Paronychia

Paronychia, more commonly known as whitlow, is an infection of the skin around the nail (i.e., the nail folds) of the finger or toe. Looking at the nail from above, there are two nail folds along each side of the nail (i.e., lateral nail folds) and a nail fold at the base of the nail called the eponychium or the proximal nail fold. Paronychia can affect any of these three nail folds. The eponychium gives rise to the cuticle, which is the semi-circular layer of dead cells that covers the area where the nail meets the skin, preventing pathogens from entering. Below the nail plate (the hard part of the nail) lies the nail bed, which is a special layer of epidermal tissue that attaches the nail plate to the distal finger. 

The cause of paronychia depends on the type of paronychia and whether it is acute or chronic.  Acute paronychia is one of the most common hand infections and usually occurs when bacteria enter the skin through an open wound like a split in a cuticle causing an infection. It is more common after nail trauma, such as nail-biting or picking at a hangnail, getting a manicure, ingrown toenails, or a direct blow or injury to the fingernail. The most common pathogen to cause paronychia is Staphylococcus aureus, although most infections are caused by more than one bacteria. A unique form of acute paronychia is herpetic whitlow, a blistering infection of the fingertip caused by herpes simplex virus (HSV) that occurs if the finger comes in contact with an active lesion, typically located around the mouth or genital area.

In contrast to acute paronychia which is most frequently caused by a bacterial infection, chronic paronychia is usually caused by irritation from occupational or environmental exposures.  Sometimes fungal infections, especially due to Candida spp. (e.g., Candida auris) can also cause chronic paronychia. Risk factors for chronic paronychia include having frequent wet hands and coming into constant contact with soaps and detergents, as is the case for house cleaners, dishwashers, cooks, or swimmers. Immunosuppression (e.g., diabetes mellitus, HIV, malignancy) can also predispose an individual to chronic paronychia. 

More rarely, medications such as retinoids, antiretrovirals (e.g., indinavir, lamivudine), chemotherapeutic agents (e.g., taxanes) and epidermal growth factor receptor (EGFR) inhibitors (e.g., cetuximab, gefitinib, erlotinib) can cause inflammation of the cuticle and nail folds resulting in chronic paronychia.

Initial symptoms of acute paronychia include a warm, red, and swollen area on the skin around the fingernails or toenails. If left untreated, the infection can progress to an abscess, which is when a collection of pus accumulates under the skin. In cases where the abscess is close to the nail plate, pus may find its way through the nail fold and drain spontaneously or upon light pressure. Acute paronychia develops rapidly over the course of a few hours or days and typically affects one finger. 

With chronic paronychia, there can also be swelling and redness of the nail folds, however, it is more gradual in onset and typically less painful. It lasts more than six weeks and the growing nail may become affected resulting in the development of horizontal nail ridges, thickening of the nail plate, and a yellow or green discoloration. Over time, the nail folds may lift off the nail plate, allowing easier entry of pathogens and irritants. If not treated, long-standing chronic paronychia can result in the development of squamous cell carcinoma, a form of skin cancer. 

Paronychia is diagnosed clinically based on the medical history and physical exam. Routine cultures or other laboratory tests are generally not necessary unless the infection is chronic or resistant to treatment. Failure of treatment should prompt specialized culture techniques, referral to a dermatologist, and/or a biopsy to rule out malignancy.

Treatment of acute paronychia depends on the extent of the infection. In cases where there is minimal inflammation, the infection can resolve with simple measures like soaking the finger in warm water several times a day along with a course of topical or oral antibiotics, like clindamycin or amoxicillin-clavulanate. 

Surgery may be necessary if an abscess has formed or if there is no response to conservative treatment. A common surgical technique known as incision and drainage involves inserting a sterile blade under the nail fold so that pus may drain without the need for a skin incision. This technique is preferred if the abscess is small and is close to the nail plate. Alternatively, a surgeon may perform a procedure known as the Swiss roll technique in which a skin incision is made at either side of the nail fold allowing it to be lifted from the nail bed. The nail fold is then rolled over a wound dressing and secured to the skin with removable sutures. After the infection has healed, the sutures are removed and the nail fold is returned to its original position. The Swiss roll technique has the advantage of preserving the nail plate, however, complete nail removal may be necessary in rare cases where an abscess has caused the nail plate to separate from the nail bed.  

Finally, supportive treatment of chronic paronychia may include the use of topical corticosteroids during flare-ups or treating pathogens grown on cultures with topical or systemic antifungal medication (e.g., itraconazole and terbinafine). However, the most important part of the treatment is avoiding any predisposing factors by wearing gloves when coming in contact with irritants and avoiding nail trauma. 

Paronychia is a common infection of the nail folds. Most cases are preceded by a history of nail trauma, such as nail biting, a direct nail injury, or an ingrown nail. Initially, paronychia presents as a painful warm, red, and swollen area that can progress to the formation of an abscess. The diagnosis is clinical and no additional tests are generally needed. For treatment, mild cases can be managed by soaking the affected finger in warm water along with antibiotics. In severe cases, a minor surgery may be necessary to resolve the infection.