Skip to main content
Dermatology Diagnosis

When A Patient Presents With Interdigital Erosion, Mild Erythema And Scaling

Good medical practice is all about prevention. Early detection of simple disorders can lead to prevention of secondary complications as well as a reduction in healthcare costs.

A 48-year-old patient presents with a superficial erosion between his toes. He denies pain but reports intermittent bouts of scaling and clear drainage between his toes. Previous treatments included a variety of over-the-counter remedies with some improvement but without complete resolution. He has not applied any treatments in the last week. Last year, he was admitted to the hospital for a two-day stay for right limb cellulitis, which resolved with systemic antibiotics.

The patient notes hay fever symptoms that are controlled with over-the-counter loratadine. He denies any allergies to mediations. Current medications are metformin, losartan (Cozaar, Merck) and atorvastatin (Lipitor, Pfizer). His type 2 diabetes is controlled with a recent HbA1C of 6.0%. The patient takes 81 mg of aspirin daily for myocardial infarction and transient ischemic attack (TIA) prophylaxis. His review of systems is essentially non-contributory. The patient specifically denies having skin rashes, claudication, numbness or tingling.

On physical examination, no lymphadenopathy is present. The exam detects no current limb edema or redness. He reports that foot and ankle swelling preceded his hospitalization for cellulitis. Calf, ankle and foot circumferences are symmetrical. Muscle strengths are 5/5 with normal range of motion of all joints. No swollen or erythematous joints are visible. He has size 13 feet but he is wearing size 12 slip-on, narrow toe box dress shoes. Between his right third and fourth toes, there is a 3 cm oval erosion surrounded by mild erythema and scaling with a small amount of clear drainage.

Key Questions To Consider

1. What are the differential diagnoses?
2. What is the most likely diagnosis?
3. What are the key characteristics of this condition?
4. What tool can make the diagnosis easier?
5. What is the proper treatment and follow-up?

Answering The Key Diagnostic Questions
1. Intertrigo, tinea pedis, Candidiasis, erythrasma and possible contact dermatitis to OTC medications
2. Intertrigo with secondary erythrasma
3. An oval erosion surrounded by mild erythema and scaling with a small amount of clear drainage
4. Wood’s lamp
5. Topical therapy, erythromycin and patient education

What You Should Know About Intertrigo

In thin patients, there is little soft tissue apposition and plenty of air flow between the toes. Obese patients tend to have more digital adipose, which closes their pedal web spaces and obstructs airflow. In addition, narrow toe box shoes, which were worn by the patient in this case, can close the web spaces, creating a dark moist environment, akin to an Amazon rainforest that never really dries out.

Many patients with lower extremity lymphedema have interdigital maceration. The wet toe web is an ideal site to grow a variety of bacteria, fungi and yeasts, causing intertrigo. Perpetually damp skin diminishes its barrier function and may allow superficial bacterial colonization to become an actual infection. Superficial cracks in the skin from interdigital tinea open the door to limb cellulitis in patients with lymphedema. In fact, the causative bacteria of limb cellulitis are often the same species cultured from the toe webs of the patient.1

Pertinent Pearls On Treating The Patient’s Intertrigo And Secondary Erythrasma

Our patient has undiagnosed intertrigo as well as erythrasma that predisposed him to a bacterial cellulitis infection, resulting in an expensive hospital admission. The secondary erythrasma is common in the toe and finger webs of patients with diabetes. It tends to favor the third and fourth web spaces of the foot. Dress style shoes that are not the correct size may close toe web spaces. Predisposing factors include the high humidity of the juxtaposed skin surfaces of the toe spaces.

The causative organism is Corynebacterium minutissimum. A Gram stain of erythrasma shows delicate Gram-positive rods and filaments.2 Typically, Corynebacterium minutissimum requires tissue culture medium with fetal bovine serum in the agar and is not easily grown on conventional culture and sensitivity media.3 Molecular analysis of peripheral skin scraping is a newer approach to confirm the mix organisms in complex cases.

The most effective treatment for significant erythrasma infection is 500 mg of erythromycin twice daily for 14 days with cure rates as high as 100 percent.3 Milder cases clear with thorough daily cleansing, drying between the toes and applications of imidazole creams that suppress the weaker superficial bacteria as well as treat tinea and interdigital yeast infections. Drying agents like aluminum chloride hexahydrate and microporous cellulose powder and miconazole (Zeasorb-AF, Stiefel Laboratories) along with daily use of cotton cast padding folded between the closed toe webs helps to prevent the primary problem of excessively moist skin.4 Erythromycin 2% gel is also a good choice if available on the patient’s formulary.

