Overly dilated capillaries can cause port-wine stains. These enlarged capillaries allow blood to collect in them anywhere in the body, giving port-wine stains their pinkish or purplish color.

A port-wine stain is a pink or purple birthmark on the skin. It’s also referred to as nevus flammeus.

In most cases, port-wine stains harmless. But occasionally, they may be a sign of an underlying health condition.

Read on to learn more about port-wine stains, including what causes them and when they might be a sign of something else.

Port-wine stains generally don’t cause any symptoms, aside from their appearance. They usually start out as red or pink. Over time, they can darken to a purple or brown color.

Other characteristics of port-wine stains include:

  • Size. They can range in size from a few millimeters to several centimeters.
  • Location. Port-wine stains tend to appear on one side of the face, head, and neck, but they may also affect the abdomen, legs, or arms.
  • Texture. Port-wine stains usually start out being flat and smooth. But over time, they may become thicker or slightly bumpy.
  • Bleeding. The skin of a port-wine stain may be more prone to bleeding when scratched or injured.

Port-wine stains are caused by an issue with capillaries, which are very small blood vessels.

Usually, capillaries are narrow. But in port-wine stains, they’re overly dilated, allowing blood to collect in them. This collection of blood is what gives port-wine stains their distinctive color. Port-wine stains may become larger or change shape as capillaries grow larger.

Port-wine stains on the scalp, forehead, or around your eyes, may be a symptom of a condition called Sturge-Weber syndrome.

This condition happens when there are unusual blood vessels in the skin and the surface of the brain, which affects the flow of blood to the brain.

Learn more about Sturge-Weber syndrome.

When port-wine stains appear on the arms or legs, they may also be a symptom of Klippel-Trenaunay syndrome. In this case, they usually appear on just one limb.

This rare genetic condition causes changes in the blood vessels of the affected leg or arm. These changes can cause the bone or muscle of that limb to grow longer or wider than usual.

Port-wine stains usually don’t require treatment. But some people choose to have them faded for cosmetic reasons. This is usually done with laser treatments that use a pulsed dye laser.

Other laser and light treatments include:

  • Nd:YAG
  • bromide copper vapor
  • diode
  • alexandrite
  • intense pulsed light

Laser and light treatments work by using heat to damage the abnormal blood vessels. This causes the blood vessel to close up and disintegrate after a few weeks, helping to shrink, fade, or possibly remove port-wine stains.

Most people will need several treatments, though the exact number depends on several factors, including skin color, size, and location.

Keep in mind that laser treatments may not completely remove a port-wine stain. But they may be able to lighten the color or make them less noticeable. Laser treatments may also cause some permanent scarring or discoloration.

Following laser treatment, your skin will be extra sensitive, so be sure to wear sunscreen and protect the affected skin following the procedure.

Most port-wine stains are harmless. But they can sometimes lead to the development of an eye condition called glaucoma if they’re located near the eyes.

Glaucoma involves high pressure in the eye, which can lead to vision loss if not treated. Up to 10 percent of people with a port-wine stain near the eye develop glaucoma.

If you or your child has a port-wine stain near the eyes, check if:

  • one eye has a larger pupil than the other
  • one eye appears more prominent
  • one eyelid is open wider than the other eye

These may all be symptoms of glaucoma, which is treatable with prescription eye drops or surgery.

Also, skin thickening and “cobblestoning” can occur as a result of malfunctioning capillaries. Early treatment of port-wine stains can help prevent this from occurring.

Port-wine stains usually aren’t anything to worry about, though in some cases, they can be a symptom of an underlying condition. Regardless of their cause, port-wine stains are sometimes removable with laser treatments.

Laser treatments may not completely get rid of port-wine stains, but they can help make them less noticeable.