In the September 2006 issue of the British Medical Journal, G.P. David Connell speculates about the unnaturally long fingers of bluesman Robert Johnson . Based on pictures and contemporary accounts, Connell thinks that Johnson had Marfan Syndrome . Marfan Syndrome is a genetic connective tissue disorder: those affected are usually tall and slim, with long fingers and flexible joints. Another sign is the slight droopiness of the left upper eyelid, seen in the photograph.
Connell speculates that Johson's early death at the age of 27 was due to the heart defects associated with the syndrome, not syphillis as others have suggested.
It could be that Johnson's long and flexible fingers contributed to his unique playing style. Eric Clapton on his Sessions for Robert J (a tribute to Johnson) remarked about the difficulty in copying some of Johnson's acoustic techniques.
Source: BMJ. 2006 September 2; 333(7566): 489