VS117 Co-ops

Thursday, February 6, 2003

Sabrina Chan

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The material on the midterm will not be tested again, so think of it as a final. Also, we will be allowed to stay a little over the normal hour and a half lecture time, if needed.

*the slides refer to the powerpoint slides from lecture that were sent out to the class

Last Tuesday:

• fixation disparity – clinical technique for evaluating effects of vergence adaptation, accommodation, convergence; dynamic & static components on accuracy of convergence response
• used to assess whether stress or demands are placed on convergence & accommodation that need some kind of assistance (optical or eye exercises)

Classical Approach – initial introduction on how to analyze patient’s clinical data to determine if they need some optical correction or eye exercise to assist them in their near work, far work, visual demands

Graphical Analysis (GA) – based on Maddox classification (tonic, proximal, accommodative, fusional vergence)

- Maddox assumption – trust that Maddox components add linearly, but ignore proximal vergence

1. Tonic Vergence – estimate from distance phoria
1. distance phoria is the error at distance
2. tonic vergence – eyes move from anatomical into physiological position of rest
3. if have too much, are eso at distance; not enough, are exo at distance
2. Accommodative Vergence – plot near phoria
1. uses the calculated AC/A ratio – comparison of near and far phorias
3. & 4. Fusional – measured by Positive Relative Convergence (PRC) and Relative Convergence (NRC)
1. PRC uses BO convergence stimuli and NRC uses BI divergence stimuli
2. How NRC works:

then doubling

- reduce prism until they see single again – called blur, break, and recovery

- this measures the range of divergence

- patient sees blur b/c the eyes are near their limits of fusion and are purposely accommodating in

error to extend fusion w/accommodative vergence; eventually it’s too much and you see double

- recovery is when you reduce the prism until fusion captures again

5. Accommodative Amplitude – min. accommodation response

(Refer to slide 4) The x-axis of the graph is describing the convergence response and stimuli (prisms). Always assume convergence response equals the amount of prism in front of the eye. Y-axis is diopters of accommodation. When all 5 clinical measures are plotted on the graph, they will define a parallelogram. The parallelogram, the zone of clear single binocular vision (ZCSBV), describes all possible combinations of lenses and prisms that we can fuse and see singly with. Basically, it shows the entire range of interactions b/w accommodation and convergence that you can perform successfully. It allows you to predict clinical data that you haven’t taken, you can see when a patient will have double vision and single vision, and you can see the effects of adding lenses and prisms on the patient’s comfort and phoria.

Interpretation of the Graph (see either handout from previous lecture or slide 4):

- 0 indicates infinity or far viewing distance with no prism in front of the eyes.

• to the right is convergence stimuli w/BO prism

• to the left is divergence stimuli w/BI prism
• vertical axis – lens power or diopter stimuli to accommodation (reciprocal of viewing distance)
• 2 dashed lines – represent near testing distance at 40cm which is 2.5D stimulus to accommodation
• vertical dashed line represents convergence stimulus at 40 cm which is 2.5MA; assume this person has a 6 cm PD; 2.5MA x 6cm PD = 15 prism diopters, which is the convergence demand at 40 cm
• first want to represent all natural stimuli to convergence and accommodation, where natural means that the accommodative stimulus is matched to the convergence stimulus (i.e. at 40 cm, accommodate 2.5D and converge 2.5MA = 15 prism diopters) – this occurs where the 2 dashed lines intersect
• at far, 0D of accommodation and 0 prism diopters of convergence are the same pt. = optical infinity
• if connect pt of optical infinity to pt where dashed lines cross, you’ll get the demand line

1. 1:1 or Demand Line – represents combination of convergence and accommodative stimuli for all viewing distances
1. this line represents the real world where those are the accommodative and convergence stimuli as a result of geometry w/o glasses
2. can use to predict phoria and fusion ranges at any distance even though clinically, you only measured at 40 cm and infinity
3. remember, based on assumption of linear components
2. Phoria Line (slide 8 or reader p.201, bottom graph)
1. example of how to plot phoria line: at distance, patient is 5 exophoric
2. put "x" at divergence value of 5 prism diopters (use bottom scale for far)
3. at near (i.e. 40 cm), patient is ortho, so put "x" at 0 prism diopters (using top scale)
4. connect those 2 pts. -> the phoria line
5. slope of the phoria line equals the reciprocal of the calculated AC/A ratio

- b/c have convergence on the horizontal axis and accommodation on the vertical axis

6. if phoria is same at all viewing distances, AC/A ratio is equal to PD in cm, then phoria line would be parallel to the demand line
7. the separation b/w the phoria line and the demand line equals the phoria (the error of convergence)
8. if you have no phoria, you would be on the demand line
9. phoria line predicts the phoria at all viewing distances
10. cap to the ZCSBV is your amplitude of accommodation; gives height to ZCSBV
3. Linear combination of tonic, accommodative, fusional vergence graph (slide 10 or reader p.200)
1. to get width of ZSCBV is to add sensory motor fusion in the convergent and divergent direction
2. at infinity, find convergence range until patient sees double, and find the divergence range until patient sees double; repeat this at near test distance
3. once connect the pts, you’ll get a parallelogram where…

- inside = zone of clear single binocular vision (ZCSBV)

Practice Plotting Points (on handout given the previous lecture) – Dr. Schor walks us through the steps

