Author: A/Prof Amanda Oakley, Dermatologist, Hamilton, New Zealand. Copy edited by Gus Mitchell. April 2018.
An atypical solar lentigo is a solar lentigo with unusual characteristics. The term may be used when a clinician is unsure whether a flat brown mark (a lentigo) is a solar lentigo (benign) or melanoma in situ (an early form of melanoma, which is a form of skin cancer). The plural of lentigo is lentigines.
Byrom et al gave the name unstable solar lentigo to a specific kind of atypical solar lentigo in sun damaged skin . Unlike the usual solar lentigo, which is predominantly keratinocytic, the unstable lentigo has areas of melanocytic proliferation on histology.
The illustrated atypical solar lentigines are large flat brown marks with irregular shape, structure and colour.
Typical and atypical solar lentigines arise in sun damaged, ageing skin. Lentigines most commonly affect people over 50 years with fair skin (Fitzpatrick skin type 1 and 2) who spend a lot of time doing outdoor work or are outdoors for recreation.
Atypical solar lentigines may also arise in people with genetic disorders in which sun damage occurs at an early age (for example, xeroderma pigmentosum), or that are immune suppressed due to disease or medication.
Other features of sun damage may be present in surrounding, mottled skin . These can include:
Atypical solar lentigines are thought to be caused by genetic changes in keratinocytes and in melanocytes due to exposure to ultraviolet radiation. A lichenoid keratosis is due to a local immune reaction.
An atypical solar lentigo arises on the face, ears, neck, hands, forearms or upper back. It is a solitary and distinct macule.
When compared to surrounding solar lentigines, an atypical solar lentigo:
The differential diagnoses for an atypical solar lentigo can include:
The main complication of an atypical solar lentigo is missing a diagnosis of melanoma in situ. Some atypical solar lentigines may also be precursors to melanoma in situ, especially the histological variant, unstable solar lentigo.
An atypical solar lentigo is evaluated clinically and by dermatoscopy. Pathological examination often leads to a more precise diagnosis, such as solar lentigo, lichenoid keratosis, unstable solar lentigo, atypical lentiginous hyperplasia, atypical junctional melanocytic naevus, or melanoma in situ.
If there is melanocytic proliferation within an atypical solar lentigo, the whole lesion should be excised and multiple levels should be examined by histology. Note that a single atypical solar lentigo may show characteristics of solar lentigo, unstable solar lentigo, actinic keratosis, and melanoma in situ (and even invasive melanoma).
An atypical solar lentigo is often excised.
Lesions that are not excised should undergo a careful follow-up, preferably using serial sequential dermatoscopy (mole mapping). Observed changes may include increase in size, and change in shape, structure and colour.
Greater asymmetry in the distribution of structures and colours on dermoscopy should lead to excision of the lesion. Stable lesions can continue to be observed.
The outcome depends on the actual diagnosis.
Solar lentigo and lichenoid keratosis are harmless. Melanoma in situ can progress to invasive melanoma.
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