Presenting symptoms / signs:One eye moves normally, while the other points in (esotropia), out (exotropia), up (hypertropia) or down (hypotropia), double vision is a common symptom.
- ‘The Cover Test’ = to aid in the diagnosis of strabismus. If the eye being tested is the strabismic eye, then it will fixate on the object after the “straight” eye is covered, as long as the vision in this eye is good enough. If the “straight” eye is being tested, there will be no change in fixation, as it is already fixated.
- ‘The Hirschberg test’ = A flashlight is shone in the patient’s eye. When the patient is looking at the light, a reflection can be seen on the front surface of the pupil. If the reflection is not in the same place in each eye, then the eyes aren’t properly aligned.
Aetiology: A lack of coordination between the extraocular muscles, which prevents bringing the gaze of each eye to the same point in space and preventing proper binocular vision, which may adversely affect depth perception.
Pathophysiology: due to a cranial nerve lesion of 3, 4 or 6 (=reduced innervation of associated muscle). Or due to increased intracranial pressure, as CN VI is particularly vulnerable to damage from brain swelling. More commonly caused by a refractive error in one or both eyes. This refractive error causes poor vision in one eye and so stops the brain from being able to use both eyes together.