What is the cause of a red nodule on the lateral aspect of the thumb?

Red Nodule on Lateral Aspect of Thumb DIP

A red nodule on the lateral aspect of the thumb most likely represents either an infectious process (such as erysipeloid, cutaneous fungal infection, or bacterial abscess), an inflammatory condition (such as neutrophilic dermatosis of the hands or nodular fasciitis), or less commonly a neoplastic lesion—and requires biopsy with culture to establish the definitive diagnosis.

Immediate Diagnostic Approach

Obtain a detailed exposure history and perform incision/aspiration with culture and biopsy to differentiate between infectious, inflammatory, and neoplastic etiologies 1, 2.

Key Historical Features to Elicit:

• Occupational exposure: Contact with fish, marine animals, swine, or poultry suggests erysipeloid, which presents as a red maculopapular lesion 1-7 days after exposure, typically on fingers or hands 2, 1
• Animal contact: Cat scratches or bites raise concern for Bartonella infection, which develops as a papule or pustule 3-30 days after inoculation 2
• Trauma history: Even minor trauma can precede neutrophilic dermatosis of the hands (NDDH), which presents as violaceous papulonodules on dorsal hand surfaces 3
• Systemic symptoms: Fever, weight loss, or night sweats suggest systemic disease including cutaneous lymphoma 4

Primary Differential Diagnoses by Clinical Pattern

Infectious Etiologies (Most Common)

Erysipeloid presents as a red maculopapular lesion with centrifugal spread and central clearing, often with a blue ring and peripheral red halo creating a target appearance 2. Regional lymphangitis occurs in one-third of cases, but systemic symptoms are unusual 2. Treatment: Penicillin 500 mg four times daily or amoxicillin 500 mg three times daily for 7-10 days 2.

Cutaneous fungal infections begin as erythematous papules that may become pustular with central ulceration and elevated borders 1. Diagnosis requires skin scraping with KOH preparation, and treatment involves topical antifungals 1.

Cutaneous abscess presents as a painful, tender, fluctuant red nodule with surrounding erythema 2. S. aureus is present as a single pathogen in only 25% of cases; most are polymicrobial 2. Treatment requires incision, thorough evacuation of pus, and probing to break up loculations 2. Gram stain and culture are indicated if multiple lesions, extensive cellulitis, or systemic symptoms are present 2.

Inflammatory/Autoimmune Conditions

Neutrophilic dermatosis of the hands (NDDH) is a localized variant of Sweet's syndrome presenting with violaceous edematous plaques, ulcers with undermined borders, or necrotic pyoderma-like lesions on the dorsal hand 3. This condition is associated with malignancies (27% of cases, most commonly leukemia and lymphoma), myelodysplasia, inflammatory bowel disease, and medications 3. Histopathology shows dense dermal neutrophilic infiltrate with leukocytoclastic debris without true vasculitis 3. Treatment includes systemic corticosteroids, dapsone, or methotrexate 3.

Nodular fasciitis is a benign reactive myofibroblastic tumor that can involve the thumb and may cause cortical bone erosion, mimicking sarcoma 5. Despite rapid growth and aggressive imaging appearance, it is treated with excisional biopsy alone 5.

Neoplastic Lesions (Less Common but Critical)

Cutaneous lymphoma, particularly diffuse large B-cell lymphoma, can present as dark reddish nodules 0.3-2 cm with firm edges 4. Skin involvement at presentation occurs in only 3.3% of DLBCL cases 4. Diagnosis requires biopsy with immunohistochemistry showing CD45, CD20, and CD10 positivity 4.

Critical Diagnostic Algorithm

1. Perform incision/aspiration with Gram stain and culture to exclude bacterial infection 2
2. Obtain skin scraping with KOH preparation to rule out fungal infection 1
3. Perform punch or excisional biopsy if infection is excluded or lesion persists despite appropriate antimicrobial therapy 3, 2
4. Request immunohistochemistry if histology suggests lymphoproliferative disorder 4
5. Screen for underlying systemic disease including complete blood count, inflammatory markers, and age-appropriate malignancy screening if NDDH is diagnosed 3

Common Pitfalls to Avoid

NDDH is frequently misdiagnosed as an infectious condition, leading to inappropriate antibiotic therapy, surgical debridement, and even amputation 3. The presence of neutrophilic infiltrate does not automatically indicate infection—histopathologic correlation with clinical context is essential 3.

Do not assume all red nodules on the hand are simple abscesses or furuncles—the lateral DIP location is unusual for typical pyogenic infections, which favor hair-bearing areas 2. This atypical location should prompt consideration of alternative diagnoses 1, 3.

Erysipeloid can be mistaken for cellulitis, but its characteristic target appearance with central clearing and occupational exposure history are distinguishing features 2. The organism (Erysipelothrix rhusiopathiae) is resistant to vancomycin, so empiric vancomycin therapy will fail 2.