Dr Chander Grover Kubba, Senior Resident Dept Of Dermatology &STD, MAMC
Onychomycosis is a fungal infection of the nail unit caused by yeast, molds or dermatophytes. Tinea unguium refers specifically to a dermatophyte infection of the nail unit and is the commonest type of onychomycosis (91% of the fungal infections of the nail )1. Trichophyton rubrum and mentagrophytes are amongst the commonest pathogens. Candida is responsible for 5.5% and non-dermatophyte molds for 3% of the cases.
An overall prevelance of 1.5 - 8% in the general population has been found2 which goes upto 90% in the elderly3. This finding has not been confirmed in various studies4. No significant differences in the prevalence of onychomycosis have been found with respect to sex distribution5. It carries a significant psychological morbidity6,7. Toenails are more commonly involved than the finger nails8. Onychomycosis is probably the most common nail disorder, accounting for upto 50% of all nail diseases9. Predisposing factors include excessive perspiration, moisture of feet (hyperhidrosis), tight fitting footwear, trauma to nails, diabetes mellitus, immunosuppression or a poor peripheral circulation.
Distal and lateral subungual onychomycosis (DLSO) is the most frequently encounterd type4,8,9. The most common causative agent is T.rubrum4. It is primarily a nail bed infection. Earliest stages show a yellowish or blackish discoloration of lateral or distal nail plate, whereas advanced cases show subungual hyperkeratosis, crumbling and disintegration of nail plate and may progress to a Total dystrophic onychomycosis (TDO). Severe cases may show yellow spikes which represent concentrated foci of fungal elements (Dermatophytoma)11. According to Zaias et al, DLSO caused by T. rubrum may have an autosomal dominant inheritance12. One hand, two feet dermatophytosis may demonstrate concomitant onychomycosis.
Proximal subungual onychomycosis (PSO) is a less common pattern, affecting toenails and finger nails equally. There is a whitish discoloration under the proximal part of the nail. The overlying nail plate is generally normal. Most common organism involved is Trichophyton rubrum. This pattern is seen with a higher frequency in AIDS patients especially proximal white subungual onychomycosis (PWSO)13. It is believed that a preceding episode of trauma plays an important role14. PSO with Fusarium oxysporum has been reported in immunocompetent patients15.
Superficial white onychomycosis (SWO) is generally seen in elderly and affects the toenails. It is seen in the form of a chalky white discoloration of the nail plate. The underlying nail bed is completely normal. T.mentagrophytes is the most common organism involved apart from Aspergillus terreus, Fusarium spp. and Acremonium spp. This is the only form that can be treated with topical medications as well, because of the superficial nature of these lesions.
Candida commonly affects the nails damaged by other conditions as in the cases of onycholysis, chronic paronychia etc. True candidal onychomycosis is a rare condition, seen in patients lacking an intact immune system, such as in chronic mucocutaneous candidiasis (CMC), HIV infection etc. Candida species is usually a poor invader of nail keratin, but in CMC, the organism can attack the nail and cause nail thickening, hyperkeratosis and candidal granulomas in the nail.
Various non-dermatophyte molds e.g. Scopulariopsis brevicaulis, Scytalidium dimidiatum16 and hyalinum, Aspergillus species17, Fusarium species, Onychocola canadensis18 etc. may invade the nail under certain conditions. It appears that the incidence of these infections is rising in recent years19. Certain features, which help in identifying the saprophytic etiology of onychomycosis, include the affection of great toenails and an absence of concomitant fungal infections of the skin.
Onychomycosis needs to be confirmed in a dystrophic nail, before starting any treatment. The sequential management of a case of suspected onychomycosis is as outlined in Fig. 1.
Special techniques required to obtain viable fungal hyphae have been described20. These if utilized properly, help in increasing the yield of KOH examination or of culture.
The process of KOH preparation can be hastened by the use of Dimethyl sulfoxide (DMSO). With the help of Chlorazol black E and Parker’s blue-black ink21, Congo red, Methylene blue etc. many false positive result may be eliminated. Fluorescent microscopy is also a useful technique22,23. Fluorescent dyes like Acridine orange or Calcoflour white may be used.
Culture is useful in microscopy negative cases. It also provides a species identification so as to enable proper treatment. Simultaneous culture should be obtained on media with and without cycloheximide so as to identify any saprophytic species. An Australian study has reported an average strike rate of 45% for nail samples examined by both direct microscopy and culture24. They examined a total of 32,000 nail samples with 14,200 reported as positive. Out of these 10-40% were positive by microscopy alone and 10-30% were positive by culture alone. Thus it is essential to perform both direct microscopy and culture on all specimens24.
