Introduction

The skin acts as the body’s largest and most effective barrier against infection. The skin consists of three layers: the epidermis, the dermis and the subcutaneous tissues. Below this lies layers of superficial and deep fascia. Between these planes there exists a cleft, containing blood vessels, lymphatics, fat and nerves. Below the fascia lies musclle and bone.

Skin infections are uncommon. The most likely causes of these infections arise when:

1. There is a break in the surface of the skin.
2. There is loss of perfusion: ischaemia, cell death and necrosis.
3. Colonising bacteria are particularly virulent.
4. The patient is, in some way, compromised:

• Diabetes
• Chronic liver disease
• Cancer
• AIDS
• Collagen vascular diseases
• Chronic renal failure
• Trauma
• Systemic sepsis (the "second hit")
• Steroids
• Advanced age
• Malnutrition

Classification of soft tissue infections

These are classified according to anatomical site:

There are two types:

Type 1 necrotising fasciitis: infection with mixed bowel organisms.

Type 2 necrotising fasciitis: caused by group A streptococci.

Hx of minor trauma, surgery, zoster infection.

Again compromised patients are more vunerable.

Usually begins in an area of cellulitis, which rapidly spreads.

Localised thrombosis of blood vessels leads to loss of perfusion and necrosis/gangrene.

There is usually extraordinary evidence of sepsis, fever, rigors, shock. Pain may not be evident due to necrosis of nerves. Crepitus may be present, due to gas production in the tissues.

Patients may rapidly go into multi-organ failure.

1. Supportive measures – inotropes, dialysis, mechanical ventilation etc.
2. Intravenous antibiotics: benzylpenicillin 2.4g 4 hourly or clindamycin 600mg 6 hourly, plus gentamycin 5mg/kg/day. Metronidazole may also be added.
3. Surgical debridement [often extensive] of necrotic tissue must be performed and repeated at frequent intervals, until the necrosis stops. Amputation of limbs may be necessary.

There is no doubt that early and aggressive surgical intervention is the essential therapy. Other fashionable remedies, such as hyperbaric oxygen therapy, have no proven benefit.

Muscle is remarkably resistant to infection.

Myonecrosis is caused by Clostridium Perfringens. This infection is often called "gas gangrene" due to the palpable crepitus, caused by liberation of gas. Commonest cause is contamination of wounds – as occurs in trauma, septic abortions, immunocompromise, surgery.

Clinical Features: sudden onset, very painful, extensive swelling, smelly purulent discharge, discolouration of skin. Gram stain of infected material reveals gram-positive rods.

Treatment is extensive surgical debridement of all infected tissue and intravenous antibiotics: benzylpenicillin 2.4 g 4 hourly plus clindamycin 500 mg 6 hourly. Again, hyperbaric oxygen may have a role, but has not been proven by prospective clinical trials.

Pyomyositis: this is any infection of muscle not caused by clostridium: usually Staph, Strep, coliforms. More common in the tropics.

Clinical features: Usually presents with an ache and induration, followed by local tissue destruction and septic metastases (eg. to heart valves). CPK is enormously raised.

Treatment: again, surgical debridement and intravenous penicillin.