Hyperkeratosis is a condition in which there is a thickening or abnormal growth of the outermost layer of the skin.

Hyperkeratosis Definition

The name is derived from word keratin which is a skin protein which in turn comes from the old Greek word keras meaning horn with the suffix in which is used to denote a chemical substance. The prefix hyper is another Greek word meaning over or above and is usually used to indicate extreme or excess. The suffix osis simply indicates a condition or state and, in a medical sense, is sometimes used to describe an abnormality. Thus it can be seen that the word keratosis has little meaning unless further qualified by the addition of other words or prefixes.

Before looking in detail at the various different forms of hyperkeratosis, it is worth considering the structure of the skin and the nature of keratin. The skin is of course the body’s largest organ and we are most familiar with the outer layer, the epidermis. There are various different types of epidermal cells but those nearing the outermost layer of the skin become increasingly flattened in shape and described as squamous. As these cells approach the skin surface they die, losing both cytoplasm and nucleus. They become keratinised forming a tough horny stratified layer which forms the surface of the skin. This layer of dead keratinised skin is usually around fifteen to twenty cells deep and the outermost cells are constantly being shed as the skin replenishes itself by growing from within as more squamous cells become keratinised forming part of the outer skin layer (stratum corneum).

What is Keratin?

Keratin is known as a type of filament protein so called because of the presence of strong long strands within the cell structure. Keratin cells bond together by employing their filaments along with other proteins known as desmomes. Such keratinocytes (keratin cells) exist throughout the skin providing structural strength and integrity. As these cells approach the skin surface, as described above, they effectively die leaving behind the remains of the cells including the tightly bonded filament links which form the stratum corneum (outer layer of the skin). In addition to the skin, keratins are also the major constituents of hair and nails and are found throughout the animal kingdom forming fur, wool, feathers and scales along with claws, hooves and even the horn of the rhinoceros. There are various different types of keratins but those found in the skin are described as being of an intermediate type. Like all proteins, amino acids form a key part in their make-up.

Mechanical Hyperkeratosis

In many cases, the thickening of the outermost layer of the skin is the body’s natural way of providing greater strength and protection where needed. As an example, those who regularly walk barefoot on hard surfaces will invariably develop a layer of strong hard skin on the bottom of the feet but in some cases, the development of patches of hard skin can be far from beneficial. Corns and calluses are perhaps the best known types of hyperkeratosis brought about by mechanical aggravation. Calluses can appear anywhere on the body that has been subjected to pressure or friction but are most common on the hands and feet. The affected area will show an area of rough hard skin often with an ill-defined boundary. Corns however tend to be more localised often appearing on the outside of the foot. They are usually seen as hard raised mounds (although soft corns also occur often between the toes). The characteristic defining feature of a corn is the fact that it has a central keratinous cone-shaped core which reaches down into the skin below. The actual mechanisms of this core formation remain uncertain but it appears that, as the body is producing a hard protective layer of keratin, pressure on the skin surface results in this keratin to be forced downwards into the skin.

Follicular Hyperkeratosis

In this condition, hyperkeratosis results in blockages of the hair follicles. The result is that small conical bumps appear at the sites of the follicles. The follicles are often blocked with crusted sebum (fluid secreted by the sebaceous glands) giving each of the spots a small white head. This condition is sometimes mistaken for acne.

Epidermolytic Hyperkeratosis

Epidermolitic hyperkeratosis, also known as epidermolytic ichthyosis or bullous congenital ichthyosiform erythroderma is a rare but very serious inherited form of hyperkeratosis. It is caused by a genetic defect connected with keratin creation with the result that the skin lacks the strength and integrity normally bestowed on it by normal keratinocyte cells and the surface stratum corneum layer fails to develop correctly. It is a lifelong condition present at the time of birth when the skin may lack much of its protective surface and exhibit erythmoderma (red skin) and blisters. A baby with this condition is very vulnerable to infections and dehydration. The condition may improve as time passes with reduced erythmoderma and blistering but there is always a marked thickening of the skin. The term ichthyosis is used to describe skin which resembles the scales of a fish but many other forms of ichthyosis cause a greater degree of scaliness.

