4.7 I see double

Sometimes, a patient self-refers because: "I see double". Double vision is quite disturbing and debilitating at the same time.

Diplopia is usually sudden-onset, and is a manifestation of numerous possible major systemic diseases. It is to be distinguished from squint in children (Section 3.3.2). We will now examine this issue closely. First, the control of the eye movement:

(from health.howstuffworks.com)

The above diagram shows the six extraocular muscles (EOMs) of a left eye. You can imagine a mirror image as the right eye. The muscles must work in coordination to maintain binocular vision, i.e., fixation at the same spot.

Let us take a simple situation, say, you are looking to the left. Then for the left eye, the lateral rectus contracts, while the medial rectus relaxes. At the same time, for the right eye, the medial rectus contracts and the lateral rectus relaxes. At all times, the two eyes are locked in for looking at the same point/object. If you now see double, apparently this coordination is broken. And the underlying reason(s) must be examined both neurologically and neuroradiologically.

Other than a direct trauma to the muscles or a result of eye surgery that is readily apparent from history, one must start from: which muscle is affected. And deduce from it, which cranial nerve is involved. Basically, the 4th nerve innervates the superior oblique, the 6th innervates the lateral rectus, and the rest by the third nerve. Again, take the lateral gaze example, if there is a 6th nerve palsy affecting one of the two lateral recti, then the eyes will not be able to look to the left or the right without seeing double. The question is then what happened to the 6th nerve on its way from its origin in the pons of the brain stem to the orbit and the lateral recti. You'll need to entertain the possibility of multiple sclerosis, mysathenia gravis, diabetes, circulation problems, hypertension, tumors, and other known and unknown causes. Indeed, any of these can disrupt the course of the 6th cranial nerve. Sometimes, the etiology remains unknown. And often the diplopia resolves without any intervention in a few months, especially if the primary cause is diabetes or hypertension.

[Note: Hypertension, with the increasing intracranial pressure affects more frequently the 6th nerve; whereas diabetic infarct, more the 3rd nerve. Diplopia, then, is a sign of poor control of blood pressure and blood glucose, respectively.]

And obviously the same workup goes for 3rd and 4th nerve palsies, except the former is usually accompanied by drooping eyelids (ptosis) and eye/head pain, while the latter is usually benign.

Similar to vertical squint/strabismus, often people with sudden 4th nerve palsy eventually learn to tilt their heads to avoid diplopia. The temporary relief of double vision can be a simple patch for one eye or a large amount of prism in the spectacles.

It is of course the eye doctor's job to perform a thorough evaluation. The patient is then referred for further consult for the subsequent management by appropriate specialists.