Snooze University
Tools

PLMS Index: How to Intrepret It

by Dale Cudmore | Updated: Mar 14, 2022

Periodic limb movements during sleep (PLMS) are often recorded on sleep studies.

They’re usually not the most important thing in sleep study results, but can be an indication of an underlying condition like sleep apnea or a neurological disorder.

Let’s go through some basic questions about what the PLMS index is, and how to interpret it correctly.

What is The PLMS Index?

During a sleep study, data on leg movements can be collected using electromyography (EMG).

A PLMS is typically counted as a series of leg movements that are 0.5 to 5 seconds in length, and are separated by 5 to 90 seconds.

The PLMS index is simply the total amount of periodic limb movements per hour of sleep. Note that

How is PLMS Measured?

While not every sleep study will include PLMS, most do.

You’ll likely see a section labeled “PLM” or “PLMS” or “PLMS Index” on your sleep study results.

What Is a “Normal” PLMS Index Score?

According to the International Classification of Sleep Disorders (ICSD) evaluation of PLMS, scores can be interpreted in 3 categories (1):

  • Normal: PLMS index of less than 5
  • Mild: PLMS index of 5-25
  • Severe: PLMS index of 50+

Note that the actual criteria for a “severe” diagnosis also includes a PLMS arousal index score of over 25; we’ll look at what this means shortly.

How Common is PLMS?

PLMS is fairly common, it’s currently estimated to occur in 4-11% of adults (2).

While PLMS is rare in people under 30 (about 2%), PLMS to some degree is extremely common in older people. It appears that PLMS occurs in over 30% of people that are 65 or older (4).

There’s no significant difference in PLMS rates between men and women.

It doesn't always cause sleep issues, and doesn’t always require treatment.

What is Periodic Limb Movement Disorder?

Periodic limb movement disorder, or PLMD, is not particularly common.

It’s the diagnosis given when someone has sleep issues like insomnia or hypersomnia along with a high PLMS index, that can’t be explained by other sleep conditions.

In most cases, a high PLMS is caused by other more common sleep conditions like sleep apnea or narcolepsy. But when all those are ruled out, PLMD is typically the diagnosis given.

Is the PLMS Index Different From the PLMA Index?

Earlier, we looked at how the classification of PLMS index scores can also depend on a PLMS arousal index score.

This is also often written as PLMA.

Arousal in this context refers to the periodic movement causing someone to gain some level of consciousness within 3 seconds of the PLM. Therefore, the PLMA index score is at most the same as the PLMS index score, but typically lower.

How Is PLMD Related to Restless Leg Syndrome?

While closely related, PLMD is a different disorder than restless leg syndrome (RLS).

PLMD requires that sleep issues also occur, while RLS can occur without significant sleep issues, and can even occur while awake during the day.

It is estimated that over 80% of patients with RLS also have PLMD (3). Typically, PLMD is seen as a symptom of RLS, rather than an independent comorbidity.

References

  1. Moderate to Severe Periodic Limb Movement Disorder in Childhood and Adolescence
  2. Periodic leg movements in sleep and periodic limb movement disorder: Prevalence, clinical significance and treatment
  3. Merck Manuals
  4. PLMS and PLMW in Healthy Subjects as a Function of Age

Medical Disclaimer: The information on SnoozeUniversity.com is not intended to be a substitute for physician or other qualified care. We simply aim to inform people struggling with sleep issues about the nature of their condition and/or prescribed treatment.


About the authorDale is the founder of Snooze University and a sleep researcher. I overcame my sleep issues and now I'd like to help you do the same by summarizing the latest sleep studies for you.