Ongoing agony of the feet

HISTORY
This 66-year-old woman has had a very itchy rash on her left foot for several months. She has tried applying a number of different OTC and prescription medications—including betamethasone dipropionate cream and econazole cream—without successful resolution of the problem.

She denies having any other skin problems, and there is no relevant family history. The patient is retired and lives alone with her cat. Medical history is remarkable for rheumatoid arthritis, for which she takes methotrexate.

EXAMINATION
The dorsum of her left foot is covered with a sharply demarcated, papulosquamous red rash. Interestingly, the interdigital areas are spared, as are the sole and the entire right foot. The patient’s elbows, knees, and scalp exhibit no significant changes. On further examination, a fine, slightly pink, powdery rash is noted on the sides of both feet (including the heels).

A KOH prep of the rim of the feet is positive for fungal elements, as is a similar microscopic examination of scrapings from the dorsum of the left foot. In fact, the fungal elements seen on the latter are so numerous and dense that they are initially difficult to see.

What is the diagnosis?

DISCUSSION
Rashes on the dorsum of the feet are almost never of fungal origin; rather, they usually represent contact (not eczematous) dermatitis. That’s partly because fungi typically require more heat and moisture than the dorsum can provide. Two things are needed to allow it to flourish in this unusual location: immune suppression and a source for the fungi.

Methotrexate and the prolonged use of a potent topical steroid were the likely culprits in terms of immune suppression. Both reduce the chemotactic response that these dermatophytes normally trigger, allowing them to multiply unchecked. But where did the fungi come from in the first place?

Traditionally, three kinds of tinea pedis have been described: the well-known interdigital type, typically affecting the space between the third and fourth or the fourth and fifth toes; the so-called inflammatory type, which presents acutely with highly pruritic vesicles and pustules on the plantar surface (most commonly on the instep); and the most common but least recognized of all, the moccasin type, which causes few if any symptoms and often flies under the patient’s radar. The rash it causes is faint and dry and covers the rim of the foot (sparing the toes).

But, given the right circumstances, it can serve as a reservoir for infection on the foot and leg. The resultant infection can be cured, but moccasin-variety tinea pedis is considered incurable, due to the ubiquitous nature of the organism and patient’s demonstrated susceptibility. The causative organism is almost always Trichophyton rubrum, by far the most common dermatophytic pathogen.

TREATMENT
Fortunately, T. rubrum responds well to oral terbinafine therapy, (250 mg/d). This patient was provided a month’s supply, along with topical miconazole to be applied twice daily until the rash clears. Use of the latter on the sides of the feet is also advised, for purposes of control.

TAKE-HOME-LEARNING-POINTS
• The performance of the KOH prep can be crucial in terms of establishing the correct diagnosis, but it also gives everyone involved confidence in the diagnosis and treatment. The only way to learn how to do KOH preps is to do them.
• Moccasin-variety tinea pedis is the most common but the least recognized of the three major types. Because it is chronic and asymptomatic, patients rarely know what it is, although it can serve as a reservoir for infection elsewhere.
• Fungal infections don’t just happen. Failure to establish a source can doom the patient to repeated episodes.
• Fungal infections (dermatophytosis) in odd places usually involve immune suppression, typically from the use of topical steroids and/or systemic immunosuppressants.

HISTORY
This 66-year-old woman has had a very itchy rash on her left foot for several months. She has tried applying a number of different OTC and prescription medications—including betamethasone dipropionate cream and econazole cream—without successful resolution of the problem.

She denies having any other skin problems, and there is no relevant family history. The patient is retired and lives alone with her cat. Medical history is remarkable for rheumatoid arthritis, for which she takes methotrexate.

EXAMINATION
The dorsum of her left foot is covered with a sharply demarcated, papulosquamous red rash. Interestingly, the interdigital areas are spared, as are the sole and the entire right foot. The patient’s elbows, knees, and scalp exhibit no significant changes. On further examination, a fine, slightly pink, powdery rash is noted on the sides of both feet (including the heels).

A KOH prep of the rim of the feet is positive for fungal elements, as is a similar microscopic examination of scrapings from the dorsum of the left foot. In fact, the fungal elements seen on the latter are so numerous and dense that they are initially difficult to see.

What is the diagnosis?

DISCUSSION
Rashes on the dorsum of the feet are almost never of fungal origin; rather, they usually represent contact (not eczematous) dermatitis. That’s partly because fungi typically require more heat and moisture than the dorsum can provide. Two things are needed to allow it to flourish in this unusual location: immune suppression and a source for the fungi.

Methotrexate and the prolonged use of a potent topical steroid were the likely culprits in terms of immune suppression. Both reduce the chemotactic response that these dermatophytes normally trigger, allowing them to multiply unchecked. But where did the fungi come from in the first place?

