What is the physiology of swallowing in a normal individual and how is it managed in patients with swallowing disorders, such as dysphagia, particularly in those with underlying conditions like Parkinson's disease or esophageal conditions?

Physiology of Swallowing

Swallowing is a remarkably complex neuromuscular process involving six cranial nerves, multiple muscle groups, and cortical and subcortical brain signals that must be precisely coordinated within a few seconds, occurring in three overlapping phases: oral preparatory, oral, pharyngeal, and esophageal. 1

Normal Swallowing Mechanism

Oral Preparatory Phase

• The bolus is masticated and mixed with saliva for moistening 1
• Salivary amylase begins the digestive process 1
• This phase is under voluntary skeletal muscle control and requires an alert, participating individual 1

Oral Phase

• Food and liquid are collected into a cohesive bolus 1
• The muscular tongue contacts the hard palate to sequentially propel the bolus toward the pharynx 1
• This phase remains under voluntary control 1

Pharyngeal Phase

• The tongue propels the bolus into the pharynx, triggering an automatic swallow response 1
• Critical protective events occur simultaneously: velopharyngeal closure, base of tongue retraction to the posterior pharyngeal wall, hyoid bone and larynx elevation, and triple-level airway closure (true vocal fold closure, false vocal fold approximation, and arytenoid cartilage contact at the base of the epiglottis) 1
• Pharyngeal muscles contract and the upper esophageal sphincter opens 1
• This phase is partially voluntary and partially involuntary 1
• The entire pharyngeal phase occurs rapidly within 1-2 seconds 2

Esophageal Phase

• A peristaltic wave of contraction moves the bolus through the esophagus 1
• This phase is entirely under involuntary control 1

Neurophysiological Control

The swallowing process involves a central pattern generator located in the brainstem swallowing network, which includes the nucleus tractus solitarius and nucleus ambiguus with reticular formation connections to cranial motoneuron pools bilaterally. 3

• The brainstem network receives descending inputs from multiple cortical regions bilaterally but asymmetrically 3
• Cortical areas can trigger deglutition and modulate brainstem sequential activity 3
• The system demonstrates neuroplasticity with continuous modulation by ascending sensory input and descending motor output 3

Dysphagia: Pathophysiology and Classification

When swallowing dysfunction occurs, it can be classified by the affected phase: oral, pharyngeal, or esophageal dysphagia, though patients frequently have impairments across multiple phases. 1

Key Pathophysiological Concepts

• Impairments may involve motor sequence planning, coordination and timing deficits, or anatomical structural displacement 1
• Penetration occurs when the bolus enters the laryngeal vestibule but remains above the true vocal folds 1
• Aspiration occurs when the bolus passes below the true vocal folds into the trachea and lungs 1
• Silent aspiration is particularly dangerous—patients with impaired laryngeal sensation do not cough or clear their throat in response to aspiration 1

Common Etiologies in Neurological Disease

• Stroke: Dysphagia occurs in at least 50% of patients with ischemic or hemorrhagic stroke, conferring a three-fold increased risk of aspiration pneumonia and significantly higher mortality 1
• Parkinson's Disease: More than 80% develop dysphagia during disease course, with pneumonia being the most frequent cause of death 1, 4
• Multiple Sclerosis: Dysphagia occurs in more than one-third of patients, increasing aspiration pneumonia risk particularly in late stages 1, 5
• ALS: Up to 30% present with swallowing impairment at diagnosis, with virtually all developing dysphagia as disease progresses 1

Management of Dysphagia

Essential Evaluation Approach

Prior to initiating any stimulation therapy, patients must receive a clinical swallow exam or, preferentially, an instrumental swallow evaluation (fiberoptic endoscopic evaluation of swallowing or videofluoroscopic swallowing study), repeated after treatment completion. 1

• For Parkinson's disease specifically, a modified water test assessing maximum swallowing volume is recommended to uncover oropharyngeal dysphagia 4
• Instrumental methods allow reliable detection of aspiration events and identification of specific impairment patterns during oral, pharyngeal, and/or esophageal phases 4

Compensatory Maneuvers (Immediate Effect)

Chin-down technique: Used for patients with decreased airway protection, delayed swallow initiation, or reduced tongue base retraction 1

• Patients bring chin to chest and maintain this posture throughout swallowing 1
• Reduces aspiration risk by approximately 50% in patients presenting with aspiration 1

Super-supraglottic swallow: For patients with reduced airway closure 1

• Patient holds tight breath, swallows while keeping airway closed, then immediately coughs 1
• Demonstrates immediate effects on swallowing physiology 1

Rehabilitative Exercises (Long-term Effect)

Expiratory muscle strength training (EMST): The most evidence-based strengthening intervention 1

• Involves exhaling forcefully into a one-way valve mouthpiece to strengthen expiratory and submental muscles 1
• Demonstrated significant effects on swallowing safety in RCT in Parkinson patients 1
• Improved swallowing safety and feeding status in RCT in subacute stroke patients 1
• Improved hyoid displacement in ALS patients 1

Mendelsohn maneuver: For patients with decreased hyolaryngeal excursion and/or decreased upper esophageal sphincter opening duration 1

• Patients keep thyroid cartilage elevated for several seconds before finishing swallow 1
• RCT in stroke patients demonstrated improved hyoid movement and upper esophageal sphincter opening 1

Effortful swallow: For patients with significant residue in valleculae/pyriform sinuses or decreased airway closure 1

• Increases hyolaryngeal excursion, laryngeal closure, and tongue base retraction pressure 1
• Small cohort study in Parkinson's disease showed improved pharyngeal manometric pressure after two weeks 1

Comprehensive Treatment Approach

The largest RCT to date (Carnaby et al., N=306 stroke patients) demonstrated that standard high-intensity swallowing intervention achieved 70% return to normal diet at six months versus 56% with usual care, with significant reductions in medical complications, chest infections, and death or institutionalization. 1

• Due to limited evidence quality, all stimulation treatments should preferentially be carried out within clinical trials 1
• Neuromuscular electrical stimulation (NMES) as adjunct to behavioral therapy shows modest effects on swallowing physiology and feeding status per three meta-analyses 1

Parkinson's Disease-Specific Considerations

Fluctuating dysphagia with deterioration during off-state should be treated by optimizing dopaminergic medication. 4

• Swallowing treatment methods must be selected according to individual dysphagia pattern 4
• Deep brain stimulation does not appear to have clinically relevant effect on swallowing function 4
• Esophageal body impairment occurs in 95% of PD patients throughout all disease stages, possibly reflecting α-synucleinopathy in the enteric nervous system 6

Critical Pitfalls to Avoid

• Never assume absence of symptoms means absence of dysphagia: Defects in all swallowing phases precede symptoms in Parkinson's disease 7
• Silent aspiration is common: Patients with impaired sensation do not cough, making clinical assessment unreliable without instrumental evaluation 1, 4
• Anticholinergic medications worsen dysphagia through multiple mechanisms and should be avoided 5
• Acetylcholinesterase inhibitors increase saliva production, potentially worsening swallowing difficulties 5