Searching for a Marker of REM Sleep Behavior Disorder: Submentalis Muscle EMG Amplitude Analysis during Sleep in Patients with Narcolepsy/Cataplexy

Abstract

Study Objectives:

To evaluate the amplitude of submentalis muscle EMG activity during sleep in patients with narcolepsy/cataplexy with or without REM sleep behavior disorder (RBD).

Design:

Observational study with consecutive recruitment.

Settings:

Sleep laboratory.

Patients:

Thirty-four patients with narcolepsy/cataplexy and 35 age-matched normal controls.

Measurements and results:

Half the patients (17 subjects) had a clinical and video polysomnographic diagnosis of RBD. The average amplitude of the rectified submentalis muscle EMG signal was used to assess muscle atonia, and the new REM sleep Atonia Index was computed. Chin muscle activations were detected and their duration and interval analyzed. REM sleep Atonia Index was lower in both patient groups (with narcolepsy patients with RBD showing the lowest values) with respect to controls, and it did not correlate with age as it did in controls. The total number of chin EMG activations was significantly higher in both patient groups than controls. No significant differences were found between the two groups of patients, although more chin EMG activations were noted in narcolepsy patients with RBD than those without.

Conclusions:

Elevated muscle activity during REM sleep is the only polysomnographic marker of RBD. This study shows that polysomnographically evident RBD is present in many patients with narcolepsy/cataplexy. This condition might be specific to narcolepsy/cataplexy, reflecting a peculiar form of REM sleep related motor dyscontrol (i.e., status dissociatus), paving the way to enacting dream behaviors, and correlated with the specific neurochemical and neuropathological substrate of narcolepsy/cataplexy.

Citation:

Ferri R; Franceschini C; Zucconi M; Vandi S; Poli F; Bruni O; Cipolli C; Montagna P; Plazzi G. Searching for a marker of REM sleep behavior disorder: submentalis muscle EMG amplitude analysis during sleep in patients with narcolepsy/cataplexy. SLEEP 2008;31(10):1409–1417.

REM SLEEP BEHAVIOR DISORDER (RBD), FIRST DESCRIBED AS A CLINICAL ENTITY IN 1986 BY SCHENCK AND COWORKERS,¹ HAD BEEN OBSERVED EVEN earlier in narcoleptic patients and termed “ambiguous sleep”² because of its “low phasic atonia with an extreme abundance of twitches and muscular discharges.” RBD is currently classified as a parasomnia related to REM sleep, characterized by loss of the stage-specific muscle atonia, and a frequent enactment of violent dream content resulting in self or bed-partner injuries. Polygraphically, tonic and phasic electromyographic activity is evident during REM sleep. When not symptomatic, this activity is called REM sleep without atonia (RWA). The importance of this altered sleep stage-dependent muscular activity was stressed in the latest international classification of sleep disorders, the ICSD-2.³ Indeed, this classification advises considering a diagnosis of RBD in the “presence of REM sleep without atonia,” but no quantitative parameters are given.

The prevalence of RBD in narcolepsy/cataplexy seems to be fairly high: according to two recent studies, it is clinically evident in 45% to 61% of patients and polysomnographically detectable in 36% to 43% of them.^4,5 Patients with narcolepsy/cataplexy are more frequently affected by RBD than those without cataplexy⁴ and in many narcolepsy/cataplexy patients RBD can be induced or aggravated by anti-cataplectic treatment (antidepressants).⁶ RBD may also be an early sign in childhood narcolepsy/cataplexy.⁷

An increased electromyographic activity during REM sleep has frequently been observed in narcoleptic patients without RBD.⁸ The prevalence of RWA, phasic EMG activity, and REM density is also higher in these patients than in controls,⁹ whereas patients with idiopathic RBD have a higher prevalence of RWA and a lower REM density than narcoleptic patients and controls.¹⁰ RBD in narcolepsy also differs from the idiopathic form because of its much earlier age of onset^5,6 and different sex ratio (in the idiopathic form, RBD mostly affects men).^4–6

