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New Felon Etiology: Occult Subungual Infection

Rupert, Ed MD

doi: 10.1097/01.EEM.0000731732.54841.17
    felon, finger

    I have treated six patients with classic felons in my 44 years of practicing emergency and urgent care medicine. Each had a subungual abscess that could not be detected on visual inspection of the nail.

    Early in the progression of a classic felon, before serious complications alter the appearance of the digit, the dorsum of the digit, nail, nail bed, and skin attached to the nail have a normal or relatively normal appearance.

    Non-classic felons, in contrast, are accompanied by paronychia, hyponychia, or visible subungual abscess. A non-classic felon arises from an infection that originates in immediate proximity to the nail. Similarly, a classic felon also occurs from an occult subungual infection that originates in immediate proximity to the nail. Occult subungual infection is another manifestation of the vulnerability to infection that exists at the junction of the nail and its soft tissue attachments.

    Bacteria enter the digit at the junction of the nail and the nail bed in classic felon cases. The infection begins in the nail bed and then spreads to the pulp through vascular channels, including lymphatics. It may also spread by direct extension.

    The felon literature offers one theory on the origin of the classic felon. Allen Kanavel, MD, made it clear in his historic work, Infections of the Hand, first published in 1912, that he was not sure how felons begin. He suggested, however, that a small pinprick or unnoticed injury might be the source. This is problematic because pinpricks and wounds that allow bacteria to be transmitted through the skin of a fingertip into the pulp are usually noticed because of pain and soreness, and some pinpricks and injuries leave visible wounds.

    Why is it that no one previously considered the possibility of an occult subungual infection being the cause of a classic felon? The answer, I believe, is multifactorial, starting with the fact that everything about the presentation of a classic felon suggests the infection begins in the pulp. The dorsum of the digit looks normal or relatively normal while the volar aspect of the distal digit is swollen, red, and hot. There is severe but poorly localized pain that emanates from the middle of the distal digit and sometimes from the entire digit, so pain is not helpful in localizing the inciting infection.

    The belief that a felon begins in the pulp is strengthened by the knowledge that draining a pulp abscess may improve or cure a felon. Some subungual abscesses are easily seen through the nail, so there is an expectation that all subungual abscesses are easily seen through the nail. When an advanced felon requires debridement, including nail removal that uncovers subungual pus, it is assumed the subungual pus is secondary to the pulp infection or abscess.

    Felons are not common. I completed medical school and an internship oblivious to their existence. I had my first encounter with one a few months after completing my internship when a man presented with a classic felon. I had no familiarity with the infection and found in Schwartz's Principles of Surgery what I thought was the information I needed to treat the man's felon.

    I performed a digital block and then, per the textbook's instructions, made a large incision across the volar aspect of the man's distal finger. I expected pus. I got blood. I am not sure why I decided to look under his nail at that point, but I think it was an act of desperation. I separated no more than 2-3 mm of the distal nail from the nail bed when pus gushed out.

    Years later, another man presented with a classic felon. I told him that the cause of his felon could be an abscess under the nail that we were unable to see. He agreed to nail removal. Once again, pus gushed out.

    I have treated six patients with classic felons, all with non-contributory histories. All of the patients were seen one to three days after onset of symptoms, and all had occult subungual abscesses with surprisingly large volumes of pus. None had a pulp abscess. All were treated with nail removal, given oral antibiotics, and recovered quickly.

    I recommend adequate exposure of the nail bed to ensure drainage of the occult subungual abscess when treating a patient with a classic felon. I think complete removal of the nail is the best and easiest way to ensure adequate exposure of the nail bed. Complete exposure of the nail bed also permits a full examination for tracks caused by direct extension of the infection.

    It is possible that simply draining the subungual abscess may be effective in achieving a cure. A pulp abscess detected on physical exam or by ultrasound should be drained too. The pad should not be incised if a pulp abscess is not detected. X-ray the digit, and check tetanus toxoid status. Initial antibiotic selection should be based on local prevalence and sensitivity data. Close follow-up is advised.

    The longer a felon goes without appropriate drainage surgery, the greater the risk of complications, including osteomyelitis, tissue destruction, joint and tendon infection, and infection spreading beyond the involved digit. Refer any patients whose felons have progressed beyond pulp abscess immediately to a hand specialist, orthopedist, or plastic surgeon. Referral or consultation should also be provided to patients who have had drainage surgery but do not improve daily.

    Dr. Rupertis a retired board-certified emergency physician in Clinton, MD.

    Copyright © 2021 Wolters Kluwer Health, Inc. All rights reserved.
    • Ed Rupert, MD1:29:45 PMThis glossary may be helpful to readers of this article. CLASSIC FELON: A classic felon is described by Allen Kanavel, MD, in "Infections of the Hand," and is the felon upon which the felon literature is based. It is the felon mistakenly believed to originate in the pulp, but, in fact, originates under the nail. NON-CLASSIC FELON: A nonclassic felon develops from a paronychia, a hyponychia, or from a visible subungual abscess, one easily seen when looking at the nail. Dr. Kanavel did not mention any of the nonclassic felon types in "Infections of the Hand." OCCULT SUBUNGUAL INFECTION/ABSCESS: An occult subungual infection/abscess is an infection/abscess located under the nail that cannot be detected by looking at the nail. Also the tissue adjacent to the nail (the paronychium, the hyponychium, and the eponychium) has a normal appearance or one so close to normal that it gives no indication of the infection under the nail. Ed Rupert, MD