Approach to diplopia: Clinical sciences

Diplopia, or double vision, is the visualization of two images of a single object. Diplopia might be the result of impaired function of the structures within the eye itself, or due to impaired ocular motility that causes misalignment of the eyes. Diplopia can be classified as monocular, if it’s present when one eye remains closed, or binocular if it’s present only when both eyes are open.

Now, if your patient presents with double vision, start with a focused history and physical exam. They might describe double vision like seeing two images of the same object side by side or one above the other. If on physical exam, these symptoms persist with one eye closed, that’s monocular diplopia. This means that there’s pathology in the eye that perceives the visual abnormality, the open eye. Next, perform a pinhole exam, in which you ask the patient to read an eye chart through a pinhole. If their vision does not improve, the likely diagnosis is macular disease. Some common causes include macular edema from diabetic retinopathy, age-related macular degeneration, and uveitis.

On the flip side, if the patient’s vision does improve with the pinhole exam, your next step is to assess for lens opacity on the fundoscopic exam. If lens opacity is present, you should diagnose a cataract. Risk factors for cataract development include advanced age, sunlight exposure, tobacco use, diabetes, and corticosteroid use.

However, if lens opacity is absent, you can diagnose refractive error. The different types of refractive error are hyperopia, otherwise known as farsightedness; myopia, known as nearsightedness; and astigmatism, which is an irregular curvature of the cornea.

Okay, let’s go back to the history and physical exam and look at some different findings. As before, patients report double vision. However, their symptoms improve with either eye closed on a physical exam, meaning that the double vision is due to misalignment of the eyes. In this case, we are talking about binocular diplopia. To find the cause you should assess the orientation of diplopia.

If your patient sees one object on top of the other, they are experiencing vertical diplopia, which is suggestive of cranial nerve IV, or trochlear nerve, palsy.

The trochlear nerve innervates the superior oblique muscle, which intorts, depresses, and abducts the eye.

Here’s a clinical pearl! In trochlear nerve palsy, the patient will likely have a head tilt away from the affected eye. If there is a history of a chronic head tilt, which might be present in photos or by report, they have decompensated congenital cranial nerve IV palsy. In this case, they were born with trochlear nerve palsy, but experience symptoms later in life. On the flip side, if history reveals a recent head trauma, you are dealing with traumatic cranial nerve IV palsy.

Next, let’s consider a patient with horizontal diplopia, meaning they see the same object side by side. Here, you’ll need to assess for pupillary dilatation. Parasympathetic innervation of the pupil comes from cranial nerve III, or oculomotor nerve, of the same side, and allows for pupillary constriction. If you see a dilated pupil that doesn’t react to light, consider cranial nerve III palsy.

The oculomotor nerve also innervates the inferior oblique muscle, and the superior, medial, and inferior recti muscles, as well as the levator palpebrae superioris, which elevates the upper eyelid.

So, if the exam shows impaired adduction, elevation, and depression of the eye; the eye position is “down and out” at rest; and there is ptosis, diagnose a cranial nerve III, or oculomotor nerve, palsy.

Time for a couple of clinical pearls! Patients with oculomotor nerve palsy might also report an oblique or diagonal orientation of their diplopia, as the muscles that control both horizontal and vertical movements are affected. Also, because adduction is impaired, patients might report worsening double vision when looking at near objects.
Now, in some cases of oculomotor nerve palsy, the pupil size and reflex will appear normal. This happens with ischemic mononeuropathy, which is caused by decreased blood supply due to microvascular damage that can occur with diabetes or hypertension. Parasympathetic fibers of cranial nerve III that innervate the pupillary sphincter and constrict the pupils run along the outer surface of the nerve, so they are less susceptible to ischemia.

On the other hand, if there is compression of the nerve causing pressure on the parasympathetic fibers, the pupil will become dilated and the pupillary reflex will be sluggish or absent. A common cause of compression is an aneurysm on the posterior communicating artery, which you can confirm with angiography.

Okay, let’s talk about cases where the pupil is not dilated, which should make you consider cranial nerve VI palsy or internuclear ophthalmoplegia.

To distinguish between the two, assess ocular motility. Remember that cranial nerve VI, also known as the abducens nerve, innervates the lateral rectus muscle.