Despite the crucial relevance of the increased chin EMG activity during REM sleep for the diagnosis of RBD, the reliability of polysomnographic criteria for the identification of RWA is largely unexplored.¹¹ There have been very few systematic attempts to measure submentalis muscle EMG activity during sleep,¹² probably because of decades of paper recordings in sleep research. Few literature reports have quantified submentalis muscle EMG activity in RBD patients,^13–15 and only one included narcoleptic patients.¹⁰

This study aimed to evaluate the quantitative amplitude of submentalis muscle EMG activity during sleep in patients with narcolepsy/cataplexy, without or with RBD, in comparison to normal controls. In addition to a visual quantitative approach following the criteria set by Lapierre and Montplaisir in 1992,¹³ we adopted a new computer quantitative analysis describing both basal and transient modifications in chin EMG amplitude during sleep in normal controls and idiopathic and symptomatic RBD patients.¹⁶

SUBJECTS AND METHODS

Subjects

Thirty-four patients with narcolepsy/cataplexy were consecutively recruited for this study at the Sleep Disorders Center of the Department of Neurological Sciences, University of Bologna, together with 35 age-matched normal controls. Clinical and demographic characteristics of patients are listed in Table 1. The diagnosis of narcolepsy/cataplexy was based on the International Classification of Sleep Disorders (ICSD-2) criteria.³ All patients had ≥ 2 sleep onset REM-sleep episodes upon the multiple sleep latency test (MSLT), cataplexy and excessive daytime sleepiness, reflected by a mean sleep latency at the MSLT ≤ 8 min, and all carried the human leukocyte antigen (HLA) DQB1*0602. Diagnosis of RBD was also based on the ICSD-2 criteria,³ including presence of REM sleep without atonia, sleep related injurious-disruptive behaviors by history—following a semistructured clinical interview described elsewhere,¹⁷ administered to the patients and, whenever possible, to their bed partners, evaluating RBD symptomatology and frequency of episodes¹⁸—and abnormal sleep behaviors documented during polysomnographic monitoring, absence of EEG epileptiform activity during REM sleep, and sleep disturbance not better explained by another sleep disorder, medical or neurological disorder, mental disorder, medication use, or substance use disorder.

Table 1.

Clinical and Demographic Characteristics of Patients

Patient	Sex	Age	Age at onset, years			SP	HH	AB	DNS	Therapy withdraw one month before admission, daily dose	Therapy at PSG time, daily dose
			EDS	Cataplexy	RBD
1	M	36	10	18	−	+	+	+	+	Valproic Acid 1,250 mg; Citalopram 20 mg
2	M	41	33	33	−	+	−	−	+	drug naive
3	F	52	49	50	−	−	−	−	−	Paroxetine 20 mg
4	M	35	13	15	−	−	−	−	+	drug naive
5	M	44	33	33	−	+	+	+	+	drug naive
6	F	33	18	n.a.	−	+	+	+	−	drug naive
7	F	20	12	12	−	+	+	−	+	drug naive
8	F	73	45	44	−	−	−	−	+	Enalapril 10 mg	Thyroxine sodium 50 mg; Furosemide 25 mg
9	F	18	10	16	−	+	+	+	−	Modafinil 100 mg
10	F	23	9	22	−	−	+	−	+	drug naive
11	M	31	18	18	−	−	−	+	−	drug naive
12	F	37	29	29	−	+	+	+	+	Modafinil 100 mg; Venlafaxine 75 mg
13	M	59	48	n.a.	−	−	−	−	−	Fluoxetine 60 mg
14	M	67	13	13	−	−	−	+	+	drug naive	Nifedipine 40 mg; Perindopril 2 mg
15	M	44	40	n.a.	−	−	−	−	−	drug naive
16	M	35	15	15	−	+	−	−	+	drug naive
17	F	34	19	19	−	+	+	−	+	drug naive
18	M	66	10	40	50	−	−	−	−	Sertraline 50 mg; Flurazepam 15 mg	Metformin 500 mg; Alfuzosin 10 mg
19	F	36	24	24	24	+	+	−	+	drug naive
20	M	25	10	10	15	−	−	+	+	Imipramine 30 mg
21	M	32	11	11	12	−	+	−	−	Modafinil 100 mg
22	F	41	15	16	20	−	+	+	+	Modafinil 100 mg
23	F	59	45	47	50	+	+	+	+	Modafinil 200 mg; Clomipramine 50 mg
24	M	75	18	20	22	−	−	−	+	Modafinil 100 mg
25	M	31	15	26	28	+	+	+	+	drug naive
26	M	55	44	44	50	−	+	+	+	drug naive
27	M	38	17	20	20	+	+	+	+	drug naive
28	M	24	7	n.a.	16	−	−	−	−	drug naive
29	M	44	22	22	19	−	+	+	+	drug naive
30	M	50	16	15	18	−	+	−	+	drug naive
31	M	30	18	20	24	+	+	+	+	Modafinil 200 mg
32	F	35	14	14	25	+	+	+	−	drug naive
33	M	36	28	n.a.	30	−	−	−	−	drug naive
34	M	20	18	19	19	−	−	−	−	drug naive

