Causes
- Athlete’s foot is the commonest of a group of topical fungal infections caused by dermatophytes, organisms that invade and proliferate on the outermost horny layer (stratum corneum) of the skin, hair and nails.
-They do not normally penetrate deeper into the skin or tissues.
- Dermatophytes tend to thrive in areas of the body that are occluded and moist.
- The common infecting organisms are Trichophyton, Microsporum and Epidermophyton species.
- The infection is easily transmitted in moist or humid locations, e.g. sports clubs, gyms and swimming-pool changing rooms, hence the common name of the condition. It is also associated with the use of occlusive footwear such as trainers.
Signs and symptoms
- Infection usually starts in the toe webs, especially in the fourth web space (next to the little toe), where the tissue can become macerated, white and cracked.
- Infection can spread to the soles, heels and borders of the foot.
- Painful itching is common.
- The skin may fissure and allow entry of bacterial infection.
- The sole may be affected, making the condition more difficult to diagnose and differentiate from psoriasis or eczema.
- With persistent infection the toenails may become involved, becoming dull, opaque and yellow in appearance. Over time the nail hardens and then starts to crumble
Treatment
- Treatments available for the treatment of athlete’s foot are antifungals. Salicylic acid is also included in some preparations.
- Terbinafine and the imidazoles are widely accepted as being the most effective treatments for athlete’s foot. Little overall difference in efficacy has been found between them, although terbinafine clears infections up to four times more quickly.
- Griseofulvin has also been found an effective treatment.
- Undecenoic acid and its derivatives are thought to be suitable for mild forms of athlete’s foot characterised by dry scaling of tissue, but are less effective where the skin is macerated and moist.
Antifungals
Compounds available are: imidazoles, terbinafine, griseofulvin, tolnaftate, undecenoates and benzoic acid.
Imidazoles
- Imidazoles licensed for treatment of athlete’s foot without prescription are clotrimazole, econazole, ketoconazole, miconazole and sulconazole.
- They act by inhibiting the biosynthesis of ergosterol, a constituent of the fungal cell membrane, resulting in disruption of the cell.
- These compounds also possess activity against Gram-positive bacteria, which is useful, as secondary bacterial infection may complicate the fungal infection.
- Application twice or three times daily is recommended, and treatment for at least a month is generally advised to ensure that this tenacious infection is eradicated.
Terbinafine
- Terbinafine is an allylamine derivative with a broad spectrum of antifungal activity.
- It is available as a 1% cream which is applied once or twice daily for 1 week,
a 1% gel which is used once daily for 1 week, and a cutaneous solution which
requires only a single application.
Griseofulvin
- Griseofulvin is exclusively active against dermatophytes, through inhibition of cellular mitosis.
- It also binds to host cell keratin and reduces its degradation by fungal keratinases.
- It may also interfere with dermatophyte DNA production.
It is available as a 1% topical spray. One spray is applied daily, increasing to three sprays daily for more severe or extensive infection affecting the sides or soles of the feet.
- Treatment should be continued for 10 days after lesions have disappeared.
- The treatment period should not exceed 4 weeks.
Tolnaftate
- Tolnaftate is believed to act by distorting fungal hyphae and stunting mycelial growth.
- It is active against all species responsible for athlete’s foot but has no antibacterial activity.
- It should be used twice daily and treatment should be continued for up to 6 weeks.
- It is well tolerated when applied to intact or broken skin, although slight stinging on application is probable. Skin reactions are rare and include irritation and contact dermatitis.
Undecenoates
- Both undecenoic acid and zinc undecenoate are used in proprietary athlete’s foot preparations.
- Zinc undecenoate has astringent properties, which helps to reduce the irritation and inflammation caused by the infection.
- Undecenoic acid, the active antifungal entity, is liberated from the zinc salt on contact with moisture on the skin.
- Up to 4 weeks’ treatment may be needed to produce therapeutic results.
- Irritation occurs rarely after application of undecenoic acid or its salts.
Benzoic acid
- Benzoic acid has antifungal activity, lowering the intracellular pH of infecting organisms.
- It is combined with salicylic acid (see below) in an emulsifying ointment base in Benzoic Acid Ointment Compound BP (Whitfield’s ointment).
- This preparation has been in use for over 90 years but more cosmetically acceptable
products are now available.
- Benzoic acid may cause irritation of the skin, and should not come into contact with the eyes or mucous membranes.
Salicylic acid
Salicylic acid alone has little or no antifungal activity but it facilitates the penetration of other drugs into the epidermis.
- Preparations for athlete’s foot containing salicylic acid therefore also contain antifungal constituents; it is present in Whitfield’s ointment and some proprietary preparations.
- At concentrations above 2% salicylic acid has a keratolytic effect, causing the keratin layer of the skin to shed.
- Keratolysis is achieved by increasing the hydration of the stratum corneum, softening the cells and facilitating dissolution of the intracellular cement that bonds the cells together so that they separate and detach (desquamate).
- Moisture is essential to this process and is provided by either the water in the formulation or the occlusive effect produced by its application to the skin.
- Although salicylic acid is readily absorbed through the skin, salicylate poisoning is highly unlikely to result from application to a small area for the limited period of treatment for athlete’s foot.
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