The experience of seeing brief flashes of light or “stars” after a sudden, forceful action like a cough or sneeze is a common phenomenon. This visual effect resolves almost instantly and is typically a normal physiological response of the eye to a rapid change in internal pressure. While the sensation may cause momentary concern, it is usually a harmless signal created entirely within your visual system. Understanding the body’s mechanics during such a forceful expulsion reveals why this visual trick occurs.
Defining Phosphenes
These flashes of light seen without an external light source are scientifically known as phosphenes. Phosphenes are not actual light rays entering the eye but are the brain’s interpretation of a signal generated by stimulating the visual pathway. This stimulation can take various forms, including mechanical, electrical, or magnetic forces.
The visual system, including the retina and the optic nerve, only sends one type of signal to the brain: light. When the system is physically disturbed, the brain interprets the resulting electrical impulse as a flash or pattern of light, similar to when you rub your eyes vigorously. Other triggers for phosphenes include low blood pressure, rapid head movements, or certain neurological events like migraines.
The Mechanics of Pressure and Vision
The core reason a cough can produce phosphenes lies in the body’s pressure-regulating system. A forceful cough or sneeze involves an involuntary action similar to the Valsalva maneuver, requiring intense strain against a closed airway. This straining causes a rapid, temporary surge in pressure within the chest and abdomen.
This sudden pressure spike is quickly transmitted to the head and neck, resulting in an elevation of both intracranial pressure and, more importantly for vision, intraocular pressure (IOP). The eye is a fluid-filled structure, and the pressure within it is sensitive to the surrounding environment. Studies simulating the Valsalva maneuver have shown that this straining can significantly raise the IOP.
This transient increase in pressure physically deforms the eyeball slightly and mechanically stimulates the retina, the light-sensitive tissue lining the back of the eye. When the specialized photoreceptor cells in the retina are physically pressed or stretched by the elevated IOP, they are mechanically activated.
The mechanical pressure acts as a non-light stimulus that forces the cells to fire an electrical signal. This signal travels along the optic nerve to the brain’s visual cortex, which is programmed to interpret any signal from the optic nerve as light. Because the pressure is momentary, the resulting phosphenes appear as fleeting flashes that disappear as soon as the body’s pressure returns to normal.
When to Seek Medical Attention
While phosphenes caused by a cough are usually harmless and resolve immediately, it is important to differentiate this normal occurrence from a sign of a more serious underlying issue. A normal phosphene lasts only a second or two and is directly related to the physical strain. You should seek immediate medical consultation if you experience flashes of light that persist or occur frequently without a physical trigger.
Warning Signs
These persistent symptoms can indicate a serious condition, such as a retinal tear or detachment. Seek prompt professional evaluation if you notice:
- Flashes of light that persist for more than a few seconds.
- A sudden increase in the number of floaters (specks or cobwebs drifting across your vision).
- The appearance of a shadow, veil, or curtain effect in your peripheral vision that does not go away.
- New visual disturbances associated with chronic conditions like severe hypertension.