Ocular Muscle Balance

Eyes should be well aligned and should have normal binocular vision. No manifest squint permissible. Convergence must be adequate. Ocular movements full and free Ocular muscle balance can be tested with the cover test, the Maddox rod or an approved vision tester.

Cover Test

• The purpose of this test is to determine whether manifest strabismus is present, or whether there is any tendency of the eyes to deviate when the two eyes are dissociated.

• The examiner stands in front of the candidate who is told to fix his eyes on a small target such as a small examining light.

• An occluder card is then placed in front of one eye and the other eye checked for movement. If there is none the card is removed and the covered eye examined to see whether it has remained fixed or whether it has moved medially or laterally and has to be re-fixated.

• The test is then repeated with the other eye covered.

• If the candidate is orthophoric no movement of the eyes will take place.

• If there is esophoria one eye will move in and then re-fixate when the occluder is removed. In exophoria the opposite is true.

Technique

• Cover one eye completely. 

• Hold the pencil vertically with the point 45 cm from the candidate’s face, between his eyes and level with the root of his nose. 

• Ask the candidate to follow its movement with the eye and move the pencil 3 or 4 times across his face, from side to side in a level plane. 

• Range of movement should be approximately 30 cm. 

• Now bring the pencil to rest level with the root of his nose and evenly between his eyes, still at a distance of 45 cm from his face. 

• Quickly remove the cover, and observe any movement of previously covered eye. The covered eye may not show any movement or it may move either inwards or outwards. 

• Now the eye, which was open, is covered and the movement of the previously covered eye is once again noted. 

• This part of the test may be termed stage 2.

• The previously covered eye may once again show no movement or may move either inwards or outwards.

Interpretation of the Results

• If there is no movement of the eyeball either in stage 1 or stage 2 of the test, it indicates that the muscle balance is normal and fusion is achieved with effort.  Such a stage is called orthophoria. 

• However if there was no movement in stage 1 but some movement in stage 2 after covering the other eye, the individual is suffering from heterotropias or manifest squint. 

• If the movement is inwards or outwards in stage 2, the case is diagnosed to suffer from divergent or convergent squint respectively. 

• If in stage 1 the eye moved inwards and there was no further movement in stage 2, the individual suffers from latent divergence with complete recovery. 

• However, if there was further inward movement in stage 2, he suffers latent divergence with incomplete recovery. 

• In the same way if the movement in stage 1 was outwards but no further movement occurred in stage 2, the candidate suffers from latent convergence deficiency with complete recovery.  In case further outward movement occurred in stage 2, the individual will be deemed to be suffering from latent convergence with incomplete recovery.

            Whenever the recovery is complete, whether divergent or convergent, the individual suffers from heterophoria.  If the recovery was incomplete, he is considered to be suffering from heterotropia. Not only the movement but the rate of recovery is also noted.  The recovery can be rapid or slow, immediate or delayed.  Now the second eye is tested in similar fashion.  The cover test is to be done for distant and near vision separately and mentioned accordingly.

The Maddox Rod Test

            This test is used to uncover latent squints (phorias). The test may be performed with a hand frame, a vision tester or a trial frame but the principle in all is the same. If a candidate is given two dissimilar targets to view at the same time, the stimulus to fusion is absent and phorias are uncovered.

• The Maddox rod is a disc of red glass in which are molded grooves.

• When a distant spot of light is viewed with the disc in front of one eye, a red line will be seen by the eye covered with the lens, whilst a spot of light will be seen with the other eye. The line will be at right angles to the grooves to that when these are horizontal the line will appear vertical.

• A candidate with no latent deviation will see the coloured line pass through the spot of light (orthophoria), whereas a candidate with latent squint will see the light source to one side of the line.

• The Maddox rod with rotating prism is held in front of the right eye and the candidate is asked to look at a point source of light 6m (20 ft.) away in a darkened room. Both eyes must be open and squinting should be avoided.

• The candidate is asked which side of the line the dot is seen. If it is to his right, esophoria is present and if to the left, exophoria.

• The candidate is then asked to “put the line on the light” by adjusting the rotating prism. The examiner reads off the degree of phoria from the scale on the device.

• The test is repeated with the disc turned to the vertical position.

• The light will now be seen either above or below the line and may be adjusted by the candidate in the same way. If the red line is above the light there is left hyperphoria, if below the light, right hyperphoria.

• The test should also be done with the spotlight at 33 cms.  If the Maddox rod is placed in front of the left eye the interpretation will change accordingly.

• The two tests results of Maddox Rod test at 6 meters and at 33 cm are recorded separately.

Red Green Test

• The most commonly used test is Worth’s “four dot test”.

• It consists of an illuminated box with four apertures for coloured glasses, one red, two green and one white.   

• The candidate at 6 meters distance wears a red glass before right eye and green before left eye, so that he sees the red with one eye, green with the other and white with both. 

• If he sees four dots (one red, two green and one pinkish green) he has binocularity; if he sees five (two red and three green) he uses both the eyes but has diplopia. 

• If he sees two reds or three greens only, he is using one eye only, in the first case right eye and in the second case left eye.

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