I began Lithium in February '08, though my neurologist was a leading voice opposing Li.
In April, my FVC measured 67% (was 100% in Nov). I wanted to blame the heavy pollen, but in May the pollen cleared, yet my breathing remained shallow. By then, I was up to 900mg/day Li and my blood finally hit the target level, but due to continued shortness of breath I quit Li.
I now use the biPap at night--it helps a lot. I continue to atrophy in the limbs. Walking, standing and climbing stairs is limited by back and neck weakness, but I can still ride my recumbent bicycle--its done wonders for my mobility and independence.
The breathing difficulties were preceded by cramping in the abdomen whenever I coughed or sneezed. Breathing may have stabilized once I stopped Lithium--hard to say for sure. For me, the whole deal started in March '05 in the same way I've seen a lot of PALS report lately: My left thenar (thumb) muscle disintegrated, turning the big fleshy part of the palm at the base of the thumb into a sunken depression.