# Differential Diagnosis of Hypopigmented Lesions

> NEET-PG Dermatology notes on hypopigmented lesions, covering naevus depigmentosus, naevus anaemicus, guttate hypomelanosis, ash-leaf macules and leprosy.

- Canonical URL: https://prepelephant.com/topics/neet-pg/dermatology/hypopigmented-disorders-differential
- Exam / course: NEET-PG · Subject: Dermatology
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-15
- Last updated: 2026-10-15
- Reference basis: IADVL Textbook of Dermatology; Fitzpatrick's Dermatology; Harrison's Principles of Internal Medicine
- How to cite: "Differential Diagnosis of Hypopigmented Lesions", PrepElephant, https://prepelephant.com/topics/neet-pg/dermatology/hypopigmented-disorders-differential

## 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.