A phoria (or latent deviation) only appears when binocular viewing is broken and the two eyes are no longer looking at the same object.
It is simple to undertake, does not require great amounts of skill by the examiner and is objective in nature.
To ensure that the test is executed accurately and that the maximal amount of information is obtained, it is paramount that the appropriate fixation targets are used for near (approximately 33 cm (13 in)), distance (3 m (10 ft)) and far distance (>3 m (10 ft)) and also that a cover consisting of a black 'paddle' is used.
Near Targets (33 cm) Light source – A penlight/light from retinoscope or ophthalmoscope could be used at the start of the near cover test assessment to observe the patient's corneal reflections, and to see whether their fixation is steady, central and maintained.
[1] Detailed target – Any small object that has the ability to stimulate Accommodation (eye) and allow the examiner to assess the patient's fixation.
[1] Snellen chart – This is the most commonly used target when assessing a patient's far component of a cover test.
Also during the cover test, the examiner must make sure that the patient is clearly seeing the fixation targets to confirm their Accommodation (eye) is controlled.
Prior to proceeding, ensure that there is sufficient light in the room so that the examiner can clearly see the patient's eyes.
Explain the purpose of the test to the patient, for example: "I am going to check how well your eye muscles are working together.
The alternate CT can't distinguish between manifest and latent deviations.
[1] An alternate cover test is performed to ensure full dissociation, observe any changes in the amount and type of movement.
[1] Lastly, repeat the cover/uncover test and note whether the eye remains deviated or returns to its original position.
Once a cover/uncover test has been performed to confirm the presence of a manifest deviation our attention is turned to the behavior of the covered eye.
[1] Finally, a cover/uncover test must be performed again to ensure the deviation has remained latent and recovery is the same.
Use of a detailed target for near fixation in both adults and children will identify the effects of accommodation on the deviation.
This practice ensures the detection of a consistent increase in deviation and DVD isn't neglected.