Cavernous hemangiomas (also called angioma cavernosum or cavernoma) are
similar to strawberry hemangiomas but are more deeply situated. They may appear
as a red-blue spongy mass of tissue filled with blood. Some of these lesions
may disappear on their own -- usually as a child approaches school age.
In most cases, a health professional can diagnose a red birthmark based on
the appearance of the skin. Deeper birthmarks can be confirmed with tests such
as MRI, ultrasound, CT scans or biopsies.
more about red birthmarks.