What are the differences between monovision and mono ocular (single eye) vision in terms of correcting presbyopia and refractive errors?

Monovision vs. Monocular (Single Eye) Vision: Key Differences

Monovision is a deliberate refractive strategy where one eye is corrected for distance and the other for near vision to address presbyopia, while monocular vision refers to functional vision using only one eye (either due to vision loss in the fellow eye or suppression). These are fundamentally different concepts with distinct clinical applications and outcomes.

Defining the Terms

Monovision is an intentional optical correction strategy for presbyopia where:

• One eye (typically the dominant eye) is corrected for distance vision
• The fellow eye is corrected for near vision, creating controlled anisometropia
• Both eyes remain functional, with the brain alternating between eyes based on viewing distance 1

Monocular vision refers to:

• Vision using only one functional eye
• The fellow eye may have significant visual impairment or be suppressed
• No binocular function is present 1

Monovision: Clinical Application and Outcomes

Patient Selection Criteria

Ideal candidates for monovision are patients over 40 years old who prioritize freedom from optical aids and can accept compromises in uncorrected distance stereoacuity 1. The American Academy of Ophthalmology guidelines emphasize:

• Patients must be willing to sacrifice some binocular function for spectacle independence 1
• A preoperative trial with contact lenses is essential to determine tolerance and preferred refractive endpoints 1
• Success rates range from 59-67% in adapted contact lens wearers, with surgical monovision showing 73% mean success 2, 3

Critical Contraindications

Exercise extreme caution in patients with previous strabismus surgery, phorias, or intermittent tropias, as these patients may develop postoperative diplopia 1. This represents a key screening requirement that distinguishes suitable from unsuitable candidates.

Eye Dominance Considerations

The traditional approach corrects the dominant eye for distance and the nondominant eye for near 1. However:

• Evidence suggests that near correction in the dominant eye ("crossed monovision") may be successful and even preferable in some patients 1
• In LASIK monovision, 85% of patients chose their dominant eye for distance correction, but crossed monovision achieved comparable success rates 4
• Ocular dominance is not fixed but rather a fluid, adaptive phenomenon in most patients 2

Degree of Anisometropia

Larger degrees of anisometropia produce better visual function at near, but smaller degrees may be better tolerated 1. The evidence shows:

• LASIK monovision with -0.50D to -1.25D of residual myopia in the nondominant eye enables good vision at all distances without significantly affecting stereovision in patients under 50 years 5
• Micro-monovision (smaller anisometropia) may be better suited for patients who can tolerate minimal distance vision compromise to attain greater intermediate vision function 6

Visual Compromises with Monovision

Contrast Sensitivity and Stereopsis

Patients with monovision correction experience decreased contrast sensitivity and stereopsis compared with bilateral distance correction 1. Specific findings include:

• Significant reduction in binocular contrast sensitivity at spatial frequencies higher than 4 cycles per degree 3
• Stereoacuity reduction of less than 50 seconds of arc correlates with monovision success 3
• Most patients do not subjectively notice impaired stereopsis in daily activities 2

Functional Limitations

Patients with monovision who function well for most daily activities may still benefit from eyeglass correction, especially in dim-light conditions while driving 1. Additional considerations:

• Many patients with low degrees of monovision can drive without difficulty 1
• Task performance shows 2-6% reduction, but minimal reduction in binocular visual acuity, peripheral vision, and visual field width 3
• Main limitations include problems suppressing the blurred image when driving at night and need for intermediate focal length (e.g., computer screens) 2

Surgical Implementation Strategies

Refractive Surgery Options

The most widely used surgical approach for monovision is excimer laser photoablation (LASIK, PRK, LASEK, epi-LASIK) or refractive lenticule extraction 1. Key outcomes:

• In LASIK monovision, only 7% of patients chose to forego monovision and enhance the near eye to distance vision 4
• However, 27.9% of monovision patients required enhancement of their distance vision eye, suggesting lower tolerance for residual refractive error in the distance eye 4

Intraocular Lens Strategies

Monofocal and extended depth of focus IOLs can be used with monovision strategies for presbyopia correction 1. Important caveats:

• It is difficult to assess ocular dominance in preoperative patients with cataracts 1
• Demonstrating proposed monovision results with contact lenses before cataract surgery is challenging 1
• Patients who demonstrated success with monovision contact lenses before cataract development are well-suited for this modality 1

Comparison with Multifocal IOLs

When comparing monovision to multifocal IOLs for presbyopia:

• Multifocal IOL patients had better near uncorrected visual acuity 5
• LASIK monovision patients had better distant and intermediate vision 5
• Multifocal IOL patients reported more visual disturbances (halo, glare) 5
• Patient satisfaction and spectacle independence were high in both groups 5

Monocular Vision: Clinical Context

Monocular vision is not a refractive strategy but rather a condition where:

• The majority of adults can tolerate up to 3.00 D of anisometropia in eyeglass correction between eyes 1
• Symptomatic aniseikonia may require undercorrection at the expense of acuity or lens modifications 1
• Eyeglasses are protective, which is especially important for monocular patients 1
• Functional monocularity increases surgical risk for intraocular procedures 7

Key Clinical Pitfalls to Avoid

1. Never proceed with surgical monovision without a successful contact lens trial 1, 2—surgically induced monovision is less easily reversed than contact lens-induced monovision 2

2. Screen for binocular vision anomalies preoperatively—monovision could cause a binocular vision anomaly to decompensate 2

3. Account for occupational factors—patients requiring exceptional distance vision or stereopsis (e.g., pilots, surgeons) may not be suitable candidates 2

4. Do not allow patients to drive or operate machinery until successfully adapted to monovision 2

5. When correcting the near eye for distance with eyeglasses, distance visual acuity and depth perception are optimized—this provides a reversible option for activities requiring binocular distance vision 1