High blood pressure (hypertension) is a condition where the force of blood against the artery walls is consistently too high. This sustained pressure affects blood vessels throughout the body, including those in the eyes. Bloodshot eyes refer to visible redness caused by enlarged or burst blood vessels on the eye’s surface. While chronic hypertension does not typically cause persistent surface redness, an acute spike in blood pressure can directly lead to a noticeable bleeding event.
Acute Redness: Subconjunctival Hemorrhage
The most direct link between blood pressure and a sudden, bloodshot eye is a subconjunctival hemorrhage (SCH). This occurs when a tiny, fragile blood vessel beneath the conjunctiva—the clear membrane covering the white of the eye—ruptures. The blood leaks out and becomes trapped, appearing as a bright red patch.
This rupture is typically triggered by activities that cause a sudden, temporary increase in venous pressure in the head. Examples include a forceful sneeze, deep cough, vomiting, or straining during a bowel movement. Uncontrolled or severely high blood pressure makes these small vessels more fragile and susceptible to bursting under such transient pressure spikes. Although the appearance can be alarming, SCH is usually painless, does not affect vision, and resolves on its own within one to two weeks as the blood is reabsorbed. A spontaneous or recurrent SCH often signals a need to check and manage overall blood pressure control.
Systemic Effects of High Blood Pressure on Vision
Sustained high blood pressure causes chronic damage to the delicate vascular structure at the back of the eye, known as hypertensive retinopathy. This damage occurs deep within the eye, affecting the blood vessels of the retina, and is not visible as surface redness. Chronic hypertension causes the retinal artery walls to thicken and narrow, restricting blood flow and oxygen supply. In advanced stages, this can lead to visible changes during an eye exam, such as retinal hemorrhages or cotton-wool spots, which indicate nerve fiber damage. This process is often silent, potentially leading to vision impairment or loss if hypertension remains untreated.
Common Non-Hypertension Causes of Red Eyes
Most instances of bloodshot eyes are caused by common and benign issues unrelated to high blood pressure. Environmental factors frequently cause the small surface vessels to dilate, making the eyes appear red. Allergies are a frequent culprit, often causing redness accompanied by itching and watering. Eye strain from prolonged digital screen use can also lead to redness by reducing the blink rate, causing dryness and irritation. Other common causes include dry eye syndrome, minor infections like conjunctivitis, and irritation from contact lenses or exposure to smoke or dust.
Urgent Symptoms Requiring Medical Attention
While a simple, painless red spot is often a subconjunctival hemorrhage, certain accompanying symptoms indicate a serious eye emergency. Immediate medical evaluation is required if you experience any of the following:
- Sudden, noticeable change in vision, such as blurriness, double vision, or the sensation of a curtain or shadow coming over the visual field.
- Severe eye pain, intense light sensitivity, or a severe headache alongside red eyes.
- Redness accompanied by a thick discharge suggesting an infection.
- Redness that does not begin to clear up within a few days.
These symptoms can be signs of conditions like acute glaucoma, uveitis, or a retinal detachment, which require urgent intervention to preserve vision.