Paronikia & Felon

01Pathophysiology

Paronychia is an infection of the perionychium, which is the soft tissue surrounding the nail plate. The perionychium includes the lateral nail folds (on either side) and the proximal nail fold (the eponychium or cuticle area). Any disruption of the seal between the nail fold and the nail plate allows bacteria or fungi to enter this space and establish infection.

In acute paronychia, the most common mechanism is a minor trauma to the nail fold, such as aggressive manicuring, nail biting, or a torn hangnail. This breach allows S. aureus (the dominant pathogen) to invade the disrupted tissue. In children or habitual nail biters, anaerobic oral flora (such as Eikenella corrodens or Bacteroides) may be co-pathogens because saliva is repeatedly introduced into the wound. The infection initially causes cellulitis of the nail fold, and if untreated, progresses to a localized abscess. Because the nail fold is a relatively superficial compartment, pus tends to track along the margin of the nail plate. If the abscess extends beneath the nail itself, it becomes a subungual abscess, which requires partial or complete nail removal for adequate drainage.

In chronic paronychia, the pathogenesis is fundamentally different. Repeated exposure to moisture and irritants (detergents, water, citrus) causes chronic maceration of the cuticle, destroying the protective seal. This allows Candida species to colonize the space beneath the proximal nail fold. The result is a chronic, low-grade inflammatory process rather than an acute suppurative infection. Over time, the nail fold becomes boggy, retracted, and thickened, and the nail plate may develop transverse ridging or discoloration from chronic inflammation of the nail matrix.

Felon is a closed-space infection of the fingertip pulp (the fleshy pad on the palmar side of the distal phalanx). The key anatomical concept here is that the pulp contains multiple vertical fibrous septae that run from the periosteum of the distal phalanx to the skin. These septae divide the pulp into small, non-communicating compartments. When bacteria are introduced (through a puncture wound, splinter, or hematogenous spread), the infection is trapped within these rigid compartments. As pus accumulates, pressure builds within the closed space because the septae prevent decompression. This rising pressure can compress the nutrient vessels that supply the distal phalanx, leading to ischemic necrosis of the bone (osteomyelitis) or necrosis of the overlying skin. This is why a felon always requires surgical drainage: antibiotics alone cannot penetrate a pressurized, walled-off abscess, and delay risks permanent damage to the distal phalanx.

02Classification and Clinical Manifestation

Paronychia Classification

Felon Classification

03Diagnostic Workup

Both paronychia and felon are clinical diagnoses. The exam will not typically test you on ordering advanced imaging for a straightforward presentation. The diagnosis is made at the bedside by history and physical examination alone.

For paronychia, the critical clinical decision is whether the infection is still in the cellulitis phase or has progressed to a drainable abscess. A cellulitic nail fold will be erythematous and tender but soft on palpation, while an abscess will demonstrate fluctuance or visible pus at the nail margin. This distinction determines management (antibiotics alone versus I&D).

For a felon, the examiner wants you to recognize the tense, swollen fingertip pulp as a closed-space infection that will not resolve with antibiotics alone. An X-ray of the affected digit should be obtained before drainage to identify a retained foreign body (such as a needle tip or thorn) and to evaluate the distal phalanx for early osteomyelitis (cortical erosion, periosteal reaction). If the felon has been present for more than a week, or if the patient is diabetic or immunocompromised, the threshold for imaging should be low.

For chronic paronychia, the workup shifts toward confirming a fungal etiology. A KOH preparation of material scraped from beneath the proximal nail fold can demonstrate budding yeast and pseudohyphae consistent with Candida. Fungal culture is more sensitive but takes days to weeks to result. The clinical picture of a chronically boggy nail fold without acute suppuration in a patient with chronic wet exposure is usually sufficient.

04Management and Treatment

Acute Paronychia Without Abscess

The initial step is conservative treatment with warm water soaks for 15 to 20 minutes, 3 to 4 times per day. This promotes spontaneous drainage and local blood flow. An oral antibiotic with staphylococcal coverage should be started concurrently. Cephalexin is the standard first-line agent. In communities or populations with high MRSA prevalence, trimethoprim-sulfamethoxazole (TMP-SMX) is preferred. If the patient is a habitual nail biter, anaerobic coverage should be added with amoxicillin-clavulanate or clindamycin to address oral flora such as Eikenella and Bacteroides. Reassess in 48 hours. If there is no improvement or if fluctuance develops, proceed to I&D.

