Partial Unilateral Lentiginosis
Partial unilateral lentiginosis (PUL), also called segmental lentiginosis, is an epidermal pigment disorder that is present at birth or appears in early childhood. It is marked by numerous small, densely packed light- to dark-brown macules (lentigines) confined strictly to one side of the body, following a dermatome or the lines of Blaschko, with a sharp cut-off between affected and normal skin at the midline.
Genetically, PUL reflects genetic mosaicism in the skin. Isolated PUL is purely a local cosmetic concern, but segmental lentigines can also be a mosaic expression of neurofibromatosis type 1 (segmental NF1).
Because the spots are so numerous and cover a whole body segment, treating PUL takes patience and carefully controlled laser energy. At Haute-Derma, we combine medical screening with a precise spot-targeted 532 nm / 1064 nm picosecond laser protocol that clears the pigmented lesions while preserving as much of the surrounding healthy skin as possible.
Pathogenesis and Cellular Mechanisms of Partial Unilateral Lentiginosis
PUL arises directly from a genetic mutation that occurs late in embryonic development:
- Somatic mosaicism during embryonic development: PUL results from a random post-zygotic mutation in a single line of neural crest cells or keratinocyte precursors during embryogenesis. Cells carrying the mutation migrate to and settle in one specific region of the body, following a dermatome or the lines of Blaschko.
- More melanocytes and melanosome build-up: Unlike café-au-lait macules or postinflammatory hyperpigmentation (PIH), each lentigo in PUL shows a clear increase in the number of melanocytes in the basal layer of the epidermis on histology, along with elongation of the rete ridges. These extra melanocytes produce large amounts of melanin that fill the surrounding keratinocytes.
- Strict midline restraint: Because they follow embryonic cell lines, the lentigines stop abruptly at the midline of the chest, abdomen or back. The other half of the body is completely normal, with no pigmented lesions at all.
Examination, Dermoscopy and Medical Screening at Haute-Derma
- Full-body examination and neurological screening
- Map the distribution of the lesions and identify the body midline.
- Look carefully for signs of mosaic (segmental) NF1: cutaneous neurofibromas or skinfold freckling (Crowe sign) on the same side as the lesions.
- Detailed dermoscopy: Each lentigo shows a well-defined dark brown pigment network with thick, evenly distributed lines and a sharp border, without vessels or blue-gray dots.
- Grading density to plan treatment
- Sparse type: The spots are well separated and easy to treat one by one.
- Dense type: The lentigines merge into what looks like a pigmented patch, so the area is divided into zones and lasered in stages.
Our Picosecond Laser Protocol at Haute-Derma
In PUL the extra melanocytes sit in the basal layer of the epidermis rather than deep in the dermis, which makes picosecond laser (532 nm / 1064 nm) the gold standard for breaking down this pigment thoroughly.
- Spot test: We treat 3–5 typical lentigines at the edge of the area with the picosecond laser first. After 3 weeks we assess light crusting, healing speed and the risk of postinflammatory hyperpigmentation (PIH) before treating the whole segment.
- Picosecond laser pigment removal (532 nm / 1064 nm)
- 532 nm wavelength: For superficial, light-brown lentigines. A small spot size (2–3 mm) is aimed precisely at each lesion until a light white frosting appears.
- 1064 nm wavelength (Fitzpatrick skin types IV–V): Whole-area picosecond toning combined with spot targeting keeps treatment safe and spares the natural pigment of the surrounding healthy skin.
- Course: 3–5 sessions, spaced 6–8 weeks apart.
- Recovery and maintenance to prevent recurrence: A soothing solution containing panthenol, Centella asiatica and epidermal growth factor (EGF) is applied right after treatment. We then prescribe maintenance topicals with azelaic acid 15% or tranexamic acid to preserve results and reduce the risk of new spots arising from the mosaic basal cells.
