What Causes Tingling Sensation , Numbness And Swelling In The Hand?

I m asking this for my husband...yesterday he was out working in the forest, riding his horse..then suddenly he felt a tingly sensation in the left hand. His left hand went numb and became a bit swollen. He lost feeling in the tip of his thumb. It started to become bruised around the palm, where the thumb connects up to the wrist. He did not injure his hand at all, this was just random. I, fact while he was riding his horse he held the reins in his right hand with his left hand hanging free. The feeling in his thumb came back but he palm up to the wrist is still bruised and is now painful...any idea of what this could be? Thank you.