A serious event that is potentially fatal. Complications include pulmonary embolism and intracranial extension of the thrombus. Central venous catheters are a common underlying cause. May also result from intravenous drug abuse, head or neck infection, surgery, a localized malignancy, or even spontaneously. Clinical symptoms may be vague and subtle and can include fever, leukocytosis, swelling, and pain among others. May be a part of a superior vena cava syndrome, which is often associated with malignancy.