Cellulitis

Cellulitis is an infection of the dermis and subcutaneous tissue that has poorly demarcated borders and is usually caused by Streptococcus or Staphylococcus but can be caused by a wide range of both aerobic and anaerobic bacteria.

• Cellulits is a common problem and a common presentation in primary care.
• Cellulitis is more common and more serious in individuals with underlying diseases such as diabetes, cancer, or immunodeficiency.¹

• The most common bacteria that cause cellulitis in patients with a healthy immune system are Group A Beta Haemolytic Streptococcus (Streptococcus pyogenes) and Staphylococcus aureus.
• Rarely, gram-negative organisms, anaerobes, or fungi may cause cellulitis. However these organisms are more common causes in children, people with diabetes and immunocompromised individuals.²
• Cellulitis in infants is usually caused by bloodborne spread of group B Streptococcus (urgent admission is required).
• In children, Haemophilus influenzae was a frequent cause prior to the introduction of the HiB vaccination.
• Cellulitis occurring around surgical wounds less than 24 hours post-operatively may result from Group A Beta Haemolytic Streptococcus or Clostridium perfringens. C. perfringens produces gas, leading to crepitus on examination.
• Cellulitis is being seen with increasing frequency in HIV-infected patients.³

• More commonly seen on the lower limbs. In many cases there is an obvious precipitating skin lesion, such as a traumatic wound or ulcer, or other area of damaged skin, e.g. athlete's foot.
• Erythema, pain, swelling, and warmth.
  
  
  
  
  
  
• Oedema and erythema often gradually blend into the surrounding skin and so the margin of the affected area may be indistinct.
• Systemic symptoms (e.g. fever, malaise) may occur.
• Red lines streaking away from a cellulitic area represent progression of the infection into the lymphatic system. Localized adenopathy is commonly observed with lymphangitis.
• Crepitus is a sign of infection most commonly observed with anaerobic organisms.

• Impetigo
• Erysipelas
• Necrotising fasciitis.

Many other conditions are mistakenly diagnosed as cellulitis. If the 'infection' does not respond to antibiotics, consider a biopsy or urgent referral for a dermatology opinion.⁴

• Most patients with cellulitis do not require further investigation.
• Full blood count: often shows raised white cell count but white cell count may be reduced in patients with toxin-mediated disease.
• Blood cultures if significant systemic symptoms or if bacteraemia is suspected.
• Fine needle aspiration: from leading edge of lesion may assist in diagnosis.
• X-rays, CT scan or MRI if there is any concern about a foreign body in situ.
• If bullae or abscesses form, culturing the fluid from inside these lesions yields an organism in more than 90% of cases.

Any patient who is at risk (e.g. young child, immunocompromised) or any concern about the underlying nature of the infection or lack of progress requires admission to hospital.

Non-Drug

• Consider need for tetanus prophylaxis.
• Immobilization and elevation of involved limb.
• Clean wound site: irrigation, debride devitalized tissue.
• Incision and drainage if deep fluctuant pocket.
• Cool sterile saline dressings decrease pain.

Drugs

• Benzylpenicillin/phenoxymethylpenicillin plus flucloxacillin (or erythromycin alone if penicillin-allergic).
• In mild cellulitis flucloxacillin may be used as single drug treatment.
• Always add phenoxymethylpenicillin if severe or rapid deterioration.
• Discontinue flucloxacillin if streptococcal infection confirmed.⁵
• Substitute treatment with broad-spectrum antibacterials if gram-negative bacteria or anaerobes are suspected.
• In facial cellulitis, use co-amoxiclav.⁶

Surgical

• Any patient with crepitus, circumferential cellulitis, or necrotic-appearing skin requires rapid surgical intervention.
• Necrotic skin requires examination of fascial planes to exclude necrotizing fasciitis.
• Crepitus requires immediate debridement of tissue.
• Pain disproportionate to the physical examination or severe pain on passive movement of the extremities may indicate necrotising fasciitis and requires prompt evaluation.

• Bacteremia: if the invading bacteria have the necessary virulence factors, bacteremia can develop into sepsis and/or seeding of internal organs and prosthetic devices.
• Circumferential cellulitis may impede blood flow or increase pressure distally, resulting in a compartment syndrome.
• Toxin release:
  • Even when infection is localized in healthy patients, some strains of gram-positive bacteria, such as group A beta-haemolytic Streptococcus and Staphylococcus aureus, may produce systemic toxins.⁷
  • Toxin-mediated disease is a serious infection causing septic shock, hypotension, and organ failure.
  • In adults, the mortality rate of toxin-mediated disease is approximately 50%.
• Local complications: abscesses, superinfection, lymphangitis (lymphatic involvement can lead to obstruction and damage of the lymphatic system that predisposes to recurrent cellulitis), thrombophlebitis, gas-forming cellulitis, necrotizing fasciitis.

• Uncomplicated cellulitis has an excellent prognosis.
• Treatment without hospital admission is effective for well over 90% of patients.
• Of those who fail outpatient therapy or require admission initially, intravenous antibiotics are very effective.