Patients are usually middle-aged or elderly and develop rough scaly areas on sun-exposed sites, which are generally asymptomatic. Many patients give a history of relapsing and/or remitting lesions, which often disappear either spontaneously or after sun avoidance and use of sunscreens.
Actinic keratoses are commonly present on light-exposed areas such as the face, scalp and back sides of the hands. The sides of the neck are involved in both sexes, but the ears predominate in men. The lower lip may also be involved, with a much higher incidence in men than women. The variants of AK include:
- Erythematous AK: The typical AK lesion is called the erythematous AK. Lesions are usually multiple and comprise of patches with a rough scaly surface resulting from disorganized skin growth and a variable degree of inflammation. Lesions vary in size from less than 1 mm to over 2 cm and are usually asymptomatic. In many individuals, the number of lesions can be better appreciated by skin palpation, which is a sensitive way of detecting the characteristic roughness associated with smaller lesions. Many of these small lesions may pass unnoticed by most patients, and the diagnostic changes often only appear later as a dry rough adherent and often yellow- or brown-colored scale. The adherent scale can only be picked off with difficulty, revealing a base with punctate bleeding points. In some cases, scaling may be prominent and in time may become thick and horny. The edge of the AK is usually sharply demarcated and the reddening is usually closely confined to the area immediately below the area of abnormal scaling. There is often associated ‘field change’, with areas of photodamage; skin atrophy, vessel dilatation and pigmentary change which surrounds and extends beyond clinical AKs.
- Hypertrophic AK: The hypertrophic AK (HAK) presents as a thicker, scaly, rough plaque that is skin-colored, gray-white, or erythematous. It can be found on any habitually sun-exposed site on the body but has a propensity for the back sides of hands, arms, and scalp. A typical erythematous AK can progress into an HAK. It can be difficult to distinguish an HAK from a skin cancer, clinically necessitating a biopsy. Biopsies must be taken to a level deep enough to ensure that the extent of proliferation can be evaluated in order to obtain an unequivocal histopathologic diagnosis. Induration or thickness, pain, and ulceration are the main clues to the transition of AK to cancer.
- Cornu Cutaneuum: Cutaneous horn, also known as cornu cutaneum, refers to a reaction pattern and not a particular lesion. In reference to AKs, a cutaneous horn is a type of HAK that presents with a conical protuberance emanating from a skin-colored to erythematous base. Classical definitions of a cutaneous horn maintain that the height is at least one half of the largest diameter. Thirty-eight percent to 40% of all cutaneous horns represent AKs. The pathology underlying a cutaneous horn can be a number of different lesions, such as AK, skin cancer, seborrheic keratosis, filiform verruca vulgaris, keratoacanthoma. The most common underlying lesion in elderly, fair-skinned persons is an HAK.
- Actinic Cheilitis: Actinic cheilitis represents confluent AKs on the lips, most often the lower lip. Persons with this condition have red, scaly, chapped lips, and at times erosions or fissures may be present. The border of the lip is often indistinct, and thickening and white patches may also be seen. Individuals with this condition often complain of persistent dryness and cracking of the lips, and the diagnosis of actinic cheilitis should always be suspected in photodamaged patients with such complaints. Persistent ulcerations or indurated areas on the lip require biopsy to ensure that cancer is not present.
A number of classifications have been proposed but none have shown accurate clinicopathological correlation. The term Keratotic Interepithelial Neoplasia (KIN) has been described with KIN1 having minimal cancerous changes at the basal layer to KIN3 showing severe cell changes. However, there is poor clinical correlation with this pathological staging.
Some dermatologists describe the clinical thickness of disease by using the term ‘mild’ with thin slightly palpable disease to ‘moderate’ where AKs are easily seen and felt, to ‘thick’, but this does not take into account any pathological changes.
Actinic keratoses are usually asymptomatic but patients may complain of local itching and discomfort. Most patients find them a cosmetic nuisance. Patients with AK are at a higher risk of concurrent skin cancer and melanoma. Quality of life (QoL) may be affected and can be determined using the AKQoL questionnaire.
The prognosis of AK includes persistence, regression, or malignant transformation into invasive SCC. The relative risk of progression to SCC depends on factors related to the AK itself, such as the length of time an AK has been present and the number of baseline AKs that are on the skin. In addition, the risk for skin cancer increases with increased UV radiation exposure and with certain individual patient characteristics, such as suppressed immune status. It was observed that before an AK progresses to skin cancer, it may become clinically tender and inflamed.
Studies in patients with AK have established, first, that there is a high probability of developing new lesions; and, second, that many lesions undergo spontaneous resolution. Rates of regression of single lesions have been reported to range from 15% to 63% after 1 year. Data available on recurrence rates of single lesions 1 year after regression indicate a recurrence rate of 15-53%. Although the rate of progression of an individual AK to invasive SCC has been estimated to be low (less than 1 in 1000/year), an individual with an average of 7.7 AKs has a probability of around 10% of one AK transforming to a skin cancer over a 10-year period. The presence of AK is therefore an important biomarker of excessive UV exposure and increased skin cancer risk.
The estimated rates of metastases arising from actinically derived SCC have ranged from 1% to 2% to over 20% depending primarily on depth of invasion, location (with the lip, ear, and scalp representing the sites of highest risk), differentiation, and the presence of invasion around neurons.
Diagnosis of AK is made on clinical grounds but a skin biopsy may be necessary to differentiate from a skin cancer or Bowen disease. More recently, non-invasive techniques such as reflectance confocal microscopy and high-definition optical coherence tomography have been used to determine the optical features of AK and differentiate these from cancer.