A cyst is a closed sac under the skin or within the body tissue, lined with epithelial cells and filled with fluid, semi-solid material, or air. These structures are common and typically grow slowly, often remaining painless and unnoticed. An inflammatory cyst indicates a change in status, where the normally benign sac becomes an acute source of pain and swelling. This rapid shift is driven by the body’s immune system responding aggressively to irritation within the cyst wall, transforming it into a symptomatic, red, and tender issue requiring attention.
Defining the Inflammatory Cyst
An inflammatory cyst is distinct from a simple, stable cyst due to the acute immune response occurring within or around its structure. A true cyst is defined by its epithelial lining, a sac of cells that continuously sheds material, such as the protein keratin, causing the cyst to grow slowly over time. This structure differs significantly from an abscess, which is a localized collection of pus without an epithelial lining, forming rapidly due to a direct bacterial infection.
When a cyst becomes inflammatory, it exhibits the classic signs of the immune response: redness, heat, swelling, and pain. This reaction occurs because the body is responding to irritation or contamination inside the sac. Inflammation often begins when the cyst’s thin wall ruptures or when bacteria colonize the contents.
This event releases the cyst’s internal debris, like keratin and sebum, into the surrounding dermal tissue. The immune system recognizes this material as foreign, triggering a flood of white blood cells and inflammatory mediators. The resulting pressure and chemical irritation cause the visible and palpable signs of acute inflammation.
Common Forms and Locations
Many types of cysts can become inflamed, but those closest to the skin’s surface are the most frequently affected. The Epidermal Inclusion Cyst (EIC) is a common example found on the face, neck, trunk, and scalp. These cysts develop from hair follicles and are filled with thick, cheese-like keratin material, making them prone to rupture and subsequent inflammation.
Nodular and cystic acne lesions are also common, though they are technically pseudocysts that lack a complete epithelial lining. These lesions form deep within the hair follicle when dead skin cells and oil become trapped, leading to severe inflammation and painful, pus-filled nodules.
The skin near the tailbone is a common site for a Pilonidal Cyst, which forms around ingrown hairs and skin debris in the gluteal cleft. Due to its location, a pilonidal cyst is highly susceptible to friction, trauma, and secondary bacterial infection, making acute inflammation a frequent occurrence.
Triggers for Acute Inflammation
A previously stable cyst becomes acute when its contents are exposed to the surrounding tissue, inciting a powerful inflammatory cascade. The most immediate trigger is the physical rupture of the cyst wall, which can occur spontaneously due to internal pressure or external trauma. When keratin or sebum is released into the dermis, it acts as a foreign body, causing a severe immune reaction even without bacteria present.
A secondary bacterial infection is another common trigger, where skin flora like Staphylococcus aureus or Cutibacterium acnes enter the cyst sac through a pore or break in the skin. Once inside, the bacteria multiply rapidly using the cyst material as a nutrient source. This colonization turns the cyst into a painful, pus-filled lesion.
Friction, rubbing from clothing, or repeated manipulation can also lead to irritation and swelling, facilitating rupture or bacterial entry. Mechanical stress weakens the thin epithelial lining, creating a pathway for external microbes to penetrate. Furthermore, blockage of the cyst’s small surface opening, known as the punctum, causes pressure to build up inside, increasing the likelihood of internal rupture.
Medical and Surgical Management
The appropriate treatment for an inflammatory cyst depends on the severity of the inflammation, the presence of infection, and the level of pain. For a cyst that is purely inflamed but not yet infected or draining pus, a healthcare provider may inject a small dose of intralesional corticosteroid, such as triamcinolone, directly into the lesion. This injection suppresses the localized immune response, reducing swelling, redness, and pain within days.
If the cyst is suspected to be secondarily infected, oral antibiotics are often prescribed to target common skin bacteria. Warm compresses can also be applied at home to promote circulation and encourage natural drainage, providing symptomatic relief. In cases of significant pain, large size, or confirmed pus accumulation, an Incision and Drainage (I&D) procedure is necessary.
During an I&D, the provider makes a small cut into the cyst and gently expresses the accumulated fluid and debris, immediately relieving pressure and discomfort. While this procedure resolves the acute episode, it often leaves the epithelial lining behind, meaning the cyst can recur once the incision heals.
To achieve permanent resolution, a complete surgical excision must be performed. This procedure is usually delayed until the acute inflammation has subsided, typically four to eight weeks later. Operating on an actively inflamed cyst is difficult because the tissue is fragile, increasing the risk of the cyst wall tearing and subsequent recurrence.