Lecture 5/6 Flashcards
how is fixation disparity different from binocular disparity?
fixation disparity is a misalignment of the visual axes and binocular disparity is non-correspondence of the retinal regions stimulated by a target located off the horopter
what is Hering’s law of binocular visual direction?
the visual direction of fused images which fall on slightly disparate on retinal points is the average of the two visual direction
what is is fixation disparity angle under exo fixation disparity?
aL > 0 and aR < 0 = n is positive (crossed binocular disparity)
what is the fixation disparity angle under eso fixation disparity?
aL < 0 and aR > 0 = n is negative (uncrossed binocular disparity)
how are fixation disparity and heterophoria’s related?
FD and heterophoria is usually correlated (exophoria = exo FD and esophoria = eso FD) - but do not always agree
what are the testing conditions for fixation disparity?
test under binocular conditions using prisms to find the associated phoria
what are the testing conditions for heterophorias?
test under monocular conditions using prisms to find the dissociated phoria (need to break binocular fusion)
what is an associated phoria?
the amount of prism needed to eliminate fixation disparity (horizontal associated phoria is usually smalelr than dissociated phoria)
what are the two common features shared by most clinical tests for FD?
binocular fusion lock (seen by both eyes with angular width of 1.5) and two nonius lines (seen monocularly)
what are some tests that assess FD and the associated phoria?
Bernell lantern, malett box, wesson card, sheedy disparometer, and Saladin card
what is fixation disparity?
a very small deviated of the visual axes during normal binocular fusion - the fixated object is still seen as fused
can a fixation disparity exist if the patient has a strabismus?
no - it is a deviation that only exists during binocular fusion
which clinical test is the most accurate for the measurement of FD?
sheedy disparometer (Saladin card uses the same method)
using the wesson card, are the polarized lines deviated or centered?
they are centered - the lines mark the intended fixation point but each eye’s visual axis misses it (appear deviated due to FD)
using the sheedy disparometer, are the polarized lines deviated or centered?
they are actually deviated - each line falls on the visual axis of that eye and appear centered due to FD
what does the sheedy disparometer display?
the location of the visual axes of each eye (actual position of OS and OD)
what does the wesson card display?
the perceived location of the fixation point relative to each visual axis (apparent location of OS and OD)
what does introducing prism do when a patient has a fixation disparity?
used to shift the images to match their visual axis (image will follow the apex)
what does BI prism stimulate?
negative fusional vergence (divergence)
what does BO prism stimulate?
positive fusional vergence (convergence)
if BI prisms are introduced in front of both eyes - what happens to the image for the OD and what does the eye do in response?
BI prism moves the image temporally (nasal retina) (for OD) - creates a small disparity, within panum’s area which stimulates disparity vergence (divergence) and foveal fixation is restored
what do you do if a patient has a 2’ exo FD?
start adding BI prism - causing the image to shift temporal (decreasing the exo FD) - eventually with the right amount of BI prism (4 BI) the FD will be zero
what happens to a 2’ exo FD if you introduce too much BI prism?
more BI prism will shift the images further to the right - beyond the visual axis and the patient will now have an eso FD
what is forced vergence?
using prisms to force the eyes to diverge or converge to eliminate an FD (using BI or BO)
what is the x-axis in the forced vergence fixation disparity curve?
the prism that you use to neutralize the associated phoria (BI or BO) where FD = 0
what is the y-axis in the forced vergence fixation disparity curve?
the patient’s fixation disparity (Eso or Exo) without any prism
what do the ends of the curve plotted represent in the forced vergence fixation disparity curve?
the break points for BI and BO prisms - after these points the patient is outside Panum’s area and will have diplopia
what is the forced vergence fixation disparity curve?
characterizes how the vergence system will respond to the demand - more dynamic measurement than a single phoria or vergence measure
what is the slope at the y-intercept indicate in the forced vergence fixation disparity curve?
how “good” the vergence system is (flatter is better/stronger)
what does a flat central part (stays around zero) indicate in the forced vergence fixation disparity curve?
there is no fixation disparity
what does a greater/longer slope indicate across the middle section in the forced vergence fixation disparity curve?
fixation disparity is slowly changing with addition of prism (eyes tend to lag behind prism when they stimulate vergence)
what is the type 1 FD curve?
the more common and considered a normal response (can be EXO or ESO)
what is the type 1 eso FD curve?
eso FD with no prism - then add BO prism to decrease eso FD until it is zero
what is the type 1 exo FD curve?
exo FD without prism - add BI to decrease until it becomes zero
what is the type 2 FD curve?
usually found in patients with large eso-phorias (curve is all in eso portion)
why does the type 2 curve stay the same no matter how much BO prism is given?
the eyes are too good at converging and they stop following the prism
what is the type 3 FD curve?
usually found in patients with large exo-phorias (curve is all in exo portion)
why does the type 3 curve stay the same no matter how much BI prism is given?
the eyes are too good at diverging and they stop following the prism
what is the type 4 FD curve?
the patient has a small exo FD - very rare (5% of population)
what happens to the type 4 curve when BI increases?
the FD quickly converts from exo to eso - the eyes are not following the prism very closely
what everyday task may increase FD, associated phoria or the FD curve slope?
sustained near visual tasks (reading) produce stress in the binocular system