Histopathology has diagnostic value in caswes with onychomycosis. The finding of hyphae or spores in a nail plate permits a diagnosis of onychomycosis. Other features, which may be present, include subungual hyperkeratosis, neutrophilic spongiosis, epidermal hyperplasia and a lymphocytic infiltrate. Extravasated RBC’s may be seen occassionally. This is a very useful method as upto 50% of the cases may be negative on KOH and culture25 because the spores or hyphae are characteristically located in the cornified layers of nail bed, deep portion of nail plate or the hyponychium. Some workers even advocate a routine histopathological examination of the nail plate specimens as it confirms that the nail plate is actually invaded and reduces the false negative direct reports where the fungus is cultured from a microscopically negative nail26. Routine histological examination has been found out to be superior to culture and KOH examination27. It has also been suggested that dermatophytes, yeasts, and molds may be differentiated by their structure and organization on the basis of a histopathological examination28. The important differentiating features include:
· Dermatophytes usually appear as regular, straight, septate, branched filaments and tend to run parallel to the nail surface.
· Yeasts may appear as small grouping of cells inside the of the nail plate or as pseudofilaments running haphazardly within the nail.
· Non dermatophytes vary in their presentation according to species, but typically have irregular, sinuous or fronded hyphae.
Two new diagnostic techniques, immunohistochemistry and flow cytometry, provide an effective means of identifying different dermatophytes, yeasts and nondermatophytic molds28. Immunohistochemistry employs antibodies to certain fungi to enable positive identification in situ; whereas flow cytometry differentiates fungi on the basis of molecular differences. These techniques provide evidence that nondermatophytic molds and yeasts can actively invade nail tissue and that mixed infections do occur28.
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11. Tosti A, Peluso AM, Tanti PA et al. Nail lichen planus: a clinical and pathological study of 24 patients. J Am Acad Dermatol 1993, 28: 724-30.
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14. Cohen JL, Scher RK, Pappert AS. The nail and fungus infections. In Elewski BE (ed): Cutaneous fungal infections. Igaku Shoin, New York 1992, pp 106-123.
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16. Elewski BE, Greer DL et al. Hendersonula toruloidea and Scytalidium hyalinum. Arch Dermatol 1991, 127: 1041-1050.
17. Torres RJM, Madrenys BN, Siddat M et al. Aspergillus versicolor as a cause of onychomycosis: a report of 12 cases and susceptibility testing to antifungal drugs. J Cur Acad Dermatol Venereol 1998, 11(1): 25-31.
18. Gupta AK, Horgan CD, Summerbell RC. Onychomycosis associated with Onychocola canadensis: ten case reports and a review of literature. J Am Acad Dermatol 1998, 39(3): 407-10.
19. Dening DW, Evans EVG, Kibbler CC et al. Fungal nail disease: a guide to good practice (report of a working group of British society for medical mycology). Br Med J 1995, 311: 1277-81.
20. Daniel CR, Elewski BE, Gupta AK. Surgical pearl: Nail microniser. J Am Acad Dermatol 1996, 34: 278-280.
21. Cohen MM. A simple procedure for staining tinea versicolor (Malassezia furfur) with fountain pen ink. J Invest Dermatol 1954, 22: 9-10.
22. Chick EW, Behar VS. A simple flouroscent method for the detection of fungi in skin and hair: a combined stain with acridine orange and KOH. J Invest Dermatol 1961, 37: 103-6.
23. Monod M, Baudroz RF, Ramelet AA et al. Direct mycological examination in dermatology: A comparison of different methods. Dermatologica 1989, 179: 183-6.
24. Ellis DH. Mycology survey report. Australian Microbiology Quality Assurance Programme; 1995.
25. Norton LA. Nail disorders: A review. J Am Acad Dermatol 1980, 2: 451-472.
26. Suarez S, Silvers D, Scher R et al. Histologic evaluation of nail clippings for diagnosing onychomycosis. Arch Dermatol 1991, 127: 1517-19.
27. Machler DC, Kirsner RS, Elgart OW. Routine histological examination for the diagnosis of onychomycasis: an evaluation of sensitivity and specificity. Cutis 1998, 61(4): 217-9.
28. Pierard GE, Arrese JE, De Doncker P et al. Present and potential diagnostic techniques in onychomycosis. J Am Acad Dermatol 1996, 34: 273-7.
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