What is idiopathic guttate hypomelanosis?

Idiopathic guttate hypomelanosis is a commonly acquired leukoderma characterised by flat porcelain-white macules on sun-exposed areas. The classical sites are the forearms, legs and trunk.

The pathogenesis of idiopathic guttate hypomelanosis is attributed to local trauma, genetic predisposition, chronic sun exposure and senile degeneration [1]. 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. 

What are the clinical features of idiopathic guttate hypomelanosis?

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 [2]. The borders may be angulated, scalloped or irregular. The diagnosis is supported by enhanced whiteness upon Wood lamp skin examination. Histopathological changes are specific.

Guttate hypomelanosis on the face

Guttate hypomelanosis on the chest

Guttate hypomelanosis on the lower leg

Porcelain-white macules on the lower leg

Porcelain-white patch extending peripherally

Guttate hypomelanosis in skin of colour

What are the dermoscopic 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].

• Amoeboid pattern: white structureless area with a pseudopod-like periphery resembling an amoeba
• Feathery pattern: white structureless area with featherlike linear extensions and irregular margins
• Petaloid pattern: the white structureless areas resemble the petals of a flower
• Nebuloid pattern: the white structureless areas have indistinct borders like a cloud. Other descriptions include ‘cloudy sky-like’ and ‘cloudy’ patterns
• Perifollicular, perilesional or diffuse pigmentation may be observed with occasional red dots [4].

Fig. 1. Amoeboid pattern

Fig. 2. Amoeboid pattern

Fig. 3. Feathery pattern

Fig. 4. Petaloid pattern

Fig. 5. Nebuloid pattern

Figure legends
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).

Macro photo of idiopathic guttate hypomelanosis

Nonpolarised view of idiopathic guttate hypomelanosis

Polarised view of idiopathic guttate hypomelanosis

What is the dermoscopic differential diagnosis of idiopathic guttate hypomelanosis?

The dermoscopic differential diagnosis for idiopathic guttate hypomelanosis include guttate vitiligo, pityriasis versicolor and lichen sclerosus.

• Vitiligo dermoscopy: white structureless areas with absent or reduced pigment network, that appear to glow due to total loss of melanocytes in the epidermis. Red dots and linear vessels are not seen See Dermoscopy of vitiligo.
• Pityriasis versicolor dermoscopy: white structureless areas, persisting pigment network, and white scale without a glow. The scale is prominent in the skin lines and separates into lines when the lesion is stretched. See Dermoscopy of pityriasis versicolor
• Lichen sclerosus dermoscopy: comedo-like openings, telangiectasia, perpendicular white lines on polarised dermoscopy, blue-grey peppering and black dots. See Dermoscopy of lichen sclerosus

Guttate vitiligo

Hypopigmented pityriasis versicolor of upper back

Lichen sclerosus on upper back

White structureless 'glow' (black stars), perilesional pigmentation (yellow arrows) and satellite lesions (black arrows)

Dermoscopy of pityriasis versicolor

Dermoscopy of lichen sclerosus

What is the histological explanation of 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 [5].