EDS = Excessive daytime sleepiness; SP = sleep paralysis; HH = hypnagogic/hypnopompic hallucinations; AB = automatic behaviors; DNS = disturbed nocturnal sleep; n.a. = not available; + = present; − = absent.

Patients were recorded on average within 18.6 years (SD 2.81, range 2–57 years) from the beginning of narcolepsy clinical symptoms, and all had been drug-free ≥ 3 weeks at the time of the polysomnographic recording.

None of the 35 normal subjects had any physical, neurological or psychiatric disorder or history of sleep problems, and none was taking medication at the time of recording. Exclusion criteria included (a) a sleep disorder diagnosis (including sleep apnea); (b) a major mental illness; (c) a significant history of cognitive difficulties; (d) prior (within one year) or current use of a neuroleptic agent or SSRIs, venlafaxine; and (e) history of alcohol or other substance abuse.

Nocturnal Polysomnography

Nocturnal polysomnography was carried out after a night of adaptation in a standard sound-attenuated (noise level to a maximum of 30 dB nHL) sleep laboratory room, under video recording monitoring. Subjects were not allowed caffeinated beverages the afternoon preceding the recording and were allowed to sleep until they awoke spontaneously in the morning. Lights-out time was based on individual habitual bedtime and ranged between 21:30 and 23:30. The following signals were recorded: EEG (at least 2 channels, one central and one occipital, referred to the contralateral earlobe); electrooculogram (electrodes placed 1 cm above the right outer cantus and 1 cm below the left outer cantus and referred to A1), electromyogram (EMG) of the submentalis muscle (bipolar derivations with 2 electrodes placed 3 cm apart and affixed using a collodion-soaked gauze pad), impedance was kept less than 10 KΩ (typically <5 KΩ), EMG of the right and left tibialis anterior muscles, and ECG (one derivation). Sleep signals were sampled at 200 or 256 Hz and stored on hard disk in European data format (EDF)¹⁹ for further analysis. The sleep respiratory pattern of each patient was monitored using oral and nasal airflow thermistors and/or nasal pressure cannula, thoracic and abdominal respiratory effort strain gauge, and by monitoring oxygen saturation (pulse-oximetry). This had been performed in all subjects in a previous recording (within one week) by means of a portable cardiorespiratory monitor or during the study recording; patients with an apnea/hypopnea index ≥ 5 were not included. Sleep stages were scored following standard criteria²⁰ on 30-sec epochs; since muscle atonia can be absent in RBD, REM sleep was scored without submental EMG atonia, using electroencephalogram and electrooculogram only. According to a method specifically developed for RBD,^10,13 onset of a REM sleep period was defined as the occurrence of the first rapid eye movement in the presence of an EEG signal characteristic of REM sleep (low amplitude mixed frequencies, absence of sleep spindles and K complexes). Offset of a REM sleep period was determined by the occurrence of a specific EEG feature indicative of another stage (K complex, sleep spindle, or EEG signs of arousal) or absence of rapid eye movements during 3 consecutive minutes. Figure 1 shows an example of REM sleep without atonia scored following these criteria. Epochs containing technical artifact or extremely elevated muscle activity causing saturation of amplifiers were carefully detected and marked for exclusion from the subsequent quantitative EMG analysis.

Figure 1.

Example of polysomnographic recording of REM sleep without atonia.

