Soft Tissue: Abscess vs Cellulitis

Soft TIssue Infection: 10 Mhz linear probe (high frequency)

case: 25yo gentleman with 5 days of erythema on upper back, no fever
indurated and tender, ?fluctuance.
choices:
- stick a needle blindly
- I&D blindly
- trial empiric antibiotics
- antibiotics and short stay ward
- admit patient
- how about ultrasound?

compared with Gold Standard (Pus on I&D, clinical improvement with Antibiotics with I&D not performed)
- clinical exam: detected abscess 86% Sn
- U/S: detected abscess 98% Sn
(Squire BT, Fox C. Abscess Study. Academic EM. July 2005)

U/S
- Normal tissue: well delineated tissue planes
normal fascial planes, sees borders of muscle and fascia
- Cellulitis: Tissue thickening + loss of distinct tissue planes
- Severe Cellulitis:
- Severe Cellulitis with Edema: pre-abscess abscess stage lakes of fluids
noted pockets of echoic fluids but no large collection drainable (antibiotics and elevation of infected area)

- Nec Fasc:
1. loss of normal tissue planes
2. edema in fascia (black echoic) = hallmark
3. hyperechoic gas pockets with comet trails at subcutaneous tissue = hallmark

Abscess
- swirls of dark anechoic collections with mixed echogenicity = pus activity
- may show pseudomembranes = linear hyperechoic structures

Pitfalls:
- high risk: neck and groin (vascular structures)
- abscess vs femoral artery pseudoaneurysm s/p cardiac cath (look for doppler flow)
- watchout for nerves and blood vessels
- chest wall abscess may track close to pleura
- abdominal wall abscess may track close to peritoneum
- lymph node

lymph nodes: typical at cervical triangle, femoral region
- multiple (vs abscess usually single)
- lima bean appearance
- hyperechoic bright stalk leading to actual node (abscess more of heterogenous)

solid mass tumours
- more of solid appearance than typical pus
- eg Ca with mets to skin