Expect diplopia and a left esotropia. Diplopia means double vision. The sixth cranial nerve, the abducens nerve, powers one muscle only: the lateral rectus. That muscle pulls the eye outward, toward the ear. When the left sixth nerve is weak, the left lateral rectus cannot do its share. The medial rectus on the inner side then wins the tug of war, and the left eye drifts inward. An in-turned eye is an esotropia.
Double vision follows because the eyes no longer point at the same spot. The brain normally fuses the two pictures into one. When alignment breaks, fusion breaks, and an adult sees two separate images. The textbook uses this exact example: an adult with a left sixth nerve palsy notices diplopia on looking to the left, the field of action of the left lateral rectus. Young children usually do not complain, because a developing brain can suppress, or ignore, the second picture. That same brain trick puts children at risk for amblyopia, which is vision loss from lack of use. Note also that the images are clear, not blurry. Each eye alone still focuses well, so blurry vision is the wrong complaint here.
Tie the nerve to the muscle and the muscle to the drift. A weak outward pull leaves the eye turned in, on the same side as the bad nerve. Linking nerve palsies to eye position is a core part of the COA ocular motility content area.
Incorrect Answers
- A. The side is wrong and so is the symptom. A left nerve palsy turns the left eye in, and the patient sees double rather than blurred.
- B. Vertical drift is not a sixth nerve problem. The lateral rectus moves the eye side to side only, never up or down.
- D. Diplopia fits, but hypertropia means an upward drift. That points to the vertical rectus or oblique muscles, not the lateral rectus.
Testing Pearls
- Cranial nerve VI, the abducens nerve, powers the lateral rectus and abducts the eye.
- A sixth nerve palsy leaves an esotropia on the same side, worst on gaze toward that side.
- Cranial nerve IV drives the superior oblique; cranial nerve III drives the other four muscles and the lid.
- Adults report diplopia when fusion breaks; children often suppress instead.
- Suppression in a young child risks amblyopia and poor depth perception.
References
- American Academy of Ophthalmology. Ophthalmic Medical Assisting: An Independent Study Course. Chapter 9, Ocular Motility.
