Differentials
Cellulitis
SIGNS / SYMPTOMS
Systemic toxicity should be absent or minimal.[2]
INVESTIGATIONS
Absence of major abnormalities in full blood count, serum biochemistry, imaging findings.
Impetigo
Erysipelas
Myositis
SIGNS / SYMPTOMS
No involvement of skin or soft tissue. Swelling over involved area is present but may not be painful. Unusual to see systemic signs/symptoms of toxicity.
Some cases of necrotising fasciitis may have associated myositis due to contiguous spread. This is more common in group A streptococcal than polymicrobial infections.
INVESTIGATIONS
Ultrasound or CT/MRI to identify focal involvement of muscle with swelling.
MRI can also identify oedema.
Cutaneous anthrax
SIGNS / SYMPTOMS
History of intravenous drug use, or contact with animals or their products (e.g. hides, wool). Painless, pruritic papule forms 2 to 5 days after exposure. Lesion becomes vesicular, evolving into a necrotic black eschar with massive surrounding oedema 24 to 36 hours later. Regional lymphadenopathy is common.
INVESTIGATIONS
Vesicular fluid/blood Gram stain and culture: gram-positive bacilli in short chains (Bacillus anthracis); flat, non-haemolytic mucoid colonies on 5% sheep's blood agar.
Punch biopsy of cutaneous lesion: necrosis of the dermis and epidermis, oedema, and mild inflammatory infiltrate; abundant bacillary fragments (prior to antibiotic therapy); Bacillus anthracis (post-antibiotics).
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