Infectious cellulitis

Classification of bacterial skin infections

Infections of the skin and soft tissues (ICTM) are classified according to the depth of the structures affected and the presence or absence of purulence, which directly affects the choice of treatment:

Risk factors

• Rupture of the skin barrier: bacterial entry point identified in 70-80 % of cellulitis cases - wound, scratch, insect bite, animal or human bite, toe intertrigo (mycosis of the feet - tinea pedis), leg ulcer, pre-existing dermatosis (eczema, psoriasis), intravenous injection ; tinea pedis is the most underestimated route of entry - its systematic treatment significantly reduces the risk of recurrence of leg cellulitis
• Chronic venous insufficiency and edema of the lower limbs: venous stasis and lymphedema create an environment conducive to bacterial proliferation and reduce local defence by diluting immune factors; major risk factor for recurrence.
• Lymphedema: history of lymph node dissection (breast cancer, melanoma), lymphatic filariasis, primary lymphedema - permanent reduction in lymphatic clearance of pathogens; risk of multiple recurrences justifying prolonged prophylactic antibiotic therapy (penicillin V or erythromycin) in patients with ≥ 2 episodes per year
• Diabetes mellitus: impaired immune response (neutrophil function, chemotaxis), peripheral neuropathy reducing wound perception, angiopathy reducing tissue vascularization - higher risk of infection with atypical germs (Gram-negative, anaerobic), particularly for diabetic foot infections
• Immunosuppression: HIV, chemotherapy, prolonged systemic corticosteroid therapy, immunosuppressants (transplantation) - more severe forms, broader pathogens (fungi, atypical mycobacteria, Gram-negative bacteria)
• Obesity: increased venous and lymphatic pressure, skin fragility in folds, reduced tissue diffusion of antibiotics - independent risk factor for recurrent cellulitis and poor response to treatment.
• History of infectious cellulitis: risk of recurrence at the same site of 8 to 20 % per year; each episode aggravates local lymphedema, increasing the risk of subsequent episodes - vicious circle justifying secondary prophylaxis

Symptoms

Infectious cellulitis typically presents acutely or subacutely, with a predominantly local picture accompanied by systemic signs that vary according to severity:

• Progressively-extending erythematous plaque: warm, ill-defined redness (with no clear bulge, unlike erysipelas), painful to the touch and sometimes spontaneously; centrifugal extension visible in a few hours to a few days - it is useful to mark the edges with a felt-tip pen during the first examination to objectivize progression
• Local edema and skin tension: swelling of affected tissues with taut, shiny skin; edema may be pitting in diffuse forms or in patients with pre-existing venous insufficiency
• Local heat: increased skin temperature at the infected site, easily detectable by palpation compared to the contralateral limb.
• Local pain: from mild to very intense, depending on severity; pain disproportionate to the appearance of the skin - particularly deep, intense pain poorly relieved by the usual analgesics - is a warning sign that necrotizing fasciitis may be present.
• Systemic signs according to severity: fever (often > 38.5°C in erysipelas; variable in cellulitis), chills, general malaise, tachycardia; severe systemic signs (fever > 39°C, hypotension, confusion) should alert to a serious infection requiring hospitalization
• Regional adenopathy and lymphangitis: painful regional lymph nodes; linear red streak running up the lymphatic pathway towards the draining lymph node (lymphangitis) - sign of lymphatic extension of the infection
• Most frequent location: lower limb (leg) in 70-80 % of cases; face, upper limb, thorax or abdomen in the remaining forms.

Diagnosis

• Clinical diagnosis first and foremost: the physical examination remains the cornerstone of the diagnosis - characterization of the plaque (sharp or blurred edges, presence of a bulge, unilaterality), search for a portal of entry, assessment of extension, signs of severity (crepitation, necrosis, hemorrhagic bullae, cutaneous hypoesthesia) and systemic signs; delineation of edges with a pen to monitor progression
• Biological workup: complete blood count (hyperleukocytosis with neutrophil polynucleosis), CRP and/or procalcitonin (markers of inflammation and bacterial severity), urea and creatinine (renal function before antibiotic therapy), blood glucose (diabetes screening); blood cultures if fever > 38.5°C, immunodepression or signs of sepsis - low yield (2-4 %) in uncomplicated cellulitis but essential in severe forms
• Computed tomography (CT) with injection: urgently indicated if necrotizing fasciitis is suspected (presence of gas in deep tissues - pathognomonic sign, although absent in 25 % of cases) or deep abscess not accessible to clinical examination; not systematically indicated for uncomplicated cellulitis.
• Soft-tissue ultrasonography: useful for detecting a purulent collection (abscess) to be drained, distinguishing an abscess from cellulitis without a collection, and ruling out associated deep-vein thrombosis; first-line examination if there is any doubt about the presence of an abscess
• Microbiological samples: swab of the entry site if the wound is open, pus sample if the collection has been drained, skin biopsy in atypical or recurrent forms; blood cultures and superficial skin samples taken by needle aspiration have a low yield in common cellulitis and are not routinely recommended.
• LRINEC score (Laboratory Risk Indicator for Necrotizing Fasciitis): score based on CRP, leukocytosis, hemoglobin, natremia, creatinine and glycemia - score ≥ 6 suggestive of necrotizing fasciitis (sensitivity 68 %, specificity 84 %); decision-support tool, never used alone

