Glomus tumor

A glomus tumor is a rare, but usually benign tumor of a certain type of skin cell that regulates the flow of blood to the surface of the skin to regulate body temperature, the glomus. They can appear anywhere there is skin, but usually appear under the fingernails. They are generally small and solitary, although in some cases patients may have multiple tumors. They are generally painful and because the glomus reacts to cold, the pain usually increases when exposed to cold, particularly cold water.
Although a glomus tumor may become malignant, this is very rare. As such, surgery to remove the tumor is only recommended if the tumor is easily accessible and solitary. Luckily, the tumors are less likely to be painful if there is more than one of them, but multiple tumors can interfere with other body functions.
Glomus tumors are slightly more common in women, although men are slightly more likely to have multiple tumors.
Glomus tumor at Wikipedia