Anomalous Retinal Correspondence
Anomalous retinal correspondence was first described by Johannes Peter Müller in 1826 – today much more is known. In order to understand anomalous retinal correspondence (ARC), it is important to first understand normal retinal correspondence (NRC). NRC occurs when the point in space that is being focused on corresponds to the fovea in each eye, producing a single image, and other points in the peripheral visual field likewise correspond to the exact same points on both retinae – a point to point relationship. ARC is an adaptation that occurs when light from the point in space that is being focused on hits the fovea of one eye and hits an extra-foveal retinal point in the contralateral eye. Under normal circumstances, having the same image stimulate two dissimilar points of the retina would produce diplopia. But in ARC, over time the fovea of the normal eye and the extra-foveal retinal point of the abnormal eye coordinate and are able to correspond to produce a single visual image.
The cause of ARC begins due to foveal misalignment which can occur due to a variety of reasons. The first major risk factor is strabismus, which is one of the most common pediatric ophthalmologic pathologies and is a risk factor for ARC. There are many causes of strabismus, most of which are due to issues with neuromuscular eye control. Eye misalignment due to strabismus in childhood can cause foveal misalignment, causing diplopia. Because the strabismus began in childhood, the maturation of the visual cortex has not yet occurred and still has plasticity; therefore, an extra-foveal corresponding point in the retina can be created, leading to ARC, which clinically would present as strabismus without diplopia. It is speculated that the crucial point in childhood at which ARC would occur is around 3 years of age. In addition, in adult onset ARC, the culprit is usually late onset retinal pathologies that pull the fovea off center. These retinal pathologies include epiretinal membranes and subretinal neovascular membrane formation due to age related macular degeneration, which can lead to foveal misalignment and disruption in foveation.
The presence or absence of diplopia and compensation with ARC first depends on the level of foveal misalignment. Microtropia is a small angle strabismus of less than 5 degrees (8-10 prism diopters) in which diplopia may not be present since ARC is able to occur. This lack of diplopia and the presence of the same image in each eye is thought to be due to points on the retina having an acceptable level of eccentricity when fixated on a single location in space, also known as a horopter. Each retinal point has a certain deviation within which it can still produce an image that matches that of the other aligned eye. This area of acceptable disparity on the retina is also called Panum’s area – if an object is in this space on the retina it will fall on different retinal areas of each eye but is still seen as a single image. If a horopter falls outside of Panum’s area and reaches vastly different areas of the retina, then this will manifest as a perception that the horopter is coming from different directions, causing a physiological diplopia. In microtropia (<5 degree angle of deviation), the level of deviation due to the strabismus is low enough and the motor misalignment is within the acceptable amount of error for each retinal point (within Panum’s area) such that diplopia is not found and ARC takes place.
As the angle of deviation of the eye in strabismus increases, ARC decreases. Therefore, in large angle strabismus, the angle of deviation is too large and falls outside of the acceptable overlapping eccentricities of the corresponding retinal points in each eye (outside of Panum’s area), and two images are formed.
For ARC to take place, a neurological adaptation in which the extra-foveal retinal point of the abnormal eye corresponds with the fovea of the normal eye occurs, resulting in corresponding vision. Before the adaptation seen in ARC takes place, the horopter is seen separately in each eye and arising from different points in space since the image is passing between instead of through the intersection of the axis of vision. The occipital cortex, in still attempting to produce a single image from the two, will suppress the retinal points with more eccentricity and worse resolution, while the other eye which produces the better image is perceptually emphasized. It is thought that the adaptation that causes ARC occurs in the occipital cortex where image fusion happens – in particular, the primary visual cortex V1, where binocular neurons are first found. It was originally thought that in strabismus, ARC is created due to neuronal axons stretching to longer lengths and being monosynaptic, but now it has been shown that instead they retain their normal length and are polysynaptic. V1 is best able to perform ARC when ocular dominance columns in each eye are less than two neuronal lengths apart. Adaptation through ARC is more likely in pediatric patients due to a higher degree of cortical plasticity. The presence of coordinated areas of the occipital cortex where the retinal topology is less defined, and where the area where binocular information is received and integrated is very large are plausible to be responsible for ARC. It is also hypothesized that areas 20 or 21 of the brain could be responsible due to their size and their incorporation of the corpus collosum.
In terms of prevalence, microstrabismus is seen in about 1% of the population. Looking at the angle of deviation of the strabismus and presence of ARC, there is a correlation. As the angle of deviation of the eye in strabismus increases, the presence of ARC decreases. Specifically, in children with angles of strabismus that are less than 5 degrees or 8-10 prism diopters, the prevalence of ARC is >90%. With angles between 15 and 30 prism diopters ARC is still more prevalent, and in children with deviation of >40 prism diopters the prevalence of ARC is <16%.
In addition, there is a higher chance of achieving ARC as an esotrope versus as an exotrope. Anatomically, there is greater input and territory in the visual cortex from the nasal retina compared to the temporal retina. In exotropes, the extra-foveal retinal point will be more temporal, creating a higher degree of suppression instead of the rewiring that occurs in esotropes when the extra-foveal retinal point falls in the more prioritized nasal retina.
The type of ARC depends on the objective and subjective angles of anomaly. The objective angle is the motor deviation, or the angle of deviation of the eye that is seen. The subjective angle is the sensory deviation or how far off the extra-foveal retinal point is from the actual fovea. There are three different types of ARC: harmonious, unharmonious, and paradoxical. In harmonious ARC, the objective angle is equal to the subjective angle. In unharmonious ARC, the subjective angle is less than the objective angle. If the localization of the subjective and objective angles is crossed or uncrossed it is called paradoxical ARC.
