Detection of ocular misalignment.
Optometrists COMMUNICATION BINOCULAR VISION Dispensing opticians COMMUNICATION REFRACTIVE MANAGEMENT
1 CET POINT
Fusion is the core principle upon which the binocular vision system is based whereby the image received by each eye is joined to produce a single 'percept.' Fusion has two components: sensory and motor. Sensory fusion is the result of retinal correspondence (see 'Retinal correspondence' (opposite) and video link (page 79), and occurs within the visual cortex. In order for sensory fusion to occur, the eyes must be aligned and the images received by the visual cortex must be of similar shape, size and clarity. The visual pathways must also be intact, and have been exposed to normal visual stimuli during the early phases of development. (1) Sensory fusion is itself comprised of two components: central and peripheral. Central fusion is driven by the foveae and forms the basis of fine stereopsis. Peripheral fusion is driven by the outer areas of the retina and gives rise to gross stereopsis.
Once sensory fusion has been achieved, motor fusion maintains the single image over a range of eye movements, which is stimulated by retinal disparity. These can be conjugate movements, where the eyes move in the same direction (version movements), or disjugate, where the eyes move in opposite directions (vergence movements). Convergence is described as positive fusionai amplitude, and divergence as negative fusionai amplitude. (2)
There are two principal types of vergence movement, namely fast and slow. Fast movements give rise to 'step' fusion, employed during changes in convergence and divergence. In the case of convergence, these comprise:
* Fusionai convergence--which is stimulated by retinal disparity (see video links)
* Accommodative convergence--whereby every dioptre (D) of accommodation produces an associated amount of convergence according to the individual's accommodative-convergence/accommodation (AC/A) ratio. In most cases, ID of accommodation gives rise to 2-4 prism dioptres (A) of convergence; however, a high AC / A ratio can cause a large esotropia with even small amounts of accommodation. This will be discussed in more detail later.
Slow vergence movements are used to maintain a given level of vergence, and include:
* Proximal convergence--the stimulus to converge in response to a near object
* Tonic convergence--the inherent resting state of the extraocular muscles, which helps sustain vergence.
The motor fusion system is designed to overcome a deviation of the visual axes and retain binocular single vision, resulting in heterophoria or latent strabismus.
Duke-Elder (1950) describes heterophoria as: 'The condition wherein the eyes in their conjugate movements are maintained on the fixation point only under stress with aid of corrective fusionai reflexes.' In other words, single vision is achieved, but with effort. (3)
Heterophoria may be horizontal, vertical or torsional, and is caused by a number of factors, including:
* Anatomical--for example, a wide pupillary distance can give rise to exophoria. Abnormal positions of the orbital check ligaments, congenital muscle weakness and facial asymmetry are other common causes
* Refractive--due to the relationship between accommodation and convergence, the act of accommodating to overcome hyperopia can lead to esophoria. Anisometropie spectacle corrections can induce vertical prismatic effects when looking away from the optical centre of the lenses, which can cause hyperphoria. Particularly high, or less commonly low, AC/ A ratio may be another cause.