1. distance phoria: ortho
1. plot an "x" at 0 on the bottom scale
2. near phoria: 2 exo
1. plot "x" 2 prism diopters to the left of the vertical dashed line (use top scale for near)
2. always shift away from the demand line the amt of the phoria
3. Far PRC: 6/13/10 Þ blur pt/double/single or recovery
1. add convergent stimuli and person starts to blur at 6 prism diopters
2. plot a dot at 6 prism diopters to the right on the bottom scale
4. Far NRC: 8/6
1. when stimulate divergence, blur at 8 prism diopters

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2. plot dot at 8 prism diopters to the left of the demand line (on bottom scale)
5. Near PRC: 8
1. plot pt. 8 prism diopters to the right of the demand line, on the horizontal dashed line for near test distance
6. Near NRC: 6
1. plot pt. 6 prism diopters to the left of the demand line, on the horizontal dashed line
7. then just connect the dots
1. connect the 2 "x"s to get the phoria line
2. the right line is the Base Out Limit
3. the left line is the Base In Limit
8. BO and BI limits are generally parallel to the phoria line – this is what you would expect if there is a linear summation of fusional vergence with accommodative vergence
9. phoria line – shows how much accommodative vergence it evokes and at any distance, you can add a constant amt of convergence or a constant amt of divergence

1. the tilt (slope) of the demand line is equal to the AC/A ratio
2. if the demand line is vertical, the AC/A is 0
3. if the demand line is horizontal, the AC/A is infinite
4. if it’s ideal, the slope of the demand line equals the slope of the zone (parallelogram)
5. as long as the demand line is inside the zone, the person can fuse and see single in the real world
6. if the demand line exits outside the zone anywhere, that person either has strabismus at that distance or constant blurred vision that they can’t correct; don’t have clear single binocular vision
7. demand line is same for everyone w/same PD
8. the phoria line will differ, though
9. fusional vergence – measure from phoria line
10. relative vergence – measure from demand line

Blur and Break Points (slides 11, 12)

1. blur point - when you run out of fusional vergence, but want to keep converging so start accommodating and begin to use accommodative convergence to bring eyes in

a. you accommodate enough to go outside the depth of focus and detect blur

2. break point – are using accommodative convergence after you have run out of fusional vergence until the eyes see double; there is too much blur and you just give up
3. recovery – reduce convergence stimulus until your fusional vergence kicks in again
4. measure recovery b/c as add prism, are adapting phoria temporarily

1. if adapt phoria a lot, when you double, it won’t take much reduction in prism to see single again
2. if you lose all your phoria after you see double, you’ll have to go back to 0 before you see single again
3. high recovery is a good sign– means that you are able to hold any adapted change in vergence, able to use it to assist in fusion
4. the higher the recovery, the more sustained vergence adaptation is; the patient has very adaptable convergence and divergence
5. recovery – tells how much system can cope w/stress put on it

1. find out range of viewing distances where demand line is inside the zone
1. if AC/A doesn’t equal the ideal, that means that everyone will eventually be strabismic at some viewing distance (demand line will exit zone at some pt.)
2. predict affects of lenses and prisms on phoria
1. prism changes the physical stimulus of the world

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Effects of Lenses and Prisms on the Demand Line – (slide 15)

1. BO prism shifts demand line to the right
2. BI prism shifts demand line to the left
3. (-) lens shifts demand line up, increases stimulus to accommodation
4. (+) lens shifts demand line down, decrease stimulus to accommodation
5. ZCSBV does not change, but the demand line changes position inside the zone
6. when you add lenses and prisms, you’re not changing the eyes, but you’re changing the world
1. if the eyes are crossed, you’re crossing the world so that the targets are on the fovea in both eyes
2. you’re reducing the demands the world places on the visual system, you’re changing the accommodative stimulus and vergence stimulus
7. if person has lot esophoria at all viewing distances, the best thing to do is give them BO prism to reduce esophoria at all viewing distances
8. if person is only esophoric at near and are ortho at far, moving the whole demand line will reduce the eso at near, but will produce exo at far – why we give bifocals

1. very narrow zone – either have vertical phoria which makes it hard to fuse BI, BO prism or have sensory problem that gets in way of fusion
1. treatment – correct vertical phoria, then eye exercises
2. tilted zone – indicates high AC/A ratio
1. treatment – bifocals
3. demand line close to lateral limits of zone
1. will cause patient discomfort
2. (slide 17) zone of comfort = middle third of ZCSBV; demand line should lie w/in middle third of zone

1. don’t prescribe if there are no patient problems

1. take a good case history to reveal any problems
2. listen to your patients

1. correct all vertical phorias before measuring horizontal fusion ranges – will get a narrow zone if you don’t
1. measure vertical phorias before measure BI and BO limits of convergence and divergence
2. demand line should be inside middle third of ZCSBV

1. CA/C ratio
2. proximal convergence
3. prism, lens adaptation
1. recoveries give you a covert peek at vergence adaptation
2. more adaptation ability have, the higher the recoveries will be
4. dynamic properties of AC/A and CA/C ratios
1. when person moving around, fusing moving target, there is no indication of how well they can do that
5. why it’s important to supplement GA with a measure of fixation disparity (how much stress on binocular system) and associated phoria (how much prism need to neutralize that stress)
1. either by themselves is inadequate, but together they are thorough