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Vertical Vergences at Distance
PURPOSE To measure, through the uniocular application of base or base down prism，the patient’s vertical fusional vergence abilities．
Vertical Vergences (Sra- and Infravergences)
When testing sravergence (base down，BD)and infravergence (base ，BU)，the examiner is looking for two subtest findings．These are：
1．Break point：The break represents the point when the patient has used all his vertical vergence and can no longer maintain single vision．
2．Recovery point：The recovery indicates that the retinal disparity induced by the prisms has been decreased to the point that the patient can access the vertical vergence system and regain single vision．
Note：There is no blur finding for vertical vergence testing as accommodation does not change during vertical vergence movements．
EQUIPMENT
·Phoropter．
·A distance acuity chart that can isolate letters．
SET-UP
·The phoropter should contain the patient’s distance correction and be set for the patient’s distance PD．
·Expose an isolated letter one line larger than the patient’s best corrected visual acuity in the poorer seeing eye．
·The Risley prisms, set to 0，are positioned before both eyes so that vertical prism may be introduced(see Figure 4-6)．
STEP-BY-STEP PROCEDURE
1．Ask the patient to open both eyes and report what he sees．He should see one clear image．
2．Instruct the patient to look at the letter and to tell you when it doubles．
3．At about 1 prism diopter per second introduce base prism before the right eye．This measures right infravergence．
Note：Unlike the measurement of horizontal vergences, it is only necessary to move the prism before one eye．A sravergence measured in one eye is equal to an infravergence in the other．The second prism is placed，with no prism power，before the contralateral eye to equalize the quality of the image presented to the two eyes．
4．Note the amount of base prism before the right eye at the break point．
5．Overshoot the break point slightly by adding a little more prism in the same direction．2 t0 3Δ is usually sufficient．
6．Instruct the patient to tell you when the target is single or becomes single again．
7．Reduce the prism until the patient reports the target is single．This is the recovery point．Note the amount of base prism before the right eye．
8．Repeat steps 2 through 7 starting with base down prism over the right eye．This measures right sravergence．
RECORDING
·Record the technique and the distance at which it was performed．distance or near.
·Record the eye over which the prism was introduced．
·Record the orientation of the prisms (BU and BD)along with the corresponding results．
a) Each result should contain two values, break and recovery．
b) If the recovery values are in the direction opposite to what you expect(eg，testing BU but had to go into BD for recovery to occur)，record as a negative value．
EXAMPLES
·Distance vertical vergences：OD inffa 4/2, sra 2/-1
·Distance vertical vergences：OD BU 6/4，BD 3/1
EXPECTED FINDINGS
·Break：3Δ-4Δ
Recovery：1.5Δ-2Δ
·The numbers provided are population norms．These serve as a general indicator．Precise interpretation of the vergence findings require that they be viewed in relation to any vertical heterophoria that may exist． |
