Onychomycosis pathophysiology
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Overview
The pathophysiology of onychomycosis depends on the histological subtype.
PATHOGENESIS
Onychomycosis is usually preceded by a dry hyperkeratotic tinea pedis.Moist environment and micro traumatic pressure on the nail unit break the hyponychial seal, which allows penetration of the dermatophyte into the nail bed. The dermatophytes producekeratinases that begin the infection between the lesser toes, spread to the hyperkeratotic sole, and gradually extend to the distal hyponychial space of micro-traumatized nail units. Once the distal nail hyponychium is breached, the dermatophytes infect the nail bed, spreading proximally as onycholysis and subungual hyperkeratosis.
The acute infection occurs in the nail bed with a low-grade inflammatory response and progresses to a chronic phase as total dystrophic onychomycosis. Onychomycosis secondarily damages the viable nail matrix and invades the overlying nail plate, detaching and distorting it over time. The nail plate becomes elevated and misaligned as the infection becomes chronic. Studies show high levels of cytokines IL-6- and IL-10-positive cells in few cases.
Distal Subungual Onychomycosis
Distal subungual onychomycosis (DSO)is characterized by invasion of the nail bed and underside of the nail plate beginning at the hyponychium .[1]
Proximal Subungual Onychomycosis
Proximal subungual onychomycosis (PSO) occurs when organisms invade via the proximal nail fold through the cuticle area. It migrates distally through newly formed nail plate.
White Superficial Onychomycosis
White superficial onychomycosis (WSO) occurs when certain fungi invade the superficial layers of the nail plate directly. It forms well-delineated opaque white islands on the external nail plate. As the disease progresses, they coalesce and spread. The nail becomes rough, soft, and crumbly. As viable tissue is not involved, inflammation is minimal is seen. The toenails are primarily involved.
Total Dystrophic Onychomycosis
Total dystrophic onychomycosis may be the end result of any of the four main patterns of onychomycosis. The entire nail unit becomes thick and dystrophic.
Candida Onychomycosis
Candida onychomycosis can be divided into three general categories.
(i)Infection (whitlow) begins as a paronychia .Invasion by Candida spp., unlike dermatophytic invasion, penetrates the nail plate only secondarily after it has attacked the soft tissue around the nail . After the nail matrix gets infected, transverse depressions (Beau’s lines) may appear in the nail plate. The nail becomes convex, irregular, and rough and, ultimately, dystrophic.
(ii)Candida granuloma is common in patients with chronic mucocutaneous candidiasis and immunocompromised patients. The organism invades the nail plate directly and may affect the entire thickness of the nail, resulting in swelling of the proximal and lateral nail folds, ultimately giving the pseudo-clubbing or “chicken drumstick” appearance to the digit .
(iii)Candida onycholysis can occur when the nail plate has separated from the nail bed. Distal subungual hyperkeratosis can be seen as a yellowish gray mass lifts off the nail plate, mostly in hands. The lesion resembles that seen in patients with DSO.
Histologic pathology
Histologically, the acute lesion resembles psoraisis and manifests as spongiosis, acanthosis, papillomatosis with edema, hyperkeratosis and a dense inflammatory infiltrate. At the chronic stage of the infection, there are large amounts of compact hyperkeratosis, hypergranulosis, acanthosis, onychylosis and papillomatosis with sparse perivascular infiltrate. Dermatophytosis and subungual seromas can occur.
References
- ↑ Elewski BE (1998). "Onychomycosis: pathogenesis, diagnosis, and management". Clin Microbiol Rev. 11 (3): 415–29. doi:10.1128/CMR.11.3.415. PMC 88888. PMID https://www.ncbi.nlm.nih.gov/pubmed/9665975 Check
|pmid=value (help).