Hyperkeratosis Tongue

Although the skin covering most parts of the body has a protective keratinised outer surface, some internal skin surfaces do not and the squamous cells remain soft and moist. The surface of the tongue is unusual however and normally described as being of specialised mucosa due to the presence of papillae. Some parts of the tongue are keratinised and other parts are non-keratinised. Hyperkeratosis of the tongue can therefore occur and has the appearance of white patches. There are many different causes of this including simple friction and pressure irritation, similar to calluses in other parts of the body, to serious malignant or pre-malignant conditions. The habit of pushing the tongue against the teeth can give rise to this condition but smoking is also a common cause. As there is no obvious visual difference between the harmless and the serious types of lesions, it is essential to obtain expert medical advice.

Hyperkeratosis Pilaris

This condition, also commonly known as keratosis pilaris, is a common inherited form of follicular hyperkeratosis. The word pilaris is derived from the Latin word pilusmeaning hair and, as the name suggests, this is a condition affecting the hair follicles. Perhaps the best description of affected skin is to liken its appearance to “chicken skin” or permanent “goosebumps”. These areas of bumpy skin are usually to be found on the upper arms and thighs but may also appear in other locations. The condition runs in families and is very common with perhaps a third of the population being affected to some extent. The good news is that it is of no medical significance and is simply regarded as being a particular type of perfectly normal skin.

Seborrheic Keratosis

This is another condition which may give rise to concern but is in fact completely harmless. Seborrheic keratosis is the name given to the condition when waxy, often light brown, lesions appear on the skin. It is a very common condition usually arising in middle age, rarely before the age of thirty. Single lesions may occur but the appearance of several, or many, is more common. They have a flaky, wart-like appearance and can look very worrying especially when darkly pigmented. Removal is not normally necessary other than for cosmetic reasons or if they are constantly being caught by clothing and there is a risk of infection. They are normally simply shaved off the skin surface under local anaesthetic and usually leave no noticeable scar although there may be a light scar on dark skin.

Actinic Keratosis

The alternative name of solar keratosis gives a clue as to the cause of these areas of thickened dry scaly skin. They normally arise as a result of years of damage due to sun exposure. They can vary in size from a few millimetres to several centimetres and can be brown, red or pink in colour. Sometimes they may project from the skin resembling small spikes. In many cases, they remain harmless but they can occasionally develop into squamous cell carcinoma so it is important to to have them medically accessed to determine whether any further treatment is required.

Other Causes of Hyperkeratosis

There are many other conditions giving rise to hyperkeratosis including warts and verrucas (plantar warts) usually caused by the human papillomavirus, lichen planus which is regarded as being an autoimmune problem and chronic eczema.

Hyperkeratosis Treatment

The method of treatment of hyperkeratosis will, of course, depend on the cause of the condition, if known, but will often involve the use of salicylic acid to break down the keratin. Creams containing urea are also often used. Corticosteroid creams and ointments are also often highly effective and frequently employed in the treatment of chronic eczema and lichen planus. Specialist treatment may be needed to facilitate the effective removal of corns and warts and, of course, the simple avoidance of any physical causes may be sufficient in the case of mechanical hyperkeratosis. In most cases, the prognosis is good but occasionally expert medical evaluation is essential as detailed above.

Hyperkeratosis in Dogs’ Paws etc.

Finally, it is worth remembering that keratin appears throughout the animal world in everything from budgies’ beaks to the plates of baleen whales and we may come across cases of hyperkeratosis in many animals other than human. Our canine friends, for example, may sometimes exhibit the condition on the skin of their noses or on the pads of their paws. The latter sometimes takes on the appearance of fronds or hairs growing out of the dog’s paw pad but is of course simply an excessive production of keratin. Just as with many human forms of hyperkeratosis, the cause of hyperkeratosis paw is not fully understood but it is believed to be largely hereditary.



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