Hi Ray,
This is a really nice observation,
It might be a nice tip and trick for the website,
I think when you extend the fingers you tighten the ulnar bursa and see less flow,
Gentle flexion of the fingers minimizes this pressure and the flow is more easily seen.
It makes sense !
Another thing we have discussed before is the level at which you look for vascularity in the carpal tunnel. The median nerve is nearly always at its most swollen at the lunate level as this is immediately prior to the transverse carpal ligament where it becomes compressed. If you do your vascularity assessment at the level of the lunate you will see more flow.
So we now have 2 new tricks.
1: Slightly flex the fingers
2: Make your Doppler assessment at the level of the lunate rather than more distally within the tunnel.
This is a great tip and trick Ray,
Steve