Flashes are transient, sudden sensory or physical events. In a health context, this term most commonly refers to two distinct phenomena: sudden bursts of light perceived in the visual field (visual flashes) and abrupt sensations of intense heat felt throughout the body (hot flashes). Although both are called “flashes,” their origins and underlying mechanisms are completely different, one being an ocular event and the other a systemic, thermal response. Understanding the source of these occurrences is the first step toward determining their significance and whether they require medical attention.
Understanding Visual Flashes
Visual flashes, medically known as photopsia, are perceptions of light that occur without an external source. They result from the mechanical stimulation of the retina, the light-sensitive tissue lining the back of the eye. When the retina is stimulated by force, its photoreceptor cells send a signal to the brain that is interpreted as a flash.
The most frequent cause of photopsia is a change in the vitreous humor, the clear, gel-like substance filling the eyeball. As the vitreous shrinks and liquefies with age, it can lead to posterior vitreous detachment (PVD). During PVD, the shrinking vitreous pulls away from the retina, and this traction generates the perceived flash of light.
Patients often describe these flashes as lightning streaks or arcs of light, typically appearing in the peripheral vision and lasting less than a second.
The Physiology of Hot Flashes
Systemic flashes, commonly known as hot flashes or vasomotor symptoms, are the body’s rapid heat-dissipation response. They are characterized by a sudden feeling of intense heat, often starting in the face and chest and spreading across the body, typically followed by sweating and sometimes chills. A single episode usually lasts between one and five minutes.
The primary mechanism involves the thermoregulatory center in the hypothalamus, which maintains stable core body temperature. Hormonal fluctuations, especially the decline in estrogen during menopause, affect this center’s function. This decline causes a narrowing of the thermoregulatory zone—the temperature range where the body does not need to activate cooling responses.
When this zone narrows, even a small rise in core body temperature triggers the hypothalamus. This immediately initiates a heat-dissipation sequence, including peripheral vasodilation, which causes flushing, and sweating. The subsequent rapid drop in core temperature often results in the sensation of chills following the heat.
Visual Events Confused with True Flashes
Floaters
Floaters are often seen alongside flashes but are fundamentally different. They are shadows cast onto the retina by tiny clumps of protein or debris suspended within the vitreous gel. Unlike flashes, floaters appear as specks, webs, or threads that drift across the field of vision, especially noticeable against a bright background.
Migraine Aura
Another distinct event is the migraine aura, or scintillating scotoma, which is neurological, not retinal, in origin. Migraine aura typically presents as shimmering, zigzagging lines or geometric shapes that move across the visual field. These patterns often affect both eyes and last significantly longer, usually 20 to 30 minutes, which differs from the rapid, peripheral flashes of photopsia.
Circulatory Changes
Visual changes related to orthostatic hypotension can also be misinterpreted as flashes. When a person stands up too quickly, a temporary drop in blood pressure reduces blood flow to the brain and eyes. This circulatory event may result in a brief “graying out,” tunnel vision, or a temporary dimming of the visual field. These symptoms are associated with postural change and light-headedness, confirming they are circulatory and not caused by mechanical stimulation of the retina.
Identifying Red Flags for Immediate Care
While many flashes are benign, certain associated symptoms require immediate evaluation by a healthcare professional.
For visual flashes, a sudden, dramatic increase in the number of flashes or a new “shower” of floaters are red flags. These changes can indicate a retinal tear, a serious complication of vitreous traction. The most concerning visual symptom is the perception of a shadow or dark curtain covering part of the visual field. This strongly suggests a retinal detachment, requiring urgent medical intervention to preserve vision.
For systemic hot flashes, medical assessment is needed if they are accompanied by severe, non-typical symptoms. Red flags include hot flashes alongside unexplained weight loss, a persistent high fever, or significant shortness of breath. Hot flashes that occur with severe chest pain or radiating pain to the arm, neck, or jaw also warrant immediate attention. These combinations suggest a potential underlying condition, such as an infection or a cardiac issue.