I. Introduction: Cover test, unilateral and bilateral; prisms, base in, out, up or down...choose your poison; retinoscopy, static or dynamic...doesn't this sound familiar? We offer another look at these indispensable skills! Mixed in this review may be something new; a clinical insight or two. These skills are, after all, the backbone of our attempts to determine the binocular vision skills of a patient.
II. UNILATERAL COVER TEST: interrupts fusion as one eye is covered then uncovered. On uncover, the eyes have every opportunity to make fusional movements.
PEARLS:
1. Occlusion should be about 2 seconds before uncovering to ensure dissociation
2. As you occlude one eye, watch for movement in the other. Movement of the fixating eye indicates a strabismus
3. Watch BOTH eyes as you uncover the selected eye. You will observe one of the following:
i no movementii a version eye movement
iii a vergence eye movement
4. A unilateral cover test is valuable to determine phorias as well as strabismus:
a. vergence eye movements as you uncover an eye indicate that fusional effort is being made. The amount of time it requires to complete the vergence movement (to restore bifoveal fixation) is correlated to the integrity of the fusional vergence system
b. fusional recoveries of greater than one second is considered problematic and is correlated with a low fusional recovery on vergence testing
5. It is sometimes useful to conduct a delayed cover test. Occlude for 30 seconds or more before uncovering to allow for complete dissociation to occur.
III. ALTERNATING (DUANE) COVER TEST: interrupts fusion as occlusion is alternated between eyes. There is no opportunity for fusion during the testing.
PEARLS:
1. Make observations of the magnitude of eye movements throughout the dissociation
2. Watch each eye and compare movement as you alternately occlude. Some individuals will have asymmetric eye movements
3. An alternating cover test is valuable to determine phorias as well as the full magnitude of a strabismus. This is why we neutralize a strabismus with an alternating cover test.
IV. Dynamic Retinoscopy: A group of procedures conducted at near point to:
-assesses the dynamic relationship between the vergence and accommodation
-determines the accuracy of accommodative response (lag of accommodation)
-assesses the suitability of a tentative add
-assesses the refractive balance of static retinoscopy
A. Bell retinoscopy
1. METHOD : This test is done binocularly
a. a distance of 20" is marked on a yard stick; this is the initial target distance where the "bell" is held. The patient holds the yard stick, with the "0" at the side of his/her head, even with the outer canthus
b. the doctor (you) holds the bell and a retinoscope at the 20" distance, while assessing the retinoscopic response. There should be "with motion" at this point. If initially against, the test is over.
-RETINOSCOPY SHOULD CONTINUE THROUGHOUT THE TEST!
-THE RETINOSCOPE SHOULD ALWAYS BE 20" AWAY FROM THE PATIENT!
c. the target is slowly moved towards the patient to point where against motion is first noted. It is very important to remember that this is a dynamic test. Therefore, continue doing retinoscopy and moving the target after seeing the "shift" from with to against motion!
d. after noting on the yardstick the point at which against motion was first seen, the target is now moved away from the patient to a point where with motion is seen again. This is known as the release.
e. It is important to monitor the two principal meridians in each eye. This procedure may uncover some previously undetected astigmatism @ near point or a significant difference between far point and near point astigmatism.
f. probe lenses (usually +1.00 D) should then be used (binocularly,...remember?) to assess plus acceptance.
2. RECORD:
a. the point of first against motion (AM)
b. the point of release (return to with motion) (WM)
c. in the case of neutral motion at 20", record N at 20"
V. Monocular Estimate Method (MEM): This test is done binocularly
1. METHOD:
a. working distance should be habitual reading distance
b. an age-appropriate MEM graded cards should be used (they attach to the retinoscope).
c. with or against movement is quantified by BRIEFLY introducing plus (or minus) lenses in front of the eye being observed. It is important to monitor the two principal meridians in each eye. This procedure may uncover some previously undetected astigmatism @ nearpoint. or a significant difference between far point and near point astigmatism
d. plus probe lenses may be used to assess plus acceptance. These lenses should be placed over both eyes and repeating the MEM test. Sometimes a patient can wear the probe lens for 5 minutes before repeating the MEM.
2. RECORD:
a. a quantitative lag or lead of accommodation
b. any differences between the 2 eyes
c. any differences in the lag of accommodation in the principal meridians
d. the effect of plus lenses on lag (or lead) of accommodation
VI. A WORD ABOUT PROBE LENSES AND PLUS ACCEPTANCE:
1. Probe lenses: a tentative add for the patient.
-lenses may be worn for a few minutes while the patient reads before re-measuring
- if lag of accommodation is normalized (plano to +0.50), the tentative add may be appropriate to prescribe
- if a lead of accommodation worsens with probe lenses, the tentative add may not be appropriate to prescribe. Other options, such as vision therapy, should be discussed
- if a high lag of accommodation (over +0.75) remains high with a probe lens, it may be necessary to
1. try a higher power probe lens
2. initiate a vision therapy program for accommodative dysfunction
2. Plus acceptance: a numeric value indicating the change in accommodation response made by the patient when probe lenses are applied binocularly.
- when observing a near point target, a certain amount of accommodation occurs
- plus probe lenses add additional power to the refractive system
- the lag measured through the probe lens is the difference between the overall accommodative demand (near point working distance) and the sum of the dioptric values of accommodation and the probe lenses
Example 1: Complete Plus Acceptance: the accommodative response is reduced by the amount of the probe lens, resulting in no change in the MEM
Original Lag +0.50
Probe lens +1.00
Lag With Probe Lens +0.50
Example 2: No Plus Acceptance: the accommodative response does not change in the presence of the probe lens, resulting in a shift in the MEM to a lead of accommodation
Original Lag +0.50
Probe lens +1.00 Lag
With Probe Lens -0.50
- if the lag of accommodation is restored to (or remains at) normal levels with the probe lenses, an add may be appropriate (example 1)
- if the lag of accommodation is out of normal levels with the probe lenses, the add is not appropriate (example 2)
VII. KINETIC COVER TEST: measures the dynamic changes in phoria as the target is moved towards, then away from the patient. This test is especially valuable to gain information on the binocular status of young children. Although it was originally intended as a test for heterophoria, it is also very useful as a diagnostic tool in accommodative dysfunction.
1. METHOD:
- a fixation target is held at a habitual working distance (usually 40 cm.) while an alternate cover test is performed (point A). The phoria is observed.
- the cover test continues as the target is moved inward along the patient's midline. The eye movements are observed as the target moves to 10 cm in front of the patient. (point B)
- the cover test continues as the target is moved outward along the patient's midline. The phoria is again observed as the target reaches the original fixation distance (usually 40 cm). (point C)
2. RECORD:
- the shift in the phoria at Point C relative to Point A
- KCT is positive if phoria shifts esoward by 5 prim diopters or more