Differential Diagnosis of Hypopigmented Lesions
On this page
Direct answer
Hypopigmented lesions are patches of reduced skin colour, and their differential is sorted by border, distribution, age of onset and sensation. Naevus depigmentosus is a stable, melanin-poor patch present from early childhood, naevus anaemicus is a pale patch with abnormal vascular response, and idiopathic guttate hypomelanosis is a multiple, sun-related lesion of adults. Ash-leaf macules signal tuberous sclerosis, while any hypopigmented patch with sensory loss must be checked for leprosy.
What you must remember
- Naevus depigmentosus: a stable, melanin-poor hypopigmented patch present from early childhood, typically unilateral and non-progressive.
- Naevus anaemicus: a pale patch that does not show friction-induced erythema, reflecting abnormal vascular reactivity rather than missing melanin.
- Idiopathic guttate hypomelanosis: multiple small, discrete, well-defined porcelain-white macules on sun-exposed lower legs and forearms of adults, usually harmless.
- Ash-leaf macules: oval or lanceolate hypopigmented macules present from birth, a major criterion of tuberous sclerosis, best seen under Wood's lamp.
- Pityriasis alba: ill-defined, slightly scaly patches on the face of children, often linked with atopy and tending to resolve.
- Leprosy: hypopigmented or erythematous patches with loss of sensation, reduced sweating and thickened nerves.
- Vitiligo: chalk-white, sharply demarcated patches without melanocytes, which look brightest under Wood's lamp.
- Tinea versicolor: fine scaling hypopigmented or hyperpigmented macules, with yellow-green fluorescence under Wood's lamp and short hyphae with clusters of round yeast on potassium hydroxide preparation.
Sorting hypopigmented patches by key clues
A 6-year-old child has a pale patch on the thigh present since birth and a second patch with fine scale on the trunk. The table lists the commonest diagnoses, and the discriminating clue for each is what usually decides the diagnosis.
| Condition | Key feature | Discriminating clue |
|---|---|---|
| Vitiligo | Chalk-white, sharp-edged, no melanocytes | Brightest under Wood's lamp; may show Koebner phenomenon |
| Naevus depigmentosus | Stable, present from early childhood | Non-progressive, unilateral course |
| Naevus anaemicus | Pale patch with normal pigment | Failure of friction-induced flare |
| Ash-leaf macule | Lanceolate, present from birth | Accentuated under Wood's lamp; look for other features of tuberous sclerosis |
| Leprosy | Hypopigmented anaesthetic patch | Loss of sensation to light touch and pinprick; thickened nerves |
| Pityriasis alba | Ill-defined, scaly patches on the face | Atopy and spontaneous resolution |
| Tinea versicolor | Fine scale, variable pigment | Yellow-green fluorescence; short hyphae and yeast on KOH |
The sensation test and Wood's lamp examination separate most of these lesions quickly. Leprosy is the diagnosis that cannot be missed, because a sensory deficit within a patch demands nerve examination and a slit skin smear.
Where the exam tries to catch you
Hypopigmented patch questions test sensation, Wood's lamp findings and the logic of the differential.
- Trap: calling a sensory-loss hypopigmented patch pityriasis alba. Correct: loss of sensation within a hypopigmented patch is leprosy until proven otherwise, and it needs nerve examination and a slit skin smear.
- Trap: diagnosing naevus anaemicus from paler colour alone. Correct: the distinguishing feature is the absence of friction-induced flare, which reflects abnormal vascular response rather than reduced melanin.
- Trap: ignoring Wood's lamp in a child with ash-leaf macules. Correct: ash-leaf macules glow more clearly under Wood's lamp, and their detection should prompt a tuberous sclerosis evaluation.
Frequently asked questions
Which hypopigmented patch needs a leprosy workup?
Any hypopigmented or erythematous patch with reduced sensation to light touch, pinprick or temperature needs a leprosy evaluation. Thickened peripheral nerves, reduced sweating and a positive slit skin smear strengthen the suspicion of leprosy.
How is naevus anaemicus differentiated from naevus depigmentosus?
Naevus anaemicus is pale because of abnormal vascular response, and its diagnostic sign is the failure of friction-induced flare on stroking. Naevus depigmentosus is a stable, melanin-poor patch that is present from early childhood and does not reflect abnormal vascular reactivity.
What is idiopathic guttate hypomelanosis?
Idiopathic guttate hypomelanosis presents as multiple small, discrete, well-defined hypopigmented macules on the sun-exposed lower legs and forearms of adults. It is thought to relate to chronic sun exposure and is harmless, so reassurance is usually the main management.
Why are ash-leaf macules important in dermatology?
Ash-leaf macules are oval or lanceolate hypopigmented patches present at birth, and they are a major diagnostic criterion of tuberous sclerosis complex. They are best seen under Wood's lamp, and their detection should prompt a search for other features of the syndrome.