Overview
Cellulitis is a bacterial infection of the dermis and subcutaneous tissue, usually secondary to a wound/ulcer infection, skin lesion or trauma. It is most commonly caused by Streptococcus pyogenes and can also be caused by Staphylococcus aureus.
Cellulitis
Risk Factors
Most cases are due to skin breaks. Disruption of the skin barrier can allow entry of microorganisms to enter and infect subcutaneous tissues. Predisposing factors can include:
- Trauma – bites, burns, lacerations
- Ulcers and their causes (e.g. chronic venous insufficiency)
- Immunosuppression – including HIV, pregnancy, and immunosuppressant drugs
- Comorbidities – diabetes mellitus, chronic kidney disease, chronic liver disease
- Concomitant skin conditions – such as eczema, chickenpox
- Oedema – lymphoedema and leg oedema
- Obesity
- Prolonged water contact – can increase the risk of Pseudomonas aeruginosa infections (e.g. hot tubs)
Presentation
Overview
Cellulitis tends to affect the lower limbs, however, other places can be affected as well. Its features include:
- Pain, heat, swelling, and erythema over the affected area that develops over a few days
- The erythema may have well-defined or blurred margins
- Associated systemic upset – including fever, malaise, nausea, rigours
- There may be a source of entry (e.g. a wound, ulcer, and other breaks in the skin).
In severe cellulitis, the following may be seen:
- Blisters and bullae
- Skin necrosis
- Lymphangitis – suggesting the spread of infection into the lymphatic system
- Abscess – a fluctuant, tender mass suggests its presence
Eron Classification and Hospital Admission
Overview
Cellulitis is diagnosed clinically, however, investigations may be requested in secondary care if a patient is admitted (e.g. blood cultures for sepsis). The Eron classification system guides hospital admission:
- Class I – no signs of systemic upset, no uncontrolled comorbidities
- Class II – systemically well/systemically unwell with comorbidities that may complicate or delay resolution of the infection (e.g. peripheral arterial disease, chronic venous insufficiency, morbid obesity)
- Class III – significant systemic upset (e.g. confusion, tachycardia, tachypnoea, hypotension) or unstable comorbidities that may interfere with treatment response (e.g. acute limb-threatening ischaemia)
- Class IV – sepsis or severe life-threatening infection (e.g. necrotising fasciitis)
Admission
The following people should be admitted to hospital for intravenous (IV) antibiotics:
- Eron Class III or IV
- Severe/rapidly worsening cellulitis (e.g. extensive skin involvement)
- Very young (<1 year old) or frail
- Immunocompromised
- Significant lymphoedema
- Facial cellulitis (unless very mild) or orbital/periorbital cellulitis
- Symptoms of a more serious illness, such as septic arthritis or osteomyelitis
- Eron Class II – however, if facilities for community IV antibiotics are available, this may be done instead of admission
Differential Diagnoses
Deep vein thrombosis (DVT)
- DVT risk factors may be present, such as hypercoagulable states, immobility, contraceptive use, hormone replacement therapy use
- DVT is associated with pain and swelling of the calves, tenderness of the affected vein, and erythema is less significant
Superficial thrombophlebitis
- Erythema, heat, and tenderness may be present in a similar manner to cellulitis, however, these tend to follow the course of the thrombosed vein, which itself is also palpable and tender
- Oedema and systemic features are not usually present
Erysipelas
- Erysipelas tends to present more acutely with sharply-marked erythema, oedema of the affected area, and systemic upset (e.g. fever, malaise, and nausea)
Necrotising fasciitis
- Characterised by rapidly progressing pain that is out of proportion to clinical findings
- Over time, crepitus and gangrene develop
- Symptoms may worsen despite giving oral antibiotics
- Patients are systemically unwell
Chronic venous insufficiency
- The skin is dry and flaky, the calves may be swollen but the ankles are narrow beneath (similar to an ‘inverted champagne bottle’)
- Dependent rubor may be seen – the legs become dusky and red when sitting/standing
Venous eczema
- Lower limb venous oedema can lead to eczematous skin, which is not usually as erythematous, hot, or oedematous as cellulitis
- The skin may be itchy, dry, and scaly
Management
Overview
Management depends on the Eron Classification, comorbidities, and type of cellulitis:
- Eron Class I – oral antibiotics are used:
- 1st-line: oral flucloxacillin
- If allergic: oral clarithromycin, oral erythromycin (for pregnant people), or oral doxycycline
- Eron Class II – admission for IV antibiotics is indicated, however, if facilities for community IV antibiotics are available, this may be done instead of admission
- Eron Class III-IV – admit to hospital for IV antibiotics:
- 1st-line: oral/IV co-amoxiclav or clindamycin, or IV cefuroxime or IV ceftriaxone
Complications
Necrotising fasciitis – infection of deeper subcutaneous infections, fascia, and muscle can lead to necrosis and gangrene and is life-threatening. See Necrotising Fasciitis.
Severe infection – including abscess formation and sepsis
Chronic complications – including lymphoedema due to lymphatic inflammation and damage, recurrent cellulitis, and persistent leg ulcers.
Prognosis
- Most cases resolve with treatment and without major complications, however, recurrence is common and with each recurrence, the probability of future recurrence and hospital admission duration can increase.
