Bulla
What is it?
A bulla is a large blister filled with clear fluid that is bigger than 1 centimeter (larger than a fingernail). It's essentially a big version of a vesicle - imagine a large water bubble under the skin. Bullae (plural) can form from burns, severe allergic reactions, autoimmune conditions, or infections. Because they're large and have thin roofs, they often break easily, leaving raw, weeping areas that need careful attention.
What does it look like?
Bullae have distinctive characteristics:
- Large fluid-filled blisters, from fingernail-sized to palm-sized
- Contents: Usually clear fluid, may become cloudy or blood-tinged
- Shape: Dome-shaped when tense, wrinkled when flaccid (loose)
- Roof: Thin and translucent or thick and tense
- Appearance: May look like a large bubble under the skin
- Fragility: Often rupture easily due to size
- Base: When roof breaks, reveals moist, red, raw skin underneath
Where does it appear?
Bullae can develop on:
- Trunk and back: Bullous pemphigoid (elderly patients)
- Hands and feet: Burns, friction blisters, drug reactions
- Mouth and lips: Pemphigus, Stevens-Johnson syndrome
- Sun-exposed areas: Severe sunburn, phototoxic reactions
- Pressure points: Diabetic bullae, coma blisters
- Anywhere: Severe drug reactions, autoimmune blistering diseases
Associated features
Bullae may be accompanied by:
- Pain or burning sensation
- Surrounding redness and inflammation
- Multiple bullae appearing over days
- Itching before bullae form (pemphigoid)
- Fever and feeling unwell (drug reactions)
- Mouth sores with skin bullae
- Healing with dark marks or scarring
- Nikolsky sign (skin sloughs with gentle rubbing)
When to see a doctor
Seek immediate medical attention for:
- Multiple new bullae appearing rapidly
- Bullae with fever or feeling very unwell
- Bullae in the mouth or on lips
- Bullae after starting new medication
- Large areas of skin peeling or sloughing
- Signs of infection (pus, red streaks, increased pain)
- Bullae in children or elderly patients
- Any unexplained bullae - these always need evaluation
References
Primary Sources
Description of Skin Lesions - Merck Manual Professional Edition (2024)
- URL: https://www.merckmanuals.com/professional/dermatologic-disorders/approach-to-the-dermatologic-patient/description-of-skin-lesions
- Key findings: Bullae as fluid-filled blisters >10mm in diameter
Terminology in Dermatology - DermNet NZ (2024)
- URL: https://dermnetnz.org/topics/terminology
- Key findings: Bulla defined as large blister >1cm containing liquid
Blistering Skin Conditions - American Academy of Dermatology (2024)
- Key findings: Clinical approach to bullous disorders
Additional Sources
- Primary Skin Lesions - UTMB Dermatology - URL: https://www.utmb.edu/pedi_ed/CoreV2/Dermatology/page_07.htm - Accessed: 2025-09-30
- Stanford Medicine 25 - URL: https://stanfordmedicine25.stanford.edu/the25/dermatology.html - Accessed: 2025-09-30
Research Notes
- URL: https://pubmed.ncbi.nlm.nih.gov/ - Topic researched: Bullous disorders, autoimmune blistering diseases
- URL: https://dermnetnz.org/ - Topic researched: Bulla vs vesicle distinction, bullous drug reactions