Blurry vision occurring in only one eye, known as monocular blurry vision, indicates a localized problem within that specific visual pathway. Unlike blurriness affecting both eyes, which is often related to general fatigue or minor refractive changes, unilateral blurring points to a specific issue in the affected eye or its connection to the brain. The cause is almost always an identifiable disruption in the complex process of sight. The source of the blurriness can originate from the transparent front surface of the eye, the light-sensing structures at the back, or the neural pathways that transmit the image.
Blurring Due to Changes in the Eye’s Surface and Focus
The front structures of the eye gather and bend light rays to focus them onto the retina. When the shape of the cornea or the lens changes, the resulting refractive error can cause blurriness that affects one eye more than the other. This difference is called anisometropia, where one eye progresses in conditions like nearsightedness, farsightedness, or astigmatism independently of the other.
The cornea, the eye’s clear outer dome, is vulnerable to localized issues that cause blurring. A corneal abrasion (a scratch on the surface) or keratitis (an infection or inflammation) can significantly distort the image entering the eye. These conditions disrupt the smooth surface required for proper light transmission, resulting in blurred vision, often accompanied by pain or light sensitivity.
Tear film instability, known as dry eye syndrome, can selectively affect one eye more severely, leading to fluctuating vision that often improves with blinking or lubricating drops. When the tear film evaporates too quickly or is produced inadequately, the cornea cannot maintain its smooth optical surface, causing temporary blurriness. Similarly, the gradual clouding of the eye’s lens, known as a cataract, typically progresses unevenly, causing one eye to experience blurring, glare, and halos before the other.
Sudden Vision Loss from Posterior Eye Structures
When blurriness or vision loss is sudden and affects the posterior segment of the eye, it signals a serious structural failure requiring immediate medical attention. Retinal detachment is one such emergency, occurring when the light-sensitive tissue pulls away from its underlying support layer. Symptoms include a sudden increase in floaters, flashes of light (photopsia), and the perception of a shadow or curtain moving across the field of vision.
Damage or fluid leakage affecting the macula, the small central part of the retina, can result in sudden, distorted central vision. This occurs in the wet form of age-related macular degeneration (AMD) when abnormal blood vessels leak fluid or blood beneath the macula. A vitreous hemorrhage, which is bleeding into the vitreous gel, causes sudden, dense cloudiness or a shower of dark spots that obscure vision.
Acute angle-closure glaucoma is a sudden-onset condition caused by a rapid buildup of pressure inside the eye. This occurs when the iris blocks the drainage angle, preventing the outflow of aqueous humor. The resulting high pressure can quickly damage the optic nerve, causing severe eye pain, headache, nausea, and rapid blurring or the perception of halos around lights in the affected eye.
Neurological and Vascular Causes of Unilateral Blurring
Blurriness in one eye can arise from issues affecting the blood supply or the neurological pathways that relay visual information. Optic neuritis involves inflammation of the optic nerve, the bundle of fibers that transmits visual signals from the retina. This inflammation causes sudden vision loss, dimness, or “washed-out” color perception, often accompanied by pain that worsens with eye movement. Optic neuritis is associated with autoimmune conditions, particularly multiple sclerosis.
Vascular occlusions, referred to as “eye strokes,” occur when blood flow to the retina is blocked, leading to rapid monocular vision loss. A Central Retinal Artery Occlusion (CRAO) involves a blockage in the main artery supplying the retina, causing instantaneous and profound vision loss. A Central Retinal Vein Occlusion (CRVO) involves a blockage in the vein draining the retina, resulting in blood and fluid back-up that causes less severe but still sudden blurring.
Amaurosis fugax, or transient monocular vision loss, presents as a temporary “shutter” or curtain coming down over the vision, usually lasting only a few minutes. This is caused by a temporary blockage, such as a tiny embolus from the carotid artery, that briefly impedes blood flow to the retina. While vision often recovers spontaneously, this event is a warning sign that the individual may be at risk for a future stroke or transient ischemic attack (TIA). Less commonly, a visual aura associated with an ocular migraine can cause temporary, shimmering or blurred vision that may be perceived as affecting only one eye.
Determining the Urgency of Unilateral Blurry Vision
The timing and accompanying symptoms of monocular blurry vision are the primary factors determining the speed of medical care. If the blurring is a sudden, noticeable event, it should be treated as an emergency. Immediate care is required if the vision loss is accompanied by severe eye pain, headache, nausea, or the appearance of a dark curtain or shadow over the vision. These symptoms may signal acute angle-closure glaucoma, retinal detachment, or a vascular event, all of which can lead to permanent vision loss if treatment is delayed.
If the sudden blurring is paired with neurological symptoms such as weakness, numbness on one side of the body, difficulty speaking, or confusion, it may indicate a stroke or TIA, requiring an immediate call to emergency services. Conversely, if the blurriness has been gradual and slowly worsened over weeks or months, it warrants a scheduled appointment with an eye care professional. Conditions like a slow-progressing cataract, mild refractive error changes, or chronic dry eye fall into this category. When seeking care, describe the onset—whether the change was instantaneous or developed slowly—to help the provider assess the urgency.