134.1 🎓 醫學生版

134.1.0.1 📌 一頁重點

• 層次分類:
  • 表淺: impetigo (epidermis), erysipelas (sup. dermis + lymphatics)
  • 中: cellulitis (deep dermis + subcutaneous)
  • 深: necrotizing fasciitis (fascia + subcutaneous fat)
  • 肌肉: myositis / pyomyositis, gas gangrene (Clostridium perfringens)
• 病原:
  • Gram +: S. aureus (MSSA, MRSA), S. pyogenes (GAS)
  • Gram -: Pseudomonas (folliculitis hot tub), Vibrio vulnificus (海水, 肝病)
  • Anaerobes / Mixed: necrotizing, perineum, post-op
• MRSA prevalence rising in community → 經驗性 cover 取決於 local epidemiology
• Necrotizing fasciitis: pain 不成比例 + skin necrosis + crepitus + septic shock → 緊急 OR debridement

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134.1.0.2 1️⃣ 表淺感染

134.1.0.2.1 Impetigo

• 表皮，多兒童
• Non-bullous (more common): GAS or S. aureus → honey-colored crusted erosions
• Bullous: S. aureus exfoliative toxin → flaccid bullae
• Treatment: topical mupirocin 2% for limited; PO dicloxacillin or cephalexin if extensive

134.1.0.2.2 Folliculitis / Furuncle / Carbuncle

• Folliculitis: hair follicle infection (S. aureus most; Pseudomonas in hot tub folliculitis)
• Furuncle (boil): single follicle abscess
• Carbuncle: multiple coalescing furuncles
• Treatment: warm compress, incision/drainage if abscess; PO antibiotic if surrounding cellulitis or systemic symptom

134.1.0.2.3 Erysipelas

• Sharply demarcated, raised, bright red plaque with shiny induration
• Face or extremity
• GAS (most), occasionally S. aureus
• Fever, lymphangitis (“streaks”), regional lymphadenopathy
• Treatment: PO penicillin V or cephalexin × 5-10 d; IV if severe

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134.1.0.3 2️⃣ Cellulitis

• 皮下組織感染 — diffuse, poorly demarcated (vs erysipelas), warm, tender, erythematous
• 多 GAS or S. aureus
• Risks: lymphedema, skin breakdown (tinea pedis = portal entry), DM, obesity, IVDU

134.1.0.3.1 Treatment

嚴重度	治療
Mild, no MRSA risk	Cephalexin 500 mg QID OR Dicloxacillin × 5-7 d
MRSA risk (recurrent SSTI, IVDU, healthcare)	TMP-SMX 1-2 DS BID OR Doxycycline 100 mg BID OR Clindamycin
Hospitalized	IV cefazolin (MSSA) OR vancomycin (MRSA) OR linezolid alternative
Severe / sepsis	+ Pip-tazo / Cefepime if broader cover (Pseudomonas), + clindamycin if NF suspected

134.1.0.3.2 Duration

• 5-7 days uncomplicated (shorter if rapid response)
• 10-14 d if slower or moderate-severe

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134.1.0.4 3️⃣ Abscess (subcutaneous)

• 多 S. aureus (MSSA + MRSA)
• Treatment: incision + drainage (I&D) = MAINSTAY
• Antibiotic ADDED if:
  • Lesion > 2 cm
  • Multiple lesions
  • Surrounding cellulitis
  • Immunocompromised
  • Failed I&D alone
  • High-risk anatomic (face, hand, perineum)
  • Systemic signs
• Antibiotic of choice: TMP-SMX or Doxycycline (cover MRSA)

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134.1.0.5 4️⃣ Necrotizing Fasciitis (NF) — 致命急症

📖 中文概念說明：NF 是 SSTI 中唯一「每延遲一小時死亡率明顯上升」的急症。臨床判斷三步：(1) 是否 pain 不成比例於 skin findings (早期關鍵)；(2) LRINEC score ≥ 6 視為高度懷疑；(3) 不要等影像 (MRI / CT) 確認再 OR — 高度懷疑就直接 explore + debride。Clindamycin 是 NF empirical 中不可省的一支 (Eagle effect: 抑制 toxin 合成, 對 high-inoculum 感染特別重要), 國考經常考「為什麼 NF 要給 clindamycin」。Fournier’s gangrene (會陰 NF) 是 NF 的特殊解剖位置變型，多 polymicrobial + DM 病人。Vibrio NF (Type III) 在台灣沿海要警覺 — 海水或生海鮮接觸 + 肝病人 (cirrhosis、hemochromatosis) 出現急速 worsening cellulitis 就要往這想，doxy + ceftri + debride 立即啟動。

134.1.0.5.1 Types

Type	特徵	病原
Type I (polymicrobial)	DM, post-op, perineum (Fournier’s gangrene)	Mixed aerobic + anaerobic (Streptococcus + Enterobacteriaceae + Bacteroides)
Type II (monomicrobial)	Healthy adult, often extremity	GAS (S. pyogenes) ± S. aureus
Type III (Vibrio)	Coastal exposure, raw seafood, chronic liver	Vibrio vulnificus
Type IV (Clostridial)	Trauma, surgery	C. perfringens (gas gangrene)

134.1.0.5.2 Diagnosis

• Pain 不成比例 of skin findings (early!) — most reliable
• Late signs: skin necrosis, hemorrhagic bullae, crepitus, “dishwater” pus
• LRINEC score (Laboratory Risk Indicator for NF): CRP, WBC, Hb, Na, Cr, glucose; ≥ 6 = high risk

134.1.0.5.3 Treatment

• 緊急 OR debridement — 必須 6 hr 內
• Empirical antibiotic:
  • Vancomycin + Pip-tazo + Clindamycin
  • Vibrio suspect → add Doxycycline + Ceftriaxone
  • Clostridium suspect → High-dose penicillin G + Clindamycin (clinda inhibits toxin production)
• Repeat debridement q24-48 h until margins clean
• IVIG: consider for streptococcal TSS (IDSA 2014 guideline allows)
• Mortality 20-40% even with treatment

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