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4Δ Base out Test
PURPOSE To confirm or rule out the presence of a small central scotoma．
INDICATIONS
A small central spression zone should be suspected in a patient with a slight reduction in visual acuity(20/25 to 20/40)in one eye and mildly reduced stereopsis(30 to 80 seconds of arc)．Monofixation syndrome secondary to a small angle strabismus is the most likely cause of this central spression．Macular diseases affecting the foveal area will create a similar clinical presentation．
EQUIPMENT
·Loose 4Δ prism．
·A distance visual acuity chart that can isolate letters
SET-UP
·The patient wears his best distance correction．
·The room illumination must be sufficient to allow the examiner to observe the patient’s eye movements．
·The examiner must be in a position to see the patient’s eyes easily without interfering with the patient’s view of the target．
·Isolate a single letter, one line above best VA in the patient’s poorer eye, on the distance visual acuity chart．
STEP-BY-STEP PROCEDURE
1. Instruct the patient to fixate continually on the target even if it appears to move，and to attempt to keep the target single at all times．
2．Hold the 4Δ prism between your thumb and forefinger so it will be positioned base out when placed in front of the patient’s better seeing eye．
Note：Always test the patient’s better seeing eye first．If you are testing the patient’s right eye, the prism should be held in your left hand with its base pointing to your left．If you are testing the patient’s left eye，the prism should be held in your right hand with its base pointing to your right．
3．Quickly insert the prism in front of the better seeing eye while watching the opposite eye for movement．A normal response is a concomitant movement outward，then an inward refixation movement．Keep the prism in front of the eye for several seconds to allow time for the refixation movement．Step 3 may be repeated as often as necessary to ascertain the movement of the poorer seeing eye．
4．Turn the prism so it will be positioned base out for the poorer seeing eye．Quickly insert the prism in front of the poorer seeing eye while watching the better seeing eye for movement．A normal response is a concomitant movement outward，then an inward refixation movement of the better seeing eye．Keep the prism in front of the eye for several seconds to allow time for the refixation movement．Step 4 may be repeated as often as necessary to ascertain the movement of the better seeing eye．
RECORDING
·Write“4Δ base out test”or“4BO．”
·Record“positive”if the test results indicate spression of an eye．Indicate which eye is spressing．
·Record“negative”if the test results do not indicate spression．
EXAMPLES
·4B0—positive，OD spressing
·4B0一negative，no spression either eye
EXPECTED RESULTS
·Normal result(indicating no spression of either eye)．When the prism is placed in front of the right eye, the left eye will make a concomitant outward movement as predicted by Hering’s law．The patient will then see double, so the left eye will make an inward refixation movement to avoid diplopia．When the prism is placed over the left eye, the right eye will make an outward movement followed by an inward refixation movement(see Figure 4-25)．
·Abnormal result(indicating spression of the poorer seeing eye)．When the prism is placed over the better seeing eye, the opposite eye will make a concomitant outward movement consistent with Hering’s law．However, because the eye without the prism is spressing，the patient does not see double．Therefore, the eye does not refixate in order to avoid diplopia．When the prism is placed in front of the spressing eye, it will not be aware of the retinal image shift．There will be no movement of either eye(see Figure 4-26)．
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