# Bioelectrical Signals for the Diagnosis and Therapy of Functional Gastrointestinal Disorders
## 2 Oropharynx and Esophagus
### Diagnostics  
Bioelectrically based methods to investigate swallowing include assessment of either or both central inputs from the cerebral cortex and peripheral inputs at the local oropharyngeal and esophageal musculature [22]. For decades, electromyography (EMG) has been used to investigate the peripheral inputs and activation of musculature involved in swallowing [23]. The intrinsic or extrinsic biopotential of the local musculature results in myoelectric signals that can be measured using either needle-based electromyography (nEMG) or surface electromyography (sEMG). The intramuscular placement of nEMG electrodes into the target tissue allows localized assessment of muscle activity, which was found to have 95% concordance between the acquired signal and muscle activity [24]. Similarly, non-invasive sEMG can identify contraction profiles of muscles during swallowing. A study showed that activity monitored on healthy and stroke patients with three electrodes placed on the period, masseter, and infrahyoid muscle provided information on the contraction sequence of the three selected muscle sites [25]. Moreover, the dysphagia patients compared to healthy controls produced higher and more variable amplitude characteristics (37 vs. 18 mV) over a shorter duration (1.2 vs. 1.9 s) with less coordination when swallowing [25]. More recently, high-resolution sEMG has been utilized to differentiate motor activities across different bolus volumes, bolus viscosities, and head postures [26]. The results have shown that high-resolution sEMG recordings can provide detailed spatial and temporal properties of the muscle electrical activity that can help identify the muscle contractions closely related to swallowing function [26]. Surface electrodes are also used in bioimpedance recordings of swallowing. Applied bilaterally across pharyngeal musculature, current injection and sensing electrodes have been shown to detect the sequential superior or inferior closure of the pharynx in 64% of swallows [27]. Measured concomitantly with pharyngeal manometry, a reasonable correlation was found between bioimpedance and manometric features [27]. A recent study on an ALS patient measured bioimpedance magnitude and phase while simultaneously performing fluoroscopy imaging [28]. The impedance phase and magnitude correlated to the chronological series of anatomical events; however, no significant difference was found between different swallowing tasks, giving no diagnostic advantage of swallowing functionality over the traditional fluoroscopy method. Swallowing motor control can also be assessed via cortical neural pathways as the initiation of swallowing is voluntary and engages the motor areas of the cerebral cortex. The integrity of the cortical afferent pathway can be assessed by repetitively applying sensory stimulation to swallowing muscles to create sensory-evoked potentials (SEP) which are then measured at the cortex using electroencephalography (EEG). A study on post-stroke dysphagia patients using SEP showed that there was a significant correlation between latency shortening of SEP and severity in dysfunction of biomechanical swallowing events, suggesting that a disrupted pattern of cortical activation to pharyngeal inputs is associated with the stroke severity [29]. Similarly, non-invasive single-pulse transcranial magnetic stimulation (TMS) to evoke a transient muscle evoked potential (MEP) at associated swallowing musculature has revealed the important role of the motor cortex in the regulation of the complex swallowing process [30]. A study showed that unilateral stroke patients with dysphagia had a smaller pharyngeal response than stroke patients with no dysphagia when TMS was applied to intact cortical hemisphere (mean amplitudes of 64 vs. 118 \(\upmu\)V), and had similar responses when applied to the affected hemisphere [31]. Pharyngeal motor responses were also shown to correlate with swallowing recovery, where an increased pharyngeal response was observed after 3 months (17 vs. 9 sites) as functional reorganization occurred [32].