Anomalous Retinal Correspondence (ARC) is a sensory adaptation phenomenon associated with strabismus. It refers to a condition where, when light from the fixation point reaches the fovea of one eye, it reaches an extra-foveal retinal point in the other eye, and that extra-foveal point acquires the same visual direction as the fovea of the opposite eye. It was first described by Johannes Peter Müller in 1826.
In Normal Retinal Correspondence (NRC), the foveas of both eyes share a common visual direction, and retinal points at equal distances from the fovea on the temporal and nasal sides correspond, establishing a “point-to-point” relationship with a common visual direction.
The following conditions are necessary for normal binocular vision to develop:
The horopter is the set of points in space (Vieth-Müller circle) that project onto corresponding points of the two retinas. Panum’s fusional area is the region near the horopter where sensory fusion is possible. The width is narrow near the fixation point and wider in the periphery.
Microstrabismus is found in about 1% of the population. The smaller the angle of strabismus, the higher the incidence of ARC; in children with deviation less than 5 degrees (8–10 PD), ARC is present in over 90%. ARC is also common with deviations of 15–30 PD, but drops to less than 16% when deviation exceeds 40 PD.
The smaller the strabismus angle, the higher the incidence of ARC. In microstrabismus with a deviation angle of less than 5 degrees, ARC is observed in over 90% of patients. Conversely, in large-angle strabismus exceeding 40 prism diopters, the prevalence of ARC drops to less than 16%. It is thought that suppression is more likely to occur than ARC when the deviation angle is large.
Rarely, binocular triplopia, in which normal and abnormal correspondence coexist in the same eye, may occur. It can also be seen after strabismus surgery correction or during the transition period of treatment.
The most common cause is childhood strabismus. When foveal misalignment occurs during the period of visual cortical plasticity (around age 3 is a critical period), it can develop into ARC. Esotropia is more likely to lead to ARC than exotropia. This is thought to be because the input area from the nasal retina in the visual cortex is wider than that from the temporal retina, making neural rewiring easier. In exotropia, the extrafoveal point is located on the temporal retina, so stronger suppression tends to occur rather than rewiring.
In adults, the following diseases can displace the foveal position and cause ARC.
This condition is called “dragged-fovea diplopia syndrome” 1). The prevalence of epiretinal membrane and macular disease is about 2% in those under 60 years old and up to 12% in those over 70, of which 16–37% present with binocular central diplopia 1).
The larger the deviation angle, the lower the incidence of ARC.
| Strabismus angle | Approximate ARC prevalence |
|---|---|
| Less than 5 degrees (8–10 PD) | Over 90% |
| 15–30 PD | Moderate (still common) |
| Over 40 PD | Less than 16% |
For diagnosing ARC, it is important to use tests that reflect everyday vision and tests that completely separate the two eyes. The closer to everyday vision, the easier suppression and abnormal correspondence occur. The further from everyday vision, the easier normal correspondence occurs. The test method and conditions should be selected based on whether you want to know the binocular vision state in everyday life or the potential binocular vision ability.
Bagolini test
Principle: Examines retinal correspondence and simultaneous perception in a state closest to everyday vision. A plano lens (one eye 45°, the other 135°) has fine parallel line scratches, causing a penlight source to be perceived as an X-shaped streak.
Judgment: If the patient sees an “X” crossing despite having strabismus, ARC is suggested. Testing distance is 30 cm or 5 m in front of the eyes.
Worth 4-Dot Test
Principle: The red-green filter separates the two eyes to test simultaneous perception, fusion, and retinal correspondence. With a red filter, green targets are not visible, and vice versa, utilizing complementary colors.
Judgment: Understanding eye position is important for evaluating retinal correspondence. Used to differentiate suppression, ARC, and normal correspondence.
Afterimage Test
Principle: Occlude one eye at a time, and shine a vertical light on one eye and a horizontal light on the other to create afterimages on the fovea (Bielschowsky afterimage test).
Judgment: If central fixation is present, measurement is possible regardless of eye position. In strabismus with NRC or ARC, it appears as a cross shape.
Major Amblyoscope
Principle: The synoptophore separates the two eyes and projects different images to each eye. It can be adjusted horizontally, vertically, and torsionally.
Judgment: ARC is determined by the difference between the subjective and objective angle of deviation (anomalous angle). If the simultaneous perception test result differs from the objective strabismic angle, abnormal retinal correspondence may be present. It is also useful for predicting binocular visual function after strabismus surgery.
This is an auxiliary test to detect suppression or ARC. ARC is suggested when esotropia patients report crossed diplopia and exotropia patients report uncrossed diplopia.
A specific test for foveal displacement due to epiretinal membrane is the “lights on/off test” 1). In a completely dark room, central fusion allows a small white background with black letters to be seen as single, but when the room light is turned on, peripheral fusion is restored and diplopia reappears. This is considered a pathognomonic finding for this condition.
