What is the diagnosis and treatment for an elderly patient with a chronic itchy rash on the right side of the abdomen to the back?

Chronic Unilateral Itchy Rash in Elderly Patient: Diagnosis and Management

Most Likely Diagnosis: Herpes Zoster (Shingles)

A one-month itchy rash localized to the right side of the abdomen extending to the back in an elderly patient is herpes zoster until proven otherwise, and you must examine the skin carefully for any vesicles, crusted lesions, or post-herpetic changes. 1

Why Herpes Zoster is the Primary Concern:

• Unilateral dermatomal distribution from abdomen to back strongly suggests a thoracic dermatome involvement, which is classic for herpes zoster 1
• The one-month duration is consistent with healing or post-herpetic phase, where vesicles may have already crusted over or resolved, leaving only erythema and persistent itch 1
• Elderly patients are at highest risk for herpes zoster due to age-related decline in cell-mediated immunity 2, 3
• Even without obvious vesicles now, early presentations may show only erythema, and you may be seeing the aftermath 1

Critical Examination Points:

Look specifically for:

• Any grouped vesicles on an erythematous base, even if subtle 1
• Crusted lesions or healing erosions in the distribution 1
• Scarring or post-inflammatory hyperpigmentation in a dermatomal pattern 1
• Excoriations from scratching that may obscure the primary lesion 4

Differential Diagnosis: Asteatotic Eczema

While asteatotic eczema (xerotic eczema) is the most common cause of pruritus in elderly patients, it typically presents bilaterally, though asymmetric presentations can occur 4, 1. The strictly unilateral distribution makes this less likely but not impossible.

Immediate Management Algorithm

Step 1: Topical Therapy (Start Immediately)

Apply high-lipid content emollients at least twice daily to all affected areas plus 1% hydrocortisone cream 3-4 times daily for 2 weeks to treat potential asteatotic eczema while investigating for herpes zoster 4, 1, 5

• Elderly skin has severely impaired barrier function and increased transepidermal water loss, making emollients essential 6, 1
• Hydrocortisone will help whether this is post-herpetic inflammation or eczematous changes 4, 5
• Avoid hot water bathing and harsh soaps, as these worsen xerosis 6, 1

Step 2: Systemic Therapy for Itch Control

Add a non-sedating antihistamine: fexofenadine 180 mg once daily OR loratadine 10 mg once daily for symptomatic relief 4, 6, 1

CRITICAL PITFALL: Never prescribe sedating antihistamines (diphenhydramine, hydroxyzine, chlorpheniramine) in elderly patients due to increased risk of falls, confusion, urinary retention, and potential contribution to dementia 4, 6, 1

Step 3: Reassessment at 2 Weeks

If no improvement after 2 weeks of emollients and topical steroids 4, 1:

• Escalate to clobetasone butyrate (more potent topical steroid) for persistent areas 1, 7
• Consider gabapentin 100-300 mg at bedtime if pruritus persists, as it has specific efficacy for elderly skin pruritus and post-herpetic neuralgia 4, 6
• Topical menthol preparations can provide additional relief through cooling effects 1, 7

Step 4: Investigation for Underlying Causes

Given the one-month duration and elderly age, investigate for:

• Herpes zoster serology if diagnosis remains uncertain 1
• Complete blood count to exclude hematologic malignancy or polycythemia vera (paraneoplastic pruritus) 6, 8
• Hepatic function tests (especially if any elevation in alkaline phosphatase or GGT) to exclude cholestatic disease 6, 8, 9
• Renal function tests to exclude uremic pruritus 8, 9, 10
• Thyroid function tests to exclude thyroid disease 8, 9
• Fasting glucose to exclude diabetes 8, 10

Heightened concern for underlying malignancy is warranted in patients over 60 years with diffuse itch less than 12 months duration 8

Step 5: Referral Criteria

Refer to dermatology if:

• Diagnostic uncertainty exists regarding zoster versus other etiologies 4, 1
• No improvement after 2-4 weeks of first-line therapy 4, 6, 1
• Skin biopsy is needed to exclude bullous pemphigoid, which can present with pruritus alone in elderly patients before blisters appear 4
• Patient is distressed by symptoms despite primary care management 4

Special Consideration: Bullous Pemphigoid

Pruritus alone can very rarely be the presenting feature of bullous pemphigoid, particularly in the elderly, and may precede blistering by weeks to months 4. If standard therapy fails, maintain high suspicion and consider:

• Skin biopsy with direct immunofluorescence 4
• Indirect immunofluorescence testing 4

Treatments to Absolutely Avoid

• Sedating antihistamines (Strength of recommendation C) 4, 6, 1
• Crotamiton cream (ineffective, Strength of recommendation B) 6, 7
• Calamine lotion for this presentation 6, 7
• Topical capsaicin for generalized or extensive pruritus 6

Key Clinical Pearls

• The unilateral dermatomal distribution is the critical diagnostic clue that distinguishes this from typical elderly xerosis 1
• Post-herpetic itch can persist for months after the acute vesicular phase has resolved 1
• Elderly patients often have multifactorial pruritus due to xerosis, immunosenescence, neuronal changes, and comorbidities 2, 3, 10
• Polypharmacy is common in elderly patients and medications (especially opioids and calcium channel blockers) can cause or exacerbate pruritus 3, 10