According to the American Diabetes Association, there are approximately 29.1 million Americans or 9.3 % of the population that suffer from diabetes [1]. Twenty-one million have already been diagnosed with the disease, and 8.1 million remain undiagnosed [1]. This number pales in comparison to the 86 million Americans who have been classified as “prediabetic,” i.e., those who are on the borderline of becoming diabetic in the future. Within this subset of patients, pedal complaints, such as painful, thickened toenails are of chief concern as onychomycosis is much more than a cosmetic issue. In the diabetic population, the diagnosis and management of the fungal toenail may be overlooked as more medically pertinent findings like concomitant renal disease, cardiovascular risk factors, retinopathy, and diabetic foot wounds usurp the practitioner’s time. However, if onychomycosis is overlooked and not properly managed in these patients, it can become a limb-threatening condition [2].

In the general population, estimations of mycotic toenail infections vary, but one epidemiologic survey reported an overall disease prevalence of 2–13 % with evidence of a steadily rising incidence worldwide [3]. The prevalence of diabetic vs nondiabetic patients with onychomycosis was shown to be statistically significant at 17 and 6.8 %, respectively. Almost a threefold increase was appreciated in those patients that had diabetes mellitus when it came to having clinically evident toenail fungus [4]. Looking at prevalence and risk factors, onychomycosis was present in 26 % of diabetics of the time [5]. Further projections were made, and approximately one-third of the overall subjects with a diagnosis of diabetes were affected [5].

General predisposing factors in developing onychomycosis include increasing age, male gender, family history of onychomycosis, concurrent intake of immunosuppressive agents, and peripheral vascular disease [6]. Onychomycosis is the most common skin issue in a diabetic patient [6]. Also, in this population, mycotic nails are often associated with hyperlipidemia, peripheral artery disease, diabetes duration, metabolic syndrome, obesity, smoking, and atherosclerosis [7]. Onalan et al. examined 127 patients with diabetes type 2 [7]. Of those patients, 48 had onychomycosis (37.8 %), and 60 had subclinical atherosclerosis of the carotid (47.2 %). After further investigation, they concluded that “the presence of toenail onychomycosis is independently associated with the presence of subclinical atherosclerosis” as seen in the carotid intima-media thickness in patients with diabetes [7]. Therefore, toenail disease in diabetics can be a marker of something much more sinister but ultimately screenable and, in theory, treatable if caught early.

In addition to it being a marker for atherosclerosis, mycotic toenails may also pose a problem when the patient is neuropathic. Long-standing hyperglycemia affects cellular immunity, which may predispose a diabetic to fungal invasion [8]. Increasing nail thickness is associated with a higher hemoglobin A1C (HgA1C) value [9]. One of the most frequent diabetic complications is a diabetic foot ulcer, especially in a neurovascular compromised patient. A sharp-edged, thickened toenail that would be perceived as such in a sensate patient could go unnoticed in a neuropathic diabetic patient. This could lead to an ulceration of the nail bed or erosions of the surrounding tissue or digits that are exposed to the jagged or sharp edges of the nail plate [8]. Depending on the vascular status of the patient, this ulceration may become gangrenous and may develop osteomyelitis or cellulitis which in theory could lead to a possible amputation of not just the digit but also the limb. Doyle et al. state that there is a higher incidence of foot ulceration and gangrene in diabetic patients with onychomycosis versus those who do not [10]. This justifies the need for proper and routine foot screenings and care, as simply debriding the nails and inspecting the pedal skin on a regular basis could be limb saving in certain cases.

Unique to the lower extremity, studies have suggested that confining shoe gear and systemic immunosuppression increase the susceptibility of onychomycosis with varying levels of morbidity in the diabetic foot [11–13]. In the confines of the shoe, faulty foot and ankle biomechanics add another risk to developing nail disease. Onychomycosis is most commonly found in the hallux toenail in comparison to the lesser toenails [14]. Normal pedal ambulation and biomechanics rely on the hallux acting in the propulsive cycle as a rigid lever arm [13]. If the first metatarsophalangeal joint is limited in either a dorsiflexory or plantarflexory direction, the hallux nail can become compromised due to chronic repetitive trauma which may lead to fungal invasion [15]. In the case of a diabetic with neuropathy, the pain that this joint limitation would typically cause in a shoe may go unrecognized. Hallux nails in this situation may develop subungual hematomas creating a negative space for microbial invasion as well as an eventual hypertrophic toenail when subject to that force chronically. Patients will often not recall the source of this nail trauma. For these patients, it is not only important to have their feet checked on a regular basis but also wear appropriate footwear that has a deep toebox and diabetic-appropriate insoles.

In a study by Al-Mutairi et al., 460 patients with diabetes and 460 patients without diabetes were evaluated. Toenails were affected in 62 % of the cases, fingernails in 23 %, and both fingernails and toenails in 14 % of the cases in the diabetic group. Distal subungual onychomycosis was found to be the most common clinical presentation seen to reflect 65 % of the patients, followed by total dystrophic onychomycosis [6]. The dominating theme was the initial presentation occurred on the feet, thus supporting the need to remove socks and shoes of those at risk for developing diabetes or are already diagnosed with in the primary physician’s or specialist’s office.

In most practice settings, taking a sample of the nail for fungal culture remains the gold standard when assessing pathogens of question in diabetics and nondiabetics alike. A review by Mayser et al. showed that dermatophytes are the most commonly implicated etiologic agents in diabetic and nondiabetics alike [16]. The most common agents affecting the nails are (in order) Trichophyton rubrum and Trichophyton mentagrophytes var. interdigitale, followed by Candida species and non-dermatophytic molds [16].

In addition to examining the diabetic patient for nail disease, it is important to examine the feet for the presence of tinea pedis. Not only can a thickened, sharp nail create a potentially serious situation but also the presence of tinea pedis that is ulcerated, fissured, or bacterially superinfected interdigitally create an equally problematic situation [15, 17]. Regarding tinea pedis, the interdigital variants, moccasin type, and vesiculobullous infection are all possible in the diabetic patient. If there is mycotic nail involvement, the practitioner should assume tinea pedis is present or will be.

As discussed earlier, diabetic patients are particularly susceptible to fungal infections due to the modifications that occur in their immunological system; therefore, there is a need for a variety of therapies to accommodate the range of nail presentation and patient disease state [16]. Traditional treatment options for this condition include toenail debridement and drug therapies ranging from oral (terbinafine, itraconazole, fluconazole, griseofulvin) to topical (ciclopirox, efinaconazole, tavaborole) antifungal medications. However, prior to initiating antifungal therapy, it is imperative to establish a diagnosis, as other skin diseases may look similar to a fungal infection [15]. If nail mycosis is present and established, concomitant tinea pedis should be treated as well.