Diagnosing And Treating A Scaling Interdigital Rash

A 39-year-old female presented to my office with the chief complaint of a rash between her big toe and second toe for a six-month duration. She described the itch as severe at times. The patient noticed tiny blisters that would pop and express a clear fluid. Ultimately, she would have scaling of skin where the blisters were located.    

The patient had tried over-the-counter creams for athlete’s foot and has used a prescription medication, ketoconazole (Nizoral, Janssen Pharmaceuticals), which her primary care physician prescribed. She said she only got minimal improvement. The patient denied any rashes in other areas of her body. She denied having a history of psoriasis or eczema.    

Her past medical history was remarkable for dyslipidemia and chronic urinary tract infections. Her daily medications included fenofibrate (Tricor, AbbVie) and norethindrone/mestranol (Necon, Watson Pharma). She would take nitrofurantoin (Macrobid, Procter and Gamble Pharmaceuticals) as needed for urinary tract infections. She denied any history of asthma, hay fever, seasonal allergies or atopy as a child. Her past surgical history was remarkable for a cholecystectomy and breast reduction. She denied tobacco or alcohol use. The patient was employed as a bank teller.    

The physical examination revealed a 5-foot-7, 195-pound female who appeared to be in good health. The vascular exam was remarkable for strong pedal pulses and brisk capillary refill to the toes. Her neurologic examination was normal with symmetric and equal deep tendon reflexes, and epicritic sensation intact to her toes. The dermatologic examination revealed a rash in the first webspace of the right foot. The webspace was erythematous and wetness was present. A fine scale was present. An element of lichenification was also visible. The orthopedic examination displayed symmetric pain-free range of motion of the ankle, subtalar and midtarsal joints.

Key Questions To Consider

1. What are the differential diagnoses?

2. What are the key symptoms of this condition?

3. What factors can aggravate this condition?

4. What is the appropriate treatment of this condition?

Answering The Key Diagnostic Questions

1. The common differential diagnosis of a scaling interdigital rash of the foot includes: acute vesiculobullous tinea pedis, contact/allergic dermatitis, pustular psoriasis, dyshidrotic eczema and pemphigus vulgaris.

2. Acute stages include vesicles on or between the fingers and/or toes, severe itching and possible stinging. Chronic cases may include peeling and scaling of skin, crusting and fissuring of skin. Severe cases may include secondary Staphylococcus infection. Lesions affecting the periungual region can cause nail dystrophy.

3. Solvents and detergents may strip the protective oils that allow the skin to dry.

4. Use a short course of topical steroids and keep skin moisturized.

A Closer Look At The Differential Diagnoses

Tinea pedis is one of the most common rashes on the foot. Typical presentations include the acute variety, which is typically wet with vesicles and blisters in the web spaces. This infection is typically caused by T. mentagrophytes. Fissuring of skin in the web spaces is common especially when there is maceration, which typically involves the third and fourth web spaces. The chronic type of tinea pedis, which is usually caused by T. rubrum, is generally dry with scale, fissures and lichenification in a moccasin distribution. Although tinea pedis can occur on the dorsal foot, the most common areas affected are the toes and plantar foot. Tinea pedis is less likely than contact dermatitis to be symmetrical.    

Contact dermatitis is a dermatologic condition in which an inflammatory reaction occurs from direct contact with a substance. There are two types of contact dermatitis. Irritant dermatitis is the most common type, which is typically caused by acids, alkalis, fabric softeners and solvents. Clinically, irritant contact dermatitis generally looks like a sunburn.    

Allergic contact dermatitis results from exposure to a substance to which the body has sensitivity or allergic reaction. The common culprits causing allergic contact dermatitis include tape and adhesives, topical antibiotics (especially neomycin), fabrics, metals (nickel), rubber/latex gloves, plants (poison ivy), and fragrances in perfumes, soaps and lotions.   

