Manual lymph drainage attenuates frontal EEG asymmetry in subjects with psychological stress: a preliminary study.

[Purpose] The purpose of this preliminary study was to investigate the effect of manual lymph drainage (MLD) of the neck on frontal electroencephalogram (EEG) asymmetry in subjects with psychological stress. [Subjects] Thirteen subjects with psychological stress participated in the study. [Methods] Subjects received MLD of the neck for 15 min.
[Results] Analysis of the frontal asymmetry index showed that the energy shift in the alpha frequency band from the left hemisphere to the right hemisphere after MLD resulted in greater left-side activation (positive asymmetry values), which could be related to the positive emotional state observed particularly in the F7-F8 area.
[Conclusion] These preliminary findings suggest that frontal EEG asymmetry was significantly attenuated after MLD.

INTRODUCTION

In the field of medicine, “stress” is one of the most common complaints among patients1). Stress occurs when an individual fails to respond appropriately to challenge. Stress may negatively influence the affective state of an individual, which in turn may exert direct adverse effects on biological processes or behavioral patterns, thereby increasing the individual’s disease risk and pathogenesis of a disease2).

Psychosocial stress is believed to contribute to musculoskeletal disorders of the neck, shoulders, and other areas. Previous studies have shown that mental stress induces a significant increase in muscle tension3). Recently, it has been demonstrated that the same motor units are activated by mental stress as by physical stress, which means that mental stress may also result in low-threshold motor units remaining active during breaks at work and outside of work3). Thus, numerous therapeutic approaches have been used to treat musculoskeletal diseases caused by psychological stress4,5,6).

The utilization of alternative medicine has been increasing in recent years, with massage documented as being one of the most frequently treatments for musculoskeletal disease7). Manual lymph drainage (MLD) is a procedure that consists of several techniques derived from traditional massage8). MLD has become increasingly popular in recent years, not least because of the enormous amount of publicity it has recieved8,9,10,11,12). Moreover, the spread of information about edema among doctors, physical therapists, and patients, and the efforts for managing its cosmetic aspects have also resulted in its popularity10,11,12).

The frontal region of the brain has been associated with affective states, and different affective states have been associated with different EEG patterns in this region13). Normal subjects experience positively valenced emotions, such as happiness and joy, together with greater relative left frontal EEG activation. In contrast, the experience of negative emotions, including depression, is associated with greater relative right frontal EEG activation14, 15). There are some studies that have investigated the effects of different manual techniques on frontal EEG activation4, 5, 16, 17); however, MLD is not one of the techniques that have been investigated. Therefore, the present preliminary study investigated the effects of MLD in subjects with psychological stress and greater relative right frontal EEG activation, with MLD expected to shift EEG patterns toward symmetry.

SUBJECTS AND METHODS

The subjects included 52 university students, chosen according to the following criteria: (1) no history of mental illness, (2) not currently taking any medication known to affect EEG signals, and (3) no known heart- or muscle-related disease. They were all free of pulmonary, cardiac, and metabolic disease, as well as other disease states, which may cause brain dysfunction. All subjects responded to the stress response questionnaire (SRI)18) and returned it to the authors. According to the results of the questionnaire, 13 female subjects aged 19 to 23 years with an SRI score >80 and a greater relative right frontal EEG activation were included. Informed, written consent was obtained from each subject after the experimental procedures had been explained. All test protocols were approved by the ethics committee of the Physical Therapy Faculty of Kangwon National University.

The participants received a 15-min session of MLD within the neck region. All interventions were completed in a supine position on a massage table with a pillow placed under the knees to relax the lower back muscles. Data acquisition and MLD were performed in a quiet, temperature-controlled environment (22–24 °C). Stimuli such as conversation, phone calls, and noise that could increase the activity of sentinel nodes were minimized, and the subject’s body was covered with a soft and thin sheet to avoid discomfort from body exposure. MLD was conducted by a well-trained certified MLD therapist, and applied twice to the neck area. Further details of the MLD procedure can be found in Dr. Vodder’s Manual Lymph Drainage: A Practical Guide8). The protocol was standardized, in that, the massage stroke category (type) and time was the same for all participants.

