Multifocal lens implants: benefits, haloes and who should avoid them

What does a multifocal lens actually do?

A multifocal intraocular lens, or IOL, replaces the eye's natural lens during cataract surgery or lens replacement surgery. It directs light to more than one focusing distance. A trifocal is a type of multifocal, designed to help with distance, intermediate tasks such as a computer, and near tasks such as reading.

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This is different from restoring the flexible focusing of a young natural lens. The implant has a fixed optical design. While one part of its image is in focus, light intended for another distance can contribute an out-of-focus image. That helps explain both the broader range of vision and some of the unwanted light effects.

The multifocal and trifocal lens page explains the optics in more detail. The practical question is how those optics fit your eyes and your day-to-day life.

What are the benefits, and will I still need glasses?

The main benefit is doing more without reaching for glasses. Reading a phone, checking a price label, looking at a screen and then across a room may become more convenient. Compared with a standard monofocal lens set for distance, multifocal lenses generally offer better unaided near vision and less dependence on reading glasses.

They cannot promise glasses-free vision for every task. Small print, prolonged reading, dim lighting or a particular working distance may still be easier with glasses. A small remaining focusing error or astigmatism can also affect the result. Your usual reading distance matters: a lens that suits one person's phone use may be less comfortable for another person's close needlework.

For a fuller explanation of why glasses can still be helpful, see glasses after cataract surgery. The useful goal is less dependence on glasses, with a clear understanding of when you may still prefer them.

Why do multifocal lenses cause haloes and glare?

The same optics that provide several focusing distances can produce rings around headlights or streetlights. Glare is scattered brightness that makes it harder to see; starbursts are streaks extending from a light. These effects are more likely with multifocal lenses than with standard monofocal lenses.

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There can also be a reduction in contrast: the ability to pick out an object from a similar background. You might read small black letters on a bright white chart yet find a pedestrian in dark clothing harder to distinguish on a wet road at dusk. Sharpness on an eye chart and comfortable night vision are related, but they are not the same measurement.

Some people become less aware of these effects as their visual system adapts. Others continue to find them intrusive. There is no reliable promise that haloes will disappear by a particular month. The design of the lens, the eye and the task all matter; findings for one model do not describe every multifocal lens.

Who should usually avoid a multifocal lens?

People whose eyes already have reduced visual quality, or whose priorities leave little room for optical side effects, often have stronger reasons to choose another design. Particular caution is needed with:

• Macular disease affecting central vision, such as age-related macular degeneration or an epiretinal membrane. The macula must make use of the image the implant provides.
• Glaucoma with meaningful optic nerve or visual-field damage. Further reduction in contrast may be unwelcome when visual function is already limited.
• An irregular or scarred cornea, including keratoconus. An uneven optical surface can add further blur and visual disturbances.
• A strong need for dependable night vision, such as frequent night driving or work in low light. This is a reason to weigh alternatives carefully, even when the examination is otherwise reassuring.
• An expectation of perfect vision at every distance, no glasses and no haloes. No multifocal implant can reliably meet that combination.

These are reasons for a careful recommendation, rather than a diagnosis-based ban for every mild finding. The severity, stability and likely future course of an eye condition matter. Equally, accepting occasional reading glasses can be a very reasonable choice when quality of vision matters more to you than the widest unaided range.

Which problems should be checked or treated first?

Dry eye is an important example. An unstable tear film can blur vision and make preoperative measurements less reliable. Treating the surface and rechecking the measurements may change the recommendation; dry eye does not automatically rule out a multifocal lens forever.

Regular astigmatism is also different from an irregular cornea. An appropriate toric lens can correct regular corneal astigmatism, and toric versions of multifocal lenses are available. Previous LASIK or other corneal laser surgery calls for extra assessment of corneal shape, optical quality and lens-power calculations; it is not an automatic yes or no.

Assessment commonly includes the tear surface, corneal measurements, a dilated retinal examination and an OCT scan of the macula when considering this type of lens. The plan for both eyes, previous surgery and how well the eyes work together also deserve attention. As a cataract and retinal surgeon, Dr Chen's assessment considers both the lens being replaced and the retina receiving the image.

What are the alternatives?

There are several ways to reduce glasses dependence, each with a different compromise. The choice is easier when you start with the task you most want to improve. The guide to choosing an intraocular lens explores those priorities.

• Monofocal lenses: one main focusing distance, usually distance. Reading glasses are generally needed, but these lenses usually have fewer optical night-time effects than multifocals.
• Extended depth of focus (EDOF) lenses: aim to extend the useful range, often from distance towards screen distance. Reading glasses may still be needed. Designs differ, and an EDOF label does not guarantee freedom from haloes.
• Monovision or mini-monovision: one eye is aimed more towards distance and the other towards closer work. This can use monofocal lenses, but may affect depth perception or the way the two eyes work together. Glasses may still help for some tasks; a contact-lens trial can be useful when feasible.

A higher-priced or more complex optic is not automatically a better match. If night driving matters most, say so before the lens is selected.

What if I already have a multifocal lens and dislike the vision?

Arrange a review rather than assuming you simply need to adapt. Dry eye, a remaining prescription, clouding of the capsule behind the implant, lens position or a retinal problem may contribute. Some of these causes are treatable. The article on blurred vision after cataract surgery explains why the cause needs to be identified first.

If symptoms remain unacceptable, lens exchange may be discussed. It is another operation with additional risks, not a simple reset. Discuss the possibility of exchange before having YAG laser treatment to the capsule: opening the capsule can make a later exchange more complex. This does not mean YAG should never be used; it means the reason for dissatisfaction should be understood first.

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Sudden loss of vision, increasing pain or redness, or new flashes, floaters or a curtain over the vision need urgent assessment. Do not put these down to multifocal adaptation. Avoid driving if your vision is not adequate for the conditions.

What should I ask before deciding?

Ask the surgeon to relate the recommendation to your own eyes and the tasks you care about:

• What might this lens let me do without glasses, at my usual distances?
• What findings in my cornea, macula or optic nerve favour or count against it?
• How does it compare with a monofocal or EDOF lens for my night driving?
• What is the plan for both eyes, and when might glasses still help?
• If I am unhappy, what would be checked, and what might further treatment involve?

A multifocal lens can be worthwhile when the eye is suitable and the wider range of vision is worth the optical compromise to you. Choosing a simpler lens is equally valid. The decision should make sense for how you want to see, including the situations in which you most need to trust your vision.