What are the differential diagnoses for a unilateral extremity rash in a 74-year-old male?

Differential Diagnoses for Unilateral Extremity Rash in a 74-Year-Old Male

The most critical diagnosis to exclude immediately is herpes zoster (shingles), which classically presents as a unilateral dermatomal rash and is particularly common in elderly patients, though infectious causes like Rocky Mountain Spotted Fever and vascular/inflammatory conditions must also be considered based on rash morphology and distribution.

Primary Differential Diagnoses by Rash Morphology

Vesiculobullous Pattern

• Herpes Zoster: The most common cause of unilateral dermatomal rash in elderly patients, presenting with grouped vesicles on an erythematous base following a single dermatome 1
• Contact Dermatitis: Can present unilaterally if exposure was localized to one extremity, though typically corresponds to area of contact rather than dermatomal distribution 2

Petechial/Purpuric Pattern

• Unilateral Linear Capillaritis: A rare variant of pigmented purpuric dermatoses characterized by linear or pseudo-dermatomal eruption on a single extremity, presenting as asymptomatic macules and scaly papules 3
• Rocky Mountain Spotted Fever: Although classically bilateral, early presentation may appear asymmetric with small blanching pink macules on ankles, wrists, or forearms that evolve to maculopapular rash with central petechiae by day 5-6 2, 4
• Vasculitis (including cryoglobulinemic vasculitis): Can present with purpuric rash, particularly in patients with underlying rheumatologic conditions, though typically bilateral 2, 4, 5

Maculopapular/Erythematous Pattern

• Adult-Onset Still's Disease: Presents with evanescent salmon-pink maculopapular eruption, predominantly on proximal limbs and trunk, though typically bilateral rather than unilateral 2
• Cutaneous T-Cell Lymphoma: Can present with erythematous patches and plaques, may be unilateral or asymmetric in early stages 2
• Psoriasis (localized): Can present with erythematous plaques with silvery scale, though unilateral presentation is uncommon 2

Critical Clinical Features to Assess

Distribution Pattern

• Dermatomal distribution: Strongly suggests herpes zoster, particularly if follows a single nerve root distribution 1
• Linear distribution: Consider unilateral linear capillaritis or contact dermatitis 3
• Distal extremity (ankle/wrist): Consider Rocky Mountain Spotted Fever if accompanied by fever 2

Associated Symptoms

• Fever: If present with rash, immediately consider Rocky Mountain Spotted Fever (fever typically >38.5°C), Adult-Onset Still's Disease (quotidian fever pattern), or infectious causes 2
• Pain or burning preceding rash: Highly suggestive of herpes zoster 1
• Pruritus: May occur with contact dermatitis, though Adult-Onset Still's Disease rash can be mildly pruritic 2

Timing and Progression

• Rapid onset (days): Consider Rocky Mountain Spotted Fever, herpes zoster, or contact dermatitis 2, 1
• Gradual onset (weeks to months): Consider unilateral linear capillaritis, cutaneous T-cell lymphoma, or localized psoriasis 2, 3

Diagnostic Approach

Immediate Evaluation

• Complete blood count with differential: Assess for thrombocytopenia (suggests Rocky Mountain Spotted Fever or vasculitis), leukocytosis (suggests Adult-Onset Still's Disease or infection) 2, 4
• Inflammatory markers (ESR, CRP): Elevated in Adult-Onset Still's Disease, vasculitis, and Rocky Mountain Spotted Fever 2
• Electrolytes including sodium: Hyponatremia common in Rocky Mountain Spotted Fever and Adult-Onset Still's Disease 2

Specific Testing Based on Clinical Presentation

• If vesicular rash in dermatomal pattern: Direct fluorescent antibody or PCR for varicella-zoster virus 1
• If petechial/purpuric with fever: Immediate empiric doxycycline while awaiting rickettsial serology, as Rocky Mountain Spotted Fever has 5-10% mortality if untreated 2, 4
• If persistent purpuric rash: Skin biopsy to evaluate for vasculitis or pigmented purpuric dermatosis 5, 3
• If erythematous plaques: Skin biopsy with flow cytometry if concern for cutaneous T-cell lymphoma 2

Critical Pitfalls to Avoid

Do Not Wait for Classic Presentations

• Rocky Mountain Spotted Fever: Only a minority of patients present with the classic triad of fever, rash, and tick bite; rash may be absent in up to 20% of cases and typically appears 2-4 days after fever onset 2
• Herpes Zoster: Pain may precede rash by several days; do not exclude diagnosis based on absence of vesicles if patient presents early 1

Age-Specific Considerations

• In elderly patients (age 74): Herpes zoster is significantly more common and may present with atypical features; immunosenescence increases risk of severe disease 1
• Rash may be difficult to discern in darker-skinned individuals: Particularly for erythematous or petechial rashes 2

Treatment Urgency

• If Rocky Mountain Spotted Fever suspected: Initiate doxycycline immediately without waiting for confirmatory testing, as delay in treatment significantly increases mortality 2, 4
• If herpes zoster suspected in elderly patient: Early antiviral therapy (within 72 hours) reduces risk of postherpetic neuralgia 1