Quantification of Submentalis Muscle EMG Amplitude

For visual quantitative analysis, we first quantified the 2 parameters proposed by Lapierre and Montplaisir,¹³ adapted to our recordings, in REM sleep of all groups. First, each 30-sec epoch was scored as tonic or atonic, depending on whether tonic chin EMG activity was present for > 50% of the epoch; then, we calculated the percentage of total REM epochs scored as tonic. Second, we evaluated phasic EMG density as the percentage of the total number of 2-sec mini-epochs of REM sleep containing phasic EMG events (defined as any burst of EMG activity lasting 0.1–5 s, with an amplitude exceeding 4 times the background EMG activity).

For computer quantitative analysis, the submentalis muscle EMG signal was digitally band-pass filtered at 10–100 Hz, with a notch filter at 50 Hz and rectified. Subsequently, each sleep epoch included in the analysis was divided into 30 1-sec mini-epochs. The average amplitude of the rectified submentalis muscle EMG signal was then obtained for each mini-epoch. The values of the submentalis muscle EMG signal amplitude in each mini-epoch were used to draw normalized distribution histograms for each sleep stage (REM, S1, S2, and SWS) of the percentage of values in the following 20 amplitude (amp) classes (expressed in μV): amp ≤ 1, 1 < amp ≤ 2, …, 18 < amp ≤ 19, amp > 19. In these graphs, muscle atonia is expected to be reflected by high values of the first left column while phasic and tonic activations are expected to increase the value of the other columns.¹⁶ As proposed in our recent validation study,¹⁶ an index summarizing the degree of preponderance of the first column in these graphs in a single value was used in REM sleep:

Sleep Atonia Index = amp ≤ 1 /(100 − 1<amp ≤ 2).

Mathematically, this index can vary from 0 (absence of mini-epochs with amp ≤ 1), i.e., complete absence of EMG atonia, to 1 (all mini-epochs with amp ≤ 1) or stable EMG atonia in the epoch.

Finally, we also counted all sequences of consecutive mini-epochs exceeding the value of 2 μV and calculated their number per hour for REM sleep. These data were also used to draw normalized distribution histograms within the following 20 duration (dur) classes (expressed in s): dur = 1, dur = 2, …, dur = 19, dur > 19.

Statistical Data Analysis

All data were coded and analyzed blind to the subject group upon completion of recruitment. Comparisons between groups were carried out using the nonparametric Mann-Whitney test for independent data sets, with the Bonferroni correction for multiple comparisons when needed. The correlation between REM sleep Atonia Index and age of subjects was evaluated by the nonparametric Spearman rank correlation coefficient. Finally, the Chi-square test was used for the comparison of the gender composition of the groups and the nonparametric Kruskal-Wallis ANOVA for the comparison of their age. Differences and correlations were considered significant when P < 0.05. The data analysis software system STATISTICA (StatSoft, Inc. 2004, version 6. www.statsoft.com) was used for statistical analysis.

RESULTS

After the application of the current criteria for the diagnosis of RBD³ and the analysis of the video-polysomnographic recordings, 17 of the 34 patients with narcolepsy/cataplexy were diagnosed with RBD (Narcolepsy+RBD) and 17 were not (Narcolepsy−RBD). Table 2 shows the details of the RBD signs observed at videopolysomnography, in each Narcolepsy+RBD patient; the complete narcolepsy symptom tetrad was present in 3 patients with Narcolepsy−RBD (21.4%) and in 4 Narcolepsy+RBD (28.6%) patients. All subsequent statistical analyses were carried out taking into account these 2 subgroups.

Table 2.

RBD Video-Polysomnographic Features of Narcolepsy+RBD Subjects Included in this Study

