Hyperkeratosis is a thickening of the outer layer of the skin. This outer layer contains a tough, protective protein called keratin.
This skin thickening is often part of the skin's normal protection against rubbing, pressure and other forms of local irritation. It causes calluses and corns on hands and feet. It can cause whitish areas inside the mouth.
Other forms of hyperkeratosis can occur as part of the skin's defense against:
Chronic (long-lasting) inflammation
Radiation of sunlight
Less often, hyperkeratosis develops on skin that has not been irritated. These types of hyperkeratosis may be part of an inherited condition. They may begin soon after birth and can affect skin on large areas of the body.
There are many examples of hyperkeratosis. They include:
Corns and calluses. Corns and calluses develop in areas of skin exposed to repeated friction or pressure. In response, thick layers of dead skin cells pile up and harden.
Corns usually develop on irritated toes. Calluses form on the soles of the feet and the palms of the hands.
For many people, corns and calluses are simply a cosmetic nuisance. But for others, they are a painful and troublesome medical problem.
Warts. Warts are small bumps on the skin that are caused by human papilloma virus (HPV) infection. Plantar warts grow on the soles of the feet.
HPV usually is spread by direct contact. It is typically spread by touching or shaking hands with someone who already has a wart. It may also be spread by coming in contact with a contaminated surface. For example, by walking barefoot on a gym floor or a pool deck or by wearing someone else's shoes.
Chronic eczema. Eczema is an inflammation of the skin. It can be triggered by allergies, irritating chemicals and other factors. Eczema is also called dermatitis.
Eczema causes itching, redness and tiny blisters. When the inflammation is difficult to control, chronic eczema can lead to:
Changes in skin color
Localized hair loss
Lichen planus. This condition may appear as a lacy white patch on the inside of the mouth. Or it may be an itchy, violet, scaly patch elsewhere on the skin. Lichen planus may be related to an abnormal reaction of the immune system.
Actinic keratoses. These are flat, red, rough, sandpaper-like spots or patches of skin. They can be as tiny as a few millimeters.
They are caused by excessive exposure to the ultraviolet radiation of sunlight. They occur on sun-exposed areas of skin. And they have the potential to develop into skin cancer.
Seborrheic keratoses. These are small, noncancerous skin growths. They can be tan, brown or black. They appear on the face, trunk, arms or legs. Seborrheic hyperkeratoses are very common. Their cause is a mystery.
Inherited conditions. Several inherited conditions cause hyperkeratosis. They cause a widespread, thick, platelike scaling of the skin. Symptoms begin either shortly after birth or during early childhood.