Etiology and Pathology
Acute cellulitis is a diffuse, purulent infection occurring in the subcutaneous tissue, subfascial spaces, intermuscular planes, or deep connective tissues. The primary causative agent is Streptococcus pyogenes, followed by Staphylococcus aureus and, less frequently, Escherichia coli or other streptococcal species. Streptococcus pyogenes can release hemolysins, streptokinase, and hyaluronidase, which inhibit the localization of the inflammation, resulting in poorly defined borders between the infected and healthy tissues. This leads to widespread inflammation of the subcutaneous tissue within a short duration, potentially causing systemic inflammatory response syndrome (SIRS). Conversely, cellulitis caused by Staphylococcus aureus tends to be more localized due to the production of coagulase by the bacteria.
Clinical Presentation
In superficial cases, the affected area initially exhibits redness, swelling, warmth, and pain. The inflammation rapidly spreads subcutaneously to surrounding areas, with noticeable swelling and severe pain. At this stage, the skin becomes erythematous with poorly defined margins that blanch upon pressure. Vesicles of varying sizes may appear, while the affected lymph nodes become swollen and tender. In severe cases, vesicles rupture, releasing serous fluid, and some areas of the skin may turn brown.
In deeper forms of acute cellulitis, skin manifestations are less apparent as the infection resides in the deep fascial layers, complicating diagnosis and treatment. Patients often present with systemic symptoms such as high fever, headache, and fatigue. In severe cases, body temperature may be extremely elevated or abnormally low, accompanied by altered consciousness and signs of severe sepsis.
Classification
Due to variations in the type and virulence of bacteria, patient conditions, and infection sites, several distinct forms of cellulitis exist:
Subcutaneous Gas-Forming Cellulitis
This is a mixed infection primarily caused by gas-forming bacteria such as Enterococcus, facultative Escherichia coli, Proteus, Bacteroides, or Clostridium perfringens. It is most commonly observed in the abdominal wall and perineum, particularly in cases of heavily contaminated and injured skin. Severe cases may progress to necrotizing fasciitis. The primary site of infection is the subcutaneous and fascial tissue, sparing the muscle layer. Early symptoms resemble those of ordinary cellulitis; however, disease progression is rapid, and a crepitant sensation may be felt subcutaneously. Lesions may emit foul odor upon rupture, and systemic conditions deteriorate quickly.
Neonatal Subcutaneous Gangrene
Also known as neonatal cellulitis, this condition is characterized by a sudden onset and rapid progression, with poorly localized lesions that may lead to extensive subcutaneous necrosis. Staphylococcus aureus is the predominant causative agent. Lesions commonly appear on the back and buttocks, occasionally involving the occipital region, shoulders, thighs, lumbar-sacral area, or perineum, which are areas prone to pressure. The condition frequently occurs in winter and is associated with poor hygiene, abrasions, pressure injuries, moisture, or fecal contamination. Early lesions manifest as erythematous skin with mild induration. As the condition progresses, central areas darken and soften, leading to separation of the skin and subcutaneous tissues with a floating sensation upon palpation. Necrotic skin turns grayish-brown or black and may rupture. Severe cases are accompanied by high fever, lethargy, coma, and symptoms of systemic infection.
Oral and Maxillofacial Cellulitis
This type is more common in children and often originates from oral or facial infections. When arising from an oral source, swelling caused by the inflammation may extend to the pharynx, resulting in laryngeal edema and airway obstruction, posing a critical risk. Examination reveals mild erythema and warmth of the submandibular region, with significant swelling, high fever, rapid breathing, dysphagia, and inability to eat. When originating from facial tissues, the affected region shows redness, swelling, warmth, and pain, along with severe systemic reactions. The infection frequently spreads to the submandibular or deep cervical regions, potentially involving connective tissues behind the platysma or even the mediastinum, causing dysphagia, breathing difficulties, or asphyxia.
Diagnosis and Differential Diagnosis
The diagnosis is generally established based on a patient’s history, clinical presentation, and elevated white blood cell counts. Smears of discharge may reveal causative bacteria, while blood and pus cultures, along with susceptibility testing, aid in diagnosis and treatment planning.
Differential diagnoses include:
Subcutaneous Gas-Forming Cellulitis vs. Gas Gangrene
The former involves fascial tissue, while the latter extends to muscles and displays intermuscular gas on imaging studies. Smear examination of pus helps distinguish bacterial morphology, and cultures confirm the causative agent.
Neonatal Subcutaneous Gangrene vs. Neonatal Sclerema
In early-stage gangrene, skin induration and redness are key distinguishing features, while neonatal sclerema does not involve erythema or fever.
Oral and Maxillofacial Cellulitis vs. Acute Pharyngitis
In cellulitis, submandibular swelling is more pronounced, whereas acute pharyngitis exhibits marked erythema and swelling in the oropharynx.
Prevention and Treatment
Maintaining skin hygiene and preventing skin injuries are critical preventive measures. Special attention to caregiving is necessary for infants and the elderly.
Antimicrobial Therapy
Penicillin or cephalosporin antibiotics are typically used for treatment. In suspected anaerobic infections, nitroimidazoles are added. Severe cases may require carbapenems. Adjustments to antimicrobial therapy are guided by clinical response or results from bacterial cultures and susceptibility testing.
Local Management
Superficial acute cellulitis may benefit from wet compresses with 50% magnesium sulfate or topical applications such as ichthammol ointment. For abscesses, timely incision and drainage are necessary. Oral and maxillofacial cellulitis often requires early decompressive incisions to prevent airway obstruction due to laryngeal edema. Subcutaneous cellulitis, particularly necrotizing fasciitis, may require multiple small incisions for decompression. Gas-forming cellulitis necessitates isolation, with wound care involving 3% hydrogen peroxide irrigation and iodine-based wet compresses. Vacuum sealing drainage (VSD) may be employed in severe cases for continuous washing and drainage.
Supportive Therapy
Measures to stabilize the patient's overall condition and maintain homeostasis are essential. Physical cooling methods may help manage hyperthermia. Nutritional and fluid balance is sustained via intravenous therapy in cases of feeding difficulties. Supplemental oxygen or assisted ventilation is provided when respiratory distress is present.