Patient	Sex	Age	Age at onset, years			Vocalizations	Simple and Complex Behaviors
			EDS	Cataplexy	RBD
18	M	66	10	40	50	talking	upper limb movements
19	F	36	24	24	24	talking, crying	reaching, crawling
20	M	25	10	10	15	talking	lower limb movements
21	M	32	11	11	12	talking
22	F	41	15	16	20	talking	Grabbing
23	F	59	45	47	50	shouting
24	M	75	18	20	22	laughing	Grabbing
25	M	31	15	26	28	talking	reaching, grabbing
26	M	55	44	44	50	shouting	arm flailing, gesturing, crawling
27	M	38	17	20	20		reaching, grabbing
28	M	24	7	n.a.	16	laughing, shouting	upper/lower limb movements, sitting
29	M	44	22	22	19	laughing, shouting	upper/lower limb movements, arm flailing, reaching, crawling
30	M	50	16	15	18		arm flailing, reaching
31	M	30	18	20	24	talking	grabbing, punching, kicking, attempts to jump out of bed
32	F	35	14	14	25	laughing, talking	Crawling
33	M	36	28	n.a.	30	laughing, talking	Grabbing
34	M	20	18	19	19		grabbing, gesturing, crawling

Age and Sex

Table 3 reports age and gender composition of the groups; only the Narcolepsy+RBD group is male predominant, with the other 2 groups having almost equal gender composition. This was due to the consecutive character of the recruitment that prevented us forming gender-balanced groups; however, the group differences in gender composition did not reach statistical significance (Chi-square test = 5.209, 0.1 > P > 0.05). The age of the subjects in the 3 groups was not statistically different at the nonparametric Kruskal-Wallis ANOVA.

Table 3.

Subjects Included in this Study

	total, n	males	females	Age, years ± SD (range)
1. Normal Controls	35	15	20	40.2 ± 16.72 (24.5–76)
2. Narcolepsy–RBD	17	9	8	40.1 ± 15.35 (18–73)
3. Narcolepsy+RBD	17	13	4	41.0 ± 15.36 (20–75)

Sleep Architecture

Table 4 shows the comparison between the different sleep scoring parameters obtained in the 3 groups of subjects. Both groups of patients showed the expected changes with respect to controls: namely longer time in bed, sleep period time, and total sleep time, shorter REM sleep latency, smaller percentage of sleep stage 2 and higher percentages of sleep stage 1 and REM sleep. No significant differences were found in sleep architecture between the 2 groups of patients with narcolepsy/cataplexy.

Table 4.

Comparison Between Sleep Scoring Parameters in the 3 Groups of Subjects

	1. Controls (n=35)		2. Narcolepsy−RBD (n=17)		3. Narcolepsy+RBD (n=17)		Mann-Whitney U test*
	mean	SD	mean	SD	mean	SD	1 vs. 2 P<	1 vs. 3 P<	2 vs. 3 P<
TIB, min	457.4	63.19	508.4	79.72	521.0	73.47	0.021	0.006	NS
SPT, min	429.9	52.11	496.6	75.12	498.6	84.49	0.005	0.007	NS
TST, min	367.4	69.50	423.9	70.69	436.6	70.70	0.03	0.007	NS
SOL, min	20.1	24.01	6.2	2.77	16.8	33.13	NS	NS	NS
FRL, min	98.0	76.49	31.2	31.29	39.2	42.71	0.0009	0.006	NS
SE%	81.4	16.91	83.9	10.22	84.1	9.93	NS	NS	NS
WASO, %	14.1	15.65	14.1	9.82	12.3	7.63	NS	NS	NS
S1, %	4.1	4.07	12.6	6.71	12.9	5.81	0.00002	0.00001	NS
S2, %	46.5	10.50	32.6	7.93	34.7	8.25	0.00009	0.0006	NS
SWS, %	17.3	9.92	18.3	6.73	17.0	8.68	NS	NS	NS
REM, %	18.1	6.93	22.4	5.57	23.0	6.17	NS	NS	NS

TIB = Time in bed; SPT = sleep period time; TST = total sleep time; SOL = sleep onset latency; FRL = 1st REM latency; SE%= sleep efficiency; WASO = wakefulness after sleep onset; S1 = NREM sleep stage 1; S2 = NREM sleep stage 2; SWS = slow wave sleep; REM = rapid eye movement sleep.

^*Bonferroni corrected P values.

Visual Analysis of Submentalis EMG Amplitude

The comparison between the chin EMG amplitude visual analysis parameters obtained in the 3 groups of subjects is reported in Table 5. Both groups of patients with narcolepsy/cataplexy showed an increased percentage of REM sleep mini-epochs containing phasic EMG events (defined as any burst of EMG activity lasting 0.1–5 s, with an amplitude exceeding 4 times the background EMG activity).¹³ No significant difference was found between the 2 patient groups for this parameter or among all groups for the percentage of tonic REM sleep epochs.