Traditionally, three kinds of tinea pedis have been described: the well-known interdigital type, typically affecting the space between the third and fourth or the fourth and fifth toes; the so-called inflammatory type, which presents acutely with highly pruritic vesicles and pustules on the plantar surface (most commonly on the instep); and the most common but least recognized of all, the moccasin type, which causes few if any symptoms and often flies under the patient’s radar. The rash it causes is faint and dry and covers the rim of the foot (sparing the toes).

But, given the right circumstances, it can serve as a reservoir for infection on the foot and leg. The resultant infection can be cured, but moccasin-variety tinea pedis is considered incurable, due to the ubiquitous nature of the organism and patient’s demonstrated susceptibility. The causative organism is almost always Trichophyton rubrum, by far the most common dermatophytic pathogen.

TREATMENT
Fortunately, T. rubrum responds well to oral terbinafine therapy, (250 mg/d). This patient was provided a month’s supply, along with topical miconazole to be applied twice daily until the rash clears. Use of the latter on the sides of the feet is also advised, for purposes of control.

TAKE-HOME-LEARNING-POINTS
• The performance of the KOH prep can be crucial in terms of establishing the correct diagnosis, but it also gives everyone involved confidence in the diagnosis and treatment. The only way to learn how to do KOH preps is to do them.
• Moccasin-variety tinea pedis is the most common but the least recognized of the three major types. Because it is chronic and asymptomatic, patients rarely know what it is, although it can serve as a reservoir for infection elsewhere.
• Fungal infections don’t just happen. Failure to establish a source can doom the patient to repeated episodes.
• Fungal infections (dermatophytosis) in odd places usually involve immune suppression, typically from the use of topical steroids and/or systemic immunosuppressants.

HISTORY
This 66-year-old woman has had a very itchy rash on her left foot for several months. She has tried applying a number of different OTC and prescription medications—including betamethasone dipropionate cream and econazole cream—without successful resolution of the problem.

She denies having any other skin problems, and there is no relevant family history. The patient is retired and lives alone with her cat. Medical history is remarkable for rheumatoid arthritis, for which she takes methotrexate.

EXAMINATION
The dorsum of her left foot is covered with a sharply demarcated, papulosquamous red rash. Interestingly, the interdigital areas are spared, as are the sole and the entire right foot. The patient’s elbows, knees, and scalp exhibit no significant changes. On further examination, a fine, slightly pink, powdery rash is noted on the sides of both feet (including the heels).

A KOH prep of the rim of the feet is positive for fungal elements, as is a similar microscopic examination of scrapings from the dorsum of the left foot. In fact, the fungal elements seen on the latter are so numerous and dense that they are initially difficult to see.

What is the diagnosis?

DISCUSSION
Rashes on the dorsum of the feet are almost never of fungal origin; rather, they usually represent contact (not eczematous) dermatitis. That’s partly because fungi typically require more heat and moisture than the dorsum can provide. Two things are needed to allow it to flourish in this unusual location: immune suppression and a source for the fungi.

Methotrexate and the prolonged use of a potent topical steroid were the likely culprits in terms of immune suppression. Both reduce the chemotactic response that these dermatophytes normally trigger, allowing them to multiply unchecked. But where did the fungi come from in the first place?

Traditionally, three kinds of tinea pedis have been described: the well-known interdigital type, typically affecting the space between the third and fourth or the fourth and fifth toes; the so-called inflammatory type, which presents acutely with highly pruritic vesicles and pustules on the plantar surface (most commonly on the instep); and the most common but least recognized of all, the moccasin type, which causes few if any symptoms and often flies under the patient’s radar. The rash it causes is faint and dry and covers the rim of the foot (sparing the toes).

But, given the right circumstances, it can serve as a reservoir for infection on the foot and leg. The resultant infection can be cured, but moccasin-variety tinea pedis is considered incurable, due to the ubiquitous nature of the organism and patient’s demonstrated susceptibility. The causative organism is almost always Trichophyton rubrum, by far the most common dermatophytic pathogen.

TREATMENT
Fortunately, T. rubrum responds well to oral terbinafine therapy, (250 mg/d). This patient was provided a month’s supply, along with topical miconazole to be applied twice daily until the rash clears. Use of the latter on the sides of the feet is also advised, for purposes of control.

TAKE-HOME-LEARNING-POINTS
• The performance of the KOH prep can be crucial in terms of establishing the correct diagnosis, but it also gives everyone involved confidence in the diagnosis and treatment. The only way to learn how to do KOH preps is to do them.
• Moccasin-variety tinea pedis is the most common but the least recognized of the three major types. Because it is chronic and asymptomatic, patients rarely know what it is, although it can serve as a reservoir for infection elsewhere.
• Fungal infections don’t just happen. Failure to establish a source can doom the patient to repeated episodes.
• Fungal infections (dermatophytosis) in odd places usually involve immune suppression, typically from the use of topical steroids and/or systemic immunosuppressants.