Solar keratosis is caused by skin damage from long-term exposure to sunlight. Specifically, ultraviolet (UV) rays from the sun damage the DNA of cells in the skin (known as keratinocytes). The damage interferes with the growth of keratinocytes, producing thick and scaly keratoses.

In some cases, keratinocytes grow uncontrollably and form tumours. This is how a solar keratosis becomes a squamous cell carcinoma – a type of cancer.

Solar keratoses are small, dry, scaly patches that are raised above the surface of the skin. They can range in size from 3 mm to 3cm wide. They are often red in colour, but may also be pink or brown.

Yellow-scaly crusts may form on top of the keratoses. In some cases, skin from the keratoses can grow into a thick horn-like projection – this is known as a cutaneous horn.

Solar keratoses may be accompanied by other symptoms of sun-damage, including:

• reddening of the surrounding skin
• yellow, thickened skin – known as solar or actinic elastosis
• telangiectasia – small networks of veins that appear on the surface of the skin

Areas of the body with the most exposure to the sun are more likely to develop solar keratoses. Accordingly, solar keratoses are most frequently found on the:

• face
• ears
• neck
• scalp – particularly bald patches
• back of hands
• forearms
• chest, back and legs – particularly in those who frequently sunbathe.

Development of solar keratoses

Solar keratoses start off as small spots with a rough surface.  Initially, they may be hardly visible, but easily felt with your fingertips. The spots tend to first occur between 30 and 60 years old.

Typically, only one spot will develop in a sun-exposed area. Over time and with greater sun exposure, more spots will appear and grow larger.

Solar keratoses may become redder and flare up whenever the immune system is supressed – for example, when taking immunosuppressive medication.

Solar keratosis and squamous cell carcinoma

The development of solar keratosis suggests that the skin has accumulated underlying damage from years of sun exposure. This underlying damage increases the chance of skin cancers.

Solar keratosis may itself also develop into a type of skin cancer – squamous cell carcinoma. Studies show that about 10% of keratoses will turn into squamous cell carcinoma over time. Keratoses that are red, hard and thick are more likely to develop into squamous cell carcinomas. Continued sun exposure also increases the likelihood of a solar keratosis becoming cancerous.

For a single solar keratosis patch that does not cause any symptoms, your doctor may feel that no treatment is necessary. The doctor may simply ask you to observe the patch and return in the event the patch changes in appearance or becomes painful. In addition, the doctor may prescribe an emollient to moisturise the skin. It has been shown that about 26% of all keratoses will go away on their own.

Due to the chance of a solar keratosis developing into squamous cell carcinoma (a type of skin cancer), keratoses are often treated and removed. They may sometimes also be removed for cosmetic reasons.

There are various treatment options available.

Creams or gels

Creams and gels (which are thinner than creams, transparent and water based) can be applied onto solar keratoses. They cause the keratinocytes in the keratoses to die, allowing the skin to heal.

There are four main types of cream or gel prescribed for solar keratosis.

5-fluorouracil cream is applied daily to the affected area of skin for a period of 4 weeks. It has been shown to clear solar keratoses for up to 12 months. The cream does cause side-effects, including soreness, inflammation and blistering around the skin. Your GP may prescribe a steroid cream (e.g. 1% Hydrocortisone cream) to help reduce these side-effects after the initial treatment.

Imiquimod cream is applied 3 times a week for a period of 4 weeks. Similar to 5-flourouracil cream, it can cause inflammation and soreness of the skin. In some people, it may also temporarily produce flu-like symptoms.

Ingenol mebutate gel is a newer treatment that requires fewer applications to the skin compared to imiquimod and 5-fluorouracil creams. For keratoses on the face and head, it only needs to be applied for 3 consecutive days. For keratoses on the torso and limbs, treatment is only 2 consecutive days.

Diclofenac gel contains an anti-inflammatory drug and is applied to the skin twice daily over 12 weeks. This medication may be less effective than the previous three medications and so is used for less severe keratosis.

Photodynamic Therapy

Photodynamic therapy uses a specialised lamp and a light-sensitive cream to destroy keratoses. Methyl-5-aminolevulate cream is applied to the affected areas before a light is directed to them. The cream becomes activated by the light, destroying the keratoses within a single treatment.

Photodynamic therapy produces good cosmetic results and may be more suitable for widespread keratosis. Some GP surgeries may have the facilities for photodynamic therapy, but they are more often conducted in dermatology departments.

Cryotherapy

Cryotherapy involves freezing solar keratoses using a jet of liquid nitrogen. It may be more suitable for thicker keratoses. During freezing, the low temperature destroys the cells in the keratosis, causing it to fall off a few days later. The procedure leaves a small scab, but skin usually completely heals over a period of weeks.  It is also possible for the surrounding skin to blister, but these generally disappear. For thicker keratoses requiring longer freezes, cryotherapy carries the risk of leaving a permanent small white spot on the skin.

