Bullae

A bulla is a large blister filled with clear fluid, blood, or, less often, pus. The plural is bullae. A bulla is usually defined as being more than about 1 centimetre across. Smaller blisters are called vesicles.

Bullae form when layers of skin separate and fluid collects in the space between them. They can appear after friction, a burn, an allergic or irritant reaction, an infection, or inflammation within the skin.

A bulla is a symptom rather than a diagnosis. Its size, location, appearance, and accompanying symptoms can help identify the cause.

A bulla may be soft and loose or firm and tense. The surrounding skin may look normal, red, swollen, or darker than usual. On darker skin, redness can be less obvious, so warmth, swelling, and tenderness may be more useful signs of inflammation.

Depending on the cause, bullae may:

• Feel painless, tender, itchy, or burning
• Contain clear, cloudy, yellow, or blood-stained fluid
• Appear alone or in groups
• Break open and leave moist, sensitive skin
• Develop a crust as they heal

Blisters in the mouth can make eating or drinking painful. Those near the eyes may cause redness, light sensitivity, or changes in vision. Genital bullae can make urination uncomfortable.

Friction is a frequent cause, especially on the feet or hands. Repeated rubbing from shoes, tools, sports equipment, or prolonged activity can separate the upper skin layers.

Minor burns can produce bullae. Causes include hot objects, liquids, steam, sunburn, or cold injury. The depth and size of a burn matter more than the presence of a blister alone.

Contact reactions happen after the skin touches an irritant or something that triggers an allergy. Examples include cleaning products, adhesives, cosmetics, metals, and certain plants. The area is often itchy or burning.

Other possible causes include insect bites, severe swelling of the legs, and inflammatory skin conditions such as eczema.

Less commonly, bullae can result from:

• Skin infections, including bullous impetigo
• Autoimmune conditions, in which the immune system mistakenly attacks skin structures
• Reactions to medicines
• Inherited conditions that make skin unusually fragile

Sudden widespread blistering may be part of a serious burn, infection, or medicine reaction. These patterns need urgent assessment rather than home treatment.

Seek same-day medical care if bullae are spreading quickly, very painful, or accompanied by fever, increasing warmth, swelling, pus, red streaks, or worsening tenderness. These can be signs of infection or significant inflammation.

You also need urgent assessment for blisters involving the eyes, mouth, or genitals, especially if several areas are affected. Get urgent help if widespread blistering or skin peeling begins after starting a medicine. Bring a list of prescription medicines, over-the-counter products, and supplements.

Call emergency services for:

• Trouble breathing, facial or tongue swelling, faintness, or confusion
• Extensive painful blistering or peeling with fever
• A large or deep burn, an electrical burn, or a chemical burn
• Blistering after smoke inhalation or with breathing difficulty

Arrange a routine appointment if bullae keep returning, have no clear cause, heal slowly, or leave repeated sores. Seek advice promptly for a foot blister if you have diabetes, poor circulation, reduced sensation, or a weakened immune system.

A healthcare professional will ask when the bullae began and whether you had friction, heat exposure, new skin products, illness, or a recent medicine change. They may examine the blister roof, fluid, surrounding skin, and other parts of your body.

Sometimes no tests are needed. If infection is possible, a sample of fluid or skin may be tested. Repeated or unexplained bullae may require blood tests or a skin biopsy. A biopsy removes a very small piece of skin for examination under a microscope.

Treatment depends on what caused the bullae. Friction blisters often heal after the rubbing stops. Burns, infections, allergic reactions, and immune-related skin conditions may need different treatments.

For an uncomplicated bulla:

• Protect it with a clean, soft dressing
• Keep the area clean and dry
• Avoid the activity, footwear, or product that triggered it
• Do not deliberately peel away the blister roof

It is usually best not to burst a bulla yourself. The intact skin helps protect the tissue underneath from infection. If it breaks, wash gently with soap and water, let the fluid drain, and cover it with a non-stick dressing. Do not pull off attached skin.

A clinician may drain a large or painful bulla while preserving its roof. Antibiotics like amoxicillin or doxycycline are used only when a bacterial infection is diagnosed or strongly suspected. Immune-related blistering conditions may require prescription treatment and specialist care.

Do not restart a product that clearly caused a contact reaction. If blistering began after a medicine, seek urgent advice rather than making several medication changes on your own.

A small friction or minor-burn bulla may improve over several days and heal within one or two weeks. Deeper injury, infection, poor circulation, or an ongoing skin condition can delay healing.

The fluid is gradually reabsorbed or drains after the roof breaks. New skin forms underneath. Temporary colour changes are common, particularly after inflammation. Scarring is more likely when the injury extends into deeper skin or becomes infected.

Bullae that recur may continue until the underlying trigger or condition is identified and managed.

Is a bulla different from an ordinary blister?

Bulla is the medical term for a blister larger than about 1 centimetre. A smaller fluid-filled blister is called a vesicle.

Are bullae contagious?

Bullae themselves are not contagious. Some causes, such as certain bacterial or viral infections, can spread to other people. Friction, burns, contact allergies, and autoimmune conditions are not contagious.

Should I cover a bulla?

Covering it can protect it from rubbing, dirt, and accidental tearing. Use a non-stick dressing and change it if it becomes wet or dirty.

What if the fluid is bloody?

Blood can collect when friction or injury damages small blood vessels. It does not always mean the injury is serious, but unexplained, very painful, or recurring blood-filled bullae should be assessed.