Glare, haloes and night driving: what surgery can and cannot fix

Glare and haloes around lights at night are one of the most common reasons people first ask about eye surgery, and one of the most common reasons they are still unhappy afterwards. The reason is that ‘my eyes are bad at night’ is not one complaint. It has at least five distinct causes, only some of which surgery treats, and working out which one applies has to come before recommending anything at all.

The description people give is often similar whatever the cause — headlights that seem to bloom, streetlights with rings around them, a general difficulty judging distance in the dark — which is exactly why the symptom on its own is not a useful guide to treatment. Two people can describe the same night drive and be looking at entirely different underlying problems.

A cataract scattering light

A cataract clouds the natural lens, and a clouded lens scatters incoming light rather than focusing it cleanly. This is experienced as haloes, starbursts and glare from oncoming headlights, often before reading vision is noticeably affected. It is the cause surgery treats most reliably: removing the cataract removes the scattering source, and night glare from this cause typically improves, often markedly, once the lens has been replaced.

A cloudy capsule after earlier cataract surgery

Months or years after successful cataract surgery, the thin capsule left in place to support the implant can itself become cloudy — posterior capsule opacification — producing much the same glare and haze as the original cataract did. It is not the cataract returning; the lens is gone and cannot cloud again. A YAG laser capsulotomy, a brief outpatient procedure at a slit lamp, opens the clouded capsule and resolves it, usually with vision improving within a day or so.

Dry eye and an unstable tear film

An unstable or insufficient tear film scatters light across an irregular surface in much the same way a cataract does, and is a common but frequently missed cause of night glare, particularly after long screen use or in air-conditioned environments. This is not a surgical problem. It is managed with lubricating drops, attention to eyelid hygiene, and sometimes punctal plugs. Operating on an eye where the real cause is a dry ocular surface will not fix the glare, and can leave the eye less comfortable than before.

A large pupil with a multifocal lens

This is the one case where surgery itself is the cause rather than the cure. Multifocal intraocular lenses work by splitting light between two or more focal points, which can produce rings or haloes around lights at night, particularly in people whose pupils dilate widely in the dark. This is a known trade-off of the lens design rather than a fault, and it is exactly why pupil behaviour is assessed before deciding whether a multifocal lens is a sensible choice for a given person at all.

This possibility is discussed before a multifocal lens is chosen, not discovered afterwards, which is why the assessment for one includes measuring the pupil in dim light rather than only in a normally lit consulting room. Someone whose pupils dilate widely after dark may be steered towards a monofocal lens instead, or towards a multifocal design with a smaller optical trade-off, precisely to avoid trading reading glasses for headlight haloes.

Uncorrected astigmatism

An irregularly shaped cornea focuses light unevenly, which can produce streaking or smearing around point light sources such as headlights and streetlamps — a different appearance from the rounder haloes a cataract or a multifocal lens produces. Astigmatism is correctable, with spectacles, a toric lens at the time of cataract surgery, or laser treatment, but only once it has actually been identified as the source of the complaint rather than assumed.

Small amounts of astigmatism are common and often go uncorrected for years because they cause no obvious blur in daylight, only a subtle degrading of point sources of light after dark. It is easy to miss on a standard reading-chart test and is picked up properly only with corneal measurement, which is one reason a night-glare complaint is worth a fuller assessment than a quick vision check.

More than one cause at once

These causes are not mutually exclusive, and in practice they often overlap. A person in their sixties with an early cataract may also have an unstable tear film from years of contact lens wear and mild astigmatism that was never quite significant enough to correct on its own. Treating only the most obvious of the three can leave the complaint only partly resolved, which is sometimes mistaken for a disappointing surgical result when the surgery itself did exactly what it was meant to.

Why the diagnosis has to come first

The overlap between these causes is exactly why a description of the symptom is not enough to plan treatment around. A proper examination — checking the lens, the tear film, pupil behaviour under different lighting, and the corneal surface — identifies which of these is responsible, sometimes more than one at once, before any operation is discussed. Treating the wrong one leaves the original complaint unresolved and adds a procedure that was never going to fix it.