Being a cavalryman, a fall from his horse as part of the capture sequence is possible, and may not have left any overt injury. Bouncing a head around will cause such swelling. As an aside, the medical practice of trephinning, drilling a hole in the skull, was used from Greek and Egyptian times to relieve such pressure. The practice was part of the practice of WBTS medicene. I have seen the instruments; they look like hole saws. Meningitus would be another cause. My background is not medicine, but photography will sometimes lead one down strange and interesting paths. Stan