What is the most likely cause of a rash under a menstrual pad and how should it be treated?

Menstrual Pad Rash: Cause and Treatment

A rash under a menstrual pad is most commonly caused by irritant contact dermatitis from moisture, friction, and occlusion, though allergic contact dermatitis to pad components (particularly acrylates, colophony/rosin, or fragrances) should be considered if the rash recurs with specific products.

Most Likely Causes

Irritant Contact Dermatitis (Primary Cause)

• Moisture and occlusion from prolonged pad contact create an environment that disrupts the skin barrier, leading to erythema, maceration, and inflammation 1
• Friction and pressure from the pad against vulvar skin exacerbates irritation, particularly during movement 1
• This mechanism mirrors other occlusive dermatoses and does not require specific allergen exposure 1

Allergic Contact Dermatitis (Secondary Consideration)

• Acrylates in absorbent gel materials can cause severe allergic reactions if incompletely polymerized; patients may show dramatic patch test positivity 2, 3
• Colophony (rosin) in adhesive components has been documented to cause marked sensitivity (3+ reactions on patch testing) 4
• Fragrances including α-isomethyl ionone, benzyl salicylate, hexyl cinnamaldehyde, and heliotropine can exceed acceptable exposure levels and induce sensitization, particularly in scented products 5
• Allergic reactions typically recur with re-exposure to the same product brand 4, 3

Infectious Causes (Must Rule Out)

• Candidiasis presents with thick white discharge, severe vulvar itching, and vaginal pH ≤4.5; diagnosis requires KOH preparation showing yeast or pseudohyphae 1, 6, 7
• Bacterial vaginosis causes homogeneous white discharge, fishy odor (positive whiff test), clue cells, and vaginal pH >4.5 6, 7, 8
• Trichomoniasis produces frothy green-yellow discharge with marked itching and pH >4.5; wet mount shows motile trichomonads 6, 7, 8

Diagnostic Algorithm

Step 1: History

• Temporal relationship: Does the rash appear only during pad use and resolve when pads are discontinued? 4, 3
• Product specificity: Does switching pad brands change symptoms? Recurrence with the same brand suggests allergy 4, 3
• Associated symptoms: Vaginal discharge, odor, or itching suggests infection rather than contact dermatitis 6, 7

Step 2: Physical Examination

• Distribution: Rash confined to areas of pad contact (labia, perineum) suggests contact dermatitis; vaginal involvement suggests infection 1
• Morphology: Erythema with maceration indicates irritant dermatitis; vesicles or sharp demarcation suggest allergic contact dermatitis 1

Step 3: Office Testing (If Discharge Present)

• Vaginal pH: >4.5 indicates bacterial vaginosis or trichomoniasis; ≤4.5 suggests candidiasis 6, 7, 8
• Whiff test: Fishy odor with 10% KOH confirms bacterial vaginosis or trichomoniasis 6, 7, 8
• Wet mount: Identifies clue cells (bacterial vaginosis), motile trichomonads (trichomoniasis), or yeast/pseudohyphae (candidiasis) 6, 7

Step 4: Patch Testing (If Allergic Dermatitis Suspected)

• Perform patch testing with the actual pad (moistened inner and outer surfaces) and standard allergen series including acrylates, colophony, and fragrance mix 4, 2, 3
• Test only if rash recurs with specific products or persists despite conservative management 4, 3

Treatment Approach

First-Line: Conservative Management for Irritant Dermatitis

Immediate measures:

• Discontinue the current pad and switch to unscented, hypoallergenic products without gel absorbents 5
• Avoid moisture and occlusion: Change pads every 3-4 hours; avoid overnight use of thick pads 1
• Gentle cleansing: Use mild soap and warm water; avoid hot showers, excessive soap, and douching 1, 6, 8

Topical therapy:

• Barrier protection: Apply petrolatum to affected areas to prevent moisture evaporation and provide a protective film 1
• Mild topical corticosteroid: Hydrocortisone 1% cream applied to affected areas 3-4 times daily for erythema and inflammation 9
  • Clean area with mild soap, rinse thoroughly, and pat dry before application 9
  • Avoid prolonged use (>7 days) without dermatology supervision to prevent skin atrophy 1

Avoid:

• Greasy occlusive creams that may worsen folliculitis 1
• Alcohol-containing lotions or gels that cause further drying 1
• Manipulation or scratching due to infection risk 1

Treatment for Confirmed Infections

Candidiasis (if pH ≤4.5, yeast on KOH prep):

• Clotrimazole 1% cream 5g intravaginally for 7-14 days (treats vaginal infection and relieves external itching) 1, 10
• Alternative: Fluconazole 150mg oral single dose 1, 7
• Topical azoles achieve 80-90% symptom relief and negative cultures 1

Bacterial vaginosis (if ≥3 Amsel criteria present):

• Metronidazole 500mg orally twice daily for 7 days (95% cure rate) 6, 7, 8
• Avoid alcohol during and for 24 hours after treatment 6
• Partner treatment is not recommended 6, 7, 8

Trichomoniasis (if motile trichomonads on wet mount or positive NAAT):

• Metronidazole 2g oral single dose (90-95% cure rate) 6, 7, 8
• Sexual partner must be treated simultaneously to prevent reinfection 6, 7, 8

Escalation for Persistent or Severe Cases

If no improvement after 7 days of conservative management:

• Refer to dermatology for patch testing to identify specific allergens 4, 2, 3
• Consider short-term topical steroid (prednicarbate cream 0.02%) under dermatology supervision for severe inflammation 1

For pruritus grade 2-3:

• Oral H1-antihistamines (cetirizine, loratadine, or fexofenadine) for symptom relief 1

Common Pitfalls

• Do not assume all pad rashes are simple irritation: Recurrence with the same product brand strongly suggests allergic contact dermatitis requiring patch testing 4, 3
• Do not overlook infection: Always measure vaginal pH and perform wet mount if discharge is present; treating contact dermatitis alone will fail if infection coexists 6, 7
• Do not use topical steroids for prolonged periods without supervision: Risk of perioral dermatitis and skin atrophy 1
• Do not recommend OTC antifungals without confirming candidiasis: Inappropriate use delays diagnosis of other etiologies and worsens outcomes 1
• Fragrance allergens in scented pads can exceed safe exposure levels: Women should be counseled to avoid scented products if sensitization is suspected 5