What is the differential diagnosis (DDx) for blurry vision that improves with one eye closed?

Differential Diagnosis: Blurry Vision Improving with One Eye Closed

The most likely diagnosis is binocular retinal diplopia (dragged-fovea diplopia syndrome), where macular distortion creates dissimilar foveal images that cannot fuse binocularly, but closing one eye eliminates the conflicting image and restores clarity. 1, 2

Primary Diagnostic Consideration

Binocular Retinal Diplopia (Dragged-Fovea Diplopia Syndrome)

This condition occurs when macular disease creates images too dissimilar between the two eyes to fuse, even though peripheral retinal images can still align. 1, 2

Key mechanism: Peripheral fusion dominates over central fusion when viewing binocularly, forcing peripheral retinas into alignment while leaving the foveas misaligned—this central-peripheral rivalry prevents proper visual function with both eyes open but resolves immediately when one eye is closed. 2

Epidemiology and risk factors:

• Epiretinal membranes or subretinal neovascular membranes affect 2% of individuals under age 60, increasing to 12% over age 70 1, 2
• Between 16-37% of patients with these maculopathies develop binocular central diplopia 1, 2, 3
• Symptoms typically develop within days to weeks of worsening maculopathy 1

Pathognomonic diagnostic tests:

• Lights on/off test: In complete darkness with no peripheral fusion cues, the patient sees a small test letter singly; when room lights turn on, peripheral fusion reasserts itself and the letter becomes double 1, 2, 3
• Optotype-frame test: Patient fixates an isolated letter on a monitor—if the letter is single but maintaining single vision of the monitor frame causes the letter to become double, this confirms peripheral fusion is overriding central fusion 1, 2, 3

Additional examination findings:

• Prism alternate cover testing typically shows either no strabismus or only a small vertical deviation 1, 3
• Metamorphopsia identified on Amsler grid testing and quantified with M-Charts 1, 2, 3
• Aniseikonia documented with Awaya test 1, 2, 3
• Any diplopia relief from prism correction is transient at best 1, 3

Secondary Diagnostic Considerations

Convergence Insufficiency

This presents as exophoria or exotropia at near with eyestrain, headaches, blurred vision, or horizontal diplopia during reading or near work. 1

Distinguishing features:

• Symptoms specifically worse at near, not distance 1
• Annual incidence of 8.4 per 100,000 people, representing 15.7% of new-onset adult strabismus cases 1
• Median age of new-onset is 69 years 1
• Associated with history of concussion or Parkinson's disease 1
• Closing one eye eliminates the need for convergence, improving comfort rather than eliminating true diplopia 1

Corneal Edema (Fuchs Dystrophy)

Transient blurred vision upon waking that improves later in the day due to evaporation reducing edema, though this typically doesn't require closing one eye. 1

Key characteristics:

• Diurnal pattern: worse upon waking, clearer later in the day 1
• Low humidity and modest air movement lead to visual improvement 1
• Photophobia, redness, tearing, intermittent foreign-body sensation 1
• Visual acuity may not correlate with visual function—patients may have 20/40 or better but disabling glare 1

Common pitfall: This condition improves with environmental factors and time of day, not specifically with monocular viewing, making it less likely when the patient reports improvement specifically with one eye closed. 1

Uncorrected Refractive Error with Anisometropia

Significant difference in refractive error between the two eyes can cause binocular blur that improves when the worse eye is closed. 4

Diagnostic approach:

• Pinhole testing improves vision if refractive error is the cause 4
• Anisometropia causes specific functional limitations detectable with simple refraction 4
• Refractive errors account for 21.1% of all outpatient ophthalmology visits 4

Management Algorithm

For binocular retinal diplopia (most likely diagnosis):

1. First-line treatment: Fogging one eye is the most successful long-term solution, creating a central scotoma that eliminates the foveal conflict 1, 2, 3

   • Scotch Satin tape or Bangerter foils on spectacle lenses are generally well-tolerated 1, 2, 3
   • Occlusive contact lenses are an alternative 1, 3
   • Small amount of prism plus Bangerter foil can provide better relief in difficult cases 1, 3

2. Surgical considerations:

   • Strabismus surgery is rarely helpful because it cannot resolve the mismatch of distorted macular images or the central-peripheral conflict 1, 2, 3
   • Epiretinal membrane peeling may be effective in some patients, though this is a double-edged sword as some non-diplopic patients become diplopic following retinal surgery 1, 2, 3

3. Referral pattern:

   • Coordinate care between retina specialist and strabismus specialist (pediatric ophthalmologist, orthoptist, or neuro-ophthalmologist) 1, 3
   • Retina specialist manages underlying maculopathy 3

Critical pitfall to avoid: Do not assume all "double vision" is true diplopia—many patients use "double vision" to describe blurred vision or visual distortion rather than true image separation. 3 Missing binocular central diplopia can result in failed treatment, as addressing any small-angle deviation surgically without treating the retinal distortion will not resolve the condition. 3