 The typical rash that occurs from allergic contact dermatitis includes redness of the skin, and vesicles that weep and crust. Conversely, irritant dermatitis often appears as red, dry, cracking of skin (fissures).    Psoriasis is a common condition classified as an autoimmune disorder of the skin. Plaque psoriasis is characterized by a silvery scale on an erythematous base. Itching can be severe with psoriasis. The most common areas of the body for psoriasis include the scalp, elbows, knees and back. There can be excoriations and crusting of the primary lesion. Intense scratching can lead to lichenification of skin. Pustular psoriasis is an uncommon form of psoriasis. Typically, the condition is generalized (widespread) but can be present in an isolated area. When it comes to pustular psoriasis versus plaque psoriasis, the skin is dry, red and painful initially. There may be associated systemic symptoms of malaise, fever and muscle weakness. Often, there is a preceding illness such as an infection or a drug reaction with beta blockers, lithium, or withdrawal of oral or injected steroids.    

Pemphigus vulgaris is a dermatologic condition characterized by flaccid bullae of the skin and mucous membranes. The lesions are rarely pruritic and can be painful. The disease is an autoimmune disorder caused by antibodies attacking desmoglein 1 and 3. The mean age of onset is typically between 50 and 60 years of age. The most common areas of predilection include the trunk, intertriginous areas, neck and head.    

Things to consider when making the diagnosis in this particular case are that the patient failed antifungal treatments, the rash is unilateral, there is blistering and scaling of skin, and there is intense itch. A contact or allergic dermatitis is less likely to be the cause in this case due to the unilateral presentation. Vesiculobullous tinea pedis is more of a favorable diagnosis due to the unilateral presentation and blistering of the skin. However, she did fail antifungal therapy. Pemphigus vulgaris lesions are rarely associated with pruritus but they can be painful. The lesions tend to be larger blisters with a flaccid appearance. Pustular psoriasis is rare. The lesions tend to look like white, studded vesicles/blisters. The skin tends to be dry, not wet. There may be pain versus itching.    

Dyshidrotic eczema is the correct diagnosis in this case. Dyshidrotic eczema is also known as pompholyx. This is a common type of eczema of the hands and feet. The exact causation is unknown. Acute stages include vesicles on or between the fingers and/or toes. There may be symptoms of stinging but typically the condition is associated with severe itching. In chronic cases, there may be peeling and scaling of skin, crusting and fissuring of skin. In severe cases, a secondary infection of Staphylococcus can occur. In addition, lesions affecting the periungual region can cause nail dystrophy.    

As with other types of eczema, aggravating factors can be solvents and detergents, which strip the protective oils that allow the skin to dry. As a side note, nickel allergy is commonly associated with patients who have dyshidrotic eczema.

How To Treat Dyshidrotic Eczema

The treatment of dyshidrotic eczema is similar to that of other types of eczema. Use a short course of a topical steroid. Patient education on keeping skin hydrated is very important. Discuss with the patient that dry skin has nothing to do with the lack of oils in the skin, but the lack of water/moisture. Drinking water is important to keep skin hydrated and the best time to use a moisturizing cream is within two minutes of getting out of the shower or bath. This technique traps moisture in the skin versus adding oils to the skin. Hot water is very drying to the skin and patients should avoid it. Also, salts are very drying as they pull moisture out of the skin so discourage the use of salt baths.    

Commonly prescribed topical steroids come in ointments, creams and gels. Ointments are best for dry, thickened skin, which is typically visible in chronic stages. Avoid ointments in skin folds. Ointments are more potent than creams. Steroid creams are ideal for wet and weeping eczema. Creams are better for skin folds and you should have your patient rub in the cream until it disappears. Leaving clumps of cream can increase maceration. Gels are best for wet interdigital applications due to their drying effect. Also, gels are preferred in areas of hair.

In Conclusion

This case presentation included a unilateral, itching rash in the first webspace, which was not responsive to antifungal medication. A skin scraping for a periodic acid-Schiff stain was negative for fungal infection and I made the diagnosis of dyshidrotic eczema. The treatment included a two-week course of triamcinolone 0.1% cream (Kenalog, Bristol-Myers Squibb). Once the eczematous rash resolved, the patient started proper prevention by avoiding irritants and using moisturizing creams.    

Dr. Fishco is board-certified in foot surgery and reconstructive rearfoot and ankle surgery by the American Board of Podiatric Surgery. He is a Fellow of the American College of Foot and Ankle Surgeons, and is a faculty member of the Podiatry Institute. Dr. Fishco is in private practice in Phoenix.