Table 5.

Comparison Between Chin EMG Amplitude Visual Analysis Parameters Obtained in the 3 Groups of Subjects

	1. Controls (n=35)		2. Narcolepsy−RBD (n=17)		3. Narcolepsy+RBD (n=17)		Mann-Whitney U test*
	mean	SD	mean	SD	mean	SD	1 vs. 2 P<	1 vs. 3 P<	2 vs. 3 P<
Phasic REM mini-epochs, %	1.5	0.96	6.5	2.79	8.0	3.51	0.000003	0.000003	NS
Tonic REM epochs,%	10.1	13.04	7.6	8.01	6.5	5.80	NS	NS	NS

Data expressed as a percentages of the total.

^*Bonferroni corrected P values.

Computer Quantitative Analysis of the Submentalis EMG Amplitude

Figure 2 shows the normalized distribution histograms of mini-epoch amplitude for each subject group. Small but clear differences can be seen among the 3 groups. The graph pertaining to normal controls (on the left) was characterized by a very high and prominent first column indicating that approximately 80% of mini-epochs during this sleep stage have amp ≤ 1. The amplitude of the same column was lower in the Narcolepsy−RBD and lower still in the Narcolepsy+RBD group, with a shift of the respective graphs towards the other columns representing higher values of amp.

Figure 2.

Normalized distribution histograms of mini-epochs amplitude for each subject group. (The percentage of values in 20 amplitude (amp) classes are shown (1 = amp ≤ 1, 2 = 1 < amp ≤ 2, …, 19 = 18 < amp ≤ 19, 20 = amp > 19. All values are shown as means and standard errors (whiskers). The statistical analysis of the differences in REM sleep Atonia Index is also shown.

The REM sleep Atonia Index, which summarizes the distribution of these graphs, was significantly lower in both patient groups than controls. Again, no significant differences were found in REM sleep Atonia Index between the 2 groups of patients.

Figure 3 shows the individual values of the REM sleep Atonia Index in the different subject groups. Both patient groups showed a wide distribution of their individual values, largely overlapping with those of normal controls, but more often below the arbitrary threshold of 0.7 (Chi-square test = 10.006, P < 0.01), indicated with a dotted line in this figure and already indicated as a possible supporting criterion for the diagnosis of RBD.¹⁶ Also in this figure it is possible to note that REM sleep Atonia Index did not differ between the 2 groups of patients. Figure 2 also demonstrates data from a recent study in idiopathic and symptomatic RBD (multiple system atrophy), carried out by our group;¹⁶ this shows that both narcolepsy subgroups seem to show mean values of REM sleep Atonia Index between those of young normal controls and idiopathic RBD patients, with multiple system atrophy patients showing the lowest average value.

Figure 3.

REM sleep Atonia Index in the 3 groups of subjects in this study and in patients with idiopathic RBD (iRBD) and multiple system atrophy (MSA) (*data from Ferri et al.¹⁶): values are shown as means (black-filled squares) and 95% confidence intervals (whiskers). Individual values are also shown (circles).

Figure 4 reports the relationship of REM sleep Atonia Index and age in the 3 groups of subjects. Even if this index tended to decrease with age in all graphs, the correlation coefficient reached statistical significance only for normal controls who showed individual values less scattered around the linear regression line than those of the patient groups. This result was confirmed also by the analysis reported in Table 6 in which 2 age subgroups (≤ 40 years and > 40 years) were obtained in each subject group; a statistically significant difference was found only in normal controls. The same table reports the comparison between gender subgroups in each subject group; none of these comparisons reached statistical significance.

Figure 4.

Correlation between REM sleep Atonia Index and age in normal controls (top graph), patients with narcolepsy−RBD (middle graph), and patients with narcolepsy+RBD (bottom graph). The Spearman correlation coefficient is also reported for each graph together with its statistical significance and the linear regression line.

Table 6.

