What is the differential diagnosis for a rash on the hands and feet?

Differential Diagnosis for Rash on Hands and Feet

The differential diagnosis for rash involving the hands and feet must immediately prioritize life-threatening tickborne rickettsial diseases (Rocky Mountain Spotted Fever, ehrlichiosis) if accompanied by fever, followed by consideration of infectious etiologies, inflammatory dermatoses, and contact dermatitis based on morphology and distribution patterns.

Immediate Life-Threatening Considerations

Tickborne Rickettsial Diseases (RMSF and HME)

If fever is present with rash on hands/feet, initiate doxycycline 100 mg twice daily immediately without waiting for laboratory confirmation. 1, 2

• Rocky Mountain Spotted Fever presents with small (1-5 mm) blanching pink macules on ankles and wrists appearing 2-4 days after fever onset, progressing to maculopapular with central petechiae that spreads to palms and soles 1, 2
• RMSF carries a 5-10% case-fatality rate, with mortality risk increasing when rash is absent or delayed 2
• Less than 50% of patients have rash in the first 3 days, and up to 20% never develop rash 1, 2
• Human Monocytic Ehrlichiosis causes rash in only 30% of adults, appearing later (median 5 days) and rarely involving palms/soles 1, 2
• HME carries a 3% case-fatality rate 2

Critical red flags requiring immediate doxycycline: fever + rash + headache + tick exposure OR thrombocytopenia/hyponatremia 2

Other Serious Infectious Causes

Palmoplantar involvement is NOT pathognomonic and occurs with multiple serious infections: 1

• Neisseria meningitidis (meningococcemia)
• Treponema pallidum (secondary syphilis)
• Streptobacillus moniliformis (rat-bite fever)
• Infective endocarditis
• Certain enteroviruses 1

Common Inflammatory Dermatoses

Contact Dermatitis (Allergic or Irritant)

• Foot involvement occurs in 5.2% of patients undergoing patch testing, with 70% having at least one positive reaction 3
• Most common sensitizers: rubber-vulcanizing agents, metals, and topical medications 3
• Dorsal foot involvement is statistically more common in allergic contact dermatitis 3
• Hand involvement results from chronic cumulative irritant exposure to detergents, organic solvents, soaps, or "dry" irritants like powders 1
• Occupational dermatitis accounts for 70% of occupational skin disease with hand involvement predominating 1

Psoriasis

• Palmoplantar psoriasis presents with erythematous inflammatory plaques with silvery scale 1
• Superficial exfoliation of palms and soles may occur in erythrodermic psoriasis 1
• Differential from chronic allergic contact dermatitis is difficult, especially with isolated palmoplantar involvement 4
• Psoriasis typically shows less pruritus and lacks eczematous changes like oozing/crusting compared to contact dermatitis 1

Atopic Dermatitis

• Palmoplantar vesicles occur in infants with scabies infestation, an important differential 1
• Severe atopic dermatitis may involve hands/feet but typically shows eczematous changes with oozing and crusting 1
• Contact dermatitis must be excluded through patch testing, particularly in children with hand eczema 1

Dyshidrotic Eczema

• Recurrent vesicular eruption affecting palms and/or soles 5
• Vesicles appear like "tapioca pudding" on physical examination 5
• More common in young adults, affecting men and women equally 5
• Highly pruritic with sudden onset 5

Infectious Causes (Non-Life-Threatening)

Dermatophyte Infections

• Widespread tinea corporis presents with annular papulosquamous lesions without eczematous change 1
• Diagnosis confirmed by skin scraping for microscopy and culture 1
• Important differential for foot dermatitis alongside contact dermatitis, atopic dermatitis, and psoriasis 3

Scabies

• Inguinal, axillary, and genital papules with palmoplantar vesicles and burrows in infants 1
• Must be excluded in palmoplantar dermatoses 6

Viral Exanthems

• Enteroviral infections are the most common cause of maculopapular rashes, typically sparing palms, soles, face, and scalp 2, 7
• Human herpesvirus 6 (roseola) presents with macular rash following high fever 2, 7
• Parvovirus B19 causes "slapped cheek" appearance with possible truncal involvement 2

Diagnostic Algorithm

Step 1: Assess for Fever and Systemic Symptoms

If fever + rash on hands/feet is present: 1, 2

• Obtain complete blood count (looking for thrombocytopenia, leukopenia) immediately
• Obtain comprehensive metabolic panel (looking for hyponatremia, elevated transaminases) immediately
• Obtain acute serology for R. rickettsii, E. chaffeensis, A. phagocytophilum
• Start doxycycline 100 mg twice daily immediately—do NOT wait for laboratory confirmation
• Expect clinical improvement within 24-48 hours

Step 2: Characterize Rash Morphology

Vesicular pattern: 5

• Consider dyshidrotic eczema (tapioca-like vesicles)
• Consider scabies (burrows, specific distribution)
• Rule out bullous disorders

Maculopapular pattern: 1, 2

• If palms/soles involved: consider RMSF, secondary syphilis, drug reaction
• If palms/soles spared: consider viral exanthem, drug eruption

Erythematous plaques with scale: 1, 4

• Consider psoriasis vs. chronic contact dermatitis
• Psoriasis shows less pruritus, lacks oozing/crusting
• Consider "eczema in psoriatico" (overlap syndrome)

Step 3: Obtain Exposure History

Occupational/recreational exposures: 1, 3

• Detergents, solvents, rubber products (contact dermatitis)
• Camping, tick exposure in summer months (RMSF/ehrlichiosis)

Medication history: 2, 7

• Any new medications within 2-3 weeks (drug eruption)
• Ampicillin/amoxicillin with EBV causes maculopapular rash

Step 4: Perform Targeted Testing

Patch testing indicated for: 1, 3

• Chronic hand/foot eczema unresponsive to treatment
• Atypical or localized distribution suggesting contact dermatitis
• Children with hand and eyelid eczema

Skin scraping/culture indicated for: 1

• Annular lesions suggesting tinea

Skin biopsy indicated for: 4

• Differentiation of palmoplantar psoriasis from chronic contact dermatitis when clinical diagnosis unclear
• Suspected cutaneous T-cell lymphoma (look for atypical lymphocytes)

Critical Pitfalls to Avoid

• DO NOT exclude RMSF based on absence of tick bite history—40% of patients do not report tick exposure 2
• DO NOT wait for serologic confirmation before starting doxycycline if RMSF is suspected—IgM/IgG are not detectable before the second week 8
• DO NOT assume palmoplantar rash is benign—multiple life-threatening infections present this way 1
• DO NOT rely solely on clinical features to distinguish allergic from irritant contact dermatitis—patch testing is essential 1
• DO NOT overlook occupational factors—obtain detailed work history and consider workplace visit 1