What is the diagnosis and treatment for sudden onset redness and blisters on one leg with no swelling?

Diagnosis and Management of Sudden Onset Unilateral Leg Redness and Blisters Without Swelling

The most likely diagnosis is herpes zoster (shingles), and treatment should be initiated immediately with antiviral therapy if the patient presents within 72 hours of symptom onset. The clinical presentation of sudden onset, unilateral distribution, redness with blisters, and notably the absence of swelling strongly suggests a vesiculobullous dermatosis rather than an infectious or inflammatory process affecting deeper tissues.

Key Diagnostic Features

Why This is NOT Cellulitis or Infection

• Cellulitis requires at least 2 of the following: local swelling/induration, erythema, tenderness/pain, warmth, or purulent discharge 1
• The absence of swelling (induration/tumor) is a critical distinguishing feature that argues strongly against bacterial cellulitis 1
• Cellulitis typically presents with edema and induration as prominent features, which are explicitly absent in this case 1

Clinical Pattern Suggesting Herpes Zoster

• Sudden onset with unilateral distribution is the hallmark of herpes zoster, which follows a dermatomal pattern 2
• Blistering (vesicular) eruption is characteristic of viral infections, particularly varicella-zoster virus reactivation
• The combination of redness and blisters without significant swelling points away from bacterial soft tissue infection and toward a vesiculobullous process

Differential Diagnoses to Consider

If Blisters Are Tense and Widespread

• Bullous pemphigoid should be considered if blisters are large, tense, and the patient is elderly, though this typically has a more gradual onset 1
• However, bullous pemphigoid usually presents bilaterally and with more chronic evolution, making it less likely given the sudden onset 1

Other Conditions to Exclude

• Contact dermatitis (allergic or irritant) can cause blistering but typically has a clear exposure history and may be bilateral 2
• Stasis dermatitis presents with chronic bilateral leg changes, edema, and pigmentation changes—not consistent with sudden unilateral presentation 3, 4, 2
• Acute Charcot neuro-osteoarthropathy in diabetic patients presents with unilateral redness, warmth, and swelling, but the absence of swelling and presence of blisters makes this unlikely 1, 5

Immediate Management Approach

If Herpes Zoster is Suspected

• Initiate antiviral therapy immediately (valacyclovir 1000 mg three times daily or famciclovir 500 mg three times daily for 7 days) if within 72 hours of rash onset
• Pain management is essential, as post-herpetic neuralgia is a significant complication
• Consider ophthalmology referral if lesions involve the nasal tip (Hutchinson's sign), indicating potential ocular involvement

If Diagnosis is Uncertain

• Obtain a detailed history focusing on:

  • Prodromal symptoms (pain, tingling, burning in the affected area before rash appeared)
  • Dermatomal distribution of the rash
  • Immunosuppression status
  • Recent trauma or exposure history 2

• Physical examination should document:

  • Precise distribution of lesions (dermatomal vs. non-dermatomal)
  • Character of blisters (grouped vesicles on erythematous base suggests herpes zoster)
  • Presence or absence of lymphangitis or lymphadenopathy
  • Vascular status (pulses, capillary refill) 5

Critical Red Flags Requiring Urgent Evaluation

• Systemic symptoms (fever, tachycardia, hypotension) would suggest severe infection requiring immediate hospitalization 1
• Rapidly progressive symptoms or development of skin necrosis should raise concern for necrotizing fasciitis, though the absence of severe pain and swelling makes this unlikely 6
• Diabetic patients require special attention as neuropathy may mask pain, and infection risk is higher 1

Common Pitfalls to Avoid

• Do not automatically prescribe antibiotics for unilateral leg redness without confirming infection criteria—this leads to unnecessary antibiotic use 3, 4, 6
• Do not dismiss the absence of swelling—this is a key clinical clue that argues against typical bacterial soft tissue infection 1
• Do not delay antiviral therapy if herpes zoster is suspected, as efficacy decreases significantly after 72 hours
• In diabetic patients, do not assume all red legs are infected—consider Charcot neuro-osteoarthropathy, but the presence of blisters and absence of swelling makes this less likely 1, 5

When to Consider Alternative Diagnoses

• If bilateral involvement develops, reconsider systemic causes or bilateral contact dermatitis 2
• If swelling develops subsequently, reassess for secondary bacterial infection or evolving cellulitis 1
• If no improvement within 48-72 hours of appropriate antiviral therapy, consider alternative diagnoses including autoimmune blistering disorders 1