Wood's Light Examination of Hypopigmented Nevi
Hypopigmented nevi do not enhance or become more prominent under Wood's lamp examination, unlike true depigmented lesions such as vitiligo which show marked accentuation. This is because Wood's light specifically delineates areas of complete pigment loss (depigmentation) rather than reduced pigmentation (hypopigmentation) 1.
Understanding Wood's Light Principles
Wood's lamp examination uses ultraviolet A radiation to assess pigmentary disorders by exploiting how different depths and degrees of melanin loss appear under UV illumination 1.
Key principle: Wood's light increases contrast in epidermal pigmentation while decreasing contrast in dermal pigmentation 2. This means:
- Complete depigmentation (vitiligo): Areas appear bright white and significantly larger under UV than in visible light 1
- Hypopigmentation (nevi): Areas show minimal to no enhancement because residual melanin remains present 2
Expected Findings for Hypopigmented Nevi
Hypopigmented melanocytic nevi will appear similar under Wood's lamp as they do in ambient light, without the dramatic accentuation seen in vitiligo 1, 2.
The hypopigmented areas within nevi result from:
- Decreased junctional melanocytes around hair follicles 3
- Reduced pigment within keratinocytes 3
- Perifollicular hypopigmentation patterns 3
These areas retain some melanin production, preventing the bright white fluorescence characteristic of complete melanocyte loss 2.
Clinical Differentiation
Wood's lamp is most useful for distinguishing hypopigmented nevi from true depigmenting conditions in patients with fair skin (Fitzpatrick types I-II) 1.
Vitiligo vs. Hypopigmented Nevus:
- Vitiligo: Bright white enhancement, appears larger under UV, sharp demarcation 1
- Hypopigmented nevus: Minimal change from ambient light appearance, no dramatic enhancement 2
Important Clinical Caveats
Wood's lamp examination has limited utility in darkly pigmented skin where the contrast between normal and hypopigmented areas is already visible clinically 1.
The examination is primarily valuable for:
- Detecting subtle depigmentation in fair-skinned individuals 1
- Assessing extent and activity of vitiligo 1
- Monitoring repigmentation during treatment 1
Common pitfall: Do not confuse the lack of enhancement in hypopigmented nevi as a "negative" finding—it is actually the expected result that helps differentiate these lesions from true depigmenting disorders 2.