RLS, Restless Leg Syndrome, is not the same as PLMD, Periodic Limb Movement Disorder. (Sometimes referred to as PLMS, periodic limb movement during sleep)
RLS is a daytime, "voluntary" thing.
PLMD is a nighttime, "involuntary" thing.
RLS is a strong urge to move the legs (to be overly simplistic). It is a nervous system disorder. The urges make one tense up, move, twitch, etc but all of it is voluntary. It can happen during any point of the day but is worse while at rest, including when laying down to sleep.
PLMD is more sudden and only happens during sleep. It is considered a sleep disorder. It can be violent or mild. It can happen to those without other sleep disorders such as sleep apnea. Many of us with sleep apnea have the diagnosis of PLMD as a side effect of our OSA. The twitches happen because of our apnea events. The worse the event, the worse the twitch.
A person can have both RLS and PLMD but the two are not the same.
While RLS is not considered a sleep disorder, it can disrupt sleep, especially in the beginning. I can see how a medication to treat RLS would improve sleep which would, in turn, lower the AHI. Because you are sleeping without being woke up, the machine has a chance to treat the apnea events and keep the airway open. Before, it never really had a chance to do so. This is my assumption.
I did some Google foo can found this interesting article
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2671939/
