Most red legs are cellulitis and get antibiotics, elevation and an ink mark. A few are necrotising fasciitis, where the skin looks unimpressive while the fascia underneath is dying — and the tell is pain out of proportion to what you can see. Sort the findings, then run one deteriorating patient hour by hour, and finish with the drug eruption where the whole treatment is stopping a drug.
Three infections, one red limbCellulitis lives in the deep dermis and fat. Erysipelas is its shallower, sharper-edged cousin. Necrotising fasciitis travels along fascia and kills the skin from below. Assign each finding to the diagnosis it points at.
Finding
What do you do?
Bed 3 · a painful rash on day 14A 61-year-old has had a flu-like prodrome, and now has painful dusky targetoid macules on the trunk, erosions in the mouth and on the conjunctivae, and epidermis that shears off under your gloved thumb — Nikolsky sign positive. Several medications were started in the last four weeks. Work the list, set the detachment, then choose the one action that changes mortality.
Step 1 · Tick every drug you suspect
Step 2 · Body surface with epidermal detachment
Detachment6%
Step 3 · The single most important action
Tell them apart at the bedside
Cellulitis
Warm, tender, oedematous erythema
Indistinct borders that fade into normal skin
Strep pyogenes or Staph aureus through a break in the skin
Non-purulent: cephalexin PO, cefazolin IV
Purulent with abscess: incision and drainage is the treatment, antibiotics are the adjunct, plus MRSA cover
Erysipelas
Sharply demarcated, raised, fiery red
Face or shin; you can feel the edge
Classically streptococcal
Spikes fever early
Superficial dermis and lymphatics — shallower than cellulitis
Necrotising fasciitis
Pain out of proportion to the visible findings — the cardinal early sign
Progresses hour by hour; oedema beyond the erythema
Dusky or grey mottling, haemorrhagic bullae, crepitus, “dishwater” drainage
Anaesthesia over the area as cutaneous nerves die
Emergent debridement plus vancomycin, piperacillin–tazobactam and clindamycin for antitoxin effect — imaging never delays theatre
Every shift, either way: outline the erythema with a skin marker, write the date and time on the skin, elevate the limb above heart level, and reassess. Advancement past the mark on appropriate antibiotics is an escalation trigger, not a charting detail.