Exo deviations

An exo deviation is the outward turn of an eye.

An exophoria is a latent squint.

This means the eyes are straight without dissociation (covering of the eyes) and corneal reflections will be symmetrical.

• On cover-uncover test, the fixing eye will not move.
• On alternate cover test, both eyes will move with an inwards motion to fix – meaning they have a tendency to turn out.

An exotropia is a manifest squint.

This means one eye is turning outwards all the time and corneal reflections will show a misalignment.

• On cover-uncover test, the eyes will move in to fix.
• On alternate cover test, the inwards movement may increase.

Some of the main causes of an exotropia (turn out of the eye) include:

• Intermittent distant exotropia – a childhood squint. There is normal, binocular function at near, but an exotropia on relaxing the eyes to look in the distance.
• Decompensated exophoria – when a latent (hidden) squint breaks down to become manifest.
• Convergence insufficiency/paralysis – the inability to pull the eye inwards results in the eye turning outwards.
• 3rd cranial nerve palsy – the medial rectus muscle is weak so the eye can’t be turned inwards fully and may sit outwards.
• Internuclear Ophthalmoplegia (particularly bilateral) – a limitation of the medial rectus muscle results in an exotropia. These patients will also have abducting nystagmus in the contralateral eye.
• Secondary to pathology – if sight is lost after 18 months of age, the eyes don’t work together and you will develop an exotropia.
• Consecutive/residual – the surgical overcorrection of an esotropia, or undercorrection of an exotropia.

It is the role of an Orthoptist to differentially diagnose these causes when an exotropia is presented in clinic.

Many people have a small exophoria. This can be completely normal.

• Try out an alternating cover test on a friend today! You can use the tip of a pen as a near target (at 1/3rd of a metre) and your hand as an occluder.