Essential Insights On Dermoscopy Of Plantar Pigmented Lesions

Could the use of dermoscopy save lives? Given the stark statistics with melanoma and delayed diagnosis, this author says dermoscopy could be a valuable tool in discerning benign from malignant lesions in the lower extremity in a more expeditious fashion.

Arguably, one of the most important clinical decisions a podiatrist must make on a nearly daily basis is to differentiate between benign and malignant presentations. Is the presenting change a variation of normal or a new problem that needs monitoring? Is the risk of a potential delay in diagnosis and increased risk of mortality justified?

Adding to the difficulty of the decision are the sheer number, variation in appearance and potential lethality of the ubiquitous plantar melanocytic nevus. A screening examination for skin lesions by a qualified observer takes only a few moments and can uncover melanomas on the back and posterior legs, areas the patient cannot easily view.¹

Here is the problem. One person dies of melanoma every hour and an estimated 76,100 new cases of invasive melanoma were diagnosed in the United States in 2014.² Malignant melanoma is still the most common primary malignant tumor arising on the foot.³ Thankfully, despite the annual growth in prevalence, survival rates are also improving. If physicians detect melanoma in time, the survival rates have improved from 49 percent (1950 to 1954) to 92 percent (1996 to 2003).⁴

The risk of death from melanoma naturally increases with age and solar exposure, but too many of the melanoma deaths still occur and too often during the most productive years of life. From 1973 to 2004, the melanoma incidence among males 15 to 39 years of age actually increased by 61 percent.⁵ The incidence among females more than doubled. Melanoma is the most common form of cancer for young adults 25 to 29 years old and the second most common form of cancer for young people 15 to 29 years old.⁶

Unfortunately, the overall five-year melanoma survival rate for African-Americans is only 77 percent versus 91 percent for Caucasians.⁷ Beyond the melanoma deaths, melanoma treatment costs our country $3.3 billion a year.⁸ Risk factors for developing melanoma are both environmental and genetic. About 10 percent of melanomas are familial and atypical nevi increase risk. A nevus count of more than 25 is strongly associated with developing melanoma.⁹

We can attribute about 86 percent of melanomas to exposure to ultraviolet (UV) radiation from the sun.¹⁰ Melanoma risk increases with sun exposure during childhood and tanning bed exposure before the age of 35. On average, a person’s risk for melanoma doubles if he or she has had just five sunburns.¹¹ Traits of light skin pigmentation, red or blond hair, and light eye color all increase melanoma risk.⁹ About 14 percent of melanomas arise in skin that receives little solar radiation.⁹ Despite feet being generally protected from sun exposure by shoes, melanoma can still arise spontaneously.

A Closer Look At The Prevalence Of Melanoma In The Foot
Pedal melanoma represents 3 to 15 percent of all cutaneous melanomas and is the most common primary malignant tumor of the foot.³ The prognosis of acral melanoma is generally poor, due primarily to a delay in diagnosis.¹² Along with the back, the posterior calf is at higher risk for undetected melanoma because it is difficult for patients to inspect these areas.

In populations with darker skin, melanoma occurs less frequently overall and has a particular predilection for the soles. Overall, darker skinned individuals’ mortality rates are higher than in the general population.⁹ Among Japanese patients, almost one-half of cutaneous melanomas occur in acral areas and approximately 30 percent affect the sole.¹³

The three typical types of pedal melanoma are superficial spreading, nodular and acral lentiginous melanoma. Acral lentiginous is the most common form of melanoma and occurs most often on the sole and beneath the toenail. Even though the foot receives less solar exposure, it turns out pedal melanoma is more likely to kill patients than more proximal thigh melanoma. Although the five-year survival rate for melanoma of the calf is 94 percent, the five-year survival rate for foot melanoma is only 77 percent.¹⁴

It would seem that the more proximal tumor would metastasize more readily. What accounts for this significant survival difference? Metzger and colleagues reviewed delayed diagnosis and found that many patients with acral melanomas initially presented to non-dermatologists because they did not suspect the problem to be a melanoma.¹⁵

This can also lead to misdiagnosis. Reports of initial misdiagnosis can range between 33 and 60 percent. Physicians have mistaken acral melanomas for fungal infection, onychomycosis, ulceration, granuloma and hematoma.¹⁵

There naturally is a “WNL” (“we did not look”) factor. It is essential for podiatrists to survey the lower extremity skin from the tibial tubercles to the tip of the toes as well as between the toes and posterior calves. Also, a patient’s inability to see his or her soles may delay diagnosis. Richards and colleagues found this to be a factor in their review of 590 melanomas.¹⁶ Unfortunately, any significant delay in diagnosis allows for the melanoma to grow thicker. The thicker the melanoma is at the time of diagnosis, the poorer the prognosis. Since melanoma treatment options may have limited efficacy, improving patient survival means earlier detection to decrease morbidity and mortality.