Unfortunately, clinical workup may not be diagnostic in all cases of intertrigo. In complex cases, Kizny and colleagues found that real-time polymerase chain reaction (PCR) targeting the chitin synthase gene of dermatophytes or in scrapings of suspected lesions is highly specific.5 Interdigital Pseudomonas infections glow green-white due to pyoverdine.

One can treat intertrigo with interdigital cotton and absorbent powder. In addition, silver nitrate or 5% acetic acid solution, silver sulfadiazine cream or applying Castellani paint daily between the toes until the infection resolves often are effective. An oral quinolone effectively reduces the duration of infection.6,7 If empiric therapy is appropriate, econazole 2% cream is a good initial choice to counter interdigital bacteria, yeasts and dermatophytes.5

In addition to appropriate topical therapy, our patient needs problem-centered patient education as well as a shoe prescription for a longer shoe with a wider toe box.

Key Insights On The Value Of The Wood’s Lamp Examination

While many doctors are familiar with the Wood’s lamp, they may only use it occasionally because the lamp may not be readily available, typically needs to be plugged into an electrical outlet and can be costly as well as a bit bulky. A screening Wood’s light examination can be easier with a mini-LED ultraviolet lamp.

The Wood’s lamps available today vary with cost and ease of use. Actual medical device lamps generally are large plug-in devices that can cost $400. Modern black light sources may be specially designed fluorescent lamps, mercury vapor lamps, light-emitting diodes or incandescent lamps. Fluorescent black light tubes have a dark blue coating on the tube, which filters out most visible light.2

With the advent of the light emitting diodes and electronic miniaturization, an ultraviolet lamp powered by several watch batteries is suitable for simple screening, can be as small as an index finger and is available on the Internet for as little as $4. These lamps can fit in a coat pocket and often are coupled with both a small red laser pointer and white LED lamp. Once the mini-LED ultraviolet light detects fluorescence, one can complete a more thorough examination with a medical grade Wood’s lamp.

A traditional Wood’s lamp is a low-output mercury arc covered by a Wood filter (barium silicate and 9% nickel oxide), and emits wavelengths between 320 and 450 nm (peak 365 nm).8 Current medical Wood’s lamps utilize coated fluorescent tubes, which can be expensive to replace. The Wood’s light fluorescence offers a colored glow that is visible when certain substances such as collagen and porphyrins absorb the black light, and emit it again at a longer wavelength in the visible spectrum.2,9

In lower extremity dermatology, the primary purpose of the Wood’s lamp is to aid in the clinical diagnosis of interdigital scaling and secondary infection of wounds. While the most common causes of interdigital scaling are dermatophytes, yeasts and bacterial, all three appear similar in ambient light. Although interdigital tinea pedis and Candidiasis do not glow upon Wood’s light examination, erythrasma does, leading to a more accurate diagnosis and a refined treatment plan.  

In addition to Corynebacterium, Pseudomonas and Propionibacterium, all fluoresce different characteristic colors. Many other conditions may fluoresce. (See the table “A Quick Guide To Diagnosing Conditions With The Wood’s Lamp Examination” at right.) Head lice (Pediculus humanus capitus) fluoresce green. Secondary Pseudomonas infections in burn patients and hot tub folliculitis both fluoresce green. Oral tetracycline deposits well in skin, including the lunula of the nail unit, causing it to fluoresce yellow. Fluorescein dye attaches to the basement membrane of corneal abrasions and turns them yellow, and the dye is enhanced with a Wood’s lamp examination.10 The sharpness of the borders of hypopigmented vitiligo patches and the ash leaf patterns of tuberous sclerosis are enhanced by Wood’s light.

Researchers reported a very interesting case of white fluorescence of palmar and plantar spiny keratoses in a patient with hereditary punctate parakeratotic keratoderma.5 It is an autosomal dominant disorder with variable penetrance. Erkek and Ayva described the white fluorescence as resembling “stars under the moonlight.”11 A punch biopsy revealed compact vertical columns of parakeratosis within the epidermal invaginations, which lacked a granular cell layer. Both hereditary and sporadic forms of punctate parakeratotic keratoderma have been associated with internal malignancies so regular follow-op visits have been recommended.

