What are the differential diagnoses for a patient presenting with swelling, pain, and warmth in the left thumb?

Differential Diagnoses for Left Thumb Swelling, Pain, and Warmth

The most likely diagnoses are paronychia (nail fold infection), felon (fingertip pulp abscess), septic arthritis, or flexor tenosynovitis, with paronychia being the most common hand infection requiring immediate assessment for abscess formation. 1, 2

Immediate Life- and Limb-Threatening Diagnoses to Exclude First

Septic Arthritis of the Thumb

• Requires at least 2 of the following: local swelling, erythema, local tenderness, local warmth, or purulent discharge 3
• Fever, elevated white blood cell count, or elevated C-reactive protein suggests infection 3
• Joint aspiration with culture is often necessary for definitive diagnosis, as imaging cannot reliably distinguish infected from noninfected joints 4
• Delay can lead to permanent joint destruction 1

Pyogenic Flexor Tenosynovitis

• This is a surgical emergency requiring parenteral antibiotics and sheath irrigation 1
• Look for Kanavel's four cardinal signs: fusiform swelling of the digit, flexed posture of the digit, tenderness along the flexor tendon sheath, and pain with passive extension 1
• Often requires urgent surgical intervention to prevent tendon necrosis 1

Clenched-Fist Injury (Fight Bite)

• Usually results from an altercation and often involves injury to the extensor tendon, joint capsule, and bone 1
• Requires wound exploration, copious irrigation, and appropriate antibiotics 1
• High risk for severe complications if not treated aggressively 1

Most Common Diagnoses

Acute Paronychia (Nail Fold Infection)

• The most common hand infection, typically precipitated by localized trauma 1, 2
• Caused by polymicrobial infections after the protective nail barrier has been breached 5
• Presents with swelling, erythema, warmth, and pain around the nail fold 5
• Common predisposing factors include finger-sucking (24%) and nail-biting (19%) 6
• Treatment consists of incision and drainage if abscess is present, warm-water soaks, and sometimes oral antibiotics 1, 5
• Antibiotics are not necessary after adequate surgical drainage in uncomplicated cases without risk factors 7

Felon (Fingertip Pulp Abscess)

• An abscess of the distal pulp of the fingertip, often extremely painful due to complex fibrous septa limiting swelling 1, 2
• Early felon may be amenable to elevation, oral antibiotics, and warm water soaks 1
• More advanced felon requires incision and drainage 1, 2
• If septa are destroyed, spread into bones or flexor tendon sheath is possible 2

Herpetic Whitlow

• A painful viral infection caused by herpes simplex virus 1
• Critical to distinguish from bacterial infection because incision and drainage is contraindicated 1
• Look for grouped vesicles on an erythematous base 1
• Early treatment with oral antiviral agents may hasten healing 1
• Cytologic examination with Tzanck smear may be useful diagnostically and prevent unnecessary antibiotics and surgical drainage 6

Less Common But Important Diagnoses

Chronic Paronychia

• Characterized by symptoms of at least 6 weeks' duration and represents an irritant dermatitis 5
• Common in housekeepers, dishwashers, bartenders, florists, bakers, and swimmers 5
• Treatment aimed at stopping irritation while treating inflammation with topical steroids or calcineurin inhibitors 5
• May take weeks to months to resolve 5

Osteomyelitis

• May present with redness, swelling, and tenderness overlying bone 4
• Early acute osteomyelitis may show only mild soft tissue swelling on radiographs 4

Gout

• Consider in patients with acute onset, severe pain, and history of hyperuricemia or prior attacks 4

Critical Diagnostic Algorithm

Step 1: Assess for Surgical Emergencies

• Examine for Kanavel's signs (flexor tenosynovitis) 1
• Check for joint involvement (septic arthritis) 3
• Inquire about mechanism of injury (clenched-fist injury) 1

Step 2: Localize the Infection

• Dorsal/nail fold involvement suggests paronychia 2, 5
• Palmar fingertip involvement suggests felon 1, 2
• Look for grouped vesicles suggesting herpetic whitlow 1, 6

Step 3: Determine Presence of Abscess

• Fluctuance indicates need for incision and drainage 1, 5
• Early infections without abscess may respond to elevation, warm soaks, and oral antibiotics 1

Step 4: Identify Host Risk Factors

• Immunocompromised, diabetic, or cardiac valve prosthesis recipients require antibiotics even after drainage 7
• Otherwise healthy patients with uncomplicated paronychia or felon do not require antibiotics after adequate surgical drainage 7

Critical Pitfalls to Avoid

• Do not incise and drain herpetic whitlow—this worsens the condition and spreads infection 1
• Do not assume antibiotic-resistant acute paronychia is always bacterial; consider viral, fungal, drug-induced, or autoimmune causes 6
• Do not delay surgical consultation for suspected flexor tenosynovitis or septic arthritis 1
• Do not prescribe antibiotics empirically without adequate drainage in uncomplicated cases 7
• Do not miss chronic paronychia by treating repeatedly with antibiotics when the cause is irritant dermatitis 5

When to Refer Urgently

• Any suspicion of flexor tenosynovitis requires immediate hand surgery consultation 1
• Suspected septic arthritis requires urgent orthopedic or hand surgery evaluation 3, 1
• Clenched-fist injuries require urgent surgical exploration 1
• Felons with deep extension or failure to improve with initial drainage require specialist evaluation 2