Subjects were allowed to rest comfortably for at least 5 min prior to the baseline recording procedure. EEG data were acquired for 5 min before, and immediately following, MLD. A total of 6 channels of EEG were recorded: inclusive of Fp1–Fp2, F3–F4, and F7–F8. There were two other electrodes, a ground electrode and reference electrode, which were placed on both zygomatic bones. Participants were then asked to close their eyes and refrain from talking, falling asleep, or making exaggerated body movements, in order to observe the cortical electrical activity without any external stimuli, minimizing possible visual artifacts during EEG measurement. The EEG signal was acquired over a 5-min period, followed by computerized Fourier analysis of the EEG waves using the TeleScan software package (LAXTHA, Daejeon, South Korea). The signal was sampled at a rate of 256 Hz and was digitally filtered using a 1–50-Hz band-pass filter. After data acquisition and storage, all statistics were computed to extract asymmetry values for the alpha frequency band in the frontal area of the cerebral cortex19,20,21). The asymmetry index was calculated by subtracting the log-transformed absolute alpha power of the left hemisphere from the analogous log-transformed right hemisphere alpha power (log right − log left). As alpha power is inversely associated with cortical activation, a negative asymmetry score, which denotes greater alpha activity on the left and less alpha power on the right, would suggest greater right-sided activation. On the other hand, a positive score would represent greater activation on the left14, 21).

All variables were tested for normality using the one-sample Kolmogorov–Smirnov test and demonstrated a normal distribution. The asymmetry index was compared prior to, and after, MLD application using a paired t-test, and the collected data were analyzed using a statistical package program (SPSS v. 19.0). A two-tailed probability of p < 0.05 was considered statistically significant.

RESULTS

MLD generally caused an activation shift from the right hemisphere to the left hemisphere in all frontal electrode pairs analyzed in this study. Statistical analysis comparing pre- and post-MLD asymmetry values in channels F7–F8 showed a statistically significant increase in hemispheric asymmetry: Fp1–Fp2, p = 0.082; F3–F4, p = 0.065; and F7–F8, p = 0.032. Electrophysiological results from the three frontal electrode pairs are shown in Table 1.

Table 1.

Alpha asymmetry scores for each region

Area	Before- MLD	After- MLD
Fp1–Fp2	−0.13±0.06	0.03±0.18
F3–F4	−0.15±0.05	0.02±0.16
F7–F8	−0.12±0.04	0.09±0.25 *

All variables are means ± SD, MLD: manual lymph drainage, *p<0.05

DISCUSSION

In this preliminary study, frontal lobe EEG activity patterns in response to MLD of the neck were examined to better understand alpha wave processing in subjects with psychological stress. Many studies have shown that EEG asymmetry can be attenuated by massage, relaxation techniques, and musical therapies22,23,24). However, to date, there are no studies investigating the effects of MLD on frontal asymmetry.

MLD application acts as a very light stimulator against psychological stress; we predicted that MLD would be associated with frontal EEG asymmetry and state, and specifically result in greater left frontal hemisphere activation in the alpha frequency band. In this study, the alpha band (typically in the range of 8–13 Hz) was chosen as an electrophysiological marker because, as previously discussed, it is relatively stable21, 25) and inversely related to activation26,27,28). The measurement of alpha asymmetry has been shown to be effective and reliable in discriminating positive and negative emotions23, 29). Since previous investigations have demonstrated a relationship between brain electrophysiology in the frontal area and measures of affective states19, 20, 30, 31), we have also focused our analysis on the three anterior electrode pairs, Fp1–Fp2, F3–F4, and F7–F8.

Recent studies have suggested that greater left-sided anterior activation, indexed by decreased alpha activity in the left hemisphere, is associated with a higher degree of positive affectivity, a feeling of well-being, and a reduction in anxiety28, 32). Our results indicate that MLD is associated with a significant increase in left-sided anterior activation, thus supporting our hypothesis that MLD can attenuate frontal EEG asymmetry. These results are also in line with findings from previous studies that have demonstrated the therapeutic effects of massage4, 5, 20).

Although this study showed demonstrable effects of MLD, there are several limitations and suggestions for future research. Firstly, this study only examined the acute electrophysiological changes produced after MLD in subjects with psychological stress. In addition to replicating the present findings with a bigger sample size, further studies are needed to investigate the effects of MLD on other clinical populations across different conditions and diseases under a more controlled experimental design.

In conclusion, our preliminary study showed that MLD can significantly increase left-sided anterior activation. These results provide evidence to support the hypothesis that MLD gives rise to positive affectivity. Further investigations will focus on future replication and extension of these preliminary findings.