Dermabrasion

This procedure uses a special rotating tool that removes the top layers of skin. Dermabrasion under local anaesthetic may be used to remove solar keratoses on the face or scalp.

Chemical peels

Chemical peels use corrosive chemicals to remove affected patches of skin. Like dermabrasion, chemical peels may be used on keratoses affecting the face or scalp. Solar keratoses may recur six months after chemical peel treatment, so it may not be effective as a long-term treatment.

Laser resurfacing

Laser resurfacing uses high-energy beams of light to remove keratoses.

Surgical treatments

Surgically removing the keratoses might be suitable for thicker keratoses and squamous cell carcinomas in the early stages. It is also a treatment option for solar keratosis that does not respond to medication. Surgical removal of keratoses also allows the examination of removed keratoses under a microscope. This may aid diagnosis. Surgical treatments are mainly performed using local anaesthetic, whereby the skin to be operated on is numbed by injection.

There are two main types of surgical procedure:

Curettage – uses a sharp spoon-shaped tool called a curette to scrape out the keratosis. The subsequent wound is then sealed with heat treatment (cautery). Curettage may leave a small scar.

Excision – uses a scalpel to cut out the keratosis. The subsequent wound is then closed with stitches. The procedure leaves a permanent scar.

In order to prevent solar keratoses from developing and to protect against the risk of skin cancer, it is important to protect yourself from the sun’s UV rays. The following measures can help:

• Avoid exposure to strong sunlight, particularly between 11am – 3pm (when the sun is at its highest)
• Apply sun-cream with a sun protection factor (SPF) of 30 or above
• Wear clothing that reduces exposure to sunlight e.g. hats, long sleeved clothes.

 It may also help to apply a moisturising emollient to prevent the skin from becoming dry.

 The risk of solar keratosis is related to how much exposure your skin has to UV light over time. Several small exposures to sunlight over years can cause a build up of skin damage – leading to solar keratosis. Doctors term this “cumulative damage”.

Solar keratosis is therefore more common in people with long-term exposure to sunlight and is more frequently seen in:

older people – as you get older, your total amount of exposure to sunlight increases. Older people are thus more likely to have had years of sun-damage to their skin and therefore more prone to develop solar keratosis. In the UK, between 19 and 25% of people over the aged of 60 have solar keratosis.

men – solar keratosis affects more men than women. A study of people aged over 70 years living in England found that 34% of men had solar keratosis compared to 18% of women. A probable reason for this difference is that men are likely to have occupations that involve working outdoors and thus have greater exposure to the sun.

people living closer to the equator – have greater exposure to sunlight and more at risk fo solar keratosis

people with outdoor lifestyles – have greater exposure to sunlight.

Other major risk factors include:

Fair skin – people with fairer skin, which does not tan easily, are at increased risk of developing solar keratoses. This is particularly true for people with blue or green eyes and blonde or red hair. The reason for this is that fairer skin has less of a skin pigment called melanin. Melanin protects the skin from damage caused by UV light.

Fair-skinned people living in areas with high sun exposure have higher rates of solar keratosis – 40% of Australians over 40 years old have the condition.

Immunosuppression – people on medication which suppress the immune system (such as chemotherapy drugs or drugs given to people after organ transplantation) are at increased risk of both solar keratosis and squamous cell carcinoma (a type of skin cancer). Those with HIV infection, which attacks the immune system, are also at increased risk.

This reason for this is because a weakened immune system is less able to repair skin cells that have been damaged by the sun.

A GP may diagnose solar keratosis from a simple examination of the skin. In some cases, a small sample of the keratosis may need to be taken. This is known as a skin biopsy. The sample is sent to a laboratory to be analysed under a microscope, which can help confirm a diagnosis.

Referral to a dermatologist

In some cases, your GP may need to refer you on to see a dermatologist. You may be referred to a dermatologist in the event:

• the GP requires a second opinion on your condition
• the GP suspects that a keratosis may be cancerous or at high risk of becoming cancerous
• the keratoses are widely spread over your body
• you take immunosuppressive medication
• your current treatment is ineffective

Actinic keratosis. Patient.info – Professional Reference Article. Available online at: http://patient.info/doctor/actinic-keratosis

Actinic keratoses (solar keratoses). NHS Choices. Available online at: http://www.nhs.uk/conditions/solar-keratosis

Actinic keratosis. Primary Care Dermatology Society. Available online at: http://www.pcds.org.uk/clinical-guidance/actinic-keratosis-syn.-solar-keratosis

Berman, B., & Cockerell, C. J. (2013). Pathobiology of actinic keratosis: ultraviolet-dependent keratinocyte proliferation. Journal of the American Academy of Dermatology, 68(1), S10-S19.

Chetty, P., Choi, F., & Mitchell, T. (2015). Primary care review of actinic keratosis and its therapeutic options: a global perspective. Dermatology and therapy, 5(1), 19-35.

Mcintyre, W. J., Downs, M. R., & Bedwell, S. A. (2007). Treatment options for actinic keratoses. Am Fam Physician, 76(5), 667-71.