In ARC, the patient will usually present with a strabismus or ocular deviation which can be found using prism testing and would normally produce diplopia in the patient if uncorrected. A patient with strabismus and ARC will display normal vision without diplopia.
In addition, if ARC is present, the severity can differ. In some patients, the ARC will be very obvious to testing, and in others it will be very superficial or will come and go based on the circumstances. This depends on how strong the connection to the extra-foveal retinal point is – there is a greater magnitude of ARC found in patients who have dealt with ARC for a longer time and have a stable deviation with a primary extra-foveal retinal point that is used (as opposed to the point constantly changing). When the point changes frequently, this can lead to a rare occurrence of a patient presenting with normal and anomalous correspondence in the same eye as well as normal vision in the other eye, causing binocular triplopia. In binocular triplopia, the foveas of both eyes and the extra-foveal retinal point are being used and are competing with each other, producing three images. Binocular triplopia is also commonly seen in the transition during treatment or after surgical corrections as the aberrant eye is adjusting to using both foveas again.
The Worth 4-dot test can assess binocular vision can evaluate for peripheral sensory fusion and suppression of the fovea. In NRC the patient would see all four dots and in ARC the patient would be diplopic with the deviation adjustable by prism.
Bagolini striated glasses have no power to them, and the visual acuity of the patient is unchanged. The lenses are placed at right angles at 135 degrees in the right eye and 45 degrees in the left eye with narrow striations that run parallel to each other. Seeing the lights cross as an X means that the patient has normal vision and NRC. Therefore, if it is known that the patient has strabismus and they also report an X – this would indicate ARC.
The cover test can be used to detect the presence and qualities of strabismus that is always present. The occluder is held in front of and then removed from each eye at a time. When the preferred eye is covered, the non-preferred eye may be deviated and will adjust to fixate indicating a tropia. When the non-preferred eye is covered, the preferred eye should not move. Further testing for ARC is warranted if misalignment with normal vision is seen.
Testing with a synoptophore can be used to assess the subjective and objective angle of deviation and any anomalies in binocular vision when an image is presented to each eye. A difference between the subjective and objective angles (the angle of anomaly) indicates that the patient has a form of ARC.
The red filter test can be used to unmask a diplopia in a patient, as well as suppression and ARC. A red filter is placed over the patient’s eye and the patient is asked to focus on the white circle. If an esotropic patient reports crossed diplopia, or an exotropic patient reports uncrossed diplopia, this could indicate ARC. If the diplopia remains with use of a prism, this could also indicate ARC.
The after image test can also assess for ARC and can directly ascertain the foveal direction of the eye that is offset. A vertical light is beamed in one eye, and a horizontal light is beamed into the other eye. If the patient has NRC they will see an after image that looks like a cross, and if the patient has ARC with a known strabismus they will also see a cross.
The major concern with treatment of ARC is that correction of the strabismus for cosmetic purposes can cause new-onset diplopia. This finding occurs because the extra-foveal retinal point that was acting as the focal point would no longer correspond with the fovea of the contralateral eye. Surgical correction for ARC is there not usually indicated, and NRC rarely returns following surgery. However, there are still ways to treat ARC. The treatment modalities center around a few principles that work towards improving the sensory relationship between the two foveas and each retina. These methods are to increase fixation through the fovea of the deviated eye, and to correct any refractive errors.
In ARC, because the extra-foveal corresponding point is in the peripheral retina, there is suppression between the periphery and the fovea. One treatment modality is to change this sensory pattern through occlusion of the preferred eye to usage of the deviating eye. Occlusion is best done in children given the increased plasticity in childhood. Occlusion reduces reinforcement of the use of the extra-foveal retinal spot and increases stimulation of the actual fovea.
Prisms that direct light onto the fovea can be used for small deviations but not for major correction of ARC. This is because in a patient with strabismus, the patient will simply adjust for the prism and look to where the light is again directed towards the extra-foveal retinal point.
Another method is to use a red filter on the deviated eye. Since the macula contains the highest concentration of cones, using a red filter would only stimulate these cones creating increased use of the macula and fovea instead of the extra-foveal retinal point.
Another method is through the use of an ambyloscope which uses flashing lights at the patient’s objective angle of strabismus and uses a monocular diplopia routine. This method stimulates the fovea of both eyes first with large flashing targets which then over time reduce in size to a foveal size and increase in intensity. Slowly the lights are advanced further into the area of suppression and the macula towards the fovea, “massaging the macula”, and slowly as the two lights will converge the patient is asked to hold the images fused together for longer and longer periods. This method works best when the ARC is not extremely firmly set yet.
A newer potential therapy for ARC is the use of botulinum which can be injected into the various muscles of the eye to correct for deviations. In addition, with the advent of virtual reality headsets and lenses, it can be conceived that virtual technology can also be incorporated into ARC treatment in the future.
The pathophysiology of anomalous retinal correspondence is multifaceted and is still being understood. Though uncommon, it is important for the ophthalmologist to be able to diagnosed, treat, and most importantly prevent the development of anomalous retinal correspondence. Even though clinically a patient’s vision can appear normal, upon further inspection especially in patients with strabismus, anomalous retinal correspondence should be a consideration. There are a variety of tests and therapies that can be utilized by the ophthalmologist in patients that present with ARC that can aid in its diagnosis and treatment.
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