Erythema and purpura are both skin conditions that cause redness, but they differ significantly in their underlying causes and characteristics.
Key Differences Explained
| Feature | Erythema | Purpura |
|---|---|---|
| Cause | Dilation of blood vessels in the skin. | Bleeding under the skin. |
| Appearance | Redness that disappears when pressed. This is known as blanching. | Red or purple spots that do not disappear under pressure. |
| Temperature | Skin temperature is usually normal unless there is arterial dilation. | Skin temperature is normal. |
| Blanching | Blanches (redness disappears with pressure). | Does not blanch (redness persists with pressure). |
Detailed Explanation
-
Erythema:
- Erythema is characterized by a reddening of the skin due to increased blood flow in the superficial vessels.
- It's essentially a vascular reaction, making the skin look flushed or red.
- When pressure is applied to the affected area, the redness disappears momentarily (blanching) as blood is temporarily pushed out of the vessels.
- Examples include heat rash, sunburn, or allergic reactions that cause vessel dilation.
- There is no change in skin pigmentation.
-
Purpura:
- Purpura results from the extravasation of red blood cells into the skin from broken or damaged blood vessels.
- The spots are usually reddish-purple, and can be pinpoint-sized (petechiae) or larger (ecchymoses).
- These spots do not blanch when pressed, because the color is from blood outside the vessels, not blood inside them.
- Purpura is often associated with issues related to blood clotting or blood vessel fragility.
- Pigmentation can occur because of the bleeding.
- Examples of conditions causing purpura include blood disorders, vasculitis, or trauma.
Practical Insights
- Simple Test: A quick test to differentiate between erythema and purpura is the "glass slide test." Applying pressure to the skin with a clear glass slide will show if the redness disappears (erythema) or persists (purpura).
- Temperature: While erythema can sometimes be accompanied by a slight increase in skin temperature due to vessel dilation, purpura does not affect temperature.
- Underlying Cause: It's critical to determine the underlying cause of the skin changes as they differ significantly. Erythema usually points to inflammation or irritation, whereas purpura often suggests a problem with blood vessels or clotting.
In summary, erythema is a temporary flushing of the skin that disappears with pressure, while purpura involves bleeding under the skin, leading to non-blanching reddish-purple spots. The key difference lies in whether the redness is due to vessel dilation or extravasated blood. The reference information states: Erythema disappears on finger pressure (blanching), whereas purpura or bleeding in the skin and pigmentation do not. There is no temperature elevation, unless it is associated with the dilation of arteries in the deeper layer of the skin.
