see also:

• streptococcus / streptococcal infections
• necrotizing fasciitis
• cracked heels
• venous leg ulcers
• sepsis / septicaemia
• Periorbital and orbital cellulitis
• perichondritis / ear/auricle/pinna cellulitis

• cellulitis is a spreading, deep skin and subcutaneous fat infection characterized by poorly demarcated erythema, swelling, warmth, and tenderness
• there is often a linear proximal erythema of lymphangitis +/- tender regional lymphadenopathy
• it most commonly affects the legs and is usually caused by Group A beta-haemolytic streptococcal infections
• uncommonly, it may be caused by staph aureus and, in the immunocompromised, or those with unusual sources of infection by a range of microbes including Gram negatives
• it tends to be rapidly progressive and may develop into severe sepsis, particularly in the immunocompromised
• there are many differentials that need to be considered
• failure to properly identify and treat mimickers of cellulitis can result in:
  • unnecessary hospital admissions
  • failed treatment and re-admissions
  • unnecessary and prolonged antibiotic Rx with risk of Cl. difficile colitis

• infection such as cellulitis
• venous stasis dermatosis +/- venous leg ulcers
• contact dermatitis
• xerotic dermatitis
• deep venous thrombosis (DVT)
• thrombophlebitis
• vasculitis

• this is in a previously well adult with no pre-existing limb pathology such as chronic oedema, and with no immunocompromise such as diabetes, corticosteroid use, etc.
• there is usually an obvious cause such as a wound or perhaps entry point in foot such as cracked skin or tinea
• the treating doctor should exclude/address other issues such as:
  • unusual infection sources - these may require different antibiotics
    • animal or human bites
  • wounds that may need debriding such as cat bites which tend to be deep puncture wounds
  • abscess (look for fluctuance) which will need drainage
  • retained foreign body - (careful history) may need xray or USS to exclude if suspected
  • severe sepsis - life threatening requires blood cultures and early antibiotics (broad spectrum coverage if immunocompromised), and assessment and Mx as for septic shock
  • necrotizing fasciitis (expanding oedematous red plaque with blue discolouration +/- blister and anaesthesia of skin) - life threatening - requires urgent theatre and broad antibiotic coverage (urgent MRI may be useful if doesn't substantially delay theatre)!
  • erysipelas - superficial spreading sharply defined, often shiny, red, skin infection with superficial dermal oedema due to marked dermal lymphatic involvement, +/- plaques or bullae, usually of the legs or face, caused by Strept.
  • impetigo or folliculitis - most likely to be Staph. aureus rather than Strept.
  • septic arthritis - joints should be able to go through FROM without substantial pain and there should not be an effusion, if there is a possibility the joint is the source, d/w orthopaedics BEFORE starting antibiotics unless patient has severe sepsis (ortho usually want a joint aspirate or washout in theatre first)
  • acute gout
  • osteomyelitis - this is usually from a penetrating injury but may be haematogenous spread (eg. salmonella), if suspected, xray and perhaps nuclear med scan
  • thrombophlebitis erythema and tenderness tends to be linearly oriented over a vein
  • deep venous thrombosis (DVT) - if suspected, arrange an doppler USS and consider enoxaparin if there is a delay
  • bursitis
  • acute limb ischaemia although present usually as painful, pallor rather than erythema, check distal pulses
  • erythema migrans

• once doctor is happy there are no complicating factors and the above are addressed:
  • if leg cellulitis, strict elevation of leg to improve circulation and healing, and consider prophylactic enoxaparin to prevent deep venous thrombosis (DVT)
  • if the cellulitis is early, the patient is well, then the patient may be suitable for a trial of oral antibiotics (eg. flucloxacillin or cephalexin) +/- initial stat iv dose (eg. long acting antibiotic such as cephazolin)
  • otherwise, admit into short stay unit for iv antibiotics +/- hospital in the home, then if improved a course of oral antibiotics
    • usually need 5 days of iv antibiotics (eg. cephazolin 1-2g bd if managed by HITH nurses)
• remember to Rx the cause such as tinea pedis
• preferred choices of oral antibiotics
  • possible Staph:
    • flucloxacillin, or clindamycin if penicillin sensitive
    • NB. cefalexin has poor coverage for Staph aureus
  • most likely Strept:
    • cefalexin may be an option as better tolerated than the above
  • water-borne infections
    • those acquired in sea water could be caused by Vibrionaceae including Shewanella sp
      • may need doxycycline 200mg stat then 100mg bd +/- ceftriaxone if severe
    • those acquired in fresh water could be caused by Aeromonas sp
      • may need ciprofloxacin 500mg o bd or 400mg iv bd for at least 14 days +/- merepenem if severe

• these cases are likely to warrant further Ix or prolonged Rx and are usually best referred to general medicine rather than admission into a short stay unit where they are likely to block patient flow in the ED
• they should be assessed as for simple cellulitis above, looking to exclude and address potential differentials or complications.

• in the absence of trauma to both legs, rarely does cellulitis start on both legs simultaneously and rarely starts on one leg and jumps to the other
• as a general rule these patients do not have cellulitis but another disorder often grouped as “pseudocellulitis” such as:
  • patients with bilateral chronic leg oedema from the many causes of this are most likely to have florid venous dermatosis, although uncommonly could have extensive DVTs
    • venous stasis dermatosis due to chronic oedema can cause the intense red discoloration on the shins, without sharp lines of demarcation, that is the hallmark of non infectious leg oedema, unfortunately they may temporarily improve on admission but this is due to leg elevation not the antibiotics leading to further confusion
  • one should not miss classic conditions such as:
    • urticaria
    • erythema nodosum - tender discrete red nodules on mainly anterior aspects lower legs
    • sunburn
    • contact dermatitis - this is a common cause of increasing pain, erythema in patients with stasis dermatosis, especially if topical creams or antibiotics have been used
    • xerotic eczema - itchy, dry lake bed like skin lesions over shins (eg. elderly excessive bathing or legs close to heaters), may become weepy dermatitis
    • lipodermatosclerosis
    • Kaposi sarcoma (consider HIV testing)
    • septic emboli from infective endocarditis (including SBE)
    • vasculitis presents with palpable purpura such as:
      • Henoch-Schonlein purpura (HSP)
      • leukocytoclastic vasculitis (eg. due to non-steroidal anti-inflammatory drugs (NSAIDs))
      • urticarial vasculitis