A welt (urticaria or wheal) and a bruise (contusion) are often confused because both mark the skin following injury or reaction. A welt is a raised, temporary swelling caused by fluid accumulation in the upper skin layers. Conversely, a bruise is a discolored patch caused by bleeding beneath the skin’s surface. The core distinction is that a welt is fluid-based and superficial, while a bruise is blood-based and subdermal. While a welt does not transform into a bruise, the initial event can sometimes cause both injuries simultaneously.
Understanding Welts: The Skin’s Inflammatory Response
A welt is a rapid, localized inflammatory response occurring in the skin’s upper layers. This process begins when mast cells release chemical mediators, primarily histamine, into the surrounding tissue. Histamine causes small blood vessels to dilate and become temporarily more permeable. This increased permeability allows fluid (plasma) to leak out of the capillaries and accumulate in the skin, a process known as edema. This fluid influx creates the characteristic raised, often pale center with surrounding redness. Since the reaction is driven by fluid leakage and not broken blood vessels, welts typically resolve quickly, often within a few hours, as the body reabsorbs the excess fluid.
Understanding Bruises: Damage Beneath the Surface
A bruise (ecchymosis) occurs when blunt force trauma damages small blood vessels, specifically capillaries, beneath the skin. This vessel damage causes blood to leak and pool in the surrounding connective tissue, forming a contusion. The skin remains intact, trapping the pooled blood internally. The distinctive discoloration relates to the body cleaning up this trapped blood. Macrophages break down hemoglobin, the iron-containing protein in red blood cells. This breakdown causes the bruise to cycle through colors, starting as reddish-blue and progressing to purplish-black, green, and finally yellow as the pigments are metabolized and reabsorbed.
The Intersection of Welts and Bruises
A single traumatic event can result in both a welt and a bruise appearing in the same location. This occurs when the impact force is strong enough to trigger the superficial inflammatory response (welt) and simultaneously rupture deeper capillaries (bruise). For instance, a localized blow causes immediate mast cell activation and fluid release, while the same force breaks blood vessels beneath the swelling. Another scenario involves an initial welt that later becomes discolored due to scratching. Welts from insect bites or allergic reactions often cause intense itching, and aggressive rubbing can break underlying capillaries. This physical act creates the necessary subdermal trauma to form a separate contusion, resulting in a dual injury.
When to Seek Medical Advice
Most welts and bruises are minor and heal without intervention, but certain symptoms warrant prompt medical evaluation.
Welts
A welt that spreads rapidly or is accompanied by difficulty breathing, throat swelling, or dizziness suggests a severe systemic reaction like anaphylaxis. This requires immediate emergency attention.
Bruises
Consult a medical professional if contusions appear frequently without clear cause, as this may indicate an underlying issue with blood clotting or a medication side effect. Evaluation is also needed for bruising accompanied by severe, persistent pain, increased swelling, or signs of infection (warmth, spreading redness, or pus). Any bruise resulting from a significant head impact or a pattern of bruising suggestive of abuse requires professional assessment.