Skin Infections: Cellulitis, Necrotizing Fasciitis, and Herpes Zoster

Skin and soft tissue infections (SSTIs) represent a spectrum of conditions ranging from superficial cellulitis to life-threatening necrotizing fasciitis. These infections are among the most common dermatologic presentations in emergency departments and primary care settings.

Cellulitis is a bacterial infection of the dermis and subcutaneous tissue characterized by spreading erythema, warmth, and tenderness. The most common pathogens are Streptococcus pyogenes (Group A Strep) and Staphylococcus aureus, including methicillin-resistant strains (MRSA). [HIGH_YIELD] Cellulitis affects approximately 14.5 million people annually in the United States, with healthcare costs exceeding $3.7 billion.

Necrotizing fasciitis is a rapidly spreading deep soft tissue infection involving the fascial planes with secondary necrosis of subcutaneous tissue. [KEY_CONCEPT] This surgical emergency has two main types:

• Type I (polymicrobial): Mixed aerobic and anaerobic bacteria
• Type II (monomicrobial): Primarily Group A Streptococcus

The mortality rate ranges from 20-40% despite aggressive treatment.

Herpes zoster (shingles) is a viral reactivation of varicella-zoster virus (VZV) from dorsal root ganglia, causing a characteristic vesicular rash in a dermatomal distribution. [CLINICAL_PEARL] The lifetime incidence is approximately 30%, with risk increasing significantly after age 50 and in immunocompromised patients.

Cellulitis Clinical Features:

• Acute onset of localized erythema, warmth, swelling, and tenderness
• Poorly demarcated borders (unlike erysipelas)
• Common sites: lower extremities, arms, face
• Associated symptoms: fever, chills, malaise
• [CLINICAL_PEARL] Lymphangitic streaking may be present, appearing as red linear streaks extending proximally

Red Flag Signs for Severe Cellulitis:

• Rapid progression
• Systemic toxicity (hypotension, altered mental status)
• Hemorrhagic bullae
• Skin necrosis or purple discoloration
• Crepitus on palpation

Necrotizing Fasciitis Clinical Features: [HIGH_YIELD] The classic triad includes:

1. Severe pain out of proportion to physical findings
2. Rapid progression (hours to days)
3. Systemic toxicity with fever and altered mental status

Early signs (within 24-48 hours):

• Intense pain
• Erythema and swelling
• Fever >38.5°C (101.3°F)

Late signs (48-72 hours):

• "Dishwater" gray discharge
• Skin necrosis and bullae
• Anesthesia over affected area (nerve death)
• Crepitus (gas-forming organisms)

Herpes Zoster Clinical Features:

• Prodromal phase (1-3 days): burning, tingling, or shooting pain in dermatomal distribution
• Active phase: grouped vesicles on erythematous base following dermatome
• Crusting phase (7-10 days): vesicles rupture and form crusts
• [KEY_CONCEPT] Hutchinson's sign: vesicles on tip of nose indicating nasociliary nerve involvement and increased risk of ocular complications

Zoster Complications:

• Postherpetic neuralgia (most common, especially >60 years)
• Ophthalmic zoster (CN V involvement)
• Ramsay Hunt syndrome (CN VII involvement with hearing loss)

Cellulitis Diagnosis: [HIGH_YIELD] Cellulitis is primarily a clinical diagnosis based on characteristic signs and symptoms. Laboratory tests are generally not required for uncomplicated cases.

Diagnostic Criteria Checklist:

• ✓ Acute onset of erythema
• ✓ Local warmth
• ✓ Swelling/edema
• ✓ Tenderness to palpation
• ✓ Fever (may be absent)

Laboratory Studies (when indicated):

• Blood cultures: Only if systemic signs present
• CBC with differential: Leukocytosis suggests severe infection
• CRP/ESR: Elevated inflammatory markers
• Wound culture: Only if purulent drainage present

LRINEC Score for Necrotizing Fasciitis: [KEY_CONCEPT] Laboratory Risk Indicator for Necrotizing Fasciitis:

Score Interpretation:

• ≤5: Low risk (<50% probability)
• 6-7: Intermediate risk
• ≥8: High risk (>75% probability)

Herpes Zoster Diagnosis:

• Clinical diagnosis in typical presentations
• Tzanck smear: Multinucleated giant cells (not specific for VZV)
• PCR testing: Gold standard for atypical cases
• Direct fluorescent antibody: Rapid but less sensitive

Differential Diagnosis Table:

Cellulitis Treatment Algorithm:

Cellulitis Suspected ↓ Assess Severity ├─ Mild (outpatient) ──→ Oral antibiotics 7-10 days │ • Cephalexin 500mg QID OR │ • Clindamycin 300mg TID │ ├─ Moderate-Severe ────→ IV antibiotics │ • Ceftriaxone 1g daily OR │ • Vancomycin 15-20mg/kg q8-12h │ └─ MRSA Risk Factors ──→ MRSA Coverage • Vancomycin OR • Linezolid OR • Daptomycin

[HIGH_YIELD] MRSA Risk Factors:

• Previous MRSA infection
• Recent hospitalization
• Chronic kidney disease
• Injection drug use
• Purulent drainage

Necrotizing Fasciitis Emergency Management:

Suspected Necrotizing Fasciitis ↓ IMMEDIATE Actions (within 1 hour): ├─ IV fluid resuscitation ├─ Broad-spectrum antibiotics ├─ Pain management ├─ Surgical consultation (STAT) └─ ICU consultation ↓ Empirical Antibiotic Regimen: • Vancomycin 15-20mg/kg q8-12h PLUS • Piperacillin-tazobactam 4.5g q6h PLUS • Clindamycin 600-900mg q8h ↓ Surgical Debridement (within 6 hours) ├─ Aggressive debridement ├─ Multiple procedures often needed └─ Amputation if extensive

[CLINICAL_PEARL] Clindamycin is essential in necrotizing fasciitis treatment as it inhibits toxin production by Group A Streptococcus.