Early detection with a handheld dermatoscope increases the clinician’s diagnostic accuracy for pigmented lesions of the palms and soles, and may help in the recognition of acral melanoma at an early, more curable stage.^17-20

What Is Dermoscopy?  
Basically, the dermoscopic examination uses illuminated 10x magnifying lenses with a singularly important feature: The lenses are dually polarized. These lenses block incident light to allow clinical visualization of characteristic epidermal and dermal architecture of both benign and malignant lesions as well as a large number of dermatological disorders. Most dermatoscopes use multiple high performance LEDs, producing homogenous bright colors. Dermatoscopes can range in price from $250 to $2,000. There are even high quality attachments for your camera or smartphone. Dermoscopy essentially is a noninvasive clinical examination technique that one performs with a simple handheld instrument, which can lead to more frequent biopsies, not less frequent.

I am privileged to often work with student colleagues and introduce them to dermoscopy. Commonly, their first dermatoscopic examination of a real patient precipitates a “Wow!” reaction. For the first time they are able to visually link cutaneous macroscopic presentations with the characteristic epidermal and dermal features of a lesion.  

Plantar and palmar pigmented lesions are special. Since they occur in non-glabrous skin, they do not generally follow the A-E paradigm that guides the clinical diagnosis of glabrous skin. Anatomically, the palms and soles lack follicles and sebaceous glands but have a plethora of over 300 sweat ducts per square inch. Their architecture is much thicker and readily modified by repeated cycles of direct pressure of walking and grasping.

Melanocytes are special dendritic cells derived from neural crest, not ectoderm as one might think. Embryologically, melanocyte precursor cells spread over the entire surface of the body. This embryologic ability could help account for melanoma’s ability to spread across tissue planes and through blood vessels, nerves and the lymphatic system. This metastatic passport helps make melanoma more lethal than other skin cancers, which are generally limited to spreading to adjacent sites.

Plantar melanocytes are characteristically arranged in specific patterns. First, let us imagine a freshly planted garden that has mounded parallel rows of ridges containing long lines of equally spaced seedlings. Narrow irrigation channels or furrows separate these mounded rows from each other. Now think how the garden architecture resembles volar skin dermatoglyphics. The garden pattern with the mounded rows becomes the epidermal ridges, studded with regularly spaced eccrine duct openings, just like the seedlings in the garden. The more prominent epidermal ridges are separated by shallow, parallel dermal furrows.  

Most importantly, normally behaving plantar and palmar melanocytes and their pigment stay within the parallel furrows between the mounded epidermal ridges while melanoma violates the rules and spreads into adjacent ridges. Violation of the parallel furrow pattern is a very sensitive indicator for early acral melanoma.

Saida and Miyzaki reviewed 712 acral melanocytic lesions.²¹ They found that the parallel ridge pattern and irregular diffuse pigmentation findings positively predicted melanoma over 93.7 percent of the time. Conversely, they found the parallel furrow pattern lesions were benign more than 93.2 percent of the time.

Taking the dermoscopy examination a step further, there are three distinct normal pigment patterns that are characteristically present in three different zones of the foot. These three zones tend to differ in the degree and direction of weightbearing forces. These zones are the edges or periphery of the sole and toes; the direct weightbearing areas of the forefoot and plantar lateral foot; and finally the relatively non-weightbearing arch areas. These three normal sole zones have characteristically different but still normal pigment patterns.

How To Evaluate Plantar Nevi
Two studies on the early identification of foot melanoma explain and illustrate acral nevi dermoscopy technique beautifully.^14,22 We all have heard of the ugly duckling approach to cutaneous assessment. It is especially useful in the evaluation of soles that exhibit many pigmented lesions, which might overwhelm the examiner. It is best to pick out the largest or worst looking lesion, and apply a standardized examination.

