Can Fragmentary Myoclonus Increase the Total Arousal Index on PSG?
Yes, fragmentary myoclonus can increase the total arousal index on polysomnography, particularly in women, though the relationship is complex and not universally present across all patient populations.
Direct Evidence of FM-Arousal Association
The most recent and highest quality evidence directly addressing this question comes from a 2019 study that specifically investigated the relationship between fragmentary myoclonus and arousals 1. In women specifically, higher arousal index predicted higher fragmentary myoclonus index (FMI25), suggesting that FM may serve as a surrogate marker for more frequent arousals and sleep fragmentation 1. However, this association was not found in men, indicating important sex-based differences 1.
Clinical Context and Mechanisms
Historical Recognition of Sleep Fragmentation
Early observational studies from the 1980s documented that excessive fragmentary myoclonus in NREM sleep was associated with "some degree of sleep fragmentation" 2, 3. These patients presented with excessive daytime sleepiness, and the fragmentary myoclonus was noted as occurring with sleep fragmentation as an isolated polysomnographic finding 2.
Distribution Across Sleep Stages
Fragmentary myoclonus indices are significantly higher during wakefulness and lower during stage N3 sleep compared to all other sleep stages 1. This distribution pattern suggests that FM may be more prevalent during lighter, more fragmented sleep states 1. The phenomenon occurs throughout all stages of NREM sleep but shows variable intensity depending on sleep depth 2, 4.
Important Caveats and Clinical Pitfalls
Sex Differences Matter
The association between FM and arousal index is sex-dependent and primarily observed in women 1. In men, despite having higher overall FMI compared to women, the correlation with arousal index was not statistically significant 1. This means you cannot assume FM will predict arousals equally across all patients.
Confounding Variables
When interpreting FM in the context of arousal indices, consider that:
- Age independently predicts higher FMI in both males and females 1
- Lower sleep efficiency and shorter sleep period time correlate with higher FMI 1
- Higher periodic limb movement (PLM) index is associated with increased FMI 1
- FM is not associated with sleep-related breathing disorders 1
Treatment Effects
PAP therapy for sleep-related breathing disorders significantly reduces the minutes of NREM sleep with excessive fragmentary myoclonus (minNREM+EFM), though it does not significantly change the overall FMI 5. This suggests that treating underlying sleep disorders may alter FM distribution without eliminating the phenomenon itself 5.
Practical Scoring Considerations
The American Academy of Sleep Medicine Manual identifies some EFM scoring criteria but does not provide specific amplitude thresholds 4. Using a 25 μV amplitude criterion (FMI25) appears more sensitive for detecting the association with arousals compared to the traditional 50 μV threshold 1, 4. When evaluating for sleep fragmentation, the lower amplitude criterion may be more clinically relevant, particularly in female patients 1.
Clinical Interpretation
FM should be considered as one potential contributor to the total arousal index, particularly when evaluating women with unexplained sleep fragmentation and excessive daytime sleepiness 1. However, FM is likely a ubiquitous motor phenomenon that occurs spontaneously during relaxed wakefulness and sleep, primarily in men and with advanced age, rather than a primary pathological process 1. The American Academy of Sleep Medicine emphasizes that arousal-based scoring is crucial for accurately diagnosing sleep disorders and identifying sleep fragmentation 6.