How do you tell the difference between binocular and monocular diplopia?

Cover one eye: if the double vision resolves, it is binocular diplopia (caused by eye misalignment). If double vision persists in one eye, it is monocular diplopia (caused by an optical problem like astigmatism, cataract, or corneal irregularity). This simple test determines the fundamental cause.

How does prism correct diplopia?

Prism bends light, shifting the image position for one eye to align with the other eye's image. The prism base is placed toward the direction of eye deviation (base-out for esotropia, base-in for exotropia). This eliminates the doubled image without the eye needing to change position.

When should diplopia be considered urgent?

New-onset binocular diplopia should be considered urgent, especially when accompanied by headache, pupil dilation or asymmetry, eyelid droop, or pain. A CN III palsy with pupil involvement may indicate a cerebral aneurysm requiring emergency treatment. Any sudden diplopia warrants prompt medical evaluation.

Diplopia (Double Vision) for ABO

What Is Diplopia?

Diplopia is the perception of two images of a single object, commonly called double vision. It can be extremely disorienting and may indicate serious underlying conditions. For opticians, understanding diplopia helps with prism dispensing and recognizing when patients need urgent referral.

Binocular vs. Monocular Diplopia

The first step in evaluating diplopia is determining whether it persists when one eye is covered:

Type	Test	Causes	Urgency
Binocular	Resolves when either eye is covered	Eye misalignment (strabismus, nerve palsy, decompensated phoria)	May need urgent investigation
Monocular	Persists with the other eye covered	Optical (uncorrected astigmatism, cataract, corneal irregularity, lens dislocation)	Usually not urgent

If double vision goes away when EITHER eye is covered, it is BINOCULAR diplopia caused by eye misalignment. If it persists with one eye covered, it is MONOCULAR diplopia caused by an optical problem in that eye. This distinction is critical for determining the cause and urgency.

Common Causes of Binocular Diplopia

Cranial nerve palsy: CN III (oculomotor), CN IV (trochlear), or CN VI (abducens) palsy causes weakness of specific extraocular muscles
Decompensated phoria: A previously controlled latent deviation breaks down, often from fatigue, illness, or stress
Thyroid eye disease: Enlarged extraocular muscles restrict movement
Myasthenia gravis: Variable muscle weakness that worsens with use
Orbital fracture: Entrapped muscles limit eye movement
Convergence insufficiency: Eyes cannot converge adequately for near tasks

Diplopia from Spectacle Lenses

Opticians should be aware that spectacles can sometimes induce diplopia:

Incorrect PD: Unwanted horizontal prism causes horizontal diplopia
Incorrect OC height: Vertical prism from decentration causes vertical diplopia
Anisometropic vertical imbalance: Different lens powers in multifocals cause vertical diplopia in downgaze
Progressive lens distortion: Peripheral image displacement in progressives
Prism grinding error: Wrong amount or direction of prescribed prism

Prism Correction for Diplopia

When diplopia is caused by a small, stable deviation, prism in spectacle lenses can shift one eye's image to align with the other. Principles:

Prism is placed with the base toward the direction of deviation
For esotropia (inward): base-out prism
For exotropia (outward): base-in prism
For hypertropia (upward): base-down prism on the higher eye
Prism can be split between the two eyes for comfort and cosmesis

When dispensing prism, verify the prism power and base direction with a lensometer before delivering the glasses. Even small errors in prism (1Δ or less) can cause discomfort or persistent diplopia. Check both horizontal and vertical prism at the fitting cross, not just at the distance OC.

Fresnel Prism

Fresnel press-on prisms are thin, flexible plastic sheets with prism ground into ridges. They are applied to the back surface of an existing spectacle lens. Benefits:

Quick solution for trial or temporary prism needs
Available in high powers (up to 40Δ) that are impractical in ground prism
Easy to adjust or change

Limitations: reduced visual acuity (especially above 10Δ), visible ridges, and cosmetic concerns.

Dismissing a patient's complaint of new-onset double vision as "just needing new glasses." Sudden binocular diplopia, especially with headache or pupil changes, can indicate a life-threatening condition (brain aneurysm compressing CN III). Always refer new-onset binocular diplopia for urgent evaluation.

Key Takeaways

Binocular diplopia resolves with one eye covered; monocular diplopia does not
Binocular diplopia indicates eye misalignment; monocular indicates an optical problem
Spectacle errors (wrong PD, OC height, prism) can induce diplopia
Prism correction shifts images to restore single binocular vision
New-onset binocular diplopia requires urgent medical evaluation