Overview
Definition
Cellulitis is inflammation of the skin and subcutaneous tissues. It is often due to infection.
Epidemiology and etiology
- Incidence: 200 cases per 100,000 patient-years
- More common in middle-aged and older populations
- Lower extremities: most common site
- Most common agents:
- Staphylococcus aureus:
- MSSA
- MRSA
- Group A Streptococcus (Streptococcus pyogenes)
- Staphylococcus aureus:
- Less common agents:
- Pasteurella multocida (from animal bites)
- Aeromonas hydrophila and Vibrio vulnificus (after water exposure)
- Clostridium species (myonecrosis)
- Pseudomonas aeruginosa (in immunocompromised patients)
- Erysipelothrix rhusiopathiae (occupational exposure in butchers, slaughterhouse workers, farmers, veterinarians)
Risk factors
- Breach of the skin barrier (wounds, ulcers, insect/animal bites)
- Skin inflammation (psoriasis, eczema)
- Injuries contaminated with dirt or seawater
- Preexisting infection (tinea, varicella)
- Previous history of cellulitis
- Crush injuries
- Second- and third-degree burns
- Frostbite
- Lymphedema or any obstruction of lymphatic drainage
- Immunodeficiency
- Diabetes mellitus
- Obesity
- Venous insufficiency
Pathophysiology and Clinical Presentation
Pathophysiology
- Bacteria enter disrupted skin barrier:
- Through minor cuts or injuries
- May be secondary to a distant site or systemic infection
- In some cases, portal of entry is not evident.
- Invasion of dermis and subcutaneous tissue triggers cytokine release, recruiting neutrophils and other inflammatory cells to the site.
- Intrinsic bacterial factors allow the pathogen to evade initial host defenses, leading to erythema, pain, and local swelling of the skin (edema).
- In immunocompromised patients, infection is limited eventually to the area of invasion.
- If the pathogen overcomes the immune defenses, further spread (deeper and/or contiguous infection and bacteremia in immunodeficiency) occurs.
- Reduced control of infection occurs in:
- Decreased tissue vascularity and oxygenation
- Increased peripheral fluid stasis
- Poor ability to combat infections (e.g., diabetes)
Clinical findings
- Prodromal systemic symptoms:
- May have signs of toxicity (fever > 100.5℉ (38℃), chills, and tachycardia)
- Muscle and joint pain
- Headache
- Local features:
- Indurated lesions or edematous area with erythema and poorly defined borders
- Spreading redness or lesions
- Pain and tenderness in the affected area
- Tight, glossy, swollen skin or dimpling (noted in edema surrounding hair follicles)
- May present with purulent exudate (usually associated with S. aureus)
- An abscess may also be found (collection of pus within the dermis or subcutaneous layer).
- Associated signs and symptoms:
- Regional lymph node swelling and tenderness (lymphadenitis)
- Severe infections:
- Can present with general malaise, fatigue, dizziness, confusion, muscle and joint pain, and diaphoresis
- Can have blistering lesions or bullae formation
Left lower extremity cellulitis:
Image: “Cellulitis left leg” by John Campbell. License: Public Domain
Edema and erythema of the skin are noted in the left leg and foot.
Abdominal wall cellulitis:
Image: “How 40 kilograms of fluid retention can be overlooked: two case reports” by Ong HS, Sze CW, Koh TW, Coppack SW. License: CC BY 2.0
Image shows marked pitting edema and erythema with poorly demarcated borders due to cellulitis.
Lymphedema cellulitis:
Image: “A Woman with Unilateral Rash and Fever: Cellulitis in the Setting of Lymphedema” by Joseph M, Camilon M, Kang T. License: CC BY 3.0
Stagnant lymph is an ideal medium for bacterial growth, and progression can be rapid owing to a decreased ability to fight infection in the affected area. Infections are commonly caused by gram-positive cocci bacteria.
Diagnosis
- Diagnosis of cellulitis is clinical:
- Edema
- Erythema
- Warmth
- Tenderness
- Associated lymphadenopathy
- Symptoms, risk factors, and progression of the skin lesion aid in ruling out differential diagnoses.
- Laboratory tests are nonspecific:
- Leukocytosis (common in all infections)
- ↑ Inflammatory markers:
- Erythrocyte sedimentation rate (ESR)
- CRP levels
- Microbiologic studies:
- Wound culture: used to identify pathogens and guide antibiotic therapy
- Blood culture: obtained in cases suggestive of bacteremia and/or in severely immunodeficient patients
- Imaging:
- Ultrasonography (US) is used to rule out abscess.
- Complicated skin and soft tissue infections need to be further evaluated.
- Orbital and sinus CT to determine extent of orbital cellulitis and presence of an abscess
- MRI and/or bone scintigraphy may be performed in cases of severe joint pain to confirm/rule out septic arthritis or osteomyelitis.
Management and Complications
Treatment approach
- Supportive care:
- Symptomatic treatment for fever, local pain, and aches
- Adequate hydration and elevation of the affected lower limb
- Empiric therapy covers group A Streptococcus and MSSA.