If a patient with confirmed strabismus reports crossing of the “X,” ARC is suggested. If there is a strabismus angle, one of the striae should appear deviated from the center, but with ARC, the crossing of both striae is perceived. Since this test is performed under conditions close to daily vision, it reflects the everyday state of ARC well.
Treatment for ARC varies depending on the strabismus condition, age of onset, degree of ARC, and etiology.
When performing strabismus correction for cosmetic purposes, patients with ARC are at risk of developing new diplopia. This is because the extrafoveal retinal point no longer corresponds to the fovea of the opposite eye, and NRC rarely recovers after surgery.
However, in adults with childhood-onset strabismus, adaptation to ARC, including cyclotorsional ARC, is almost always achieved after surgery 1). The incidence of persistent new diplopia after surgical correction is very low. While prism correction may initially cause diplopia, surgical correction can sometimes achieve adaptation 1).
Occlusion Therapy
Method: Occlude the dominant eye to promote foveal use in the deviated eye.
Mechanism: Reduces stimulation to the extrafoveal retinal point and increases stimulation to the actual fovea. It is most effective in childhood due to high plasticity.
Prism Correction
Method: Optically corrects eye misalignment using prisms.
Limitations: Useful for small deviations but unsuitable for large corrections. Strabismus patients may adjust eye position to the prism, causing light to again fall on the extrafoveal area.
Red Filter
Method: A red filter is worn over the deviating eye.
Mechanism: The macula contains the highest density of cone cells. By stimulating only the cones with a red filter, the use of the macula and fovea is promoted, rather than the extrafoveal point.
Pleoptics training
Method: Use a monocular diplopia routine that flashes light at the objective strabismic angle. Stimulate the fovea of both eyes with a large flashing target, then gradually reduce the target to foveal size and increase intensity.
Indications: Most effective when ARC is not yet firmly established.
When caused by foveal displacement due to an epiretinal membrane, strabismus correction with prisms or surgery is not curative 1). This is because it does not resolve the mismatch of distorted macular images or the conflict between central and peripheral correspondence. The following methods reduce the awareness of diplopia.
Correcting eye position surgically carries the risk that a retinal point outside the fovea will no longer correspond to the fovea of the opposite eye, causing new diplopia. However, in adults with childhood-onset strabismus, ARC often readapts postoperatively, and persistent diplopia is considered rare 1). Adequate preoperative counseling is necessary.
ARC occurs as a sensory adaptation in the occipital visual cortex (especially V1). V1 is the first site where binocular neurons exist, integrating inputs from both eyes. In response to foveal misalignment due to strabismus, the brain adapts by suppressing input from retinal points with greater eccentricity and lower resolution, or by perceptually emphasizing the eye with the better image.
The neural structure of ARC was initially thought to be long monosynaptic, but it is now shown that axons are of normal length and polysynaptic. In V1, ARC is considered most efficient when the ocular dominance columns of each eye are within less than two neuron lengths. Due to high cortical plasticity in children, ARC occurs more readily, and areas 20 and 21 of the brain may also be involved in ARC.
Some degree of fusion is possible even with slight horizontal, vertical, or cyclotorsional misalignment.
ARC is classified into three types based on the relationship between the subjective and objective angles of deviation.
| Classification | Definition |
|---|---|
| Harmonious ARC | Objective deviation angle = Subjective deviation angle (anomalous angle = 0) |
| Unharmonious ARC | Subjective deviation angle < Objective deviation angle |
| Paradoxical ARC | Localization of subjective and objective deviation angles reversed in terms of crossing/non-crossing |
Physiological diplopia occurs when an object outside Panum’s fusional area forms images on non-corresponding points of the two retinas, but it is not noticed in daily life. Pathological diplopia occurs because the visual target fixated by the fovea of one eye is projected outside the fovea of the other eye. If there is an abnormality in retinal correspondence, “paradoxical diplopia” that contradicts the eye position may be observed.
Stereopsis is achieved when the brain detects binocular disparity and converts it into depth. Fine stereopsis results from foveal fusion, while coarse stereopsis results from peripheral fusion. Normal stereopsis enables precise depth perception, such as distinguishing an 8 cm depth difference at a distance of 10 meters.
Interocular competition in visual development begins around 2 months after birth and continues beyond 6 years of age 2). If early occlusion lasts more than 2 months, the final visual acuity becomes worse than in cases of binocular occlusion. Early recovery is driven solely by visual activity, and thereafter competitive interaction becomes the strongest determinant of recovery 2).
Correction of deviation by botulinum toxin injection into extraocular muscles is being investigated as a new potential treatment for ARC. Attempts are being made to temporarily correct strabismus and induce changes in ARC by injecting into various eye muscles.
With the advent of VR headsets and VR lenses, it has been suggested that virtual technology may be incorporated into ARC treatment in the future. VR, which can present independent images to each eye, is expected to serve as a new treatment platform to replace amblyoscope training.
