Electrophysiological Recording

In all-night sleep recording, normal control subjects and patients with PD were hospitalized for 1 night from 20:00 to 08:00. Nocturnal video-polygraphic recordings were collected using the Grass-Telefactor Beehive Millennium Video EEG Monitoring system (Grass Products, Natus Neurology, Middleton, WI). We simultaneously recorded EEG, electrooculogram (EOG), respiration (nasal airflow), electrocardiogram (ECG), electrodermal activity (EDA), vertical laryngeal movement, and electromyography (EMG) activities of the orbicularis oris (OR), masseter (MS) and suprahyoid/submental muscle group (SM). Silver cup electrodes of 10-mm diameter were used for EEG, EOG, and EMG recordings. EEG was obtained with electrodes placed over C3, C4, O1, O2, A1, and A2 according to the international 10–20 system. Laryngeal movement is recorded by two electrodes placed 2 cm apart vertically over the thyroid cartilage. The up and down movement of the larynx during swallowing can easily be measured using vertically oriented pairs of EEG electrodes. In addition, fixation and stability of electrodes during sleep were satisfactory to record the laryngeal movement during swallowing. The time scale was 30 mm/sec. The data were recorded to external memory. These values were changed in the offline analysis.

Sleep stages were scored epoch by epoch by a sleep technologist and one of us (IFU or BIT). The standard method for identifying and scoring sleep stages was used.²¹ Sleep variables including total sleep time (TST), sleep latency, sleep efficiency, duration of nonrapid eye movement (NREM) and rapid eye movement (REM) stages, REM sleep latency were calculated. In addition, numbers of awakenings, arousals, sleep stage shifts, masseter activity, breathing, and apnea were counted. Sleep latency was defined as the time until the first minute of stage N1 (NREM 1 sleep) followed by at least 10 min of this stage, not interrupted by awakenings. Sleep efficiency was defined as TST/time in bed (TIB). REM sleep latency was defined as the time from the onset of sleep until onset of the first REM sleep period. Arousal was defined as an abrupt change in sleep from a deeper stage to a lighter stage. Awakening was defined as a change from sleep to wakefulness.

The SSs were counted visually during the offline mode. We used the criteria of SS as described previously⁷: (1) The OR, MS, and SM muscles had to fire together in a burst that lasted 1–2 sec. The SM-EMG was often highest in amplitude during this activity. (2) All three aforementioned muscles had to be recorded in parallel with a synchronous, laryngeal deflexion, indicating vertical movements of the larynx during swallowing. (3) During the recording one EEG-sleep technician ensured the upward and downward movement of thyroid cartilage and swallowing movement beside the video of the subjects. (4) In all three muscles, the response amplitudes for deglutition had to be at least four times higher than baseline. (5) During deglutitional activity, all swallowing muscle should be synchronously activated by the swallowing apnea obtained from the cessation of respiratory rhythm in the nasal airflow recording (Figure 1).

Spontaneous swallowing (SS) simultaneously recorded from scalp, respiration, orbicularis oris (OR), masseter (MS), suprahyoid/ submental muscle group (SM), nasal airflow, and electrocardiogram (ECG) electrodes. Note the apnea period in respiration (RESP.) and up and down movements signs of larynx in “sensor”.

One of the major finding that differentiate patients from NCs was the increased number of peculiar type of swallowing, which was triggered with four or more swallows consecutively, bursting in one set of swallowing. This was called salvo type swallowings. Usually, NCs have solitary or two consecutive swallows (double swallow) in one trigger.⁷ The number of salvo type swallowings was also counted.

Because all the data were recorded to the external memory, the values could be changed in the offline analysis. For instance, the amplitude of each channel, the time base of all channels, or the size of video images could be altered. This allowed us to carefully compare the SS and coughs by looking at the swallowing muscles together with laryngeal and respiratory activities. In the offline mode, each swallow or the group of swallows could be analyzed by two directions of the recordings (backward and forward) together with the video monitoring images.

Coughing was also important for both of the groups investigated. All coughs were counted throughout the whole sleep recordings and video monitoring in combination. In particular, the coughs just after the swallows were counted separately because of their penetration through the larynx or anywhere in the airway.

The movements of other parts of the body including head, neck, eye, facial muscles, and limbs were also noted during swallowing movements in all cases. The SS and coughs when the subjects were awake were not analyzed.