A bulla is a large, fluid-filled blister that forms on or beneath the skin. Defined as a lesion greater than 5 millimeters in diameter, it represents a collection of fluid separating layers of tissue. The presence of bullae signals an underlying process causing a loss of structural integrity. The term applies to both the skin and the air sacs within the lungs, describing lesions that are fundamentally different in composition and implications.
Defining Bullae on the Skin
A bulla on the skin is an elevated, circumscribed lesion containing clear, serous fluid, often referred to as a large blister. The distinction between a bulla and a smaller counterpart, known as a vesicle, is based purely on size. Bullae form when the connections between the layers of the skin are disrupted, allowing fluid to accumulate.
The location of fluid accumulation determines the bulla’s physical appearance and fragility. If separation occurs high within the epidermis, the bulla will be thin-walled and flaccid, prone to rupturing easily. Conversely, bullae that form lower down, at the junction between the epidermis and the underlying dermis, tend to be tense and firm. The fluid inside is usually clear, but it may contain blood (hemorrhagic bulla) or pus, indicating a possible infection.
What Causes Dermatological Bullae
Bullae development is traced to various causes that compromise the adhesion between skin layers. One major category includes autoimmune disorders, where the immune system mistakenly attacks its own skin proteins. In conditions like bullous pemphigoid, autoantibodies target components of the basement membrane zone, anchoring the epidermis to the dermis, causing a sub-epidermal split and tense bullae.
Another autoimmune condition, pemphigus vulgaris, involves autoantibodies directed against desmogleins, proteins that act as molecular glue between epidermal cells, resulting in a split within the epidermis and characteristic flaccid bullae. Infectious causes are also common, such as bacterial infections like bullous impetigo or viral infections like herpes simplex, where pathogens damage skin cells or trigger inflammation. Physical trauma is the most common cause, with friction or thermal burns leading to a mechanical separation of the epidermis from the dermis.
Bullae can also be an adverse reaction to certain medications (drug-induced bullous eruption). This reaction is often immunologically mediated and can manifest as conditions mimicking autoimmune blistering diseases or, in severe cases, life-threatening syndromes like Stevens-Johnson syndrome or toxic epidermal necrolysis. The implicated drugs may trigger autoantibodies or initiate a direct, toxic assault on skin cells, leading to widespread loss of skin surface.
Understanding Bullae in the Lungs
A pulmonary bulla is an air-filled space within the lung tissue that measures at least 1 centimeter in diameter. These sacs form not from fluid accumulation, but from the destruction and merging of the delicate alveolar walls, the tiny air sacs where gas exchange occurs. This destruction is associated with severe forms of emphysema, a chronic obstructive pulmonary disease (COPD).
Bulla formation in the lungs is a consequence of persistent inflammation and the breakdown of elastic fibers, often due to long-term exposure to irritants like tobacco smoke. As alveolar walls disintegrate, small air spaces coalesce into larger, inefficient air pockets that cannot facilitate gas exchange. These large bullae can grow significantly, sometimes occupying a third or more of the lung space, a condition termed giant bullous emphysema.
When a bulla expands, it acts as a space-occupying lesion, compressing the surrounding healthy lung tissue. This compression impairs normal breathing function and can lead to progressive shortness of breath and decreased exercise tolerance. The thin wall of a pulmonary bulla is also susceptible to rupture, which can cause a pneumothorax (collapsed lung), a serious medical event where air leaks into the space between the lung and the chest wall.
When to Seek Medical Attention
While a small friction blister may heal on its own, certain characteristics of a bulla warrant immediate medical consultation. A healthcare provider should evaluate bullae showing signs of infection or systemic symptoms. Urgent attention is required for:
- Increasing pain, redness, swelling, warmth, pus, or foul odor.
- Systemic symptoms, including fever, chills, or widespread blistering affecting large areas of the body.
- Bullae that appear suddenly after starting a new medication.
- Bullae involving the mucous membranes of the eyes, mouth, or genitals.
- Sudden or worsening shortness of breath, sharp chest pain, or rapid decline in breathing capacity, which may indicate a ruptured pulmonary bulla.