Causes of Monocular Diplopia
Monocular diplopia is almost always caused by ocular pathology within the affected eye itself, not neurological disease, and persists when the unaffected eye is covered. The key distinguishing feature is that covering the affected eye eliminates the diplopia, while covering the normal eye does not 1, 2.
Primary Ocular Causes
The most common causes of monocular diplopia involve optical disruptions within the eye that allow a single object to stimulate two separate retinal areas with different visual directions 3:
Refractive and Optical Media Abnormalities
- Cataracts - The most frequent cause, creating multiple light pathways through irregular lens opacities 1, 2
- Uncorrected refractive errors - Particularly high astigmatism or irregular astigmatism
- Corneal irregularities - Including scarring, keratoconus, or dry eye disease 2, 4
- Lens subluxation or dislocation - Creating dual optical pathways
Iatrogenic and Post-Surgical Causes
- Laser peripheral iridotomy - Particularly when "exposed," allowing light through both the pupil and iridotomy site 3
- Post-cataract surgery complications - Including:
- Post-refractive surgery - Decentered ablations or irregular healing 1
Pupillary Abnormalities
- Traumatic corectopia - Irregular or displaced pupil after penetrating injury 5
- Polycoria - Multiple pupillary openings (true or pseudopolycoria)
- Large pupils - Exposing lens edge effects or irregular optics
Retinal Pathology
- Macular disease causing foveal displacement:
- Retinal detachment involving the macula 6
Note: While retinal causes can produce monocular diplopia, they more commonly cause binocular central diplopia when both eyes are affected, as the foveal images become too dissimilar to fuse 6.
Rare Neurological Causes
Cortical Dysfunction
- Cerebral polyopia - Perception of multiple images from occipital lobe disturbance, can occur with migraine or be accompanied by homonymous hemianopia 2
- Palinopsia - Visual perseveration from occipital lesions 2
- Idiopathic intracranial hypertension - Rarely presents with monocular diplopia, likely from papilledema-related retinal changes 4
These cortical causes are extremely rare and mechanistically uncertain 2.
Diagnostic Approach
The critical first step is confirming true monocular diplopia by having the patient alternately cover each eye - monocular diplopia resolves only when the affected eye is covered 1, 2, 7.
Key Examination Elements:
- Slit-lamp examination - Identify corneal irregularities, cataract, iris abnormalities, lens position, and anterior chamber pathology 3, 5
- Pupillary examination - Look for corectopia, polycoria, or abnormal shape
- Refraction - Assess for uncorrected refractive error or irregular astigmatism
- Dilated fundus examination - Evaluate for macular pathology, epiretinal membranes, or retinal detachment 6
- Pinhole testing - Often improves vision if refractive or optical media cause 5
Specialized Testing When Indicated:
- Amsler grid and M-Charts - Quantify metamorphopsia from macular disease 6
- Awaya test - Quantify aniseikonia 6
- OCT (optical coherence tomography) - Detailed retinal and macular evaluation
Management Principles
Treatment is directed at the underlying cause 1:
- Refractive correction - Glasses or contact lenses for refractive errors
- Prosthetic contact lenses - Can occlude aberrant light pathways in cases of corectopia or exposed iridotomies 5
- Cataract surgery - For lens opacities
- Surgical repair - For lens subluxation or traumatic iris defects
- Observation - For mild macular disease if diplopia is tolerable
- Occlusion - Last resort when other treatments fail 1
Critical Pitfall
Do not pursue extensive neurological workup for true monocular diplopia - it is almost never neurological in origin 1, 2, 7. The rare cortical causes (polyopia, palinopsia) have additional neurological signs and are diagnoses of exclusion. Ensure you have correctly distinguished monocular from binocular diplopia before proceeding with management.