The experience of closing your eyes and seeing fleeting specks, dark shapes, or shifting patterns is a common visual phenomenon. These sensations occur even without external light, prompting questions about their source. Understanding these spontaneous visual effects requires examining the internal mechanics of the eye and how the brain interprets signals. Dark dots can result from several mechanisms, including physical pressure on the eye or debris floating within the eye’s internal fluid.
Why Your Brain Creates Light (Understanding Phosphenes)
The most common and harmless cause for seeing patterns when the eyes are closed is phosphenes. Phosphenes are visual sensations of light that occur without actual light entering the eye, often described as “seeing stars” or bright flashes. These lights are triggered by non-light stimuli, such as mechanical pressure on the eyeball. When you rub your eyes, the pressure physically stimulates the photoreceptor cells in the retina.
The retina’s cells send signals to the brain that are interpreted as light, regardless of the stimulus. Since the brain receives these electrical signals, it interprets them as visual information, creating the illusion of light, swirls, or colorful dots. This is why pressure phosphenes are most noticeable after rubbing the eyes or from straining activities like coughing or sneezing.
Phosphenes can also be triggered by metabolic factors, such as low oxygenation, reduced blood glucose, or a sudden drop in blood pressure. For instance, standing up too quickly can temporarily lower blood pressure to the head, resulting in a brief visual effect perceived as dots or dimming vision. These phosphenes are transient and represent a normal physiological response of the visual pathway.
Common Benign Causes of Dark Spots and Floaters
The most common source of persistent dark spots and strands are vitreous floaters, medically known as myodesopsias. These specks drift within the vitreous humor, the clear, gel-like substance that fills the main cavity of the eye. As the eye ages, the vitreous gel begins to liquefy, a process called syneresis.
This liquefaction causes microscopic collagen fibers within the gel to clump together. These clumps of debris cast shadows directly onto the retina, which the brain perceives as dark shapes, threads, or cobwebs moving across the field of vision. Floaters are typically more noticeable when looking at a bright, uniform background, such as a white wall or a clear blue sky.
A frequent cause of new floaters, especially in adults over 50, is a posterior vitreous detachment (PVD), where the vitreous naturally separates from the back of the eye. Although the sudden onset of a large, new floater can be alarming, PVD is a common age-related change and is generally considered benign. The floaters are permanent, but the brain usually learns to ignore them over time.
Another transient cause of seeing spots is postural hypotension, also known as orthostatic hypotension. This occurs when a person rapidly moves from a sitting or lying position to standing, causing a temporary drop in blood pressure and decreased blood flow to the eyes. The resulting lack of oxygenation causes a brief loss of vision, which can manifest as blurring or a shower of dark spots. This effect resolves quickly once the body restores adequate blood pressure.
Less commonly, some people experience an ocular migraine aura. This involves visual disturbances like shimmering zigzag lines, known as scintillating scotomas, which can sometimes precede or accompany a dark spot or blind area.
When Visual Changes Require Immediate Medical Review
While most spots and floaters are harmless, certain visual changes indicate a potential medical emergency requiring immediate evaluation by an eye care professional. The most concerning symptom is the sudden, acute onset of a shower of new floaters or a rapid increase in their number and size. This rapid change, especially if accompanied by other symptoms, suggests a more serious underlying condition.
A major warning sign is the presence of flashes of light, medically termed photopsia, which often occurs simultaneously with the new floaters. These flashes are caused by the vitreous gel pulling or tugging on the light-sensitive retina. If this pulling is strong enough, it can cause a retinal tear or a complete retinal detachment.
Retinal detachment is a painless condition where the retina separates from the underlying tissue, risking permanent vision loss if not treated promptly. Other urgent symptoms include a shadow, blurriness, or a gray curtain moving across or obstructing any part of the field of vision, especially peripheral sight. The sudden appearance of black or red floaters, sometimes described as a cloud of soot, can also signal a vitreous hemorrhage, which is bleeding inside the eye.
Anyone experiencing a sudden increase in floaters, new flashes of light, or a curtain-like obstruction should seek urgent medical attention. Prompt diagnosis and treatment of a retinal tear or detachment can preserve eyesight and prevent long-term vision impairment.