What is an Abscess
An abscess is a localized collection of pus within tissue, surrounded by inflammation and granulation tissue, that forms in response to an infectious or inflammatory process. 1
Anatomical Definition and Pathophysiology
An abscess consists of purulent material (pus) confined within a cavity in the dermis and deeper skin tissues, typically appearing as a painful, tender, fluctuant red nodule surrounded by erythematous swelling. 2
The collection forms when bacteria invade tissue and trigger an inflammatory response, leading to tissue destruction, white blood cell accumulation, and the formation of a walled-off pocket of pus. 1
Importantly, infection is not mandatory for abscess formation—inflammatory processes alone can trigger abscess development, though bacterial infection is the most common cause. 3
Microbiology
Cutaneous abscesses are typically polymicrobial, containing bacteria from normal regional skin flora, often combined with organisms from adjacent mucous membranes. 2
Staphylococcus aureus is present as a single pathogen in only approximately 25% of cutaneous abscesses, contrary to common perception that it dominates all abscess infections. 2
In bacteria-positive primary skin abscesses, S. aureus is likely to dominate the microbiome, but the polymicrobial nature should not be overlooked. 3
Classification: Simple vs. Complex
Simple abscesses are defined by induration and erythema limited only to the defined abscess area without extension beyond its borders, with no extension into deeper tissues or multiloculated spread. 2
Complex abscesses include those in perianal/perirectal locations, IV drug injection sites, or those with significant surrounding cellulitis, and typically require more aggressive management. 2
Clinical Presentation
Abscesses present as localized, painful, tender, fluctuant masses with overlying erythema and warmth. 2
Systemic symptoms are usually absent in simple cutaneous abscesses, though complex or deeper abscesses may present with fever, malaise, and other systemic signs. 2
Location provides clues to causative organisms—perianal abscesses often originate from obstructed anal crypt glands, while axillary or perineal locations suggest mixed aerobic-anaerobic flora. 2
Key Clinical Pitfall
- Do not confuse inflamed epidermoid cysts with infected abscesses—these cysts ordinarily contain skin flora even when uninflamed, and the inflammation occurs as a reaction to cyst wall rupture rather than true infection. 2