What Causes Swelling In Arm With Numbness In Fingers?

My husband was admitted to the hospital today. Several Dr.s think he has an upper extremity bllo clot. He woke up Saturday AM with a swollen arm, not able to bend more than 30%, no insect bite signs, and no trauma to the arm. His left arm is twice the size as his right, 4th day of swelling, his fingers have begun to tingle and his thumb is numb, there is also a centralized ppoint of pain between the wrist and elbow on the outside of the arm and his elbow appears to have a pocket of fluid around the tissue. This was seen today on ultrasound. He thinks they are guessing, I think he is right where he needs to be. Is there anything else that this could be besides a clot?