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Horizontal Vergences at Distance
PURPOSE To measure，through the application of prism，the patient’s ability to use horizontal vergence to maintain binocular vision．Prisms which induce retinal disparity are gradually increased in power，forcing the patient’s vergence system to compensate for the disparity．
Horizontal Vergences (BI and BO Vergences)
When testing base in(BI)and base out(B0)vergences，the examiner is looking for three subtest findings．These include：
1．Blur point：The blur represents the point when the patient can no longer compensate for the prism induced retinal disparity while maintaining stable accommodation．
2．Break point：The break represents the point when the patient，using all vergence sources，can no longer maintain single vision．
3．Recovery point：The recovery indicates that the induced retinal disparity has been decreased to the point that the patient can access the vergence system and regain single vision．
EQUIPMENT
·Phoropter．
·A distance acuity chart that can isolate single letters．
SET-UP
·The phoropter should contain the patient’s distance correction and PD．
·Expose an isolated letter one line larger than the patient’s best corrected visual acuity in the poorer eye．
·The Risley prisms，set to 0, are positioned before both eyes so horizontal prism may be introduced(see Figure 4-5)．
STEP-BY-STEP PROCEDURE
1．Instruct the patient to open both eyes and ask him what he sees．He should see one clear image．If the patient sees two targets，end the test and record “diplopia．”
2．Instruct the patient to look at the target and try to keep it clear．Ask the patient to report if：
a．the target blurs (blur point)．
b．the target becomes double (break point)．
c．the target moves either left or right．This indicates an eye is being spressed．If this occurs the test is invalid and should be stopped．Record “spression” and indicate which eye．The eye being spressed may be determined by asking the patient which direction the target appears to be moving．The target will be moving toward the apex of the prism before the eye not being spressed．For example，if base out vergences are being tested and the patient reports the target is moving left, the right eye is seeing and the left eye is being spressed．This is known because the apex of the prism over the right eye is pointing left, the direction in which the target is moving．
3．Introduce base in prism equally before both eyes at a speed of approximately 1 prism diopter per second．Base in is always tested before base out．This is done because base out testing affects accommodation and convergence in a way that may carry over and modify the base in findings．
4．As prism is added note the total prism before the two eyes at each point described here．First，when the patient first reports the letter has blurred (blur point)．Second，when the letter has broken into two (break point)．For example，if the patient reported the letter became double when there was 3Δ in front of the right eye and 3Δ in front of the left eye, the break point is 6Δ．
5．Overshoot the break point slightly by adding a little more prism in the same direction．
6．Instruct the patient to tell you when the target becomes single again．
7．Reduce the prism until the patient reports the target is single again．This is the recovery point．Note the total amount of prism before both eyes．
8．Repeat steps 2 through 7 with base out prism before both eyes．
RECORDING
·Indicate that the test was performed at distance, then record the orientation of the prisms (BI and BO) along with the corresponding results．
·Each result should contain three values, blur, break，and recovery，in prism diopters．
·If no blur point is observed，an (x) is entered．
·If recovery values are in the direction opposite to what you expect (eg，testing BO but had to go into BI for recovery to occur)，record as a negative value．
EXAMPLES
·Distance Vergences：
BI x/10/4 BO 12/18/8
·Distance Vergences：
BI：Spression OD B0：4/6/-2
EXPECTED FINDINGS
·Morgan (adult, clinical population)
Distance BI： x/7/4 Standard deviation x/3/2
Distance B0：9/19/10 Standard deviation 4/8/4
·Saladin and Sheedy (adult，nonclinical population)
Distance BI：x/8/5 Standard deviation x/3/3
Distance B0：15/28/20 Standard deviation 7/10/11
·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 other functional test results．
·Typically，there is no blur point for base in vergence testing at distance．Blur on base in testing indicates a relaxation of accommodation and this should not occur if the patient has been properly refracted． |