First determine if the lesion is actually pigmented or not. Intracorneal hemorrhage can mimic acral nevi. With the dermatoscope, the examiner can readily discern a normal honeycomb pigment pattern from globules of dry blood. Applying the following three-step analysis helps to identify the pigment pattern. If the expected pigment pattern is disturbed, the clinician should be disturbed and perform a biopsy.
Step 1. Examine the lesion for the presence of the parallel ridge pattern. If the parallel ridge pattern is present in any part of the lesion, biopsy the lesion regardless of the size. If the lesion does not show the parallel ridge pattern, proceed to step two.²³

Step 2. If the parallel ridge pattern is visible in any part of the lesion, biopsy the lesion regardless of the size.²⁴ Benign acral nevi often have characteristic patterns depending upon their sole zone. It turns out acral anatomy and weightbearing shearing forces account for the three basic patterns of benign acral lesions of the sole.²³ (See the table “Insights On The Three Pigment Patterns Of Acral Lesions” at right.)

The parallel furrow pigment pattern nevi are typically present along the peripheral edge of the direct weightbearing zones of the sole. This includes the peripheral areas of the soles including the heel rim and the areas surrounding the plantar toe pads. The sensitivity, specificity and positive predictive value of the parallel furrow pattern/lattice-like patterns for melanocytic nevi are 67 percent, 93 percent and 98 percent respectively.²⁵ The lattice pigment pattern nevi are often present in the relatively less weightbearing areas of the arch and represent a pattern that is 98.3 percent benign.²⁴

These steps are based upon the high sensitivity, specificity and positive predictive value (86, 99 and 94 percent respectively) of the parallel ridge pattern for early acral melanoma.^24,26
Basically, if the pigment discoloration is leaking beyond the normal pattern, the lesion is not only highly suspicious but one can almost consider it clinically diagnostic.

Step 3. Measure the maximum diameter of lesions that do not show typical benign patterns. Excise lesions >7 mm or biopsy them for histopathologic evaluation.²⁷ For lesions ≤7 mm, one should ensure clinical and dermoscopic monitoring at three- to six-month intervals.²³

Additional criteria include abnormal coloration. Typically, benign nevi have a uniform light brown to black color. In some plantar nevi, their normal color is replaced by red, grey, blue or white discoloration. Older, benign, congenital nevi can appear to have blue coloration. The theory is that over time, melanocytes extend deeper into dermis and the overlying epidermis tends to filter out darker tones, making the lesion appear blue. Red, grey or white discolorations are highly suspicious signs of melanoma. Apparently, malignant melanocytes forget how to perform their primary job of making pigment.

Case Study: Diagnosing An Incidental Pigmented Lesion
A 48-year-old African-American male presented for regularly scheduled care of painful pressure keratoses overlying both hallucal interphalangeal joints. He had managed his foot pain with periodic paring and prescription orthotics.

The dermatological examination detected an irregularly shaped, flat, pigmented lesion, which measured 6 x 7 mm and was inferior to the lateral malleolus. Dermoscopic examination identified an abnormal parallel ridge pigment pattern clinically consistent with acral lentiginous melanoma. The next week, two 2 mm cutaneous punch biopsies revealed atypical lentiginous melanocytic proliferation. At the dermal epidermal junction, there were few nests but an unusual proliferation of solitary melanocytes with mild to moderate nuclear atypia. A Mohs surgeon performed wide excision with 5 mm margins.

Subsequent pathological examination revealed poorly circumscribed symmetrical proliferation of single hyperchromatic angulated melanocytes along the dermal-epidermal junction in a non-continuous fashion with significant pagetoid extensions. The finding favored melanoma in situ with atypical melanocytes over an atypical lentiginous junctional nevus.

In Conclusion
Documentation of the dermatoscopic examination expands evaluation and management of any skin lesion. A short description of the site, the clinical findings, the assessment and management plan are important components of documenting a dermatoscopic examination.

The very high sensitivity and specificity of the atypical parallel ridge pigment pattern makes it a key clinical sign of plantar melanoma. This dermatoscopic identification of plantar melanoma will allow earlier detection, thereby reducing diagnostic delay and mortality. In addition, differentiating seborrheic keratoses from actinic keratoses, and detecting basal cell and squamous cell carcinomas is easier. Also, differentiating verrucae from porokeratoses and pressure keratoses as well as removing superficial foreign bodies makes the dermatoscope an essential instrument for me.

I do not know who said it first but I agree with the following statement: The dermatoscope is fast becoming the podiatrist’s stethoscope.

Dr. Bodman is an Associate Professor at the Kent State University College of Podiatric Medicine. He is a Diplomate of the American Board of Podiatric Medicine. Dr. Bodman is in private practice in Ohio.
The author thanks Joan Lannoch, the Senior Graphic Designer at the Kent State University College of Podiatric Medicine, for the design of the image insets.

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