- Determination of risk factors and need for parenteral therapy:
- MRSA risk factors:
- Recent hospitalization or surgery (≤ 2 months ago)
- Hemodialysis
- Residence in a long-term care facility
- HIV infection
- Prior episode of MRSA
- Recent antibiotic use without MRSA coverage
- Indwelling medical device
- Need for parenteral therapy:
- Inability to tolerate oral intake
- Systemic signs of toxicity
- Rapid progression of erythema
- Cellulitis over a medical device (vascular graft or prosthetic joint)
- Failure to respond to prior oral antibiotics
- MRSA risk factors:
Treatment regimen
- No MRSA risk factors:
- Oral therapy (mild infection):
- Dicloxacillin
- Amoxicillin
- Cephalexin
- Clindamycin
- Parenteral therapy:
- Cefazolin
- Nafcillin
- Oxacillin
- Clindamycin
- Oral therapy (mild infection):
- With MRSA risk factors:
- Oral therapy:
- Trimethoprim–sulfamethoxazole
- Amoxicillin plus doxycycline
- Amoxicillin plus minocycline
- Clindamycin
- Parenteral therapy:
- Vancomycin
- Daptomycin
- Oral therapy:
- Other considerations:
- Bite wounds: add anaerobic coverage
- Hospitalization:
- Usually required for facial lesions because of high risk of spread of infection to the CNS (meningitis, encephalitis)
- For those with concomitant conditions such as hepatic, renal, or cardiac failure
- Severe progressive infections: might require surgical debridement of necrotic tissue or abscesses
Cellulitis in the left lower leg and knee:
Left: Local swelling with salmon-pink skin discoloration and local warmth is evident.
Right: leg after 6 weeks of antibiotic therapy
Complications
- Abscess:
- Collection of pus in the dermis or subcutaneous tissue
- Presents as a tender, fluctuant, erythematous swelling or discrete nodule
- Treatment: incision and drainage of drainable abscess detected on exam or US
- Necrotizing fasciitis:
- Deep infection associated with rapid destruction and necrosis of the fascia and subcutaneous tissues
- Life-threatening
- Septic arthritis:
- Infection in a joint, frequently secondary to bacteria
- Develops from hematogenous seeding but can also arise from extension of infection from the skin/soft tissue
- Osteomyelitis:
- Infection of the bone
- Poorly healing skin and soft tissue infections are at risk.
- Sepsis:
- Potentially life-threatening organ dysfunction caused by a dysregulated host response to infection
- Presentation can include fever, tachycardia, tachypnea, hypotension, and/or altered mentation.
- Immunocompromised patients with skin infections are at risk for this condition.
Differential Diagnosis
- Erysipelas: an acute superficial infection of the upper dermis and lymphatics, usually caused by group A beta-hemolytic Streptococcus: This condition presents as a sharply demarcated, raised skin lesion, with erythema, edema, and warmth.
- Necrotizing fasciitis: a rapidly progressive infection resulting in extensive necrosis of subcutaneous tissue, fascia, and muscle: Necrotizing fasciitis is most commonly caused by group A Streptococcus but often involves other types of bacteria in a mixed infection. The condition presents with necrosis, crepitus, bullae, and purple skin discoloration.
- Dermatitis: general term for edematous skin rash caused by an allergic reaction, irritant, or infection
- Folliculitis: a localized inflammation of the hair follicle or sebaceous glands that is primarily caused by S. aureus: Presentation includes erythema, papules, pustules, and tenderness of the affected area.
- Impetigo: a highly contagious skin infection of the upper epidermis: Impetigo commonly affects children < 5 years of age and is caused by S. aureus or group A Streptococcus. Patients present with an erythematous area covered in small vesicles, pustules, and/or honey-colored crusts.
- Staphylococcal scalded skin syndrome: a toxin-mediated blistering skin disorder caused by S. aureus. Presentation includes diffuse cutaneous erythema, tenderness, bullae formation, and superficial desquamation (sloughing off of the superficial layer of skin, leaving a red “scalded” appearance). The mucous membranes are spared.
References
- Herchline T (2019). Cellulitis. Medscape. Retrieved February 26, 2021, from https://emedicine.medscape.com/article/214222-overview#a1
- Kalyanakrishnan R, Salinas R, Higuita N (2015). Skin and soft tissue infections. Am Fam Physician. 92(6),474–483.
- Klotz C, et al. (2019). Adherence to antibiotic guidelines for erysipelas or cellulitis is associated with a favorable outcome. Eur J Clin Microbiol Infect Dis. 38(4),703–709.
- Spelman D, Baddour LM (2020). Cellulitis and skin abscess in adults: Treatment. UpToDate. Retrieved January 30, 2021, from https://www.uptodate.com/contents/cellulitis-and-skin-abscess-in-adults-treatment
- Spelman D, Baddour LM (2020). Cellulitis and skin abscess: Epidemiology, microbiology, clinical manifestations, and diagnosis. UpToDate. Retrieved January 30, 2021, from https://www.uptodate.com/contents/cellulitis-and-skin-abscess-epidemiology-microbiology-clinical-manifestations-and-diagnosis
- Stevens DL, et al. (2014). Practice guidelines for the diagnosis and management of skin and soft tissue infections: 2014 update by the Infectious Diseases Society of America. Clin Infect Dis. 59(2),e10–52.