By definition, heterophoria only occurs when the eyes are dissociated, as with both eyes open, the visual axes are directed to the object of regard. However, it is possible for there to be a small misalignment of the visual axes, such that they do not coincide at the object of regard, rather they meet slightly in front or behind. As described in 'Retinal correspondence/ the visual axes coincide at the horopter, and objects falling either side of this imaginary line induce retinal disparity, which leads to the experience of diplopia. Either side of the horopter lies Panum's fusionai space, a region in which objects still induce retinal disparity, but to a sufficiently small degree as not to cause diplopia. While such objects are still seen as single, the visual system is under stress and such situations can give rise to symptoms of asthenopia. This is referred to as associated phoria and is discussed later. (4-5)
In contrast to heterophoria, heterotropia arises when a misalignment of the visual axes cannot be controlled by the motor fusion system, and occurs without dissociation. A study of a large population of 12-year-old children found that nearly 54% of the sample had some form of ocular deviation; however, of those, only 5% had heterotropia, which clearly demonstrates the efficiency of the motor fusion system. (6-7) Due to the process of retinal correspondence and projection, such a deviation gives rise to diplopia and visual confusion. (8) If it occurs early in life, the plasticity of the visual system means the infant brain is able to ignore or 'suppress' the image from the deviating eye, thereby eradicating diplopia. It has long been thought that this was achieved through the presence of a suppression scotoma; a region of the retina which is effectively 'switched off' under binocular conditions. Recently this doctrine has been called into question and new models of suppression are being researched at present. A review of this work can be found elsewhere. (9)
There is no doubt that later onset deviations are more likely to cause persistent diplopia. However, there is some evidence that suppression may still occur well into adulthood. (10)
Heterotropia that is the result of an acquired defect to the muscle or nerve supply, whether neurogenic, mechanical or myogenic, will lead to an incomitant deviation; this aspect is beyond the scope of this article, therefore, only concomitant heterotropia will be considered here. Under certain circumstances, it is possible to achieve fusion in the presence of heterotropia. In cases where there is a small-angled heterotopia, certainly no more than 20[DELTA], but more commonly less than 10[DELTA], changes may occur within the visual cortex that alter the relationship between corresponding retinal points, giving rise to anomalous retinal correspondence. (11) In normal Optometrie practice this is an uncommon finding and as such will not be discussed in detail here.
Practical assessment of ocular deviation
The principal method used to assess ocular deviation in optometric practice is the cover test. The ability to perform a cover test well is an essential tool to discover any underlying binocular vision problem, so we are going to consider some of the key elements.
Instruct your patient to fixate a target at distance or near. The target should be chosen carefully and be appropriate for their visual acuity. The patient will need a target that is small enough to be accommodative but large enough to be resolved by each eye and enable fixation. Conventional wisdom states the target should be one line larger than the acuity of the worse eye, unless this is less than 6/12, when a spotlight is used instead. The rationale for this is that a large letter, such as 6/60, does not guarantee steady fixation, and indeed looking from one side of the letter to the other can induce a false movement, making the cover test inaccurate. It is the authors' experience that directing the patient to the apex or edge of a high contrast letter may actually give steadier fixation than a spotlight. In any case, if both eyes cannot see the target, it will affect the quality of fixation. If a target with insufficient accommodative cues is used, the subject may vary their accommodation during the test. (12) As discussed, changes in accommodation give rise to changes in convergence, according to the AC/ A ratio, thus the size of the deviation may vary.
Notwithstanding the above, when testing prepresbyopes, it may be useful to compare the size of the deviation with an accommodative target (appropriate for the patient's age and acuity) and with a spotlight. As the amount of accommodative convergence varies with accommodation, a difference in the size of the deviation with and without accommodation can give useful clues to its nature. A fully accommodative esotropia, for example, may be straight when fixating a light, but will become manifest when looking at an accommodative target. By way of contrast, a divergent deviation is likely to be smaller on accommodation than when fixating a light.
The effect of a patient's refractive error should also be taken into consideration, as hyperopic corrections relax accommodation (and reduce the size of a convergent deviation or increase the size of a divergent deviation) and myopic prescriptions stimulate accommodation (and increase the size of a convergent deviation, or decrease a divergent deviation). It may, therefore, be appropriate to measure a deviation both with and without a refractive correction.
Prior to assessing the ocular deviation, it is useful to observe your patient for evidence of an abnormal (compensatory) head posture (AHP). This can be adopted for a number of reasons, including:
* To alleviate diplopia or pain--turning the head into the field of action of an underacting muscle moves the eyes in the opposite direction and can avoid diplopia or eye strain. Tilting the head to one side can align vertically separated images, and can give a clue as to the presence of a vertical muscle imbalance. Elevating or depressing the chin can help control a horizontal phoria in the presence of an alphabet pattern. For example, an exophoria with a V pattern is smaller in downgaze, so elevating the chin can achieve the same effect.