Comparison Between REM Sleep Atonia Index Obtained in 2 Age Subgroups (≤40 Years and >40 Years), in Normal Controls, Patients with Narcolepsy–RBD, and Patients with Narcolepsy+RBD; Comparison Between Genders is also Reported in the Same Subject Groups

	Age ≤40 years			Age >40 years			Mann-Whitney U test*
	n	mean	SD	n	mean	SD	P <
Controls	24	0.937	0.077	10	0.815	0.170	0.006
Narcolepsy−RBD	10	0.747	0.300	7	0.681	0.229	NS
Narcolepsy+RBD	10	0.706	0.187	7	0.666	0.175	NS
	Males			Females
	n	mean	SD	n	mean	SD	P <
Controls	15	0.940	0.045	20	0.869	0.151	NS
Narcolepsy−RBD	9	0.710	0.221	8	0.731	0.327	NS
Narcolepsy+RBD	13	0.692	0.179	4	0.679	0.201	NS

^*Bonferroni corrected P values.

Figure 5 contains the normalized distribution histograms of consecutive mini-epoch sequences exceeding 2 μV for each subject group. These graphs show a similar type of distribution for all groups, with a tendency for patients with Narcolepsy−RBD and those with Narcolepsy+RBD to show higher values mostly for columns in the left part of the graphs (shorter activations). The same figure shows the differences in the total number of chin EMG activations detected in the 3 groups of subjects; this is significantly higher in both patient groups than controls. No significant differences were found in the total number of chin EMG activations between the 2 groups of patients.

Figure 5.

Normalized distribution histograms of consecutive mini-epoch sequences exceeding 2 μV for each subject group. The number per hour of sleep stage in 20 duration (dur) classes is shown (20 = dur>20 s). All values are shown as means and standard errors (whiskers). The statistical analysis of the differences in total number of movements per hour of REM sleep is also shown.

DISCUSSION

To our knowledge, this is the first computerized quantitative study of the EMG signal in narcolepsy, designed to evaluate polysomnographically signs of RBD in narcolepsy patients. RBD is prevalent in narcolepsy,^4,6 but it has an even higher prevalence in narcolepsy-cataplexy.^4,5 The percentage found in our narcolepsy/cataplexy patients fits with previous data. Interestingly, several findings suggest that narcoleptic patients without clinical RBD also frequently have increased electromyographic activity during REM sleep⁸ and have a higher prevalence of RWA, phasic EMG activity, and REM density than controls.¹⁰ Previous studies suggest that although it is frequent, RBD is not an every night phenomenon in narcolepsy/cataplexy^4,5 and is more frequently disclosed by questionnaires or by clinical interview than by (video)polysomnography. This finding is peculiar, and differentiates narcolepsy/cataplexy from neurodegenerative diseases (i.e., multiple system atrophy) in which the typical RBD dream enacting is easily documented by videopolysomnography.²¹ In this light, the availability of a tool to detect subclinical signs of RBD on polysomnographic tracings might be of great interest not only for research but also for clinical decision making.

The recent availability of more quantitative and computerized methods of analysis of the EMG signal, especially designed for the evaluation of the chin motor phenomena accompanying RBD^22,23 prompted us to apply this new approach to the study of RBD in narcolepsy/cataplexy.

The main finding of this study is the confirmation and quantification of polysomnographic signs of motor dyscontrol in the chin EMG of patients with narcolepsy/cataplexy. This allowed us to discover an abnormal quantity of motor activity in patients without a clear clinical complaint of RBD, as specified by the current diagnostic criteria.³ Although these signs were always more evident in subjects with clinically evident RBD, the difference from those without was not statistically significant. If we consider REM sleep Atonia Index ≤0.7 as a cut-off for RWA, 13 of our 34 (38%) narcolepsy/cataplexy patients had RWA, a figure similar to that found by others using visual scoring.¹⁰ As shown above, both narcolepsy subgroups seem to show mean values of REM sleep Atonia Index intermediate between those of young normal controls and idiopathic RBD patients, with multiple system atrophy patients showing the lowest average value; for this reason, the degree of motor dyscontrol expected in narcoleptic patients should be expected to be somewhat milder than that of idiopathic or symptomatic RBD. The increased index of motor dyscontrol in REM sleep in the 2 groups of patients with narcolepsy/cataplexy can thus be considered an intrinsic finding of the disease and a sort of “status dissociatus” (a condition characterized by ambiguous, multiple, or rapid oscillation of state-determining variables which can be observed in a wide variety of experimental and clinical situations),^24,25 opening the way to acting out dream contents (i.e., RBD).

