MBBS viva · Infectious Diseases / Dermatology
Cellulitis and skin/soft-tissue infection — recognition, severity and empirical therapy viva
A final-prof viva on a red, swollen leg: framing the differential (cellulitis vs erysipelas vs necrotising fasciitis vs mimics), reproducing the IDSA purulent/non-purulent framework, marking the border, choosing empirical antibiotics with drug/dose/route/duration, and justifying MRSA cover. Examiner expects mechanism and dose-level detail, not labels.
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Interpretation
The examiner presents a clinical photograph of a warm, erythematous, swollen, tender unilateral lower leg with poorly demarcated margins, interdigital scaling between the toes, and a pen mark around the leading edge, and asks: "What do you see, what is the diagnosis, and how would you manage this patient?"
- Lesion: spreading erythema, warmth, swelling and tenderness with poorly demarcated margins on a unilateral leg, with an evident portal of entry (interdigital tinea pedis). The pen mark shows the clinician is tracking spread and response.
- Diagnosis: cellulitis of the lower limb (acute spreading infection of the dermis and subcutaneous tissue). The sharply raised border of erysipelas is absent; the pain-out-of-proportion, systemic toxicity, necrosis and crepitus of necrotising fasciitis are absent.[1][2]
- Likely organisms: Streptococcus pyogenes and Staphylococcus aureus (including community-acquired MRSA in many regions).
Key points
The examiner will probe each of these; be ready to defend them at viva depth:
- Definition and classification — cellulitis (dermis + subcutis, poorly demarcated) vs erysipelas (superficial dermis + lymphatics, sharply raised border) vs abscess (purulent, fluctuant) vs necrotising fasciitis (deep fascia, pain out of proportion, necrosis, crepitus).[1]
- The two management axes (IDSA 2014) — purulent vs non-purulent, and mild vs moderate vs severe (presence of systemic signs). Purulent disease: incision and drainage first; non-purulent: empirical anti-streptococcal/anti-staphylococcal antibiotics.[1]
- Empirical antibiotics with drug/dose/route/duration — non-purulent mild cellulitis: oral flucloxacillin 500 mg QID for 5 to 7 days (cefalexin 500 mg QID if penicillin-allergic without anaphylaxis; clindamycin or a macrolide if anaphylaxis). Severe/systemic: IV flucloxacillin 1 to 2 g QDS (or cefazolin, ceftriaxone).[1]
- When to cover MRSA — purulent disease, severe infection, IVDU, prior MRSA, healthcare exposure, failure of first-line therapy. Oral: co-trimoxazole, doxycycline, or clindamycin; IV: vancomycin (monitor troughs), linezolid, or daptomycin.[1]
- Mark the border, elevate the limb, treat the portal of entry — antifungal for tinea pedis, compression for lymphoedema/venous disease; this prevents recurrence.[2]
- Necrotising fasciitis — pain out of proportion + systemic toxicity + necrosis/bullae/crepitus; LRINEC 6 or above is high-risk but a low score never delays surgery; management is urgent surgical debridement + broad IV antibiotics with clindamycin for toxin suppression.[1]
- Recurrence and prophylaxis — recurrent leg cellulitis is common; treat the portal of entry and consider prophylactic penicillin V (or erythromycin/clarithromycin) for frequent recurrences.[2]
References
- Stevens DL, et al. IDSA 2014 SSTI guideline. Clin Infect Dis 2014.[1]
- Rrapi R, et al. Cellulitis: A Review of Pathogenesis, Diagnosis, and Management. Med Clin North Am 2021.[2]
References2ShowHide
- [1]Stevens DL, Bisno AL, Chambers HF, et al. Practice guidelines for the diagnosis and management of skin and soft tissue infections: 2014 update by the IDSA. Clinical Infectious Diseases, 2014.PMID 24947530
- [2]Rrapi R, Chand S, Kroshinsky D. Cellulitis: A Review of Pathogenesis, Diagnosis, and Management. Medical Clinics of North America, 2021.PMID 34059247