* To control nystagmus--some types of nystagmus vary in amplitude (size) and frequency (speed) in certain positions of gaze. In some cases, there is a 'null zone' where the eyes are at rest, or close to it, which stabilises the retinal image and can improve vision. (11) Turning the head can bring the null zone into a more central position. It is also the case that an AHP may be the result of an unrelated musculoskeletal condition and has no bearing on the function of the eyes. It is, therefore, advisable to assess the ocular deviation with and without any AHP and record the effect on the size and control of the deviation.
Other features that can be noticed on general patient observation include:
* Ptosis--the position of the upper lid is determined by the relative action of the levator palpebrae superioris muscle (III cranial nerve), Muller's muscle (postganglionic sympathetic nervous system) and the orbicularis oculi muscle (VII cranial nerve).
* Facial asymmetry--as discussed already, patients with telecanthus (widely spaced eyes), or vertically misaligned orbits are likely to have developed heterophoria in order to achieve single vision.
Look for heterotropia, with the 'cover-uncover test'. With the patient viewing the fixation target, introduce an occluder to the left eye and observe the right eye for any sign of movement to take up fixation. As we have seen, heterotropia occurs without dissociation of the eyes, therefore, if the eyes are not aligned, the deviating eye will have to move to take up fixation (see Table f). If no movement is detected, there is no tropia affecting the right eye.
Repeat the process for the other eye this time observing the left eye for signs of a fixation movement. If no movement is detected, there is no tropia affecting the left eye either.
If one or both eyes have poor vision, it will take longer for the fixation movement to happen, thus it is important to keep the occluder in place for a sufficient length of time, and to ask the patient whether they can still see the target.
If there is no heterotropia, look for heterophoria with the 'alternate cover test.' As discussed earlier in the article, heterophoria occurs in the presence of fusion, therefore, the fusion must be broken, that is to say, the eyes must be dissociated for the ocular deviation to be seen. This is done with an occluder, which should be left in place sufficiently long to break the fast and slow components of fusion. Furthermore, the occluder needs to be sufficiently close to the eye so as to break both central and peripheral fusion as without doing so, deviations can be missed. It should be self-evident that something small, such as a thumb, will not give rise to proper dissociation and should be avoided.
Introduce the occluder to the left eye, and keep it in place for two to three seconds to dissociate the eyes. Now, swiftly move the occluder to the other eye, in order to maintain dissociation, and at the same time, observe the movement of the left eye the instant it is uncovered. If heterophoria is present, the eyes will no longer be aligned, and the uncovered eye has to make a movement to take up fixation, according to Table 1.
Repeat this process several times in order to achieve full dissociation as the deviation may increase in size as tonic convergence is eliminated. If there is no movement of either eye after repeated alternation, then it can be considered that there is no heterophoria. However, it has been noted by Romanao and VonNoorden (1971) that movements of less than 2[DELTA] cannot be reliably seen even by experienced examiners in ideal conditions. (13) Furthermore, vertical movements may be difficult to see because of the eyelids obscuring the movement of the eyes. One way of avoiding such errors is to ask the patient if they are able to see any movement of the target during cover testing as this can highlight even the smallest of deviations.
Look carefully for recovery. Having dissociated the eyes, it is important to see how quickly fusion is restored as this gives an indication of the quality of control of the deviation. If heterophoria is present, as soon as the occluder is removed the eyes are no longer aligned, which gives rise to retinal disparity and diplopia. As discussed earlier, retinal disparity is the stimulus for fusionai vergence, thus on removal of the cover, the non-fixing eye will need to make a compensatory movement to restore fusion. This is the recovery movement, and a rapid movement indicates a well-compensated deviation. A poorly controlled deviation will take longer for recovery to occur, and the patient may even report diplopia before fusion is restored.
