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Primary Optometry Lab 6163 (Wensveen); Fixation Disparity Practice Questions:
1) During measures taken with the Mallett box at near, which of the 4 classic components of
vergence contribute to the measured vergence position?
There is always TONIC vergence, because the measure is at near there will be
ACCOMMODATIVE convergence and convergence due to PROXIMAL vergence, but
there will also be FUSIONAL vergence because the Mallett box measures the associated
phoria which is the prism to correct for the fixation disparity which is the deviation when
you are fused.
2) The Mallett box measures the same function as the:
The Mallett box measures the associated phoria, which is prism to correct for the fixation
disparity; the other instrument that you will use to measure the associated phoria is the
Wesson Card.
3) Your lab partner has plotted her forced-vergence fixation disparity curve and has decided it is
a type III. The x-axis intercept is zero. What would her fixation disparity be?
The x-axis intercept is the associated phoria (prism to correct for the fixation disparity),
and if it is zero, then there is no fixation disparity.
4) In the lab you used the Wolff card to measure your own baseline fixation disparity. Remember
that the right eye saw the top line, and the left eye saw the array of lines on the bottom. If you
saw the Wolff card like:
l
l l l l l l l
C B A 0 1 2 3
Your fixation disparity would be: 22 arcminutes ESO
The direction of the correcting prism would be: BASE OUT
5) In the same lab we used the Wolff card to measure fixation disparities that we were trying to
elicit in our subjects. The right eye saw the top line, and the left eye saw the array of lines on the
bottom. If the subject reported seeing the Wolff card like:
l
l l l l l l l
C B A 0 1 2 3
The fixation disparity would be: 17 arcminutes ESO
The direction of prism used to create this fixation disparity would be: BASE IN
6) You are using the Wesson card to assess the ocular alignment of your patient. He sees the
black arrow with his right eye and the array of colored lines with his left eye. He tells you the
black arrow is pointing to the yellow line that is to the right of the center red line. He has an ESO
fixation disparity (kind & direction of deviation) that could be neutralized with BASE OUT
(direction) prism.
7) You decide to test the same patient with the Mallett box at near. If the right eye sees the top
line and the left eye sees the bottom line, how do you expect your patient to see the lines?
From the previous question you know that he has an ESO fixation disparity.
He should see the bottom line to the left.
8) On the same patient, with the Maddox Rod in front of the right eye you should expect him to
say that the vertical red line is:
The Maddox Rod measures a dissociated phoria; if your patient has an ESO fixation
disparity he is likely to have and ESO phoria.
He should see the red line to the right of the white light.