Treatment

• Outpatient oral antibiotic therapy (mild to moderate non-purulent cellulitis, non-immunocompromised patient, no severe systemic signs): cefalexin 500 mg 4 times/day × 5-7 days - first-line treatment in Quebec (streptococcal and MSSA coverage); amoxicillin-clavulanate in case of animal or human portal of entry; clindamycin or trimethoprim-sulfamethoxazole if allergic to beta-lactams or suspected community-acquired MRSA; clinical reassessment at 48-72 hours mandatory
• Intravenous antibiotic therapy and hospitalization: indicated if severe systemic signs (fever > 39°C, hypotension, tachycardia, confusion), rapid progression despite 48-72 hours of oral antibiotic therapy, significant immunosuppression, diabetes with foot involvement, facial or periorbital localization, ambulatory failure; cefazolin IV 1-2 g every 8 hours as first-line treatment (MRSA, streptococcus); vancomycin IV if MRSA suspected or confirmed, severe sepsis or immunosuppressed patient; standard total duration: 5 to 10 days depending on clinical response
• Surgical drainage: indispensable treatment for all fluctuating abscesses - antibiotic therapy alone without drainage is insufficient and a source of therapeutic failure; incision-drainage under local anaesthesia with cavity lavage; meching or open drainage depending on size
• Adjuvant measures: elevation of the affected limb (to reduce swelling, improve lymphatic circulation and enhance diffusion of antibiotics into tissues); rest; analgesia (NSAIDs to be avoided if necrotizing fasciitis is suspected - may mask pain and delay diagnosis); treatment of the entry site (topical antifungal if tinea pedis, local wound care).
• Management of necrotizing fasciitis: extensive emergency surgical debridement of all necrotic tissues - the only curative treatment; broad-spectrum IV antibiotic therapy (piperacillin-tazobactam + clindamycin + vancomycin); hyperbaric oxygen therapy as an adjunct in some centers; intensive care; IV immunoglobulins if streptococcal toxic shock (IVIG).
• Recurrence prophylaxis: penicillin V 250-500 mg twice/day or benzathine penicillin IM monthly for 6-12 months in patients with ≥ 2 episodes of cellulitis at the same site per year - reduction in recurrences of 50-70 % demonstrated in randomized trials; erythromycin or azithromycin in case of allergy to penicillin; concomitant treatment of lymphedema and venous insufficiency

Differential diagnoses not to be missed

• Deep vein thrombosis (DVT): unilateral calf edema with possible erythema, without fever or identifiable portal of entry; venous Doppler ultrasound essential if in doubt - the two pathologies may coexist
• Stasis dermatitis (varicose eczema): bilateral erythema of the legs, pruritus, visible varicose veins, without fever or leukocytosis; bilateral cellulitis is very rare and should cast doubt on the diagnosis of infectious cellulitis.
• Acute gout: metatarsophalangeal arthritis or hyperalgesic, red, hot ankle - may simulate cellulitis of the foot; joint puncture (urate crystals) and uricemia guide diagnosis
• Erythema migrans (Lyme disease): centrifugal erythematous plaque with central pale ring, without significant heat or pain, without leukocytosis; context of exposure to ticks in an endemic area.
• Hypersensitivity reaction or insect sting: localized, pruritic erythema, without fever or rapid progression; regression spontaneously or with antihistamines

Consult at Clinique Omicron

Clinique Omicron's physicians assess bacterial skin infections, prescribe the appropriate antibiotic therapy according to clinical severity, drain accessible superficial collections and refer to emergency or to infectious diseases or surgical specialists depending on signs of severity. Consultations are available in our Quebec branches, as well as via telemedicine throughout the province. To book an appointment, visit cliniqueomicron.ca.

The contents of this page are provided for information purposes only and do not replace the advice of a qualified healthcare professional. Any skin infection with rapid progression, systemic signs or in an immunocompromised patient requires immediate in-person medical evaluation.