Record your results. Take care to note all aspects of the cover test, which may include:
* Near versus distance
* With/without AHP
* With/without refractive correction
* Deviating eye (for heterotropia)
* Size of deviation
* Speed of fixation
* Speed of recovery (for heterophoria). As examples:
* 30[DELTA] left exotropia distance and near with and without spectacles
** No movement detected (NMD) without spectacles
** Near 8[DELTA] exophoria, good recovery without spectacles.
Measurement of the ocular deviation
Measuring the size of the ocular deviation is an important part of the binocular vision assessment, thus a consistent approach is essential. Some advocate estimating the size of the deviation by comparing it to the movement made by the eyes when looking from one end of the 6/6 line to the other, which equates to 2.5[DELTA]. It is the authors' opinion that making such estimates is prone to significant interpractitioner variability, thus it is preferable to use some form of quantifiable test, such as the prism cover test.
Prism cover test (PCT)
Every trial case or phoropter contains a set of prisms, therefore, it is easy for any optometrist to access the tools required for a PCT.
While undertaking a cover test, a prism is placed in front of one eye, and the power increased until there is no further movement of the eyes (the neutral point), or there has been reversal of the deviation (the reversal point). In the same way as for the cover test, asking the patient when they see no net movement of the target may improve accuracy further.
Johns et al (2004) compared the two end points of the PCT, namely, the first neutral point and the midpoint of first neutral and reversal points. They found with experienced examiners, either prism endpoint provides high interexaminer and intraexaminer repeatability (< 0.5[DELTA]). Although the two prism endpoints differ statistically, the differences are not clinically significant. (14)
Maddox rod and Maddox wing
The Maddox rod is a series of high-powered, red cylinders mounted in a trial lens or lorgnette. The patient is asked to view a white spot light at 6m and the Maddox rod is placed in front of one eye, for example, the right eye. The rod produces a red line perpendicular to the orientation of the cylinders, seen by the right eye, whereas the left eye sees the spot light. As the two eyes receive different images, the eyes are dissociated, but not to the same degree as complete occlusion. As such, the test measures the dissociated phoria (or heterotropia), and prism is introduced in front of the eye with the rod until the line and the spotlight intersect. As a rule, the base of the prism is placed in the direction of the deviation of the line, thus in this example, if the patient sees the line to the right of the spot, the prism is placed base out. The rod can then be rotated 90[degrees] and the process repeated to measure any vertical deviation. It is important to keep the room illumination as natural as possible; however, sometimes the light must be dimmed to stop the red streak from being supressed.
The Maddox wing is a handheld device which presents a vertical and horizontal scale of numbers to one eye, and a vertical and horizontal arrow to the other. The eyes are effectively dissociated as a result, and the subject is instructed to say which number on the scales the arrows point towards. It is of limited use clinically as the numbers are not small enough to control accommodation, therefore deviations can appear to vary wildly in size during the test. It can only assess the ocular deviation at near; however, unlike other test methods, it can measure torsional deviations.
Although the tests above are useful to measure the total size of the deviation, the dissociated phoria, they do not indicate how well the deviation is controlled. For example, it is possible for a patient to have a large deviation that is well compensated, and another to have a very small deviation that gives rise to significant symptoms of asthenopia. (15) To establish the amount of prism required to alleviate symptoms caused by decompensating heterophoria, a measurement of the associated phoria is required.
Associated phoria and fixation disparity
As discussed earlier, objects that lie within Panum's fusionai space give rise to retinal disparity, but can still be fused with a so-called associated phoria. This allows binocular single vision and fusion to occur without precise bifoveal fixation. (5) Fixation disparity is a deviation from the 'intended state of vergence' and associated phoria can be considered as a measure of the phoria size when both eyes are open. In the UK, the associated phoria is detected using the Mallett unit.
The Mallett unit has a number of different forms, but each design has monocular markers (nonius strips) in a binocular environment. The different markers are presented to each eye through the use of polarised or coloured filters; however, both eyes see the remainder of the test area, usually a row of letters such as OXO, simultaneously. This serves to minimise the amount of dissociation and ensure only the associated phoria is being measured. Some versions of the near Mallett unit have text around the markers, which induces fusion within the peripheral retina, meaning minute misalignments can be measured. A recent study has shown the instructions given to the patient can influence the results and this correlated better with the patient's symptoms. (16) Instructions for using the Mallett unit (adapted from Pickwell's Binocular Vision Anomalies) are as follows: (17)
* Check each eye can resolve the fixation disparity OXO
* Show the patient the fixation disparity target without the visor--check the nonius lines are aligned and ask: 'Can you see the green lines one above and one below the X? Are they exactly in line?'
