Anne,
I think Ampyra is prescribed to treat walking problems in people diagnosed with M.S.
Ampyra works by blocking potassium channels -- located on the surface of the nerves -- that become exposed when the nerves' protective coating, myelin, is damaged in M.S.
Normally, potassium ions, which enable nerve impulses to flow along the nerve fibers, work through these channels. When the channels are exposed, however, potassium ions leak from them, interrupting the flow of nerve impulses.
By blocking the potassium channels and improving the conduction of nerve impulses, Ampyra increases communication between damaged nerve cells to improve nerve function.
Since this is about Potassium (K+1) ions blocking and, being M.S. classified as an auto-immune disorder that affects the Myelin stealth of the nerve cells whereas PLS, ALS and other MND is the dead of the Motor Neurons (whether these latter ones are auto-immune or not has yet to be determined) I am not sure how this drug could help you with PLS.
Although, I might be wrong or mistaken.
Why don't you ask your Neuro about Baclofen?
Regards,