Monocular Diplopia – Causes

Monocular diplopia (double vision in one eye only, persisting even when the other eye is closed or covered) is almost always caused by problems in the eye's optical system — specifically anything that distorts, scatters, or diffracts light before it reaches the retina in a single, clean focus.

It is usually benign (not dangerous to life or brain) and treatable — unlike binocular diplopia (double vision only when both eyes are open), which can signal serious neurological issues.

Main Causes

• Refractive errors (most common, especially uncorrected or irregular)
• Astigmatism — irregular corneal curvature scatters light → ghosting/shadowing of images
• Uncorrected myopia/hyperopia or presbyopia — less common but can contribute
• Corneal irregularities (very common)
• Dry eye syndrome — uneven tear film causes transient ghosting (often improves with blinking or artificial tears)
• Keratoconus — progressive corneal thinning and cone shape → severe irregular astigmatism
• Corneal scars, edema, dystrophies, pterygium, or post-LASIK irregularities
• Lens abnormalities (extremely common, especially in older adults)
• Cataract — clouding/opacities in the crystalline lens scatter light → classic monocular ghosting, halos, or multiple images
• Lens subluxation/dislocation (e.g., after trauma or in Marfan syndrome) — lens shifts → distorted focus
• Retinal or macular problems (less common but important)
• Epiretinal membrane (macular pucker) — wrinkling distorts retinal surface
• Macular edema or central serous retinopathy — fluid elevation changes focus
• Macular degeneration (wet type) — can rarely cause metamorphopsia + ghosting
• Other rare causes
• Posterior vitreous detachment/floaters — large floaters can cast shadows or cause transient diplopia
• Cerebral/cortical polyopia — very rare bilateral monocular diplopia from occipital lobe lesions
• Medication side effects or intoxication (e.g., alcohol, sedatives) — temporary disruption

Key Diagnostic Clues

• Pinhole test: If diplopia improves or disappears when looking through a pinhole → almost always refractive/corneal/lens cause
• No improvement with pinhole → consider retinal or (rarely) cerebral cause
• Improves with blinking → strongly suggests dry eye/tear film issue

When to seek help

Monocular diplopia is rarely dangerous, but sudden onset, associated vision loss, pain, or new floaters/flashes warrant urgent ophthalmology evaluation to rule out retinal detachment, acute glaucoma, or other serious eye conditions.

If this is happening to you or someone you know, see an eye doctor (optometrist or ophthalmologist) — most causes are fixable with glasses, drops, or surgery.