Onychomycosis

By Kira Mayo

Onychomycosis is a fungal infection of the nail. It may involve any part of the nail unit, including the nail plate, nail bed, or nail matrix. Roughly one in ten American adults has onychomycosis, making it the most common nail disease. In fact, 48 percent of us will have experienced onychomycosis by the time we are 70 years old!

Toenails are 10 times more commonly affected than fingernails. Risk factorsinclude age, male gender, peripheral vascular disease, diabetes, and smoking. Additionally, about 30 percent of those with onychomycosis also have athlete’s foot, which is more commonly referred to as tinea pedis. It is thought that the infection begins as athlete’s foot before extending to the nail bed, where it becomes more difficult to cure. The nail bed then serves as a reservoir for recurrent infections, particularly in the setting of a hot and humid environment (think sweaty shoes or hiking in the tropics).

About 60 to 80 percent of cases of onychomycosis are caused by dermatophytes, a parasitic fungi that obtains nutrients from keratinized material and can cause infection of the skin, hair and nails. The remaining cases are caused by nondermatophyte molds and yeasts

When nails are struck by this infection, they typically become deformed and discolored – they can be white or yellow. There are four clinical types of onychomycosis:

*Distal subungual onychomycosis (DSO) is the most common form of onychomycosis. The fungus spreads from the surrounding skin and invades the junction between the free edge of the nail and the skin of the fingertip (the hyponychium). The nails yellow and thicken, and keratin accumulates under the nail, causing the nail to separate from the nail bed (onycholysis).

*Proximal subungual onychomycosis (PSO) starts as a white spot on the part of the nail by the cuticle. The infection then spreads throughout the nail. PSO is usually seen in those who have suppressed immune systems.

*White superficial onychomycosis (WSO) is only seen in the toenails. The fungus directly invades the surface of the nail and it is recognized as white to dull-yellow patches anywhere on the surface of the toenail.

*Candidal onychomycosis is rare, occurring primarily in those with mucocutaneous candidiasis. The lesions resemble those seen in DSO (thick, rough, opaque or darkened). Unlike DSO, however, the skin around the nail is often infected along with the nail itself.

Diagnosis of onychomycosis is by appearance, wet mount, culture, or a combination. Differentiation from psoriasis or lichen planus is important, because the therapies differ, so diagnosis is usually confirmed by microscopic examination and culture of scrapings. Scrapings are taken from the nail as close to the cuticle as possible, examined for hyphae on potassium hydroxide wet mount, and cultured.

Many cases of onychomycosis are not treated. This condition is usually asymptomatic and mild, and the oral drugs that are most effective can potentially cause serious side effects. Indications for treatment include: previous cellulitis on same side as affected foot; diabetes or other risk factors for cellulitis; bothersome symptoms; psychosocial impact.

Treatment options include:

*Topical treatment. Although rarely effective as a primary treatment, topicals can improve cure rate when used as an adjunct with oral drugs.

The only effective topical agent for tinea unguinum is ciclopirox (eight percent lacquer) applied daily for 48 weeks. When used for mild to moderate disease, it gets rid of the fungus 34 percent of the time, and restores the nail back to its original color seven percent of the time. Because of its relatively low cost, ciclopirox is considered to be a good cost effective treatment.

*Oral treatments. These are used for severe onychomycosis that does not improve with topical therapy.

Terbinafine kills dermatophytes by blocking the enzyme the fungus needs to build its cell membrane. While a course of 250 mg daily for six weeks is effective for most fingernail infections, a twelve-week course is required for toenail infections. Most side effects involve the GI tract (for example nausea, vomiting, abdominal pain, etc).

Itraconazole stops the growth of dermatophytes, nondermatophyte molds and yeasts. A course of 200 mg daily for one-month is used to treat fingernail infections, and a three-month course is used for toenail infections. While this drug achieves a cure rate of 60-75 percent , the recurrence rate is estimated to be as high as 10-50 percent. This drug does have a black box warning—never start it without a doctor’s supervision.

Don’t worry about stopping the medications while there is still abnormal nail present; these drugs remain bound to the nail plate and continue to be effective after oral administration has stopped. While the already-affected nail will not revert to normal, newly growing nails will appear normal.

To prevent recurrences, you should: trim your nails short; dry your feet after bathing; wear absorbent socks; use antifungal foot powder; and buy news shoes! Old shoes may harbor a high amount of spores and, if possible, should not be worn.

Reference: Rehmus, MD, MPH, Wingfield E. “Onychomycosis.” Merck Manual. Oct. 2009. Web. <http://www.merck.com/mmpe/sec10/ch125/ch125c.html?qt=onychomycosis&alt=sh&gt;.

Originally written for DermHub.com

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