A Closer Look At Pustular Tinea Pedis

When tinea pedis presents in its pustular or inflammatory form, it can be quite alarming. Multiple tense pustules on a mildly erythematous base can be visible throughout the plantar aspects of one or both feet.¹ Plantar skin may also demonstrate scaling plaques, papules, vesicles, fissures and/or hyperkeratoses, characteristics that are more typical of tinea pedis.² Upon drainage of the pustular lesions, the consistency of the purulent material is thicker than what one would expect with a bacterial abscess.

Trichophyton mentagrophytes is the most common causative organism present in the inflammatory type of tinea pedis.³ Although rare, the formation of pustules is in theory caused by the appearance of neutrophils in the epidermis or dermis.¹ These lesions are often painful with resolution of pain following their drainage.³

The differential diagnosis is similar to that of the more classic tinea pedis and should include allergic contact dermatitis, atopic dermatitis, psoriasis, erythrasma, xerosis and eczema.² Moreover, if a patient presents with pustules to the plantar foot, one must preclude tinea pedis.¹

Rule out a concomitant bacterial infection to ensure proper treatment and resolution of the infection.³ One can do this simply by obtaining a Gram stain as well as culture and sensitivity on one of the pustular lesions. One should not be surprised, however, to find a culture that is negative for bacteria.

In a case study by Hirschmann and colleagues, a Gram stain of pus demonstrated many neutrophils and branching hyphae, but no bacteria.¹ The authors also noted that a subsequent potassium hydroxide (KOH) preparation test revealed “enormous numbers of hyphae.” Finally, the study found bacterial cultures of the lesions in this study to be sterile while the fungal media grew Trichophyton rubrum.  

Clinicians can diagnose pustular tinea pedis infection in a variety of ways. As Hirschmann and colleagues showed us, a simple in-office test that one can perform is the KOH preparation.¹ Obtain specimens by simple scraping of the pustular lesions onto a slide. If fungal hyphae are present, this is enough to begin treatment for tinea pedis.³ Fungal and bacterial cultures, while not routine for tinea pedis, may be more useful for the pustular type of tinea pedis and in evaluating for concomitant bacterial infection. Sabouraud dextrose agar is the most frequently used isolation medium for dermatophytes.³

References

1. Hirschmann JV, Raugi GJ. Pustular tinea pedis. J Am Acad Dernatol. 2000; 42(1):132-133.
2. Field LA, Adams BB. Tinea pedis in athletes. Int J Dermatol. 2008; 47(5):485-492.
3. Kutlubay Z, Yardımcı G, Kantarcıoğlu AS, Serdaroğlu S. Acral manifestations of fungal infections. Clin Dermatol. 2017; 35(1):28–39.  
4. Drake LA, Dinehart SM, Farmer ER, et al. Guidelines of care for superficial mycotic infections of the skin: Tinea corporis, tinea cruris, tinea faciei, tinea manuum, and tinea pedis. J Am Acad Dermatol. 1996; 34(2):282-286.
5. Al Hasan M, Fitzgerald SM, Saoudian M, Krishnaswamy G. Dermatology for the practicing allergist: Tinea pedis and its complications. Clin Mol Allergy. 2004; 2(1):5.