[Histological diagnosis of onychomycosis].
It is mandatory to establish a firm diagnosis before starting systemic antimycotic therapy because of potential side effects and relatively high therapy costs. Direct microscopy and fungal culture are the most widely employed diagnostic tools. Not infrequently, a strong clinical suspicion cannot be proven by even repeated direct microscopic examinations and fungal cultures. In these cases histologic examination is a simple alternative. We identified 32 cases of histopathologically proven onychomycosis; of the 29 where a fungal culture was performed, only 14 (48%) were positive. Direct microscopy was performed only in 12 cases since most had repeatedly negative direct microscopy results before being referred to us; 5 cases (42%) were positive. Histopathology for onychomycosis has several advantages in addition to its sensitivity--particularly in difficult to diagnose cases: lack of the danger of contamination, a permanent preparation with the possibility to demonstrate fungal invasion of the nail organ and to identify other or concomitant nail disorders, simplicity of the procedure; quicker results as compared to fungal culture. Exact specification of the fungus is not possible with histology. Even though histological examination is not mentioned in the current guidelines for onychomycosis of the German Dermatological Society, it should be kept in mind as a simple but sensitive diagnostic tool.