Onychomycosis is probably the most common condition treated by podiatrists, but there is widespread lack of understanding about the disease. There is a tremendous body of literature about the condition, with at least a dozen papers being published each month. Unfortunately, many podiatrists have not really bothered to keep up on the science. Onychomycosis is written off as more of a chronic condition that just needs occasional debridement, rather than an infection that needs to be treated. A DPM does not think twice about treating a bacterial infection with an antibiotic…why is there hesitation to treat a fungal infection with an antifungal? Make no mistake about it; onychomycosis is an infection that needs to be treated with an anti-infective.

The vast majority of these have neither been diagnosed by a physician nor treated with a prescription medication. The same patients who have onychomycosis will, more than likely also have tinea pedis, as these two are inexorably linked. The natural history of the disease has the infection starting as tinea pedis and, through some trauma that breaks the hyponychial seal, invading the nail bed stratum corneum.

Predisposing factors to both tinea pedis and onychomycosis are essentially the same. They include older age, diabetes, peripheral arterial disease, poor hygiene, close quarter living such as in the military or a long term care facility, immunocompromise and even a genetic predisposition, to name but a few. Likewise, the organisms that cause both tinea pedis and onychomycosis are the same, with greater than 90% being caused by Trychophyton rubrum.

In 1972, dermatologist Nardo Zaias, MD devised a classification system for onychomycosis still widely used today in the scientific literature. He categorized the infection into four major subtypes:

1.  Distal subungual – now known as distal lateral subungual (DLSO).
    This is by far the most common type seen in everyday practice.
2.  White superficial – usually caused by T. interdigitale (formerly T. mentagrophytes)
3.  Proximal subungual – associated with HIV/AIDS or trauma to the eponychium
4.  Candida onychomycosis – more common in the hands than the feet

Since the original classification, there have been a number of additional types of onychomycosis identified including total dystrophic, mixed pattern, secondary and a unique form called “endonyx”.

It is often stated that 50% of all dystrophic nails are not, in fact, fungal but rather caused by other conditions such as psoriasis, lichen planus, congenital abnormalities, etc. For this reason, it is important to make sure that the diagnosis is correct before prescribing treatment. Certainly, treating a non fungal infection with an antifungal will lead to frustration and dissatisfaction for both the patient and the prescriber. Traditionally, laboratory diagnosis has been considered the sine qua non for confirming the presence of fungus. Many insurance payers require it before covering a prescription for an antifungal. If oral terbinafine is being considered as a treatment, it is actually stated in the product’s package insert that the diagnosis should be confirmed through laboratory testing before using the drug. Unfortunately, current laboratory testing suffers from poor sensitivity, specificity and accuracy. A fungal culture can be falsely negative almost 50% of the time. Periodic acid Schiff stains are very sensitive but not specific. Newer molecular technique, which may represent the “wave of the future”, are extremely sensitive but maybe too much so.

Although generally dismissed, clinical diagnosis has been shown to be highly accurate. When a clinician suspects onychomycosis and the patient presents with concomitant plantar desquamation (representing tinea), at least one study has shown the accuracy of clinical diagnosis to be 91%, far higher than laboratory testing.

The treatment of onychomycosis will be covered in a separate eZine and lecture.