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Author: Prof. Balachandra Ankad, Dermatologist, S. Nijalingappa Medical College, Karnataka, India. DermNet New Zealand Editor in Chief, Adjunct A/Prof Amanda Oakley, Dermatologist, Hamilton, New Zealand. December 2019.
The pathogenesis of idiopathic guttate hypomelanosis is attributed to local trauma, genetic predisposition, chronic sun exposure and senile degeneration . Lesions can also be observed in sun-protected sites such as the trunk, and it can affect younger age groups including, rarely, children.
‘Idiopathic’ means the cause is unknown, ‘guttate’ means resembling tear-drops, and ‘hypomelanosis’ refers to the lighter colour of the affected areas.
Idiopathic guttate hypomelanosis presents as stable, smooth, discrete and asymptomatic white macules. The macules are 2–6 mm in size but larger lesions up to 2.5 cm may occur . The borders may be angulated, scalloped or irregular. The diagnosis is supported by enhanced whiteness upon Wood lamp skin examination. Histopathological changes are specific.
Clinical features of idiopathic guttate hypomelanosis
The dermoscopic features of idiopathic guttate hypomelanosis are characterised by white structureless areas in which pigment network is absent. They extend peripherally with irregular borders and shapes.
White structureless areas appear to ‘glow’ due to total loss of melanocytes in the epidermis. This glow is not as uniform as the glow seen in the dermoscopy of vitiligo and there may be various shades of white.
Metaphoric terms such as amoeboid, feathery, petaloid and nebuloid shapes have been described [3,4].
Dermoscopic features of idiopathic guttate hypomelanosis
Figures 1,2. Amoeboid pattern. Diffuse white structureless area (black stars) with well-defined borders which are extending like pseudopods (black arrows), hence the name. Note the perifollicular (yellow arrows) and perilesional (red arrows) pigmentation.
Figure 3. Feathery pattern. White structureless area (black stars) which extends peripherally in linear strands (black arrows) like a feather. Perifollicular pigmentation (yellow arrows) is well appreciated.
Figure 4. Petaloid pattern with a white structureless area (black stars) and peripheral extensions resembling petals. Perifollicular (yellow arrows) and perilesional (black arrows) pigmentation are also noted.
Figure 5. Nebuloid pattern. White areas (black stars) with indistinct borders (black arrows) and subtle pigmentation (black circles).
The dermoscopic differential diagnosis for idiopathic guttate hypomelanosis include guttate vitiligo, pityriasis versicolor and lichen sclerosus.
Dermoscopic differential diagnosis for idiopathic guttate hypomelanosis
The histopathology of guttate hypomelanosis shows hyperkeratosis, flattening of rete ridges, epidermal atrophy and acanthosis. There are areas of absent and retained melanocytes; these may explain the white structureless areas and the perifollicular and perilesional pigmentation .
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