It is a diagnostic procedure where a glass slide or other transparent object is pressed against the skin to observe changes in the lesion's color.
Diascopy is used to -
1. Differentiates vascular lesions (e.g., erythema, which blanches) from non-vascular ones (e.g., purpura, which does not).
• Erythema blanches under pressure because it is caused by dilated blood vessels.
• Purpura, which is due to extravasated blood (leakage of blood into the skin), does not blanch.
2. See apple jelly nodules in lupus vulgaris and sarcoidosis
3. Differentiate nevus anemicus (blends with surrounding skin) from depigmentosis (remains distinct).
• Nevus anemicus:
1. Caused by localized vasoconstriction.
2. On diascopy (applying pressure with a glass slide), the lesion blends with surrounding skin as both are pale.
• Depigmentosis (e.g., vitiligo):
1. Caused by loss of melanocytes or melanin production.
2. On diascopy, the lesion remains distinct from surrounding skin as it is genuinely hypopigmented and unaffected by blood flow.