Viral warts on the feet: differential diagnosis

Human papillomaviruses (HPV) are DNA viruses that replicate in the cells of the epidermis.
HPVs are very common, with around 70% of the population having the virus at least once in their lifetime.

Types of papillomavirus

To date, about 200 types of papillomavirus have been isolated, each of which has been assigned an identification number (e.g., HPV-1, HPV-2 etc). Of these viruses, 40 cause genital tract infections and 15 (high-risk types) are implicated in the development of invasive cervical cancer.

Papillomavirus infection

The human papillomavirus replicates itself within the epidermis
(keratinocytes).

• It penetrates the epidermis through microabrasions to eventually reach and infects the stem cells of the basal layer.
• Papillomavirus infections can trigger a variety of other secondary conditions of different severities.

• The histologic photo below shows how the virus has subverted the structure of the epidermis. For example, there is hyperkeratosis, the epidermis has grown papillomatous and the blood vessels are dilated but contained within the dermal papillae.

Plantar warts are often incorrectly described as a proliferation of many blood cells within the dermis, as the drawing below shows. This misdiagnosis means the wart is never properly treated.

The fact that the wart appears only in the epidermis and never the dermis means that any surgical treatment must be limited exclusively to the epidermis itself so as not to cause unnecessary damage and scarring to other tissue.

Warts of podiatric interest

Warts of specific podiatric interest are:

1. Plantar warts
2. Mosaic warts
3. Periungual plantar warts

Warts on the foot can be identified by their circular, raised appearance, their hard to the touch texture and the pain they cause when pressure is applied. Sometimes hemorrhagic points are present.

1. Plantar warts

Hemorrhagic spots

Hemorrhagic spots occur because of dermal blood vessels between the interpapillary saws, which are very close to the supporting surface. Through walking and other activities, the blood vessels can rupture from pressure and create small subcorneal hemorrhages that will appear dark brown or black in color.
If the hyperkeratotic part of the wart is removed with a scalpel, the dermal papillae is amputated and causes punctiform bleeding, as in the image below.

Plantar warts appear as multiple, detached welts or as a cluster of welts.
In the latter case, a diagnosis of mosaic warts should be made.

Confusion with tylomas

The diagnosis of plantar warts is clinical and is generally straightforward. However, sometimes they can be misdiagnosed as tylomas.
Tylomas are made solely of keratin arranged in an inverted cone shape. They only arise at the site of pathological pressure points (e.g. ball of the foot, side of the toe) and are flat sections of very hard skin with no hemorrhagic spots.
In contrast, the plantar wart can arise anywhere on the foot, has a raised welt and may show hemorrhages. When the layer of hyperkeratosis is removed, the wart appears soft in texture.

Pictured below is a callus that may be easily confused for a wart. However, the location above the 4th metatarsal and the compact keratin component conclude that it is indeed a tyloma.

In the case below, the differential diagnosis between callus and wart is more difficult. Despite the fact the lesion occurs in a non-suppurative location, the presence of hyperkeratosis indicates this, too, is a tyloma.

Diagnosis here is also problematic
due to the presence of black dots in the lesion which could be assumed to be hemorrhagic spots. This is, however, another example of a tyloma and the black dots are likely dirt particles caught in the stratum corneum.

Inveterate multiple warts

Plantar warts tend to regress spontaneously within 12 to 24 months. However, in some individuals, the warts continue to grow, likely due the immunocompetent system failing to recognise their presence. The growing warts invade adjacent tissue to form large masses as shown in the photo below. If the lesion is not treated appropriately, it can degenerate into spinocellular carcinoma.

2. Mosaic warts

Mosaic warts should be considered as a separate entity to plantar warts. Mosaic warts are caused by HPV type 2 and have a peculiar clinical behavior whereby they do not spontaneously regress, but continue to expand into the periphery with new small warts like the pieces of a mosaic, hence the name.
In the early stages of development, mosaic warts may go completely unnoticed as the individual lesions are barely visible and do not cause pain.
Over time, however, the warts will become painful and this will usually be the only sign something is wrong. Unfortunately, by this point the mosaic warts are well established with hundreds of wart elements adhered to each other.
In this case, any destructive intervention is impractical.

In the image below, the individual pieces that make up mosaic warts are clearly visible.

Sometimes there is a peculiar hyperkeratotic warty appearance especially in non-supportive areas of the foot.

Confusion with pitted keratolysis

Mosaic warts are easy to recognize clinically.
The only clinical picture which may cause some confusion in diagnosis is pitted keratolysis, a disorder due to the development of particular bacteria.

3. Periungual warts

Periungual warts are the third variant of warts of podiatric interest. They occur, as the name implies, in the vicinity of the nail, but sometimes invade the periungual and nail bed which can deform the nail plate.
Periungual warts are considered as separate from plantar warts as, like mosaic warts, they do not respond well to destructive treatments.
Being able to identify the different types of wart is important in order to choose the right course of proposed treatment.