Periodic limb movement disorder (PLMD) is repetitive cramping or jerking of the legs during sleep. It is the only movement disorder that occurs only during sleep, and it is sometimes called periodic leg (or limb) movements during sleep. “Periodic” refers to the fact that the movements are repetitive and rhythmic, occurring about every 20-40 seconds. PLMD is also considered a sleep disorder, because the movements often disrupt sleep and lead to daytime sleepiness.
PLMD may occur with other sleep disorders. It is often linked with restless leg syndrome, but they are not the same thing. Restless leg syndrome is a condition involving strange sensations in the legs (and sometimes arms) while awake and an irresistible urge to move the limbs to relieve the sensations. At least 80% of people with restless legs syndrome have PLMD, but the reverse is not true.
When PLMD was first described in the 1950s, it was called nocturnal myoclonus. Nocturnal means night, and myoclonus is a rapid, rhythmic contraction of a group of muscles similar to that seen in seizures. PLMD movements are not myoclonus, however, and the original name is not used today.
PLMD can occur at any age. Like many sleep disorders, PLMD is more common in middle-aged and older people.