Vitiligo or guttate hypomelanosis: understanding the essential distinctions.

Although the vitiligo and guttate hypomelanosis are both types of leucoderma and share common characteristics, there are several significant differences between them.

• The white patches of vitiligo and droplet hypomelanosis have different origins.

  The origin of lesions in vitiligo and guttate hypomelanosis is based on distinct biological mechanisms, involving different processes in the loss of pigmentation. Vitiligo is an autoimmune disease where the immune system actively destroys melanocytes, the cells responsible for the production of melanin, the pigment that colours the skin. This process, partly genetic, is also intensified by several factors, such as oxidative stress and inflammation. This results in a total loss of pigmentation, visible as sharp white patches.

  Guttate hypomelanosis, on the other hand, is linked to a degeneration of melanocytes, primarily attributed to ageing. Unlike vitiligo, it is not an active destruction but a gradual reduction in the functionality of melanocytes. In addition to the natural decrease in the number of active melanocytes in the epidermis, estimated between 10 and 20% per decade after 30 years, this senescence is thought to be due to prolonged and repeated exposure to UV rays and partly based on genetic and autoimmune mechanisms. Some studies also associate guttate hypomelanosis with a defect in the uptake of melanosomes by keratinocytes.

  Although further research is required to fully understand the underlying mechanisms of vitiligo and guttate hypomelanosis, it is clear that these two pigmentation disorders have distinct pathogeneses.

• Vitiligo and guttate hypomelanosis typically do not manifest at the same age.

  The timing of lesion onset is another significant difference between the vitiligo and guttate hypomelanosis. Indeed, vitiligo is often an early-onset disease, typically beginning before the age of 30, with a majority of cases diagnosed during childhood. Although late-onset cases are possible, they are rare and mainly concern localised forms. Guttate hypomelanosis, on the other hand, primarily manifests after the age of 40, with a prevalence that significantly increases over the decades.

• Vitiligo lesions and guttate hypomelanosis do not follow the same distribution.

  The distribution of white spots also helps to differentiate vitiligo from guttate hypomelanosis. Indeed, although it is not systematic, vitiligo often manifests in a symmetrical manner. The most affected areas include the face, hands, feet and skin folds, such as the armpits. However, it should be noted that there is a segmental form of vitiligo, which is rarer, causing a unilateral distribution of spots along a dermatome, an area of the skin where all the sensory nerves originate from a single spinal nerve root.

  With regards to guttate hypomelanosis, it primarily manifests on the areas exposed to the sun, particularly the forearms, legs and décolletage. The lesions are often evenly dispersed across the affected areas, giving a "raindrop" appearance. They generally do not merge to form patches, which distinguishes them from vitiligo.

• The macules of vitiligo and hypomelanosis in drops have different morphologies.

  The appearance of the lesions also varies slightly, depending on whether it is vitiligo or guttate hypomelanosis. Vitiligo spots present themselves as well-defined white patches, with clear and regular contours. They are uniformly white, reflecting the total absence of melanin, and are sometimes accompanied by leucotrichia, corresponding to a loss of hair pigmentation. In the case of guttate hypomelanosis, the lesions tend to take the form of small rounded or oval macules, 2 to 5 mm in diameter. The edges are blurred and the spots are not always uniformly white, revealing the presence of residual melanin. No leucotrichia is observed.

• Vitiligo and guttate hypomelanosis progress in different ways.

  Finally, the progression of vitiligo and guttate hypomelanosis are not similar. Vitiligo lesions are often unpredictable, they can remain stable for years or progress rapidly, even merging together to form larger patches. Moreover, stress and skin trauma can accelerate the progression. Guttate hypomelanosis macules, on the other hand, appear progressively and do not tend to merge or widen. Their number simply increases over time, especially after the age of 40, correlating with repeated UV exposure and the natural ageing of the skin.