Diagnosing Onychomycosis: What's New?

Abstract

An overview of the long-established methods of diagnosing onychomycosis (potassium hydroxide testing, fungal culture, and histopathological examination) is provided followed by an outline of other diagnostic methods currently in use or under development. These methods generally use one of two diagnostic techniques: visual identification of infection (fungal elements or onychomycosis signs) or organism identification (typing of fungal genus/species). Visual diagnosis (dermoscopy, optical coherence tomography, confocal microscopy, UV fluorescence excitation) provides clinical evidence of infection, but may be limited by lack of organism information when treatment decisions are needed. The organism identification methods (lateral flow techniques, polymerase chain reaction, MALDI-TOF mass spectroscopy and Raman spectroscopy) seek to provide faster and more reliable identification than standard fungal culture methods. Additionally, artificial intelligence methods are being applied to assist with visual identification, with good success. Despite being considered the 'gold standard' for diagnosis, clinicians are generally well aware that the established methods have many limitations for diagnosis. The new techniques seek to augment established methods, but also have advantages and disadvantages relative to their diagnostic use. It remains to be seen which of the newer methods will become more widely used for diagnosis of onychomycosis. Clinicians need to be aware of the limitations of diagnostic utility calculations as well, and look beyond the numbers to assess which techniques will provide the best options for patient assessment and management.

Keywords: artificial intelligence; diagnosis; microscopy; onychomycosis; polymerase chain reaction; spectroscopy; tomography.

Figure 1

Examples of outputs produced for visual methods of identification. (A) Standard KOH exam photo showing hyphae; (B) KOH exam of hyphae using fluorescent microscopy; (C) Septate black hyphae in the nail plate (periodic acid-Schiff staining ×400) from Figure 2b in [7]; (D) Illustration of dermoscopy onychomycosis signs in the nail plate: jagged proximal edge (black outline), longitudinal striae (red arrows); (E) Optical coherence tomography: digital illustration of output through nail plate showing hyphae; (F) Confocal microscopy: illustration of visualized hyphae.

Figure 2

Examples of outputs produced for organism/molecular identification. (A) Illustration of Lateral Flow tests trips; (B) Example of RFLP output after PCR processing showing 3 samples being processed; (C) Digital illustration of MALDI-TOF spectrum output; (D) Digital illustration of Raman spectrum output (species 1—red; species 2—blue).