As podiatrists, we are often confronted with dermatophytes as the most common cause of foot infection. The main complication of chronic tinea pedis is fungal nail infection secondary to dermatophyte infection on the skin. Onychomycosis has always presented a clinical challenge as the dermatophyte is safely lodged between the nail bed and nail plate making topical and sometimes, even oral therapy less effective.
What is a dermatophytoma?
In 1998, British dermatologists, Evans and Roberts [1] first described a common clinical presentation we see in podiatry with fungal nails:
“Clinically, either a dense white linear area or a round white area is seen. The nail overlying these areas is onycholytic and when it is cut back a thick hyperkeratotic mass is revealed. Histology of this mass shows a densely packed clump of dermatophyte hyphae which are thick walled and somewhat abnormal looking. This mass is not particularly adherent to either the underside of the nail plate or the nail bed and can be readily removed. Clinically and histologically this lesion is a subungual dermatophytoma”.
Essentially, the dermatophytoma is a description of a dense fungal mass under the nail plate – difficult to distinguish from regular onychomycosis such as distal lateral invasion (DLSO). Dermatophytoma as described as being typically white, yellow or orange patches evident under the nail. They can form streaks or patches and may or may not be continuous with the free nail edge (hyponychium) Figure 1 [2].
Common on the first toenail
The initial paper suggested around 80% of cases to be located in the 1st toenail [1] and dermatophytoma was a histological anomaly that was responsible for the failure of topical and sometimes oral agents, owing to its loose attachment to the overlying nail plate and underlying nail bed. The dermatophytoma is often located in an “air gap” or onycholytic portion of the nail.
As with all fungal nail infection that requires treatment, diagnosis is key. Nail tip clippings for analysis commonplace in medical settings and, as we are aware, frequently negative as viable the fungus is located more proximally, and so traditional Potassium hydroxide (KOH) microscopy often fails. A paper in the Journal of Dermatology recommends the use of the Dermatophyte Test Strip as a more sensitive test for identifying dermatophytes in the dermatophytoma [3].
The dermatophytoma can be visualised as a densely packed mass of fungal hyphae and spores surrounded by a extracellular matrix that acts as a shield to antifungal treatment [4]. Their poor response to treatment has been compared to biofilms [5].
Implications for Practice
The dermatophytoma was described as being “a less reported” presentation of onychomycosis, but as a podiatrist, I suspect that much of the 1st toe onychomycosis that we see fits the medical description “dermatophytoma”. Many of the patients we see have this presentation and have had it for many years. The difficulty in treating dermatophytoma is well known to podiatrists.
Topical lacquers applied to the top of the nail do not penetrate effectively. However, we have the means to eradicate these lesions.
Research has shown physical means which remove or breach the overlying nail plate to expose the fungal mass may make it much more readily accessible to treatment. Reduction or thinning of the nail drill has been investigated in fungal nails undergoing treatment has shown benefit [6-8]. The use of fenestration with lasers or specialist drilling devices [9-11] has also been successful. Although less researched, surgical removal of the toenail is also often undertaken as a means of permitting access to fungal elements under the nail [12].
Ultimately, podiatrists have known about the dermatophytoma far longer than the dermatologists (before it was even named) and consequently have been using these physical modalities to improve outcomes for patients.
References
1. Roberts, D.T. and E.G.V. Evans, Sub-ungual dermatophytoma complicating dermatophyte onychomycosis. British Journal of Dermatology, 1998. 138: p. 189-190.
2. Gupta, A.K., T. Wang, and E.A. Cooper, Dermatophytomas: Clinical Overview and Treatment. J Fungi (Basel), 2022. 8(7).
3. Ishida, H., S. Noriki, and T. Mochizuki, Diagnostic clue for dermatophytoma using the dermatophyte test strip. The Journal of Dermatology, 2022. 50(2): p. e87-e88.
4. Costa-Orlandi, C.B., et al., In vitro characterization of Trichophyton rubrum and T. mentagrophytes biofilms. Biofouling, 2014. 30(6): p. 719-727.
5. Burkhart, C.N., C.G. Burkhart, and A.K. Gupta, Dermatophytoma: Recalcitrance to treatment because of existence of fungal biofilm. Journal of the American Academy of Dermatology, 2002. 47(4): p. 629-631.
6. Sumikawa, M., et al., Effects of foot care intervention including nail drilling combined with topical antifungal application in diabetic patients with onychomycosis. J Dermatol, 2007. 34(7): p. 456-464.
7. Harkless, L., et al. Study design of the IRON-CLAD trial:efficacy and safety of 12 weeks of oral terbinafine with and without nail debridement in the treatment of toenail onychomycosis. in Poster presentation at FIP World Congress. 2004. Boston, USA.
8. Malay, D.S., et al., Efficacy of debridement alone versus debridement combined with topical antifungal nail lacquer for the treatment of pedal onychomycosis: a randomized, controlled trial. J Foot Ankle Surg, 2009. 48(3): p. 294-308.
9. Borovoy, M. and M. Tracy, Noninvasive CO 2 laser fenestration improves treatment of onychomycosis. Clin Laser Mon, 1992. 10(8): p. 123-4.
10. Cordoba Diaz, D., M.E. Losa Iglesias, R. Becerro de Bengoa Vallejo, and M. Manuel Cordoba Diaz, Transungual Delivery of Ciclopirox Is Increased 3–4-Fold by Mechanical Fenestration of Human Nail Plate in an In Vitro Model. Pharmaceutics, 2019. 11(29).
11. Bristow, I.R., R. Baran, and M. Score, Rapid Treatment of Subungual Onychomycosis Using Controlled Micro Nail Penetration and Terbinafine Solution Journal of Drugs in Dermatology, 2016. 15(8): p. 974-978.
12. Grover, C., et al., Combination of surgical avulsion and topical therapy for single nail onychomycosis: a randomized controlled trial. British Journal of Dermatology, 2007. 157(2): p. 364-368.
Comments