Herpes Zoster Treatment Guidelines:

Antiviral Therapy (most effective within 72 hours):

• Acyclovir: 800mg 5x daily × 7 days
• Valacyclovir: 1000mg TID × 7 days (preferred)
• Famciclovir: 500mg TID × 7 days

Pain Management Algorithm:

Acute Zoster Pain ↓ First-line: ├─ Gabapentin 300mg TID (titrate up) ├─ Pregabalin 150mg BID └─ Tricyclic antidepressants ↓ Additive therapy: ├─ Topical lidocaine 5% ├─ Capsaicin 0.075% └─ Short-term opioids if severe ↓ Postherpetic Neuralgia (>90 days): • Continue neuropathic agents • Consider nerve blocks • Capsaicin 8% patch

Prevention Strategies: [KEY_CONCEPT] Zoster Vaccination (Shingrix):

• Recommended for adults ≥50 years
• Two doses, 2-6 months apart

• 90% effective in preventing zoster

• Recommended even with prior zoster history

Cellulitis Complications:

Local Complications:

• Abscess formation (requires drainage)
• Lymphangitis with lymphadenopathy
• Chronic lymphedema (especially lower extremity cellulitis)
• Recurrent cellulitis (up to 30% recurrence rate)

Systemic Complications:

• Sepsis and septic shock
• Necrotizing soft tissue infection
• Acute kidney injury (especially with vancomycin use)

[CLINICAL_PEARL] Recurrent cellulitis prevention includes addressing predisposing factors like tinea pedis, chronic edema, and maintaining proper skin hygiene.

Necrotizing Fasciitis Complications:

Early Complications (24-48 hours):

• Septic shock (most common cause of death)
• Acute respiratory distress syndrome (ARDS)
• Acute kidney injury
• Disseminated intravascular coagulation (DIC)

Late Complications:

• Amputation (15-25% of survivors)
• Chronic pain and disability
• Psychological trauma and PTSD
• Reconstructive surgery needs

[HIGH_YIELD] Mortality Predictors:

• Age >60 years
• Delay in surgical intervention >24 hours
• Shock on presentation
• LRINEC score ≥8

Herpes Zoster Complications:

Acute Complications:

• Secondary bacterial infection of lesions
• Ophthalmic zoster (10-15% of facial zoster)
  • Keratitis, uveitis, glaucoma
  • Permanent vision loss risk
• Motor weakness (1-5% of cases)
• Meningoencephalitis (rare but serious)

Chronic Complications:

• Postherpetic neuralgia (PHN) - most common
  • Affects 10-15% of patients
  • Risk increases with age (>50% in patients >80)
  • Can persist for months to years
• Scarring and hypopigmentation
• Cranial nerve palsies

Monitoring Parameters:

Cellulitis Prognosis:

• Uncomplicated cellulitis: Excellent prognosis with appropriate antibiotic therapy
• Response to treatment: Usually evident within 48-72 hours
• Recurrence rates: 8-20% within 3 years, higher with predisposing factors

Prevention Strategies:

• Proper wound care and hygiene
• Treatment of predisposing conditions:
  • Tinea pedis (athlete's foot)
  • Chronic venous insufficiency
  • Lymphedema
  • Diabetes mellitus
• Skin barrier maintenance with moisturizers
• Prompt treatment of minor skin injuries

[CLINICAL_PEARL] Antibiotic prophylaxis may be considered for patients with ≥3 episodes of cellulitis in the same location within 12 months.

Necrotizing Fasciitis Prognosis:

• Overall mortality: 20-40% despite optimal care
• Early recognition critical: Delays >24 hours significantly worsen outcomes
• Functional outcomes: Many survivors require rehabilitation and reconstructive surgery

Prognostic Factors:

Good Prognosis:

• Early recognition and treatment (<12 hours)
• Young age
• Type II (monomicrobial) infection
• Extremity involvement (vs. trunk)

Poor Prognosis:

• Advanced age >65 years
• Comorbid conditions (diabetes, immunosuppression)
• Shock on presentation
• Truncal involvement

Herpes Zoster Prognosis:

• Acute phase: Typically resolves in 2-4 weeks
• Age-related outcomes: Worse prognosis with increasing age
• Immunocompromised patients: Risk of dissemination and prolonged course

Postherpetic Neuralgia Risk Factors:

• Age >50 years (strongest predictor)
• Severe acute pain during rash phase
• Extensive rash
• Immunocompromised state
• Ophthalmic involvement

Prevention Strategies:

[HIGH_YIELD] Zoster Vaccination (Shingrix):

• Primary prevention: Adults ≥50 years (regardless of prior varicella history)
• Secondary prevention: Even after prior zoster episode
• Efficacy: >90% reduction in zoster incidence
• Duration: Protection maintained >4 years

Contraindications to Vaccination:

• Pregnancy
• Severe immunodeficiency
• Active, untreated tuberculosis

Special Populations:

Immunocompromised Patients:

• Higher risk for severe disease and complications
• Antiviral therapy recommended regardless of timing
• Extended duration of treatment may be needed
• Hospital monitoring often required

Long-term Follow-up:

• PHN management: Requires multidisciplinary approach
• Psychological support: Important for quality of life
• Vaccination counseling: For eligible patients post-recovery