What is the key question to distinguish monocular from binocular diplopia?

Ask the patient: 'Does the double vision go away when you close one eye -- either eye?' If closing one eye eliminates the double vision, the diplopia is binocular (the two images come from the two eyes being misaligned). If double vision persists with one eye closed (in the open eye), the diplopia is monocular and is caused by an optical abnormality within that eye.

What causes monocular diplopia?

Monocular diplopia is caused by an optical imperfection within one eye that creates multiple focal points or distorted images. Common causes include irregular corneal astigmatism (keratoconus), cataracts (especially posterior subcapsular cataracts with uneven opacities), subluxated lens, large pterygium, or iris abnormalities. It is not caused by neurological disease.

What is amblyopia and what causes it?

Amblyopia ('lazy eye') is reduced visual acuity in one or both eyes that cannot be fully corrected with glasses, resulting from abnormal visual development during a critical period in early childhood. The brain fails to develop normal visual processing for the affected eye. Causes include strabismus (the brain suppresses the deviating eye's image), anisometropia (significantly different refractive error between eyes), and visual deprivation (dense cataract or ptosis blocking vision during development).

How does the Worth 4-dot test detect suppression?

The Worth 4-dot test uses a box with 4 illuminated dots (2 green, 1 red, 1 white) and red-green goggles. The red filter is placed over the right eye and the green filter over the left. If the patient sees 4 dots, they are fusing normally. If they see only 3 dots (2 green + 1 red-green), the right eye is suppressed. If they see only 2 dots (both red), the left eye is suppressed. If they see 5 dots (2 red + 3 green), they are experiencing diplopia.

What is suppression in strabismus?

Suppression is the brain's adaptive mechanism to avoid diplopia when the eyes are misaligned. The visual cortex actively ignores or reduces the input from the deviating eye, preventing the patient from consciously perceiving double vision. While it eliminates the symptom of diplopia, suppression prevents binocular vision and depth perception, and in children, it leads to amblyopia if the suppressed eye is not used. Adults with acquired strabismus typically cannot suppress and therefore experience constant diplopia.

Diplopia and Binocular Vision for the CPOA Exam

What Is Diplopia?

Diplopia is the perception of two images of a single object -- commonly called double vision. It is a common patient complaint that can range from a benign, long-standing phoria decompensation to a sign of acute neurological emergency. The first step in evaluating diplopia is determining whether it is monocular or binocular.

Monocular vs. Binocular Diplopia

Monocular Diplopia

Monocular diplopia persists when the fellow eye is covered -- the patient still sees double with only one eye open. It is almost always caused by an optical problem within that eye:

Irregular corneal astigmatism (including keratoconus)
Cataract (especially a posterior subcapsular cataract with a notched opacity)
Subluxated lens (partially dislocated crystalline lens)
Large pterygium distorting the corneal surface
Iris coloboma or polycoria

Monocular diplopia is almost never caused by neurological disease.

Binocular Diplopia

Binocular diplopia disappears when either eye is covered. This means the two images come from the two eyes, and the eyes are misaligned -- classic strabismus. Binocular diplopia is almost always caused by a misalignment of the visual axes, which can arise from:

Cranial nerve palsies (CN III, IV, or VI)
Mechanical restriction (thyroid eye disease, orbital fracture)
Decompensated phoria (long-standing latent deviation suddenly breaking down)
Myasthenia gravis (variable weakness of extraocular muscles)

💡 Clinical Tip: When a patient reports double vision, the most important screening question is: "Does the double vision go away when you close one eye?" If yes -- binocular diplopia, potential neurological urgency. If no -- monocular diplopia, likely optical cause, less urgent.

Normal Binocular Vision

When the visual systems of both eyes work together properly, the brain fuses the two slightly different retinal images (due to the separation of the two eyes) into a single three-dimensional percept -- a process called stereopsis. This is the basis of depth perception.

Normal binocular vision requires:

Both eyes aligned (no significant strabismus)
Clear retinal images in both eyes
Similar image sizes in both eyes (similar acuities)
Intact visual cortex and fusion centers

Suppression

When the eyes are misaligned and both eyes are open, the brain receives two conflicting images. Rather than seeing double, the brain often learns to suppress (ignore) the image from the deviating eye. Suppression is an adaptive response that eliminates diplopia but does so by sacrificing binocular vision.

Suppression is common in children with strabismus. Adults who develop strabismus later in life typically experience diplopia because their brain does not suppress as readily.

Amblyopia (Lazy Eye)

Amblyopia is reduced visual acuity in one (or both) eyes not correctable by optical means, caused by abnormal visual development during a critical period (typically the first 8-10 years of life). The brain suppresses or fails to develop the visual pathway for the affected eye. Common causes:

Strabismic amblyopia: Chronic suppression of a deviating eye
Refractive amblyopia: Significant uncorrected refractive error in one eye (anisometropic amblyopia)
Deprivation amblyopia: Obstruction of the visual axis during development (dense cataract, severe ptosis)

Treatment (before the critical period closes) involves:

Correcting the underlying cause (glasses, surgery)
Penalizing the stronger eye with patching or atropine drops to force use of the weaker eye

Binocular Vision Tests

The CPOA may assist with or administer binocular vision tests:

Worth 4-dot test: Uses red-green glasses and a box with 4 lights to detect suppression and fusional ability. Results: 4 dots = normal fusion; 2 dots = suppression of right eye; 3 dots = suppression of left eye; 5 dots = diplopia.
Stereoacuity testing (Randot, Titmus fly): Measures depth perception in seconds of arc. Normal stereoacuity with Titmus is 40 seconds of arc (1:40). Reduced stereoacuity indicates binocular vision compromise.

⚠️ Common Mistake: When administering the Worth 4-dot test, placing the red glasses over the wrong eye. The red glass must go over the right eye and the green over the left. If reversed, suppression findings will be attributed to the wrong eye. Always confirm the filter color before recording results.

Key Takeaways

Monocular diplopia persists with one eye covered; caused by optical problems within that eye (cataract, corneal irregularity).
Binocular diplopia disappears when one eye is covered; caused by misalignment (cranial nerve palsy, strabismus).
Suppression: the brain ignores the deviating eye's image to avoid diplopia; common in children with strabismus.
Amblyopia is reduced vision in an eye from abnormal visual development during the critical period; treated with glasses and patching.
Worth 4-dot test detects suppression; stereoacuity tests measure binocular depth perception.
Red glass over right eye, green glass over left eye in Worth 4-dot testing.