Cellulitis

Clinical meaning

Cellulitis is an acute, spreading bacterial infection of the deeper layers of the skin, specifically the dermis and subcutaneous tissue. It is one of the most common skin infections encountered in clinical practice and a frequent reason for hospital admission. Unlike superficial skin infections such as impetigo, cellulitis involves deeper tissue planes and can progress to life-threatening complications if not treated promptly.

The infection typically begins when bacteria enter through a break in the skin barrier. Common entry points include cuts, abrasions, surgical wounds, insect bites, dermatitis, tinea pedis (athlete's foot between the toes), venous stasis ulcers, and intravenous catheter insertion sites. Even microscopic breaks in the skin that are not visible to the naked eye can serve as portals of entry. The most common causative organisms are Group A Streptococcus (Streptococcus pyogenes) and Staphylococcus aureus, including methicillin-resistant S. aureus (MRSA) in healthcare and community settings.

Once bacteria penetrate the skin, they multiply in the dermis and subcutaneous tissue. Bacterial virulence factors (enzymes such as hyaluronidase, streptokinase, and DNase) break down connective tissue components, facilitating rapid lateral spread through tissue planes. This explains the characteristic diffuse, spreading erythema without sharp borders that distinguishes cellulitis from an abscess (which is a walled-off collection of pus).

The host inflammatory response produces the cardinal signs: erythema (redness from vasodilation and increased blood flow), warmth (from increased metabolic activity and blood flow), oedema (from increased capillary permeability causing fluid extravasation into tissues), and pain/tenderness (from inflammatory mediator stimulation of nociceptors and tissue swelling compressing nerve endings). Lymphatic involvement is common, with bacteria and inflammatory cells tracking along lymphatic channels, producing lymphangitis (visible red streaks extending proximally from the infection site toward regional lymph nodes) and lymphadenopathy (swollen, tender regional lymph nodes).

Exam relevance

Risk factors: - Skin barrier disruption: cuts, abrasions, surgical wounds, insect bites, dermatitis, pressure ulcers, venous stasis ulcers, tinea pedis (athlete's foot is a very common entry point for lower leg cellulitis) - Lymphoedema (impaired lymphatic drainage creates protein-rich oedema that is a perfect growth medium for bacteria; strongest risk factor for recurrent cellulitis) - Chronic venous insufficiency and peripheral vascular disease (poor venous return causes tissue oedema and reduced local immune function) - Obesity (BMI greater than 30; skin folds create warm, moist environments; venous stasis; mechanical skin breakdown from friction) - Diabetes mellitus (impaired immune function, peripheral neuropathy leads to unnoticed injuries, poor wound healing, increased Staphylococcal colonisation) - Immunocompromised state: HIV/AIDS, organ transplant recipients on immunosuppressive therapy, chemotherapy, chronic corticosteroid use - Previous episode of cellulitis (recurrence rate up to 50% within 3 years if underlying risk factors are not addressed) - Peripheral neuropathy (any cause): inability to sense minor injuries that become portals of bacterial entry - Intravenous drug use (injection sites provide direct entry for skin flora into subcutaneous tissue)

Diagnostics: - Clinical diagnosis: cellulitis is primarily diagnosed clinically based on the presence of expanding erythema, warmth, oedema, and tenderness; laboratory tests support but are not required for diagnosis in typical presentations - CBC with differential: may show leukocytosis with neutrophilia (left shift) indicating acute bacterial infection; normal white cell count does not exclude cellulitis - Blood cultures: obtained in patients with systemic signs (fever, tachycardia, hypotension), immunocompromised patients, or those who fail initial antibiotic therapy; positive in only 2-5% of uncomplicated cellulitis cases - C-reactive protein (CRP) and erythrocyte sedimentation rate (ESR): elevated in infection; useful for monitoring treatment response (should trend downward with effective therapy) - Wound culture (if abscess is present and drained, or open wound): identifies specific organism and antibiotic sensitivities; swabs of intact cellulitic skin are NOT useful - Duplex ultrasonography: ordered when deep vein thrombosis (DVT) is in the differential diagnosis, as unilateral lower extremity redness, warmth, and swelling can mimic cellulitis; also useful to detect underlying abscess requiring drainage - Blood glucose and HbA1c: assess for underlying or undiagnosed diabetes in patients with cellulitis, especially lower extremity infections

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