What are the likely causes and recommended evaluation and treatment for recurrent blisters on the inner thigh of a 9‑year‑old girl?

Recurrent Blisters on Inner Thigh in a 9-Year-Old Girl

The most likely diagnosis is recurrent genital herpes simplex virus (HSV) infection, and you should initiate oral antiviral therapy with acyclovir 400 mg three times daily for 5 days for acute episodes, with consideration of daily suppressive therapy if recurrences are frequent. 1

Primary Differential Diagnosis

The recurrent nature of blisters on the inner thigh in a 9-year-old strongly suggests:

• Herpes simplex virus (HSV): Most common cause of recurrent genital/perigenital blisters, typically HSV-2 but can be HSV-1 1, 2
• Friction or contact dermatitis: Consider if related to activities, clothing, or irritants 1
• Bullous impetigo: Bacterial infection causing blisters, but typically does not follow a recurrent pattern 1
• Autoimmune blistering disease: Rare in children but includes bullous pemphigoid or linear IgA disease 1

Immediate Clinical Assessment

Examine for these specific features to narrow the diagnosis:

• Vesicle/blister characteristics: HSV presents as grouped vesicles on an erythematous base that rupture to form shallow ulcers 1, 3
• Distribution pattern: HSV commonly affects buttocks, thighs, and genital areas in children 2, 4
• Associated symptoms: Prodromal burning or tingling before blister appearance suggests HSV 1
• Lymphadenopathy: Tender inguinal nodes support HSV diagnosis 1, 4
• Mucous membrane involvement: Check oral cavity and genital mucosa—extensive involvement raises concern for severe conditions like SJS/TEN 1
• Systemic symptoms: Fever, malaise, or constitutional symptoms may indicate primary HSV or more serious conditions 1

Diagnostic Workup

For suspected HSV (most likely):

• Viral culture or PCR from fresh vesicle fluid—this is the gold standard 1, 3
• Direct fluorescent antibody testing if available 3
• Tzanck smear showing multinucleated giant cells (less sensitive but rapid) 1

If diagnosis uncertain or atypical features present:

• Skin biopsy with routine histology and direct immunofluorescence to exclude autoimmune blistering diseases 1
• Bacterial culture if secondary infection suspected 1, 5

Treatment Algorithm

For Confirmed or Highly Suspected HSV

Acute recurrent episodes:

• Acyclovir 400 mg orally three times daily for 5 days 1
• Alternative: Acyclovir 200 mg orally five times daily for 5 days 1
• Start treatment within 24 hours of lesion onset or during prodrome for maximum benefit 1

Suppressive therapy (if ≥6 recurrences per year):

• Acyclovir 400 mg orally twice daily continuously 1
• This reduces recurrence frequency by >75% 1
• Safety documented for up to 6 years of continuous use 1
• Reassess need annually, as recurrence frequency often decreases over time 1

Blister Management During Active Episodes

• Leave intact blisters alone—the blister roof serves as a natural biological dressing 1, 5, 6
• If drainage needed: Pierce at base with sterile needle (bevel up), apply gentle pressure with sterile gauze 1, 5
• Never deroof blisters—this increases infection risk and delays healing 1, 5, 6
• Apply bland emollient (50% white soft paraffin/50% liquid paraffin) to support barrier function 1, 5
• Cover with non-adherent dressing if needed 5, 6

Pain Management

• Acetaminophen or ibuprofen for discomfort 6
• Administer 20 minutes before wound care if needed 1

Infection Prevention

• Daily gentle cleansing with antimicrobial solution 1, 5
• Monitor for signs of secondary bacterial infection: increased erythema, purulent discharge, fever, worsening pain 1, 5, 6
• Obtain bacterial cultures before starting antibiotics if infection suspected 5, 6
• Apply topical antimicrobials only to clinically infected areas, not prophylactically 5, 6, 7
• Use systemic antibiotics if spreading cellulitis or systemic signs develop 1

Critical Counseling Points for Family

• HSV is a chronic, recurrent viral infection that cannot be cured but can be managed 1, 8
• Recurrences are common, especially in the first year, but typically decrease in frequency over time 1
• Asymptomatic viral shedding can occur between outbreaks 1
• Avoid contact sports and activities that may traumatize affected areas during active lesions 1
• In children, consider non-sexual transmission routes (autoinoculation from oral herpes, contact with infected family members) but also be alert to possibility of sexual abuse requiring appropriate evaluation 1, 2

Red Flags Requiring Urgent Dermatology Consultation

• Rapid progression of blisters within 24 hours 7
• Involvement of multiple mucosal sites (eyes, mouth, genitals) suggesting SJS/TEN 1, 7
• Blisters covering >10% body surface area 6
• Systemic symptoms with extensive skin involvement 1
• Positive Nikolsky sign (skin sloughing with minimal pressure) 1
• Failure to respond to appropriate antiviral therapy within 7-10 days 1

Common Pitfalls to Avoid

• Do not assume simple friction blisters if they recur in the same location—this pattern strongly suggests HSV 2
• Do not delay antiviral therapy waiting for culture results if clinical presentation is consistent with HSV 1
• Do not routinely deroof blisters—this is the most common error in blister management 1, 5, 6, 7
• Do not apply topical antivirals—they are substantially less effective than oral therapy and their use is discouraged 1
• Do not overlook the possibility of abuse in pediatric genital HSV, though non-sexual transmission is also common 1, 2