SOLAR LENTIGO

Solar lentigo (plural lentigines), also known as actinic lentigines, age spots, or sun spots, are benign hyperpigmented lesions frequently found on the skin of photo-exposed areas of the body, such as the face, upper back, and dorsum of the hands, wrists, and forearms. The prevalence of solar lentigines increases with age, and they are most commonly seen in fair-skinned, older adults who have had excessive exposure to the sun, making these lesions a hallmark of photoaged skin. Due to their appearance, solar lentigines must be distinguished from lentigo maligna and other potentially malignant lesions, making careful clinical evaluation a top priority.^1-4

Pathophysiology

• Solar lentigines develop in response to ultraviolet (UV) radiation from sun exposure. Repeated UV radiation causes a local proliferation of melanocytes and an accumulation of melanin in the skin cells (keratinocytes), which results in a solar lentigo.²
• The molecular mechanisms involved in the pathophysiology of solar lentigines are not clearly understood, but activation of keratinocyte-growth factors, stem-cell factors, and endothelin-1 signaling pathways are known to be involved. An upregulation of genes involved in inflammation and fatty-acid metabolism and a downregulation of keratinization-related genes have also been identified.¹
• The constituents of ambient air pollution are another environmental factor that may contribute to the development of solar lentigines.⁵

Related Anatomy

• Dermis 
• Epidermis
• Melanocytes
• Melanin
• Keratinocytes
• Dermo-epidermal junction zone

Incidence and Related Conditions

• Solar lentigines typically appear in middle age and slowly increase in number and size until they eventually coalesce to form larger patches. More than 90% of Caucasians >60 years are affected by solar lentigines.⁶
• Lentigo maligna melanoma
• Lentigo simplex

Differential Diagnosis

• Ephelides
• Lentigo maligna melanoma 
• Lentigo simplex
• Lichen planus-like keratosis
• Melanocytic nevus 
• Pigmented actinic keratosis 
• Seborrheic keratosis

A typical patient is a 72-year-old woman with extremely fair skin (Fitzpatrick skin type 1). She grew up in northern New Jersey and spent most of her summers living in a beachside town on the Jersey Shore, where she often sunbathed on the beach, sometimes without any sunscreen. Over time, this repeated exposure to the sun resulted in several lesions that primarily developed on the dorsum of both of her hands and wrists. Initially, the lesions were <1 cm in diameter, tan in color, and had irregular borders, but as she aged, they eventually began to form larger patches. Concerned that these patches could be a sign of a more serious skin condition, she consulted with a dermatologist to undergo a skin examination.

• Multiple lentigines syndrome

• Solar lentigines do not typically improve and will likely persist indefinitely unless addressed with treatment.
• Several studies have shown that lasers are one of the most effective treatments for solar lentigines, and patients generally prefer laser therapy over other commonly used therapies. Q-switched Nd:YAG lasers have been shown to be more effective than fractional CO₂lasers, with the primary caveat being that they are more painful and require a longer healing time.²

• Macules
  • Range from tan to dark brown in color
  • Irregular borders, often described as ‘moth-eaten’
  • Several millimeters to just over 1 cm in size
• Solar lentigines are characterized by bud-like extensions of hyperpigmented epidermis that protrude into the dermis.¹
• Lesions are frequently seen on the dorsum of the hands, wrists, and forearms, but can occur almost anywhere on the body, particularly other sun-exposed areas such as the face, back, feet, and shoulders.²
• In addition to looking for characteristic signs of a solar lentigo (ie, a tan to dark brown macule up to 1 cm in diameter with an irregular border), the surface may also be rough on palpation.⁴
• It may be difficult to differentiate between solar lentigines and other hyperpigmented lesions like macular seborrheic keratosis and lentigo maligna.¹
  • Lentigo maligna also presents as an irregularly pigmented dark brown to black macule, but is usually darker in color than a solar lentigo.⁴
• Pathological examination is not typically needed, but if performed, will reveal an increased number of melanocytes within the epidermis and increased pigmentation within the keratinocytes.³
• Dermatoscopy of solar lentigines will display diffuse brown pigmentation, light-brown fingerprint-like structures, and a fine regular network. These observations may also reveal an accumulation of melanin in the epidermis of the lesion, especially in the basal layer. Obliteration of the follicular ostia is not observed. ^1,4
• Clinicians should also be aware of unstable solar lentigines, which are isolated, irregularly pigmented macules on a background of solar damage. They stand out as solitary lesions or are markedly different to surrounding solar lentigines, and are typically larger in size, darker in color and more irregular. These unstable lesions may display histological characteristics on a continuum between a solar lentigo and lentigo maligna.⁴ Biopsy is warranted in these instances.
• Chronic treatment with topical hydroquinone has been associated with paradoxical hyperpigmentation of the skin (ochronosis).

New and Cited Articles

1. Warrick E, Duval C, Nouveau S, et al. Morphological and molecular characterization of actinic lentigos reveals alterations of the dermal extracellular matrix. Br J Dermatol 2017;177(6):1619-1632. PMID: 28570000
2. Scarcella G, Dethlefsen MW, Nielsen MCE. Treatment of solar lentigines using a combination of picosecond laser and biophotonic treatment. Clin Case Rep 2018;6(9):1868-1870. PMID: 30214780
3. Marks JG, Miller JJ. Lookingbill and Marks’ Principles of Dermatology. Fifth Ed. London, New York: Saunders Elsevier; 2013.
4. Byrom L, Barksdale S, Weedon D, Muir J. Unstable solar lentigo: A defined separate entity. Australas J Dermatol 2016;57(3):229-234.PMID: 26832231
5. Goorochurn R, Viennet C, Granger C, et al. Biological processes in solar lentigo: insights brought by experimental models. Exp Dermatol 2016;25(3):174-177.PMID: 26739821
6. Friedmann DP, Peterson JD. Efficacy and safety of intense pulsed light with a KTP filter for the treatment of solar lentigines. Lasers Surg Med 2019. [Epub] PMID: 30681160
7. James WD, Berger TG, Elston DM. Andrews’ Diseases of the Skin.12^thEd. Philadelphia, PA. Elsevier, 2016.

Review

1. Goorochurn R, Viennet C, Granger C, et al. Biological processes in solar lentigo: insights brought by experimental models.Exp Dermatol 2016;25(3):174-177.PMID: 26739821