Periodic limb movements in sleep: irrelevant epiphenomenon, marker for a potential problem, or a disorder?

446 " Things are not always what they seem and the first appearance deceives many… " —Phaedrus by Plato, written 360 BC. A s a young sleep physician in a new field 20 years ago, it was hard not to get caught up in the excitement of periodic limb movements in sleep (PLMS) as an important indicator of poor sleep and the assumption that treatment was warranted to " normalize sleep. " After all, it was known that disruption of sleep by loud tones produced negative daytime consequences. In addition , it was found that both PLMS and insomnia increased with age—so why not assume clinical relevance? Unfortunately, first appearances were deceiving. Research found that PLMS severity did not correlate well with daytime sleepiness or with insomnia complaints. Incredibly high PLMS indexes in rapid eye movement sleep behavior disorder (RBD) and in spinal cord injury appeared to be of no clinical relevance, and small case series of PLMS treatment in narcolepsy and poorly defined periodic limb movement disorder (PLMD) cases showed no clinical benefit. Thus, PLMS were declared " not clinically significant " by some, and PLMD was to be stricken from the latest version of the International Classification of Sleep Disorders, ICSD-2. But wait— could " second appearances " be deceiving? PLMS are a common and readily identified finding on poly-somnography. Over time, it has become clear they are seen in heterogeneous conditions: obstructive sleep apnea (OSAS) (limb jerks at the termination of respiratory events), restless legs syndrome (RLS) (80%-90% of patients with RLS have 5 or more PLMS per hour), narcolepsy, RBD, and spinal cord injury. PLMS are a strong predictor of mortality in end-stage renal disease. In addition, most antidepressants have been found to induce or exacerbate PLMS. Clearly, significance and treatment considerations are much different in these different groups. For example, dopa-minergic therapy is appropriate for RLS but not for OSAS. Thus, when PLMS are found, it is appropriate for the clinician to ask, " Are these a marker for 1 of the above problems? " If so, the focus should be on that diagnosis. Sixteen years ago, we reported on a man referred for OSA, who had severe PLMS on polysom-nography, but not significant OSAS, and subsequently did meet RLS diagnostic criteria. He had notable benefit from dopaminer-gic therapy. We have continued to find RLS cases this way—not because of a lack of adequate history …