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 |
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
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.
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