A visual anomaly, often described as a dark spot or shadow appearing in the peripheral field, can be an unsettling experience, especially when triggered by eye movement. While many causes are harmless and related to the normal aging process, any sudden change in the nature, frequency, or size of this visual disturbance requires prompt professional evaluation. These spots represent a disruption in the pathway of light reaching the retina or a change within the eye’s internal structure.
Understanding the Types of Peripheral Visual Spots
The dark spots or shapes people observe in their side vision generally fall into one of three categories, each defined by its appearance and movement.
Floaters
The most common are “floaters,” which appear as specks, threads, or cobwebs that drift slowly across the visual field. These floaters are physical debris within the vitreous gel that cast shadows onto the retina. They are most obvious against a bright background like a clear sky or white wall.
Flashes (Photopsia)
Another type of disturbance is “flashes,” or photopsia, which are brief streaks or sparks of light, frequently noticed in the peripheral vision. These flashes are caused by mechanical stimulation of the retina, which interprets the physical tugging as light. Flashes are a symptom of the retina being irritated and can occur with sudden eye movements.
Scotomas
A third, more serious type of visual anomaly is a scotoma, which is a true blind spot or an area of partially or completely diminished vision. Unlike floaters, a scotoma remains fixed in the visual field and does not drift or move when the eye shifts, representing a genuine area of vision loss.
Common Causes of Fleeting or Stable Dark Spots
The most frequent cause of floaters and flashes is an age-related process called vitreous syneresis, where the clear, gel-like vitreous humor inside the eye begins to shrink and liquefy. This degradation causes the internal collagen fibers to clump together, and the shadows of these clumps are perceived as the visual floaters. This liquefaction often leads to Posterior Vitreous Detachment (PVD), which is a normal part of aging, occurring in about 75% of people by age 65. PVD occurs when the shrinking vitreous gel separates from the retina at the back of the eye.
This separation can cause an abrupt increase in floaters and the sensation of flashes of light as the gel pulls on the retinal tissue. While PVD itself is typically benign and does not cause vision loss, the symptoms often subside over several weeks as the brain learns to ignore the floating debris.
Ocular Migraines
Temporary scotomas, often described as shimmering, zigzag lines or an expanding blind spot, are commonly associated with an ocular migraine aura. This visual disturbance, known as a scintillating scotoma, originates in the brain and usually expands gradually over 5 to 20 minutes before fully resolving within about an hour.
Urgent Warning Signs Requiring Immediate Attention
A sudden and dramatic change in visual spots can signal a sight-threatening event that requires immediate medical attention. The most serious concern is a retinal tear or detachment, where the light-sensitive tissue pulls away from the wall of the eye. A sudden “shower” of new floaters, a marked increase in flashes, or the appearance of a fixed curtain or shadow obscuring peripheral vision are classic warning signs.
Vitreous Hemorrhage
If the separating vitreous gel pulls too hard on the retina, it can cause a tear, which may also rupture a blood vessel, leading to a vitreous hemorrhage. Bleeding into the vitreous cavity results in a sudden increase in dark spots due to blood cells floating in the eye. This can cause blurry vision or even sudden, complete vision loss. This bleeding can be caused by the PVD process, trauma, or underlying conditions like diabetic retinopathy.
Vascular Occlusion
The sudden onset of a large, fixed scotoma or significant vision loss in one eye can also indicate a blockage of blood flow to the retina, known as a retinal artery or vein occlusion. These vascular events, sometimes referred to as an “eye stroke,” can cause painless, sudden vision loss or a dark spot because the retinal tissue is starved of oxygen. Any new, sudden-onset symptoms must be treated as an eye emergency and require an immediate consultation with an ophthalmologist or an emergency room visit.