Hypopigmentation refers to an area of skin becoming lighter than the baseline skin color due to a decreased amount of melanin pigment in the skin, which is produced by melanocytes.
Its most severe presentation is depigmentation, in which there’s absence of all pigment.
Most causes of hypopigmentation are not serious, and diagnosis can be made based upon a detailed history and physical examination, taking into account the course of the disorder, and lesion morphology, distribution, pattern, and extent of hypopigmentation, as well as additional cutaneous and extracutaneous signs and symptoms.
Individuals may be examined under a Wood's lamp, which emits low wave ultraviolet A light that allows a better visualization of variations in skin pigmentation. This is done in a darkened room with the Wood's lamp held at 4 to 5 inches from the skin, to observe any subsequent fluorescence.
Hypopigmented lesions emit a bright blue-white fluorescence and appear sharply delineated - the brighter they appear, the lower the amount of melanin pigment.
Examination under a Wood's lamp is especially helpful in fair skinned individuals to identify hypopigmented or depigmented lesions that may not be visible to the naked eye.
Finally, cases where the diagnosis is uncertain may get a skin biopsy to evaluate the number and location of melanocytes and melanin in the affected skin areas.
One of the most common and well known hypopigmentation disorders is vitiligo.
The exact cause isn’t known, but there’s an autoimmune destruction of melanocytes, leading to complete depigmentation of well defined patches that can range in size from millimeters to centimeters and can sometimes expand and merge with other patches over time.
These patches are classified into two broad categories. There’s non-segmental vitiligo, which is the more common type that affects any age group, and it occurs at various locations that are mirrored on both sides of the body.
Non-segmental vitiligo may progress and ultimately involve the whole body, which is called universal vitiligo.
And there’s also segmental vitiligo, the least common type of vitiligo, which mostly affects children, and occurs in segments along a single spinal nerve typically on only one side of the body without crossing the midline, particularly in the face following the trigeminal nerve.