Prism Cover Test

The "prism cover test" (PCT), is an objective measurement and the gold standard in measuring strabismus, i.e. ocular misalignment, or a deviation of the eye. It is used by ophthalmologists and orthoptists in order to measure the vertical and horizontal deviation and includes both manifest and latent components. Manifest is defined by the eye deviating constantly or intermittently, where-as latent is where the deviation is normally controlled but becomes present when the eyes are dissociated. A PCT reveals the total deviation and cannot distinguish between latent and manifest strabismus as you are using an alternate cover test.

A number of different instruments are required when performing a PCT.

These include:

In order to perform a PCT, you must first perform a cover test as this gives an estimation of the size of the strabismus, thus an approximate starting point on the prism bar. You can also get an indication of presence and type of strabismus by observing the patients’ eye and observing corneal reflections, also known as Hirschbergs. It also shows whether the patient has a manifest or latent deviation. If a manifest deviation is present, it reveals which eye has the deviation or if it is alternating between both eyes.

The prism cover test must be conducted at near (33 cm), at distance (6m) and if necessary at greater than 6 metres. Before commencing the test, ensure the patient is sitting upright with their chin and head straight. Patients with a head tilt (abnormal head posture) are ‘not formally controlled by using a bite bar or chin rest’ and are always tested with their head tilt and then without their head tilt

The results leading on from the cover test will give you an indication of the type of deviation and which way you should hold your prism for the next stage of the test. Either BASE IN for an exodeviation (eye turned out), BASE OUT for an esodeviation (eye turned in), BASE UP for a hypodeviation (eye turned down) or BASE DOWN for a hyperdeviation (eye turned up).

Steps: 1. The patient should be measured in primary position first and then in any other positions of gaze of concern. For near fixation, the patient should hold an accommodative target (fixation stick) at 33 cm, ensuring it is in line with their visual axis. By having the patient read out loud the letters at the top of the fixation stick, it is easy to ensure they are maintaining their accommodation.

2. The examiner then holds the prism bar over the patients deviating eye, starting at a small strength prism, however, if the deviation on the cover test appeared to be large, a larger strength prism may be used to achieve results quicker.

3. With the prism present, an alternate cover test is performed. It is important that an alternate cover test is performed correctly in order to ensure maximum dissociation as we are determining the total deviation size which includes both latent and manifest.

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Wikipedia