Cellulitis & Necrotising Fasciitis

Exam Cram Summary

• Cellulitis is usually caused by gram +ve cocci- staphlococcus or strephlococcus

• Flucloxacillin: IV or PO is the standard antibiotic of choice

• Any area of soft tissue can become cellulitic, commonly unilateral lower limb

• Complications: Necrotising fascitis 

  • Rapidly spreading, pain out of proportion, woody examination

  • Requires surgical debridlement

• If MSRA swab growth / severe pen allergy use vancomycin

Outline

Background

Organisms (predominantly):

1. Staphylococcus aureus

2. Beta Haemolytic streptococcus

Flucloxacillin has excellent cover of both. 

Cellulitis can occur in any area of the body but most commonly in lower limbs. 

Necrotising fasciitis is the condition of concern and is commonly missed in emergency waiting rooms. Particularly, the symptom of pain out of proportion can be hard to assess in the emergency room. Nec fasc is caused by either:

1. Streph pyogenes (Group A beta-haemolytic streph)

2. Polymicrobial 

Assessment

• PMH- previous celluitis, immunocompromise/ diabetes

• DHx- immunocompromising DMARDS

• Allergies- penicillin

• SHx- functional status, longer recovery in smokers

History

• Commonly lower limb but can be any area of skin

• 1-7d of warmth, swelling, tenderness

• Anorexia, fevers, malaise

• Nec fasc: pain out of proportion

• Gram -ve: dog bites, fresh/saltwater swimming

Examination

• Area of hot, swollen, tender skin

• Bite marks

• Purulent dc

• Nec fasc: rapidly spreading, black/ necrotic tissue; woody firmness (full of pus)

Investigations

• FBC- ?neutrophilia

• Urea & creatinine ?AKI

• LFTs- prior to antibiotics

• Blood cultures

• Wound culture/ MSRA swab

• Imaging

  • • XR- some orthopods like this to look for gas

    • USS- rule out DVT, look for collection

    • CT- ? necrotizing fasciitis

    • MRI- ? osteomyelitis

Diagnosis

Diagnostic criteria: clinical diagnosis

Assessing severity

• LRINEC- score for necrotising fasciitis- poorly validated

• Necritising fascitis commonly missed - pain out of proportion, rapidly spreading, 'woody'- firm with pus

• Sweep test: small incision at bedside under local anaesthetic and examine using finger (done by orthopods)

• Sepsis: qSOFA, SIRS/ multiorgan failure

Organism

• Dictates antibiotic choice

• Swab & blood cultures, note previous too

Management

1. Disposition

   • Home: PO antibiotics (5d)

   • Medical Ward: IV antibiotics (7-10d)

   • Orthopaedics: concerns of necrotising fasciitis 

   • HDU/ ICU: support for BP or oxygen requirement

   • Hospital in the home: IV antibiotics not requiring daily medical review (7-10d)

2. Antibiotics (as per trust)

   • IV Flucloxacillin 2g QID

   • MSRA: IV Vancomycin

   • Pen allergy: IV Cefazolin 2g BD

   • Nec fasc: IV Fluclox, Benpen, Metro, Clinda, Gentamicin +- Vanc

3. IVF if hypovolaemia

4. Mark leg ?rapidly spreading

5. Investigations

   • Blood cultures prior to antibiotics

   • Wound culture if purulent, MRSA swab if dry

   • USS ?DVT, CT ?nec fasc

   • Blood sugar ?diabetes

   • FBC, EUC, LFTs +/- CRP

6. Consults: 

   • Ortho ?debridlement for nec fasc,

   • ID ?antibiotic regime, 

7. Housekeeping: vte as usual, ceiling of care, family updated, analgesia

Deeper Dive

Gram Negative Cellulitis

Cellulitis is usually caused by gram +ve bacteria covered by flucloxacillin. Here are examples of cellulitis caused by gram negative organisms. 

• Bites- pasteurella 

• River water- aeromonas

• Sea water- vibrio

Management: co-amoxiclav

Hospital at the Home (HITH)

Cellulitis is an attractive option for HITH services as commonly the main indication for admission is IV antibiotics. However, they can benefit from the daily medical review & analgesia within hospital. HITH commonly only can administer IV antibiotics once daily, so change the regime to cefazolin/ probenacid. A 24hr infusion can be done through a 'Baxter infusion', but requires a PICC line. 

DC planning

Admission is commonly 1-7days. 

May require physio assessment if affecting mobility. 

Dosing of flucloxacillin

Regular dose is:

• PO: 500mg QID

• IV: 2g QID

The oral dose can be increased to 1g QID however the patient may experience GI side effects. The higher dose can be used in those with a weight > 70kg and particularly if weight > 100kg. 

Probenecid raises blood levels of flucloxacillin by reducing excretion. Usual dose: Probenecid 500mg BD. 

"Bilateral Cellulitis"

This is an 'ick' of medical consultants who commonly state this is impossible so avoid using this term at handover. Usually one leg is worse than the other so better to go with this leg. 

Erysipelas

Definition: soft tissue infection involving the dermis layer of skin

Presentation: often on face, well demarcated

Organism: often group A streph (streph pyogenes)

References and Resources

1. Weerakkody Y, Walizai T, O'Shea P, et al. Necrotising fasciitis. Reference article, Radiopaedia.org (Accessed on 27 Jan 2025) https://doi.org/10.53347/rID-17431 

2. GGC (2024) GGC Guidelines Antimicrobial, NHS choices. Available at: https://rightdecisions.scot.nhs.uk/ggc-clinical-guidelines/adult-infection-management/secondary-care-treatment/infection-management-empirical-antibiotic-therapy-in-adults-165/ (Accessed: 27 January 2025).

Written in 2025