Demystifying Actinic Keratosis: A Dermoscopic Perspective

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Actinic keratosis, often abbreviated as AK and also known as solar keratosis, is a precancerous skin condition characterized by the emergence of scaly lesions on areas exposed to prolonged sun damage. The term actinic keratosis is derived from its strong association with ultraviolet (UV) radiation from the sun, which can potentially lead to the development of cutaneous squamous cell carcinoma.

Clinical Presentation of Actinic Keratosis

Clinically, AK manifests as irregular papules or plaques that can be skin-colored, red, or pigmented. These lesions are most frequently identified on the bald scalp, ears, face, or the back of the hands, with multiple lesions often observed simultaneously. The lesions may exhibit a white or yellow, scaly surface, and in some cases, they may be tender to the touch.

Dermoscopic Characteristics of Actinic Keratosis

Facial Nonpigmented AK:

Dermoscopy of facial nonpigmented AK typically reveals a 'strawberry pattern,' with a well-defined border that can blend into the surrounding skin due to extensive sun damage. The lesion may present a pink, featureless background, a pseudonetwork of pink to red vessels encircling hair follicles, and prominent spots that correspond to the follicular openings.

Nonfacial Nonpigmented AK:

On nonfacial skin, AK often shows an irregular shape with surface scale that can be white or yellow. Occasionally, there may be concentric yellow structures resembling an oyster shell, erythema with tiny red dots, rosettes visible only under polarized light, and short shiny streaks.

Pigmented Actinic Keratosis:

Pigmented AK shares similarities with nonpigmented AK but includes additional pigmented structures. Dermoscopic findings commonly include hyperpigmented follicular openings, brown areas lacking specific patterns, and annular-granular structures. Other features can consist of a brown background, angular brown lines on the surface, perifollicular grey halos, and grey rhomboidal structures.

Differential Dermoscopic Diagnosis

Dermoscopy plays a crucial role in distinguishing AK from other skin lesions, such as solar lentigo, cutaneous squamous cell carcinoma, and lentigo maligna. For instance, the differentiation of AK from squamous cell carcinoma can be determined by the presence of peripheral white areas surrounding a central scaly core.

Cutaneous Squamous Cell Carcinoma:

This type of carcinoma can develop from AK and is characterized by specific dermoscopic patterns that differentiate it from the original AK lesion.

Squamous Cell Carcinoma in Situ:

Dermoscopy can reveal the random scattering of dotted/coiled vessels in AK, contrasting with the more organized linear or clustered arrangement in intraepidermal carcinoma.

Lentigo Maligna:

This condition is associated with specific dermoscopic features, including hyperpigmented follicular openings and pigmented rhomboids, which are more frequently observed in lentigo maligna than in pigmented AK.

Psoriasis:

Psoriasis, known for its red scaly plaques, can occasionally be mistaken for AK. However, its uniform structure and silvery white scale distinguish it from the irregular yellowish scale of AK.

Histological Basis of Dermoscopic Findings in Actinic Keratosis

The correlation between the dermoscopic appearance and the underlying histological features of AK is not always clear-cut. For instance, the dermoscopic rosettes observed in AK are thought to be related to the interaction of polarized light with keratin-filled or fibrotic adnexal openings. The 'strawberry pattern' is associated with vessels between hair follicles and white haloed follicular openings, while the circles seen in pigmented keratosis are due to the involvement of follicular infundibula.

In conclusion, dermoscopy is an invaluable tool in the identification and differentiation of actinic keratosis from other skin conditions. It allows for a more accurate diagnosis and appropriate treatment planning, ultimately aiding in the prevention of potential skin cancers.