defined as the reduction in the eyes covered and uncovered in a latent deviation) – where the visual acuity in the absence of pathology. An turn while fixated at a given distance. axes do not intersect at the object of ‘eye turn’ properly termed ‘strabismus’ It yields precise information of eye intersection on dissociation in which but colloquially known as a ‘squint’ is alignment as well as the presence, the eyes are covered alternately to one of the major causes of amblyopia amplitude, frequency and direction of prevent sensory fusion (Figure 1b). alongside significant refractive error. an ocular deviation. The test can be The eye deviates under a cover but Surprisingly, it is held accountable undertaken grossly once a child can moves to take up fixation and regain for the loss of vision in more people fixate from three months of age using BSV on removal of the occluder. under 45 years of age than all other an appropriately brightly coloured near The test is thereby conducted in two ocular diseases and traumatic incidents target. A distance cover test can be stages: combined, prevalent in an estimated performed from two years of age and ● Cover-uncover or ‘unilateral’ CT 2-3 per cent of the general population. a more detailed examination at near – determines the presence of any It is a largely avoidable cause of partial using an accommodative target from ocular deviation and distinguishes a sight, highlighting the importance of 3.5 years of age. heterophoria from a heterotropia. It early detection and management of detects the habitual angle of deviation any obstruction to the development Uses of the cover test through minimal dissociation of the of binocularity such as an anomaly of The CT is broadly used to compare eyes the extraocular muscles or their nerve and interpret eye movements at near ● Alternating CT – completely supply. and distance and in different positions dissociates binocular fusion to elicit The first three tests of basic binocular of gaze when used in conjunction with the maximum size of a deviation or function are the cover test, ocular motility ocular motility assessment. Aside from increases the amplitude to allow small 24 | Optician | 24.09.10 opticianonline.net Continuing education CET TABLE 1 A summary of the possible observations, diagnoses and recording of results of the cover/uncover test Observation of uncovered eye Diagnosis Recording to take up fixation No movement No tropia Orthotropic Divergent horizontal movement Convergent squint Esotropia/SOT Convergent horizontal Divergent squint Exotropia/XOT movement Upward vertical movement Vertical squint Hypotropia /HypoT Downward vertical movement Vertical squint Hypertropia /HyperT Both eyes diverge to take up Alternating convergent Alternating esotropia fixation squint /Alt SOT Both eyes converge to take up Alternating divergent Alternating exotropia Figure 2 The translucent Speilman occluder fixation squint /Alt XOT Small angle movement of less Microtropia MicroT movements to be appreciated more than 10 dioptres easily. In general, the cover-uncover test is primarily used to evaluate heterotropia, while the alternate CT is used more far distance is a letter or equivalently Procedure in the detection of heterophoria or to sized picture on one line above that Once your patient is set up comfortably investigate a latent element to a small resolved by the eye with the poorest in the chair, observe for any abnormal angle heterotropia. acuity. This serves to stimulate and head postures such as a marked face stabilise accommodation while still turn/tilt, any facial asymmetry or Apparatus ensuring accurate fixation with each obvious eye turns. Adjust the room The CT requires minimal equipment eye. If the vision at far distance in one lights or supplementary lamps to ensure comprising simply an occluder and a eye is sufficiently poor, some authors that the patient’s face is adequately suitable fixation target (Figure 2). recommend the use of a spotlight illuminated to make subtle deviations A standard black ‘paddle’ occluder is target, while others prefer to direct the easier to detect. You should be seated the most widely used by practitioners patient to a specific feature of a larger facing the patient and slightly to their although the translucent ‘Speilman’ letter (eg the centre of the ‘X’ letter on side to prevent obscuring the fixation type (Figure 2) affords the practitioner the 6/60 line). target at distance (Figure 3). The test the advantage of observing the Near acuity using ‘N’ scoring can be is usually conducted first without any approximate position of the eye behind estimated by dividing the denominator spectacle correction. the cover, while its frosted surface of the distance measure in standard ● Step 1: Direct the patient to the reduces any form perception. Snellen notation by three (for example appropriate fixation target and ask them A measure of visual acuity is required 6/18 acuity in the poorest eye at to concentrate on the detail. For young for each eye to allow a suitable fixation distance equates to N6 at near) and children, it is often helpful to ask them target to be selected accordingly. Use re-checked with a reading chart for to describe the colour or details of the of single letter optotypes without each eye in turn. Direct the patient to image particularly at near to maximally crowding bars should be avoided a single letter on the reduced Snellen stimulate accommodation. wherever possible as it can significantly chart of the ‘Budgie stick’ accordingly ● Step 2: Introduce the occluder in underestimate amblyopia. and check that each eye can discern the front of the eye with better vision The recommended fixation target at target before commencing the CT. for 2-3 seconds while observing the uncovered eye. Take care as subtle deviations can easily be missed with inadequate dissociation if the test is performed too quickly; some authors recommend a significantly longer occlusion of up to 10 seconds to reveal the full deviation. Any movement of the uncovered eye to take up fixation indicates a heterotropia. Note the speed of movement to take up fixation as it provides a measure of the level of vision in this uncovered eye as well as the direction of the movement (Table 1 shows a summary of how to interpret and record your observations of the Figure 3 cover/uncover test). Practitioner NB Ensure the occluder is held and patient accurately to fully cover the eye and set-up prevent any peripheral fusion that can 26 | Optician | 24.09.10 opticianonline.net Continuing education CET otherwise elicit unexpected deviations. fixation target on alternate occlusion. as a subject moves their eyes between It can be held from below or above but A positive response is known as a Phi one end of the 6/12 line on the standard take care in the latter that your arm movement in which an image shift with Snellen chart to the other.

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