27th December 2023

Assess eye misalignment

Cover and alternate cover testing is an efficient and quick way to assess the alignment of the eyes and establish if a diplopia may be due to strabismus. Cover tests can help identify and quantify ocular misalignment, and there are three types of cover test, two of which can be performed at the bedside with little equipment.

Begin with the monocular cover-uncover test, which differentiates between tropias, where a deviation is always present, and phorias, where a deviation is present when one eye is closed or covered. Ask the patient to fixate on a point ahead, roughly 50cm or further away, and observe for any deviations. If placing an occluder over the non-affected eye causes the affected eye to fixate, this is known as a tropia. A phoria would be seen when the covered eye deviates on occlusion and returns to fixation on removal of the occluder.

The alternate cover test involves the patient fixating on a point ahead of them whilst the examiner moves an occluder from one eye to the other whilst focusing on the movement of the uncovered eye. This test allows the examiner to note whether the deviation is worse in specific gazes, i.e. when looking to the left, right, up, down or on a diagonal plane. A deviation which remains the same in all positions of gaze is classed as concomitant and incomitant if it is affected. Incomitant squints should raise concern for neurological, myogenic or neuromuscular junction disorders and may involve specialist input from neurology, whereas concomitant squint should be referred to ophthalmology and may not require further investigation.