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 - monomicrobial infection usually Group A streptococcus (GAS): occurs in any age group and in otherwise healthy people; occasionally accompanied by staphylococcal infection or caused by PVL-positive S. aureus.

Other types sometimes used:

Type 3 - Gram-negative monomicrobial infection: e.g. marine organisms like Vibrio spp. and Aeromonas hydrophila.

Type 4 - fungal infection:

Question 3

What is the aetiology of NF?

Answer 3

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 4

How common is NF?

Answer 4

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

Question 5

How does NF present (with time)?

Answer 5

Pain/tenderness /systemic illness is often out of proportion to localised physical signs. Often with multi-organ dysfunction.

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 - late signs

• 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 beyond wound margins

Days 4-5

• Hypotension and septic shock
• Confusion

Question 6

What are the risk factors for necrotising fasciitis?

Answer 6

• Skin injury by insect bite, trauma, surgical wound, seawater exposure
• Alcohol abuse,
• IV drug use
• chronic liver or renal disease
• diabetes
• malignancy
• immunosuppression a
• 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 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.

Bloods:

FBC - WCC >15.4x109/L

CRP - raised

Blood cultures

LFTs - hypoalbuminaemia

ABG - acidosis, raised lactate, metabolic 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

Imaging: has no definitive role

X ray - presence of bullae/gas

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

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%

Question 10

Name 2 differentials for NF.

Answer 10

Myositis

Cellulitis

Question 11

What is the management of NF?

Answer 11

Surgical emergency - mark any red areas and the time

• Urgent broad spectrum antibiotics e.g. pip/taz + vanc + clindamycin but check trust guidelines.
• Analgesia - opioids
• Resuscitation
• Surgical debridement +/- re-exploration - any necrotic tissue is excised until only viable bleeding tissue is left. Pack and relook in 24-48hrs
• HDU/ICU support
• +/- reconstructive surgery

Sometimes a palliative approach may be more acceptable than a disarticulation.

Question 12

What are the complications of NF?

Answer 12

Mortality - 10-40%

Skin loss and scarring

Question 13

What is the prognosis with NF?

Answer 13

Mortality is high especially in those who have end-organ damage and shock

Recurrence is rare

Significant functional and cosmetic morbidity may remain