A corn, also known as a "heloma" or "focal intractable plantar hyperkeratosis," is a type of callosity. This latter is a common, uncomfortable, thickened skin lesion that results from repeated mechanical trauma due to friction or pressure forces. In the literature, confusing terminology is often used to call different types of hyperkeratotic skin lesions. Nevertheless, a corn should be distinguished from a callus, which is a more diffuse type of callosity. Thus, a corn is a well-delimited focal area of hyperkeratosis. This condition often is seen in athletes and in patients who are exposed to unequal friction force from footwear or gait problems, including older adults, patients with diabetes, and amputees. It should be regarded as a symptom rather than an effective disease.
Corns typically result from repeated accumulated mechanical trauma as well as other contributing factors like ill-fitting footwear, the presence of bony prominences (foot deformity), and certain physical activities. They are most common on the feet: on the dorsum of the toes, in the last interdigital web space, as well as on the soles.
Individuals with darkly pigmented skin are more prone to developing corns. They have been reported to affect older age groups with a slight female predominance due to wearing narrow shoes.
The repeated friction and pressure of the skin overlying the bony prominences leads to a hyperkeratotic thickness. This latter is a protective body reaction that produces an excess of horny epithelial layer to prevent skin ulceration. This explains the preferential location of corns next to the condyles of the metatarsals and phalanges as well as the occurrence of corns in patients with a foot deformity.
Histopathologic examination of a corn is usually not needed, but on a few occasions, it might be helpful to differentiate corns and calluses from other diseases like a plantar wart. A biopsy specimen from corn will show proliferation of all epidermal layers (acanthosis), including the stratum corneum, cells at the stratum corneum layers will return their nuclei (parakeratosis) indicating premature differentiation. The granular cell layer may be diminished or absent. The dermis may often show dense fibrous tissue with hypertrophied nerves, and scar tissue may extend to the subcutaneous fat.
History and Physical
Corns are primarily diagnosed on clinical presentation. It is helpful to look at other lesions on an individual's skin to see their body response to physical trauma. It is also easier to diagnose corns by inspection and palpation because of their rough hyperkeratotic texture. History should also include an account of physical activities.
Corns usually present as flesh-colored dry, hard, rough papules with a whitish center (called the core). They are located over a bony prominence. They are painful on walking and standing but are asymptomatic to touch. As the overgrowth/thickening of keratin builds up, corns can grow and cause severe pain.
There are two main variants of corn:
Hard corn ('heloma durum') – The most common type, usually located over the lateral and dorsal aspect of the fifth toe as well as on the dorsal aspect of the interphalangeal joints of the lesser toes.
Soft corn or interdigital corn ('heloma molle') – a painful hyperkeratotic lesion, mostly seen in the interdigital web spaces, especially the fourth space. This location often leads to maceration of the corn, with sometimes, the occurrence of a bacterial or fungal infection.
Some authors add to these two variants a third one, called seed corn, which manifests as multiple nonpainful keratotic plugs within plantar calluses in non-pressure-bearing areas of the soles.
Corns sometimes are difficult to distinguish from plantar warts. In corns, the plantar skin lines may be observed within the lesions, as opposed to plantar warts. Furthermore, if the physician exerts a digital pressure perpendicularly to plantar corn, this latter will come into contact with the bony prominence, and the patient will feel pain. This maneuver is generally nonpainful in the case of warts. Finally, the absence of capillary dotting called "Auspitz's sign" after paring hard corns distinguishes them from plantar warts.
Dermoscopy may help make the diagnosis showing areas of hyperkeratosis without vascular or hemorrhagic structures that are mainly seen in warts.
Radiographs of the feet may help to show bony prominences.
Pressure studies of the feet can highlight areas of excessive plantar pressure associated with plantar corns.
Other tests, like fasting glucose level and rheumatoid factor, may be done to find the etiology of foot deformity in some patients.
The differential diagnosis of a corn includes:
Gout and pseudogout
Hypertrophic lichen planus
Lichen simplex chronicus
Keratosis punctata of palmar creases
Porokeratosis plantaris discreta
Porokeratosis palmoplantar et disseminata
Although corns and calluses are a chronic, recurrent issue, most of them gradually go away when the repeated friction or pressure causes them to be eliminated. Nevertheless, if corns do not go away or if they started to cause severe pain, then the patient should be checked by a dermatologist or podiatrist.
The following specialties are involved directly or indirectly in the treatment plan for cases of corn:
Deterrence and Patient Education
Patients with peripheral neuropathies should try to avoid the application of plasters that contain salicylic acid because these plasters might cause sloughing of normal skin if placed in a wrong way. Calluses and corns are not caused by viruses and are not contagious. Patients should be instructed to trim their nails to avoid any pressure or repeated trauma, which might lead to the development of corns.
Pearls and Other Issues
One of the frequent pitfalls seen in clinical practice by some dermatologists is using liquid nitrogen (cryotherapy) for treating cases of corns. This, unfortunately, can lead to aggravation of the problem, and it can add more to the patient's suffering since liquid nitrogen damages normal skin surrounding the corn and does not help in healing the lesion.
The management of corns is interprofessional. Patients may be treated by the nurse practitioner, dermatologist, primary care physician, or podiatrist. The key in all cases is to eliminate the continuous pressure on the foot. The treating provider should refer the patient to an orthopedic surgeon to evaluate for underlying bony abnormalities in lesions that are recalcitrant to treatment or recurrent. Foot care nurses are often involved in the diagnosis and treatment of corns. They should educate the patient as to proper foot care and follow up to ensure resolution. Pharmacists often recommend over the counter products and should also educate the patients.
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Contributed by Ahmad M. Al Aboud, M.D.
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