The experience of seeing small, dark shapes that drift across your field of vision is a common phenomenon known as myodesopsias, or floaters. These specks, which may resemble dots, threads, or cobwebs, are located inside the eyeball itself, not on the surface. They are noticed most clearly when looking at a bright, plain background, such as a blue sky or a white wall. Understanding the cause of these visual specks is the first step in knowing when they are normal and when they require immediate medical attention.
The Physics of Visual Specks
The appearance of a black dot in your vision is a result of a shadow being cast onto the retina, the light-sensitive tissue at the back of the eye. Light entering the eye must pass through the vitreous humor, a clear, gel-like substance that fills the space between the lens and the retina. The vitreous is composed primarily of water and fine collagen fibers.
When microscopic debris, such as cellular waste or clumped protein fibers, becomes suspended within the vitreous gel, it blocks the path of light. This debris casts a shadow on the retina, which the retina interprets as a dark shape—a dot, line, or ring. Because the debris floats in the gel, the shape moves slowly when the eye moves, creating the characteristic drifting appearance.
Common Causes of Floaters
The most frequent origin of these drifting specks is the natural aging process of the eye. As a person ages, the vitreous humor undergoes a change called vitreous syneresis, where the gel begins to liquefy and contract. This process causes the collagen fibers within the gel to condense and clump together, forming the visible debris.
This age-related liquefaction often culminates in a posterior vitreous detachment (PVD), where the shrinking vitreous gel separates from the retina’s surface. The floaters seen during this time are released bits of tissue that were once part of the vitreous structure. A specific type of floater, the Weiss ring, is sometimes seen following a PVD; it is a large, circular floater representing the detached ring of vitreous tissue that surrounded the optic nerve.
Other, less frequent causes for floaters include minor inflammation inside the eye, known as uveitis, or the presence of blood cells in the vitreous from a small vessel leak. People who are very nearsighted (myopic) or who have had prior eye surgery, such as cataract removal, also have an increased risk of experiencing floaters at an earlier age.
When a Black Dot Signals an Emergency
While most floaters are harmless, a sudden change in their appearance can signal a serious condition requiring immediate medical evaluation. The most concerning symptom is the rapid onset of a “shower” of many new floaters, suggesting a more forceful event is occurring within the eye than simple age-related changes.
The sudden appearance of new floaters is often accompanied by flashes of light, medically termed photopsias, which appear as brief arcs or streaks in the peripheral vision. These flashes occur because the shrinking vitreous gel is tugging on the retina, mechanically stimulating the light-sensitive tissue. If the vitreous pulls hard enough, it can cause a retinal tear, which allows fluid to pass beneath the retina and lead to a retinal detachment.
Retinal detachment is a medical emergency where the retina separates from its underlying blood supply, risking permanent vision loss. A more advanced sign of this separation is the perception of a dark shadow, like a curtain or veil, moving across the field of vision. Bleeding into the vitreous, known as a vitreous hemorrhage, can also cause a sudden onset of numerous floaters that appear as minute black or red spots. Any acute change in floaters, flashes, or the appearance of a curtain-like shadow requires an immediate visit to an eye specialist.