What could be causing this fluorescence? Consider elastin, collagen, aromatic amino acids and surface contamination. The parakeratotic columns might be responsible for this phenomenon. It is unknown if other more common forms of palmar plantar keratoderma fluoresce as well.11

How To Use A Wood’s Lamp

Wood’s light lamp emits a narrow band of UVA but this is not worrisome UVB.9 However, one could ask patients to close their eyes during the examination as a precaution. Turn out the room light and the examiner’s eyes adapt to the darkened room within moments.

As with all examinations, it is best to follow the same sequence of motions. With the gloved non-dominant hand, spread apart the right fourth and fifth toes first. Examine the base and sulcus as well as the medial and lateral aspects of the adjacent digits. The medical device Wood’s lamp is larger and heavier but brighter as well. One can hold it 10 to 12 cm away to screen the entire sulcus simultaneously. Medical device lamps may have four 15 W ultraviolet bulbs while a watch battery-powered LED is rated at less than 5 mW. Hold the mini LED Wood’s lamp closer or about 5 cm away. Examine each web space sequentially from the patient’s right to left. Even though the spaces adjacent to the great toes are least likely to be involved, examine them as well as the lesser toe webs. A close-up digital image of a positive web space can be a strong educational tool for patients.

Keep in mind that some topical creams, soaps and fibers on the skin can present as a false positive glowing. Also consider that the porphyrins of Corynebacteria are water soluble and recent washing can lower the intensity of the fluorescence.2 Both bacterial colonization and bacterial infection will fluoresce with different intensities depending the number of organisms present. Detection of colonization can help prevent clinical infection before it emerges.

In Conclusion

Performing the Wood’s light examination is quick and easy as long as the lamp is at hand. If the lamp has to be plugged in, then it is less likely to be in use regularly. I used to perform the Wood’s lamp examination with a plug-in device only rarely. However, with a mini-Wood’s lamp in my pocket, I often see patients who can benefit from the sensitivity and specificity of the Wood’s lamp. Many cases of intertrigo and erythrasma may go undiagnosed for the lack of a simple Wood’s lamp examination.

Dr. Bodman is an Associate Professor at the Kent State University College of Podiatric Medicine. He is board certified by the American Board of Podiatric Medicine.


  1. Hilmarsdóttir I, Valsdóttir F. Molecular typing of Beta-hemolytic streptococci from two patients with lower-limb cellulitis: identical isolates from toe web and blood specimens. J Clin Microbiol. 2007 Sep;45(9):3131-2.
  2. Oakley A. Wood lamp skin examination. DermNet New Zealand. Available at Published August 2014. Accessed May 5,2017.
  3. Holdiness MR. Management of cutaneous erythrasma. Drugs. 2002;62(8):1131-41.
  4. Perri AJ. Candida-Erosio interdigitalis Blastomycetica. Available at Published Sept. 26, 2010. Accessed May 10, 2017.
  5. Kizny GA, McIver C, Kim M, Murrell DF, Taylor P. Clinical application of a molecular assay for the detection of dermatophytosis and novel non-invasive sampling technique. Pathology. 2016;48(7):720-726.
  6. Kates SG, Myung KB, McGinly KJ, Leyden JJ. The antibacterial efficacy of econazole nitrate in interdigital toe web infections. J Am Acad Dermatol. 1990;22(4):583-6.
  7. Moharan M, Ahmed MA, Karpenko O. Treating secondary infection in a patient with severe tinea pedis. Podiatry Today. Available at Published September 2012.
  8. Gupta LK, Singhi M K. Wood’s lamp. Indian J Dermatol Venereol Leprol. 2004;70(2):131-5.
  9. Blasco-Morente G, Arias-Santiago S, Pérez-López I, Martínez-López A. Coral-red fluorescence of erythrasma plaque. Sultan Qaboos University Med J. 2016; 16(3):e381–2.
  10. Jacobs, DS. Corneal abrasions and corneal foreign bodies: Clinical manifestations and diagnosis. UpToDate. Available at . Accessed May 15, 2017.
  11. Erkek, E, Ayva S. Wood’s light excites white fluorescence of type I hereditary punctate keratoderma. J Eur Acad Dermatol Venereol. 2007;21(7):993-4.
Dermatology Diagnosis
Myron Bodman, DPM
Back to Top