COVER TEST
METHODS:-
THE COVER TESTS SHOULD BE PERFORMED WITH & WITHOUT GLASSES AT DISTANCE (6m) AND AT NEAR (33cm). THE PATIENT IS ASKED TO FIXATE A 6/12 VISUAL ACUITY SYMBOL.
ONE EYE IS OCCLUDED WITH THE HELP OF OCCLUDER.
IN THE COVER TEST THERE ARE 4 PARTS:
1. DIRECT COVER TEST
2. COVER-UNCOVER TEST
3. ALTERNATE-COVER TEST
4. PRISM & ALTERNATE-COVER TEST.
1. DIRECT COVER TEST:-
AIM:-
IT CONFIRMS THE PRESENCE OF A MANIFEST SQUINT.
PROCEDURE:-
PATIENT IS ASKED TO FIXATE A POINT WITH BOTH EYES OPEN. THE NORMAL LOOKING EYE IS COVERED WHILE OBSERVING THE MOVEMENT OF THE UNCOVERED EYE.
IN THE PRESENCE OF MANIFEST SQUINT, THE UNCOVERED EYE WILL MOVE IN THE OPPOSITE DIRECTION TO TAKE FIXATION.
EXAMPLE- WHEN AN EXOTROPIA IS PRESENT, THE EYE TAKING UP FIXATION WILL MOVE TOWARDS NOSE AND IN THE PRESENCE OF ESOTROPIA, IT WILL MOVE TOWARDS THE TEMPLE.
2. COVER- UNCOVER TEST:-
AIM:- IT ESTABLISHES THE PRESENCE AND TYPE OF HETEROPHORIA(LATENT DEVIATION)
PROCEDURE:-
IT IS PERFORMED, WHEN DIRECT COVER TEST HAS ESTABLISHED THAT NO MANIFEST DEVIATION IS PRESENT. TO PERFORM IT, ONE EYE IS COVERED WITH AN OCCLUDER AND THE OTHER IS MADE TO FIXATE AN OBJECT.
IN THE PRESENCE OF HETEROPHORIA, THE EYE UNDERCOVER WILL DEVIATE. AFTER FEW SECONDS, THE COVER IS QUICKLY REMOVED AND THE MOVEMENT OF THE EYE OBSERVED.
DIRECTION OF THE MOVEMENT OF THE EYEBALL TELLS THE TYPE OF HETEROPHORIA AND THE SPEED OF MOVEMENT TELLS WHETHER RECOVERY IS SLOW OR RAPID.
EXAMPLE- THE EYEBALL WILL MOVE TOWARDS THE NOSE IN THE PRESENCE OF EXOPHORIA & TOWARDS THE TEMPLE IN THE PRESENCE OF ESOPHORIA.
3. ALTERNATE- COVER TEST:-
AIM:- IT IS PERFORMED TO ESTABLISH WHETHER THE SQUINT IS UNILATERAL OR ALTERNATE AND ALSO TO DIFFERENTIATE CONCOMITANT SQUINT FROM PARALYTIC SQUINT.
PROCEDURE:-
TO PERFORM THIS TEST, PATIENT IS ASKED TO FIXATE AN OBJECT ALTERNATELY WITH EACH EYE. IT IS IMPORTANT TO PLACE OCCLUDER ALTERNATELY IN FRONT OF EACH EYE SEVERAL TIMES TO MAXIMIZE THE DEVIATION.
FURTHER, THE OCCLUDER SHOULD BE QUICKLY TRANSFERRED FROM ONE EYE TO OTHER TO PREVENT FUSION FROM OCCURRING. OBSERVATIONS MADE ARE AS FOLLOWS:
IN THE PRESENCE OF AN ALTERNATE SQUINT, EITHER EYE FIXATES AND THE OPPOSITE EYE UNDERCOVER DEVIATES AND MAINTAINS THE POSITION OF DEVIATION ON REMOVING THE COVER: AFTER REMOVAL OF THE COVER, ALWAYS THE NORMAL LOOKING EYE TAKES UP FIXATION AND THE SQUINTING EYE DEVIATES.
4. PRISM & ALTERNATE COVER TEST:-
IN PRACTICE, THIS TEST IS MOST POPULAR AND A SIMPLE METHOD OF MEASURING THE ANGLE OF DEVIATION OBJECTIVELY IN VARIOUS DIAGNOSTIC POSITIONS OF GAZE.
PRISMS OF INCREASING STRENGTH WITH APEX TOWARDS THE DEVIATION ARE PLACED IN FRONT OF ONE EYE & THE PATIENT IS ASKED TO FIXATE A TARGET WITH THE OTHER EYE. FOR LARGE DEVIATIONS, A LOOSE PRISM OF 30 or 45 PRISM DIOPTRE IS PLACED IN FRONT OF ONE EYE AND THE PRISM BAR IS USED IN FRONT OF THE OTHER EYE. WHEN HORIZONTAL AND VERTICAL DEVIATIONS COEXIST, THE PRISMS ARE PLACED HORIZONTALLY IN FRONT OF ONE EYE & VERTICALLY IN FRONT OF THE OTHER EYE.
THE TEST SHOULD BE PERFORMED FOR DISTANCE & NEAR VISION.
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