* Place the visor over the patient's eyes and ask them patient to read a line of text and then direct their attention to the X.
* Ask: 'Are both the green lines present all of the time?'
* Ask: 'Are they lined up?'
* Ask: 'Does one or other ever move to one side?' If the patient reports the line(s) disappearing then this indicates suppression. If the lines are not directly aligned it indicates a fixation disparity. Movement of one or both lines may indicate binocular instability. The aligning prism can then be established whereby the markers are brought to the correct position and made stable. The minimum amount of prism to bring the bars into alignment with the X should be prescribed.
A cover test is simple to carry out and is objective. It is the authors' opinion that this test should be carried out on every patient as it forms the basis of any binocular vision assessment and gives invaluable information as to the sensory and motor status of the patient.
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Course code: C-5822 Deadline: 7 April 2018
Be able to explain to patients About binocular vision anomalies (Group 1.1.4)
Be able to assess patients Presenting with heterophoria (Group 8.1.3)
Be able to assess patients Presenting with heterotropia (Group 8.1.4)
Be able to explain to patients about binocular vision anomalies (Group 1.2.4)
Understand the investigation of patients presenting with heterophoria and heterotropia (Group 7.1.5)
* Retinal correspondence: bit.ly/2BboyAe
* Cover test: bit.ly/2rl1rjn
Consider two eyes directed towards an object of regard. If the eyes share a common direction, the object stimulates the fovea of each eye. Under normal circumstances the fovea of the right eye corresponds to the fovea in the left eye; in other words, at a cortical level the images are joined to produce a single percept. It is considered that this combined image is projected back towards the object by an imaginary Cyclopean eye. Retinal correspondence occurs across the remainder of the retina, such that points in the temporal retina of the right eye correspond to areas in the nasal retina of the left eye and vice versa. Objects which stimulate these corresponding areas lie on an imaginary line called the horopter. In the same way as for the fovea, the image is again projected back towards the object such that the temporal retina projects nasally and the nasal retina projects temporarily. Now consider the situation where both eyes are again directed towards an object of regard. By definition, the object stimulates both foveae and as they are corresponding retinal points, Cyclopean projection of the image is in the direction of the object. If a second object is introduced some distance in front of the first then as it does not lie on the horopter, it stimulates non-corresponding retinal points. In this case, regions of the temporal retina in both eyes are stimulated. As discussed, the temporal retina projects nasally, thus both eyes project to different points in space giving rise to diplopia; this is an example of crossed physiological diplopia. In order to achieve a single image of the second object, the eyes have to converge to restore foveal fixation. In other words, the presence of diplopia has stimulated fusion through the vergence system. By the same token, the first object now stimulates non-corresponding nasal retinal points and gives rise to uncrossed physiological diplopia.
Dr Catherine Porter PhD, MCOptom and Simon Frackiewicz BSc (Hons) Optom, BSc (Hons) Orthop, MCOptom, DipTp(IP)
Dr Catherine Porter is a senior lecturer in optometry at the University of Manchester. She lectures in binocular vision and has a special interest in patients with reading difficulties.
Simon Frackiewicz is an optometrist working in private practice in Somerset. He is also a qualified orthoptist and works at Yeovil District Hospital both as an orthoptist and optometrist.
Table 1 Movements seen on cover test Phoria (look at eye Tropia (look at uncovered which was under the eye when cover introduced) cover when the cover is removed) Eso Eye moves out to fuse Eye moves out to take up fixation Exo Eye moves in to fuse Eye moves in to take up fixation Hypo Eye moves up to fuse Eye moves up to take up fixation Hyper Eye moves down to fuse Eye moves down to take up fixation