Phosphenes are the experience of seeing light, colors, or geometric patterns when no actual light is entering the eye. This phenomenon, often described colloquially as “seeing stars,” is a visual sensation generated internally within the nervous system. The flashes or shapes can range from intense blue points to shimmering, colored lines. Understanding the difference between a harmless, temporary phosphene and one signaling an underlying health issue is important.
What Are Phosphenes and How Do They Occur
Phosphenes are a form of photopsia, a general term for any perception of light originating from within the eye or brain rather than an external source. The visual system, including the retina and the visual cortex, is designed to interpret photons as light. However, this system can be artificially stimulated by other forms of energy, causing the brain to register the signal as visual input.
The visual system cannot distinguish between a signal generated by actual light and one generated by mechanical, electrical, or magnetic stimulation. When the cells of the retina or the neurons in the visual cortex are activated by non-light energy, they send the same electrical impulse to the brain. This impulse is processed as a flash or pattern of light, and the specific shape and color depend on the exact area and type of stimulation occurring within the visual pathway.
Everyday Triggers of Phosphenes
The most common phosphenes are pressure phosphenes, which result from mechanical stimulation of the retina. Simply rubbing a closed eye can mechanically stimulate the retinal cells, causing them to fire an electrical signal. These flashes are often experienced as diffuse colored patches or bright, scintillating circles that quickly disappear after the pressure is released.
Other everyday actions that momentarily increase pressure on the eye or head can also trigger these sensations. A sudden sneeze, a forceful cough, or rapid changes in head position can cause brief, localized mechanical stress that stimulates the retina. A sudden drop in blood pressure, such as standing up too quickly, can also lead to transient metabolic stimulation of the visual system, resulting in a momentary display of “stars.” These occurrences are short-lived and do not indicate a serious problem.
Phosphenes as Symptoms of Underlying Conditions
While often benign, phosphenes can signal conditions related to the eye’s structure or neurological function. One common age-related cause is Posterior Vitreous Detachment (PVD), where the vitreous gel filling the eye liquefies and shrinks. As the gel separates from the retina, it exerts brief, mechanical traction on the neurosensory layer, causing cells to fire and creating flashes of light, often described as lightning or camera flashes in the peripheral vision.
Phosphenes may also be a feature of neurological events, such as an ocular migraine, characterized by a visual aura called a scintillating scotoma. This disturbance is caused by Cortical Spreading Depression (CSD), an electrochemical wave of depolarization that slowly moves across the visual cortex. The wave temporarily alters the balance of ions, generating a signal that the brain interprets as a shimmering, zigzag pattern.
A more concerning cause of phosphenes is a retinal tear or detachment, where the mechanical pulling of the vitreous is strong enough to rip the retinal tissue. This tearing action directly stimulates the photoreceptors, leading to prolonged or recurrent flashes of light. Phosphenes can also be associated with inflammatory conditions like optic neuritis, where inflammation of the optic nerve causes flashes when the eye is moved.
Lesions or tumors affecting the occipital lobe, the region of the brain responsible for processing vision, can also cause phosphenes due to abnormal electrical activity.
Urgent Warning Signs Requiring a Doctor Visit
While an occasional phosphene is normal, certain associated symptoms warrant immediate professional evaluation by an ophthalmologist. The sudden onset of new flashes of light, especially if repetitive or persistent, should be investigated, as this suggests a new mechanical force may be acting on the retina.
A sudden increase in the number or size of floaters accompanying the flashes is a significant warning sign. Floaters are shadows cast by debris in the vitreous, and their sudden increase, along with flashes, suggests active vitreous traction that could be tearing the retina.
The appearance of a shadow or dark “curtain” that moves across or blocks a portion of the field of vision is concerning. This symptom is a classic indication of a retinal detachment, where the retina has separated from its underlying support structures.
Phosphenes accompanied by vision loss, decreased visual clarity, or significant eye pain require emergency evaluation to prevent permanent vision damage.