Necrotising fasciitis

Question 1

Define necrotising fasciitis.

Answer 1

Necrotising fasciitis is a life-threatening subcutaneous soft-tissue infection that may extend to the deep fascia, but not into the underlying muscle.

Question 2

What are the clinical types of necrotising fasciitis?

Answer 2

Type 1 - polymicrobial infection with aerobic and anaerobic bacteria; usually in patients with immunocompromise or chronic disease.(non-GAS)

Type 2 - Group A streptococcus (GAS): occurs in any age group and in otherwise healthy people; occasionally accompanied by staphylococcal infection.

Type 3 - Gram-negative monomicrobial infection: e.g. marine organisms like Vibrio spp. and Aeromonas hydrophila, following seawater contamination of wounds, fish fins or stings, raw seafood consumption - esp in chronic liver disease. Theseare particularly virulent and can be fatal within 48 hours.

Type 4 - fungal infection: zygomycetes, candida infection, rapidly progressive with high mortality.

Question 3

How common is NF?

Answer 3

• About 500 cases per year in UK
• Rare ut has high mortality and complication rate

Question 4

What is the aetiology of NF?

Answer 4

Organisms spread from the subcutaneous tissue along the superficial and deep fascial planes. Muscle is usually spared; however, myonecrosis can occur due to compartment syndrome.

Question 5

What are the risk factors for necrotising fasciitis?

Answer 5

• Skin injury by insect bite, trauma, surgical wound, seawater exposure
• Alcohol abuse, IV drug use, chronic liver or renal diseasem diabetes, malignancy, immunosuppression and possibly TB
• In children may be caused by varicella zoster

NF can occur in previously healthy people with no underlying disease, particularly where Group A streptococci are involved.

Question 6

How does NF present (with time)?

Answer 6

Very similarly to cellulitis, except that pain/tenderness /systemic illness is often out of proportion to localised physical signs. Must have a high index of suspicion.

Days 1-2

• Minor skin changes in early stages
• Unexplained limb pain
• Unresponsive to antibiotics, lymphangitis, poor margins (all unlike cellulitis)
• Systemic illness

Days 2-4

• Tense oedema beyond erythema margin
• Bullae (indicating ischaemia) - not in cellulitis
• Discoloured –> grey necrosed skin
• Wooden-hard feel subcutaneous tissue
• Crepitus (subcutaneous gas)
• Anaesthesia - pain may be gone due to destruction of nerves
• Broad erythematous tract along route of infection
• Probing edges of wound -> easy dissection of superficial fascial planes well beyong wound margins

Days 4-5

• Hypotension and septic shock
• Patients become confused and apathetic

Question 7

What is Fournier’s gangrene?

Answer 7

Rapidly progressive form of infective NF of the perineal, genital or perianal regions, leading to thrombosis of the small subcutaneous vessels and necrosis of the overlying skin

Question 8

How do you diagnose NF?

Answer 8

No definitive tests - even if negative, when there is high index of suspicion explorative surgery is undertaken regardless.

X ray - presence of bullae/gas

Surgical diagnosis - macroscopic features such as grey necrotic tissue, alck of bleeding, thrombosed vessels, “dishwater pus”, lack of resistance to finger dissection

Other:

FBC - WCC >15.4x109/L

LFTs - hypoalbuminaemia

ABG - acidosis

CK - >600U/L

Urea >18mg/dL

Bedside finger test - downward incision of 2cm to deep fascia is made and index finger is used. Signs of NF: no bleedning, dishwater pus, no resistance.

Incisional biopsy - culture and gram stain

Wound swab , blood cultures, gram stains

Question 9

What are the complications? What is the prognosis of NF?

Answer 9

• High mortality - 20-40% even with surgery
• Septic toxic shock
• Ischaemia, necrosis, damage to muscles
• Large areas of tissue loss requiring reconstrction or amputation; amputation in 22.3%