Diagnosis: Felon
The most likely diagnosis is A - Felon, given the presentation of a swollen "bulb" of the thumb following penetrating trauma with a tool, despite an intact nail bed.
Clinical Reasoning
The key distinguishing feature here is the swollen bulb of the thumb, which specifically indicates involvement of the pulp space of the fingertip rather than the periungual tissues. This anatomical localization is pathognomonic for a felon 1.
Why Not the Other Options?
Paronychia (B): This affects the nail fold and periungual tissues, presenting with swelling and erythema around the nail margins, not a swollen "bulb" of the entire fingertip 1. The intact nail bed and bulb swelling make this diagnosis incompatible with the clinical presentation.
Onychomycosis (C): This is a chronic fungal infection causing nail thickening, discoloration, and friable texture developing over weeks to months 1, 2. The acute traumatic presentation following a penetrating injury is completely incompatible with this diagnosis.
Cellulitis (D): While cellulitis can occur after penetrating trauma, it presents with spreading erythema, warmth, and edema extending beyond the fingertip into the hand or digit. The localized "bulb" swelling confined to the pulp space is more specific for felon.
Pathophysiology and Clinical Significance
A felon represents a closed-space infection of the pulp space of the distal phalanx. The penetrating trauma from the tool introduced bacteria (typically Staphylococcus aureus or Streptococcus species) into this compartmentalized space. The fibrous septa that divide the pulp space create multiple small compartments, which when infected, can lead to:
- Increased compartment pressure
- Vascular compromise
- Potential osteomyelitis of the distal phalanx if untreated
- Permanent fingertip dysfunction
Critical Management Considerations
Immediate action is required because delayed treatment can result in:
- Bone involvement and osteomyelitis 3
- Permanent loss of fingertip sensation
- Chronic drainage and fistula formation
Initial Assessment
- Obtain radiographs to evaluate for foreign body, fracture, or early osteomyelitis 3
- Assess neurovascular status of the digit
- Evaluate for fluctuance indicating abscess formation
Treatment Algorithm
- If early presentation without fluctuance: Trial of IV antibiotics with close follow-up within 24 hours
- If fluctuance present or failed conservative management: Surgical incision and drainage is mandatory to prevent complications
- Post-drainage: Continue antibiotics, elevation, and close monitoring for resolution
Common Pitfalls to Avoid
- Underestimating the severity: The intact nail bed may falsely reassure clinicians, but felon is a deep space infection requiring aggressive management 4, 5
- Delaying surgical drainage: Attempting prolonged antibiotic therapy alone when fluctuance is present leads to worse outcomes including osteomyelitis 3
- Inadequate drainage: Superficial incision without addressing all compartments of the pulp space results in treatment failure
- Missing associated fractures: Always obtain radiographs as distal phalanx fractures commonly accompany these injuries and may require additional management 3, 4, 6