Thoughts: Single-digit leuconychia totalis has not been reported in the medical literature. It is likely that prolonged ischemia may have played a role.
Dr. Eckart Haneke's comments were helpful: "Single digit leukonychia is just a description of the condition, not a diagnosis. I think this is due to the long-term poor blood supply of the finger. There is no similarity to superficial white onychomycosis. I cannot predict the future development. It may get slowly better, but this will take some more time. If you want to know something more about the situation you need a comparative demonstration of the blood circulation, e.g. by thermography or another similar method, of this and the contralateral finger. Diffuse leukonychia is sometimes seen in Raynaud's and other disturbances of blood supply."
Leuconychia in reflex sympathetic dystrophy:
a chance association? Vanhooteghem O, et.al.
Br J Dermatol. 1998 Aug;139(2):355-6.
PDF of reference.