Another important finding of this study is that the altered REM sleep atonia index in all patients with narcolepsy/cataplexy was mostly caused by an increase in short-lasting EMG activity (approximately from 0 to 5 s), as shown in Figure 4. This phenomenon, already described by Geisler et al.,⁹ is also consistent with the increased limb phasic EMG activity in NREM sleep,²⁶ and might differentiate patients with narcolepsy/cataplexy and RBD from patients with other forms of secondary RBD (i.e., multiple system atrophy), if confirmed by additional studies.

A third important finding is that the visual analysis of the same signal, following criteria established in the past,¹³ yielded results similar to those of the automatic analysis only for the count of short-duration (phasic) EMG activations, whereas analysis of the tonic REM epochs failed to disclose differences among our 3 groups of subjects. This is only partially in agreement with our previous study¹⁶ in idiopathic and symptomatic RBD and with the results reported in another study in a different group of symptomatic RBD patients using another quantitative method.²²

The results of this study need to be compared with those obtained in idiopathic RBD patients.¹⁶ RBD in patients with narcolepsy differs from idiopathic RBD: patients with idiopathic RBD have a higher prevalence of RWA than narcolepsy subjects, with smaller REM density,¹⁰ and RBD in narcolepsy has a much earlier age at onset and a different sex ratio (in the idiopathic form, RBD mostly affects men).^4,6 The present study clearly shows that the pattern of motor dyscontrol found in our narcoleptic subjects is less severe than that found in idiopathic RBD and also different from that of symptomatic RBD in patients with multiple system atrophy. MSA patients present a very high number of RWA epochs compared to idiopathic RBD. Thus, this new quantitative approach seems to be not only able to disclose differences between normal controls and patients, but also among nosologically different groups of patients who might present a different type of REM sleep motor dyscontrol (which is currently referred to as RBD in all cases). These differences might indicate different neurochemical and neurophysiological mechanisms underlying an apparently similar sleep disturbance, from a strictly clinical point of view, occurring in different conditions such as narcolepsy, idiopathic RBD, and MSA.^21,27 Besides these conditions, RBD has been reported to occur in neurological diseases such as Parkinson disease,^28,29 spinocerebellar ataxia,³⁰ supranuclear palsy,³¹ Lewy body disease,²⁸ Alzheimer disease,³² amyotrophic lateral sclerosis,³³ multiple sclerosis,^34,35 and Tourette syndrome.³⁶ Lastly, RBD can be induced by medications, especially the tricyclic antidepressants and serotonin-specific reuptake inhibitors.^33,37 For this reason, the new quantitative approach proposed in the present study may also yield insights into the specific patterns of chin EMG activation during REM sleep in these disorders.

This study also provided further evidence of a significant effect of age on our REM sleep Atonia Index in normal controls, an effect which was mild and statistically nonsignificant in both groups of patients. This seems to indicate that the age of subjects is a critical factor to take into account in these studies, and that there is a significant interaction between disease and age, with the pathological condition masking the effects of age.

In conclusion, a certain degree of polysomnographically evident RBD is present in many patients with narcolepsy/cataplexy. This disorder might be specific and correlated to the specific neurochemical and neuropathological substrate of narcolepsy/cataplexy. Our approach represents an improvement over previous visual methods of analysis of chin EMG during REM sleep and yields useful and practical indices for the quantitative and nonsubjective evaluation of EMG atonia during REM sleep, and EMG activations. The combination of atonia and activation indices allows investigation of different clinical conditions characterized by abnormal chin EMG activation during sleep. Our proposed REM sleep Atonia Index is an additional useful parameter to be used in conjunction with the other criteria for the diagnosis of RBD. Future research is now needed to verify this new method in evaluating the clinical effects of drugs for RBD treatment.

DISCLOSURE STATEMENT

This was not an industry supported study. The authors have indicated no financial conflicts of interest.