Alzheimer's disease (AD) has become one of the largest global economic burdens. According to the 2015 World Alzheimer's Disease Report, the total global cost of dementia will increase at an annual rate of more than 40% between 2015 and 2030—from 818 billion dollars to 2 trillion dollars. However, recent research suggested that China’s domestic socio-economic burden has been overwhelmingly underestimated; this will significantly affect the estimated global cost. In 2015, the cost of AD in China has reached 1.47% of the Gross Domestic Product (GDP), while the worldwide cost accounted for 1.09% of the global GDP, indicating a higher socio-economic cost in AD patients in China. Morever, the annual cost of an AD patient in China was 19 144.36 US dollars (USD), and the annual social and economic cost of AD was 167.74 billion USD, which was 5.95 times higher than the estimated value in the aforementioned report. What’s more, by 2030, total annual cost related to dementia will reach 50.749 billion dollars.
The high socioeconomic burden of AD is mainly due to caregiver demands. Caring for AD patients is highly challenging for their caregivers and family members. The primary difficulty in caregiver is caused by the decline in AD patients’ daily-living ability, specifically, the swallowing dysfunction. Clinically, the swallowing dysfunction leads to malnutrition, dehydration, weight loss, fear of eating, and other complications; consequently, it might extend hospital stay and even cause severe injury or death in extreme cases. Swallowing dysfunction or dysphagia, which is a group of clinical syndromes, can occur as a result of various diseases. Dysphagia refers to improper transfer of food from the mouth to the esophagus and the stomach, caused by the impairment of the swallowing organs (e.g., mandible, lips, tongue, soft palate, larynx, esophagus, and so on). Generally, the swallowing process includes the following four physiological stages: oral preparatory, oral, pharyngeal, and esophageal.
Significant attention is needed to address the high prevalence of dysphagia in AD patients. Previous studies indicated an incidence rate of 32–45% among mild AD patients with dysphagia, and 84–93% among moderate and severe AD patients. Abnormal swallowing function in AD patients manifested as weak tongue movement and pressure, generally occurring in the oral and pharyngeal stages of swallowing. Hence, due to abnormal swallowing function, individuals might exhibit delayed pharyngeal reflex, reduced pharyngeal muscle strength, and food residue after swallowing. Individuals with abnormal swallowing function experiencing difficulty in forming and pushing the food bolus, and hence, are at high risk of food aspiration.
Studies have reported that AD patients with dysphagia at admission are at a higher risk of malnutrition than those without dysphagia, which is associated with respiratory infection and increased mortality. When screened using the Water Swallowing Test, the incidence rate of malnutrition was 1.67 times higher, with which the severity of dysphagia was positively correlated. Besides, malnutrition also reduces the quality of life in patients. A multi-center cohort study found that swallowing dysfunction was an important cause of pneumonia and lower respiratory tract infection in 170 elderly people in a nursing home (OR = 2.000, 95%CI = 1.2–3.3, P = 0.10). Thus, optimal solutions are urgently needed to improve patients’ swallowing function and living ability.
Dysphagia treatment includes three approaches: compensatory strategies, swallowing rehabilitation, and other approaches. The first aims to reduce the effects of impaired bolus flow to ensure the safety of oral diets; it has many forms, such as postural adjustment, diet modification, swallow maneuvers, and enteral feeding. Changes in head or body posture are recommended to reduce aspiration or residue. Many postural techniques including (but not limited to) head down and lift, and side-lying, can successfully eliminate aspiration on at least one bolus volume of liquid. Logemann et al. conducted a study using thin liquids with either chin-tuck or nectar-/honey-thickened liquids in individuals with dementia or Parkinson’s disease (PD). Overall, fewer participants aspirated on nectar- (p < 0.01) and honey-thickened liquids (p < 0.01). While previous studies are conflicted about these strategies and some data suggest that postural adjustments are inferior to active rehabilitation, other approaches include chemo-denervation, pharmacological treatment, neuromuscular electrical stimulation, and non-invasive brain stimulation[11, 12]. Chemical myotomy and drugs application are not highly recommended as first-line treatment in older adults with dysphagia due to the potential risks and side effects.
Wang et al. synthesized 27 randomized controlled trials to explore the effect of noninvasive neurostimulation therapies (repetitive transcranial magnetic stimulation [rTMS], transcranial direct current stimulation [tDCS], and surface neuromuscular electrical stimulation [sNMES]), which work through magnetic or electric fields to trigger and regulate the depolarization of cortical neurons, on dysphagia patients after stroke. A positive effect of rTMS, tDCS, and sNMES was reported in the recovery of swallowing function (standardized mean difference = 0.91; 95% CI: 0.54–1.27; Z = 4.84; P < 0.00001; I2 = 86%). However, there is no recommended treatment protocol, and for implementation, a specific equipment is required.
Swallowing rehabilitation comprises exercises targeted to train specific muscles or muscle groups. Given this development, more evidence-based therapeutic exercises were introduced—instead of centering on an isolated muscle, the training program gradually became more systematic and available. Kim et al. explored the instant effect of simple oral exercise (SOE), performed two times per day for a week, on 84 older adults. Masticatory performance improved immediately by around 16% in the poor-chewing group, and the unstimulated saliva production in all subjects increased to 0.26 ml/min immediately after SOE. Kang et al. examined the impact of a bedside exercise program, which comprised oral, pharyngeal, laryngeal, and respiratory exercises. After implementing the program in 25 stroke patients for one hour per day for two months, the results showed a significant improvement, compared to the control group, in the swallowing function (at the oral phase) and depressive symptoms .
However, swallowing exercises, the fundamental rehabilitation for dysphagia, need more high-quality evidence regarding their implementation in AD. Under the neural regulation mechanism of the cerebral cortex sensorimotor, practicing swallowing exercises is effective for dysphagia with stroke/PD or older adults. Clinicians find the implementation of appropriate rehabilitation exercises challenging in patients with AD. In China, clinicians apply swallowing exercises, such as lip lordosis and adduction, mandibular opening and closing, cheek blowing, and tongue extension, generally combined with routine care and other treatments. These exercises can be effective in increasing the flexibility and coordination of swallowing organs and improving the relevant muscles’ strength. Nonetheless, whether swallowing exercises are effective in dementia is controversial. Therefore, simple and feasible swallowing rehabilitation training should be developed for patients with impaired cognition and attention deficit. Based on the current swallowing rehabilitation literature, a multi-center randomized controlled trial will be conducted in three mainland hospitals to investigate the effect of stepwise swallowing training on the function and daily-living ability in AD patients.
Based on the literature and clinical evidence, a program has been developed to improve AD patients’ swallowing function, from top to bottom—muscle training in lips, tongue, face, and neck. The study aims to evaluate the effectiveness of the Stepwise Swallowing Training (SST) in swallowing function among AD patients with dysphagia.