You are quite right in suspecting that there was a decrease in the blood supply when your index finger became white and cold. The numbness is a sign that the nerves were also affected.
The phenomenon is called Raynaud's phenomenon. In this condition, the blood vessels of either a single finger or toe or a number of fingers become varrow and constricted. This can occur in reaction to extreme cold, emotional upsets and stress of any kind. This leads to a decreased blood supply to the particular finger.
Constriction at the base of the finger could have caused this problem. And constriction can occur in diseases that affect the blood vessels such as atherosclerosis, rhematoid arthritis, Buerger's disease etc.
Since you have rheumatoid arthritis, I suspect that this is the basic cause of the Raynaud's phenomenon in your case.
Please consult your doctor. There is no specific treatment of Raynaud's, but if the problem persists, you may need to be given medicines to dilate the blood vesels.
To know more about Raynaud's phenomenon, please see:
I hope this helps.
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