Cellulitis is an acute spreading bacterial infection of the deeper dermis and subcutaneous tissue, typically of the lower leg, which may complicate a wound, ulcer, interdigital fungal infection or primary skin disorder. Predisposing factors include previous episodes of cellulitis, limb oedema and lymphoedema. Streptococcus pyogenes and Staphylococcus aureus are the commonest causative organisms.

Erysipelas is an acute bacterial infection of the upper dermis and epidermis, which may be clinically distinguished from cellulitis by a more clearly demarcated border between infected and healthy skin. Assessment and management is the same as for cellulitis.

Necrotizing fasciitis is a rapidly progressive infection of the deep fascia and muscle, and should be suspected in an ill patient with severe pain and marked local tenderness. In the later stages the skin may show blue-black discolouration and blistering. The majority of cases are polymicrobial and caused by anaerobes, Gram- negative bacilli and streptococci (not S. pyogenes). Most other cases are caused by S. pyogenes or Clostridium perfringens (gas gangrene). Management requires resuscitation, antibiotic therapy and urgent referral to a plastic surgeon for consideration of debridement.

Make a focused assessment and consider the differential diagnosis (Figure 95.1, Table 95.1). Bilateral cellulitis is very rare. Mark the margin of affected skin. Investigation required urgently is given in Table 95.2.

If necrotizing fasciitis is suspected, give IV fluid, start antibiotic therapy (Table 95.4) and seek urgent advice from a plastic surgeon, and a microbiologist.

If cellulitis is the likely diagnosis, assess the severity of the illness, on the basis of the clinical features and comorbidities, and manage the patient accordingly (Tables 95.3 and 95.4).

Any underlying skin disorder should be treated. Seek advice from a dermatologist.

Supportive care of the patient with cellulitis of the lower leg includes:

• Elevation of the limb
• Use of a bed cradle
• Analgesia
• DVT prophylaxis
• Adequate hydration
• Treatment of comorbidities, for example diabetes

Clinical improvement is usually seen after 2–3 days, and with this, patients treated initially with IV antibiotic therapy can be switched to oral therapy, to complete a 7–14 day course. If there is no improvement at this time, other diagnoses should be considered and a dermatological opinion obtained.

Patients who have recurrent episodes of cellulitis should be considered for prophylactic antibiotic therapy: seek advice from a dermatologist.

Suspected cellulitis

Painful swelling and erythema of the skin, typically of the lower leg

Key observations

Focused assessment. Mark margin of affected skin.

Consider differential diagnosis (Table 95.1)

Urgent investigation (Table 95.2)

Ill patient with severe pain and marked local tenderness?

Manage as necrotizing fasciitis Fluid resuscitation

IV antibiotic therapy (Table 95.4) Seek urgent advice from orthopaedic or plastic surgeon and microbiologist

Clinical picture typical of cellulitis?

Assess severity of illness (Table 95.3)

Antibiotic therapy (Table 95.4) Supportive care

Improvement after 2–3 days?

Change to oral antibiotics

Consider other diagnoses (Table 95.1) Refer to dermatologist