Lecture 9 and 10 Flashcards
“Objects on the visual axis of either eye in symmetrical convergence appear to be on the median plane of the head.”
In other words, images on corresponding points are seen from the cyclopean eyes
Hering’s third law:
Some avoid diplopia by developing what?
anomalous retinal correspondence (ARC) to achieve fusion.
The centers of the foveae have different visual direction (oculocentric direction).
The fovea of one eye has the same visual direction (straight ahead) as a peripheral retinal point in the fellow eye. Here, these two points become corresponding points
ARC may be present with or without eccentric fixation.
Anomalous Retinal Correspondence (ARC)
In ARC, the visual system shifts its ________ coordinate system so that the visual directions of the right fovea and left non‐foveal point __________. All the other corresponding points shift in accordingly.
n ARC, the visual system shifts its oculocentric coordinate system so that the visual directions of the right fovea and left non‐foveal point correspond. All the other corresponding points shift in accordingly.
ARC is a type of antidiplopic sensory adaption adopted by some strabismic patient. Despite strabismus, some patients are able to preserve rudimentary binocular vision.
ARC is seen in as many as half of strabismus cases.
clinical significance of ARC
ARC is an important indicator for prognosis of what type of tropia?
ET:
• ARC present – less than 50/50 chance of cure
• ARC absent – better than 50/50 chance of cure
ARC is only found in what type of strabismus?
early onset strabismus
If the onset is after 6 y.o., usually no ARC.
ARC maybe influenced by luminance and contrast, unlike _____________. The more natural the testing environment, the more likely that _____ be found.
normal correspondence, ARC
Hypothesis: ARC is a sensory adaptation to constant strabismus.
ARC wiring develops slowly through early childhood. NRC is not completely lost. Some patient retain NRC
after strabismic surgery.
Once established, ARC can be turned on and off, depends on visual stimuli and testing environment.
SENSORY THEORY OF ARC:
Most widely accepted theory of ARC
Hypothesis: ARC is the result of the registration of the abnormal vergence innervation that causes the strabismus.
– Example: A person is looking at the world with their turned eye monocularly. If an object is egocentrically straight ahead, they will see it as straight ahead even though their eye is turned and seeing it in their periphery.
The deviation itself causes the ARC – it’s not necessarily an adaptation.
Covariation is easily explained by the motor theory. Harmonious ARC is expected (and usually occurs).
Motor Theory of ARC:
Less well known and less well accepted
the horizontal objective angle of deviation. This is the angle of deviation for the eyeball itself.
– It may be measured using the cover test with prisms.
Angle H
the subjective angle of directionalization. This is the perceived angle of diplopia.
– It may be measured using the same principles as the Maddox’s rod or red lens test.
Angle S
the angle of anomaly, the amount of shift of correspondence from fovea.
– It may be directly measured by the afterimage test, or indirectly calculate from angle H and angle S.
Angle A
In normal correspondence, angle H (objective angle) and angle S (subjective angle) are WHAT?
equal
In ___________ these angles are not equal, and the difference between them is the angle of anomaly.
anomalous correspondance
angle A = (angle H ‐ angle S).
