ARC Flashcards
What is arc
The ocular sensory system in children has the ability to adapt to anomalous states (confusion and Diplopia) by two mechanisms. One of which is suppression and one abnormal retinal correspondence. These occur because of the plasticity of the developing visual system in children under the ages of 6-8 years.
What type of conditions is this?
Arc is a condition in which non -corresponding retinal elements acquire a common subjective visual direction i.e fusion occurs in the presence of a small angle manifest squint ; the fovea of the fixating eye is paired with a non-foveal element of deviated eye.
Binocular responses in ARC are never as good as in normal bifoveal BSV.
It represents a positive sensory adaption to strabismus (as opposed to negative adaption by suppression) which alllows some anomalous binocular vision in presence of heterotropia.
It is most frequently encountered in small angle esotropia (microtropia), but is less common in accommodative esotropia because of the variability of the angle of deviation and in large angle deviations because of the separation of images is too great.
Two types
Two types of ARC- Harmonious (HARC) – of the angle of anomaly = angle of strabismus
Normal retinal corespondence
Normal retinal correspondence 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.
Causes
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.[4] 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.
Angle of deviation
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, 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 neurones 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.