Acute Paronychia With Abscess

Once an abscess has formed, incision and drainage is the treatment. Warm soaks and antibiotics alone will not resolve a walled-off collection of pus. The standard technique involves placing a #11 scalpel blade or an 18-gauge needle at the point of maximal fluctuance along the nail fold, directed parallel to the nail plate, and elevating the nail fold away from the nail to allow pus to drain. A digital nerve block (using 1% lidocaine without epinephrine, or with epinephrine for improved hemostasis in healthy digits) should be performed before the procedure. The wound is left open and the patient is instructed to continue warm soaks. Antibiotics are prescribed for surrounding cellulitis.

If pus is seen tracking beneath the nail plate (a subungual abscess), a portion of the nail overlying the collection must be removed. This may require partial nail avulsion (removing one-quarter to one-third of the nail plate on the affected side) or complete nail removal if the abscess is extensive.

Chronic Paronychia

The cornerstone of chronic paronychia management is avoidance of moisture. The patient must be counseled to wear gloves during wet work, keep hands dry, and avoid habitual manipulation of the cuticle. Topical antifungals such as clotrimazole 1% cream or nystatin cream are applied to the nail fold twice daily for a minimum of 3 to 6 weeks. A short course of a topical corticosteroid can reduce inflammation during acute flares. Oral antifungals (fluconazole, itraconazole) are reserved for refractory cases. Importantly, chronic paronychia is not treated with I&D because there is no discrete abscess to drain; the pathology is chronic inflammation and fungal colonization, not an acute suppurative process.

Felon

A felon always requires incision and drainage, regardless of the stage. The standard approach is a unilateral longitudinal incision placed on the ulnar aspect of the digit (for fingers 2 through 5) or the radial aspect of the thumb, to avoid placing the scar on a contact surface. The incision is made directly over the point of maximal swelling and extended to the depth of the pulp. A blunt instrument (such as a small hemostat) is then used to break up all fibrous septae within the pulp space, converting the multiple small compartments into a single cavity that can drain freely. The wound is loosely packed with iodoform gauze and left open. Packing is typically removed at 48 hours, and warm soaks are initiated.

The previously described "fish-mouth" or "hockey-stick" incision (a transverse incision across the fingertip pad) is no longer recommended because it risks injuring the digital neurovascular bundles and can create an unstable, painful fingertip.

Oral antibiotics are prescribed for 7 to 10 days. TMP-SMX is a reasonable empiric choice given the prevalence of community-acquired MRSA in skin and soft tissue infections. If the wound culture reveals MSSA, therapy can be narrowed to cephalexin.

05Differential Diagnosis and Distractors

06Traps and High-Yield Pearls

The single most tested trap in this topic is confusing a felon with herpetic whitlow. Both present with a painful, swollen fingertip, and both are common in healthcare workers. Incising a herpetic whitlow is a classic wrong answer: it does not improve the condition, risks bacterial superinfection, and can spread the virus. The discriminating feature is the presence of grouped vesicles with clear fluid in herpetic whitlow versus a tense, uniformly swollen pulp without vesicles in a felon. If the vignette describes a healthcare worker or a child with a recent history of oral herpes who presents with a painful fingertip and small blisters, the answer is herpetic whitlow, and the management is observation with oral acyclovir, not incision.

The second major trap involves chronic paronychia. Students often select "I&D" for any swollen nail fold, but chronic paronychia is a non-suppurative inflammatory and fungal process. The correct answer for chronic paronychia is moisture avoidance and topical antifungals, not surgical drainage.

A third testable concept is the anatomy of the felon. The exam may describe a patient with a fingertip infection that was treated with antibiotics alone, now returning with worsening pain, swelling, or X-ray evidence of distal phalanx erosion. The lesson here is that a felon is a closed-space infection and will not resolve without surgical decompression. The "next best step" for a felon is always I&D, not escalation of antibiotics.

Finally, remember the incision technique for felons. If the vignette asks about the correct surgical approach, the answer is a longitudinal incision on the non-contact (lateral) surface, not a fish-mouth or transverse incision across the pad.

The core competency being tested across questions on paronychia and felon is the ability to (1) distinguish infectious from non-infectious causes of finger swelling, (2) recognize when an abscess requires drainage versus when conservative management is appropriate, and (3) identify the one condition in this differential that must never be incised: herpetic whitlow.