Halos in vision are the perception of bright circles or rings seen around a light source, most commonly noticed in low-light environments like driving at night. Unlike simple glare, which is an overall increase in light intensity, halos appear as distinct rings, sometimes with color, surrounding the central light source. This visual disturbance occurs when light entering the eye is improperly focused on the retina.
The Optics Behind Halos
The appearance of halos is primarily a consequence of light scattering or diffraction within the eye’s transparent structures. For clear vision, light must pass straight through the cornea and the lens to focus precisely on the retina. When this pathway is disrupted, light rays are dispersed across the retina instead of converging at a single point.
Diffraction occurs when light waves bend as they pass near an obstacle or through a narrow opening, such as microscopic particles or irregularities in the eye’s tissues. This bending creates the ring-like pattern that the brain interprets as a halo around the light source.
Common Causes Related to Eye Surface and Refraction
The most frequent causes of visual halos involve temporary alterations to the eye’s front surface and focusing system. The cornea, the eye’s outermost clear layer, is responsible for a significant portion of light focusing. Dry eye condition can disrupt the smooth tear film covering this surface. This irregular tear film scatters light improperly, resulting in halos, particularly noticeable at night or upon waking.
Wearing contact lenses improperly or for too long can reduce oxygen reaching the cornea. This oxygen deprivation causes temporary corneal swelling (edema), which alters the cornea’s shape and leads to light scattering. Simple focusing imperfections can also cause halos. Conditions like astigmatism, where the cornea or lens is irregularly curved, or high myopia (nearsightedness) misdirect light, resulting in the halo effect.
Post-Surgical Halos
Halos are a common and anticipated side effect following refractive surgeries, such as LASIK or PRK. Reshaping the cornea can cause temporary swelling or slight tissue irregularities. These post-surgical halos usually diminish over weeks to months as the eye fully heals and the corneal surface stabilizes. The introduction of a new intraocular lens (IOL) during cataract surgery can also temporarily cause a diffraction effect, leading to halos as the brain adjusts to the new optics.
Serious Underlying Medical Conditions
When halos are persistent or appear alongside other symptoms, they may signal a developing disease process within the eye’s internal structures. The development of cataracts is a frequent cause, typically affecting older adults. A cataract involves the progressive clouding of the eye’s natural lens, which sits behind the iris. This clouding causes light entering the eye to be scattered haphazardly rather than focused clearly on the retina.
The scattering of light by the cloudy lens creates the perceived halo around bright sources, often making night driving challenging. Cataract-related halos generally worsen slowly as the lens opacity increases. Halos can also arise from corneal edema caused by conditions other than contact lens overuse, such as Fuchs’ dystrophy. This inherited disorder causes cells in the inner corneal layer to die, leading to chronic fluid buildup and swelling that severely compromises the cornea’s clarity.
Acute Angle-Closure Glaucoma
A far more urgent cause is acute angle-closure glaucoma, which is considered a medical emergency. In this condition, the eye’s drainage system becomes suddenly blocked, leading to a rapid increase in intraocular pressure (IOP). This pressure spike causes the cornea to swell and become hazy, resulting in the sudden appearance of colored halos or rainbow rings around lights. These halos are typically accompanied by severe eye pain, headache, nausea, and rapid vision loss.
When to Seek Medical Attention
Understanding when halos are benign versus when they signal serious disease is crucial for eye health. If halos are occasional, faint, and resolve after simple actions like blinking, they may relate to temporary dry eye or fatigue. Halos occurring only in the weeks following surgery like LASIK are usually a temporary part of the healing process.
Immediate medical attention is necessary if halos appear suddenly and are accompanied by severe symptoms. The combination of halos with intense eye pain, a throbbing headache, nausea, vomiting, or a rapid decrease in vision suggests acute angle-closure glaucoma. This condition requires treatment within hours to prevent permanent damage. For halos that are persistent, gradually worsening, or associated with increasing cloudiness or reduced night vision, a routine and prompt examination by an eye care professional is warranted. A comprehensive eye exam, including a pressure check and slit lamp analysis, allows a doctor to diagnose the underlying cause.