Nail conditions in children are either congenital or acquired, with the acquired conditions, like onychomycosis, appear similar, but occur with less frequency than in adults [1]. Even though nails grow at a continuous rate throughout life, the speed of growth of a pediatric nail is increased until age 10–14, where it becomes similar to that of an adult [1]. Besides the greater speed of growth in children, other thoughts as to why their nails aren’t frequently as affected as adults are less exposure to higher humidity environments, less trauma due to a smaller nail surface area, and lower incidence of the often concomitant tinea pedis [2]. Even with these facts, onychomycosis is one of the most common nail diseases in the pediatric population, with adolescents being affected more than younger children.

The literature concurs that pediatric onychomycosis is less common than in adults (0.3 % vs 2–13 %), but its existence is starting to rise [3]. Plausible causes for pediatric onychomycosis to be increasing include the presence of genetic predisposition to developing the dermatophyte infection in the presence of an affected family member, juvenile diabetes, Down’s syndrome, immunocompromised disorders such as a transplant recipient and HIV, and lifestyle issues as increased wearing of occlusive shoe gear [4]. In addition to these, other risk factors to developing onychomycosis in a child are living in a rural area, hypoxemia, metabolic alterations, repetitive trauma, and malignant neoplasms [2]. A study of 72 children, ages 2–18 years old, at an oncology clinic in Mexico, showed the frequency of onychomycosis in this immunosuppressed population as one child or 1.3 % [2]. The authors theorized that the frequency of nail mycosis in this immunosuppressed population wasn’t greater than average due to the almost protective benefits of pediatric nail growth (not allowing dermatophytes to colonize due to the rapid growth) and possibly the cytotoxic effect of the chemotherapeutic agents the children were exposed to [2]. In this particular population, the lack of increased nail mycosis does not mean they won’t have a susceptibility to disseminated fungal infections and certainly should be examined for such.

As in adults, the most common type of onychomycosis seen in children is the distal lateral subungual type, but proximal subungual and superficial white types can occur with less frequency [4]. Both the deep and classic types of superficial white occur more in younger children [1]. Candida onychomycosis is rare but, if present, coexists with a paronychia [4]. The total dystrophic type is the rarest in children. In a review of 59 cases of pediatric onychomycosis in children 0–18, 66.1 % had toenail versus fingernail onychomycosis with distal lateral subungual being the most common type [5]. The older children were the highest cohort to see mycotic nails, with a male predilection [5]. In agreement with the literature, the most common dermatophyte, Trichophyton rubrum, affects the nail the most; followed by C. albicans, C. parapsilosis, and C. tropicalis [5–7]. Interestingly, 14 of the 59 children had concomitant tinea pedis, and eight of the parents of these children had either tinea pedis or onychomycosis [5]. In another review of 16 cases of nail mycosis in children under the age of 2, Bonifaz et al. found the majority with distal subungual onychomycosis had Down’s syndrome, with one having concomitant tinea capitis and two being preterm [8]. Although not a common risk factor, preterm children have shorter nail plates which may account for this increased prevalence [8].

As in adults, dermatophytes can produce pigmentation that may appear as a melanocytic lesion. Besides completing a thorough history and physical, it is important to get mycological confirmation as onychomycosis in a child may mimic various nail issues. Differential diagnoses of onychomycosis in a child would include the following: trauma, psoriasis, lichen planus, alopecia areata, atopic dermatitis, various congenital nail disorders, and nevi [4]. As described in previous chapters, a specimen for fungal culture should be submitted prior to starting any antifungal therapy. Wiping the nail with alcohol prior to trimming the nail back to the mycelial front (the area of subungual activity), and then obtaining the hyperkeratotic subungual material, is what is best for specimen collection [4].

As with an adult, consideration of the patient age, nail involvement, pathogen, drug-drug interactions, and medical history are factors when choosing a treatment plan. As of this writing, there are no FDA-approved treatments for pediatric onychomycosis. In time, this might change as there is a phase IV efinaconazole 10 % solution trial involving children underway and a new oral antifungal, VT-1161, which is being investigated for a range of fungal diseases, including onychomycosis. That said, most practitioners will prescribe oral or topical antifungal therapy for children, while some might choose avulsion, laser, or other medical devices even though there is no evidence to support their pediatric use [9].

In their meta-analysis of systemic antifungal use in children under the age of 18, Gupta and Paquet showed a complete cure rate of 70.8 % when systemic antifungals were solely used [3]. This complete cure rate was increased to 80.0 % with the addition of a topical antifungal in several cases [3]. Ultimately, terbinafine, itraconazole, fluconazole, and griseofulvin had safety and efficacy profiles similar to those reported for adults [3].

As with adults, systemic antifungals are generally chosen where 50 % of the distal nail plate is affected, multiple nails are involved, and topical therapy may not be useful. With a mycological cure of 70 % in toenails, terbinafine is FDA approved for tinea capitis in children but is off-label for pediatric onychomycosis. That said, the adult dose of 250 mg daily in an adult is not recommended in children weighing less than 40 kg. The pediatric dosage regimen is as follows: 62.5 mg for children weighing less than 20 kg, 125 mg in those weighing between 20 and 40 kg, and 250 mg in those who weigh greater than 40 kg. Besides being available as a tablet, it is also available as an oral granule, which is helpful in children who are unable to swallow pills. In adults, the complete cure rate for terbinafine is 38 % in toenails, but in a systematic review in its use in children, a pooled complete cure rate of 78.8 % has been reported [3]. Adverse events seen in the pediatric population when taking terbinafine have included acute urticaria, anorexia, tiredness, vesiculopustular eruption, and agranulocytosis [3]. As in adults, the measurement of AST and ALT is recommended before starting this therapy in a child, and of course, dispensing of this medication in those with pre-existing liver disease should be avoided.