49Japanese Journal of Comprehensive Rehabilitation Science (2020)

Editorial

Differences between drinking and eating from the viewpoint ofdysphagia rehabilitation
Hitoshi Kagaya, MD, DMSc1
Department of Rehabilitation Medicine I, School of Medicine, Fujita Health University

1

Kagaya H. Differences between drinking and eatingfrom the viewpoint of dysphagia rehabilitation. Jpn J
Compr Rehabil Sci 2020; 11: 49-51.
Dysphagia rehabilitation in Japan
Dysphagia rehabilitation holds a very prominent
position in rehabilitation medicine in Japan. The
Japanese Society of Dysphagia Rehabilitation (JSDR)
was established in 1995 as a “conference” and becamea “society” from 1996. In 2020, the number ofmembers exceeded 14,000. Among the members of
the JSDR speech-language-hearing therapists accountfor one third, followed by dentists. Many patients withdysphagia have problems not only with swallowing
but also with the meal itself. Therefore, many medical
professionals are expected to be involved including
physicians, dentists, nurses, speech-language-hearing
therapists, physical therapists, occupational therapists,
dental hygienists, nutritionists, and medical socialworkers. However, this is not possible in practice, sothe role of transdisciplinary teamwork must beadjusted to satisfy the patient’s needs and problems[1]. Thus, in a transdisciplinary team the role ofmedical professionals changes depending on thepatient’s needs. We require a wide range of common
fundamental knowledge beyond the traditional job
role. The JSDR has a certification system and 3,211
people are currently certified, with the largest number
being speech-language-hearing therapists, followed inorder by dentists, nurses, and physicians. The certified
members who have passed the examination after
taking the e-learning system are expected to work
beyond their traditional roles.

Correspondence: Hitoshi Kagaya, MD, DMSc
Department of Rehabilitation Medicine I, School ofMedicine, Fujita Health University, 1-98 Dengakugakubo,
Kutsukake, Toyoake, Aichi 470-1192, Japan.
E-mail: hkagaya2@fujita-hu.ac.jp
Accepted: March 12, 2020.
Conflict of Interest: The author declare no conflict ofinterest.
©Kaifukuki Rehabilitation Ward Association 2020
doi.org/10.11336/jjcrs.11.49

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Drinking and eating
The main difference between drinking and eating is
the absence or presence of mastication. Traditionally,
the four-stage sequence model is advocated forswallowing [2, 3] (Figure 1). In the past, liquid wasused for the bolus to minimize the exposure time dueto the large irradiation dose by X-ray fluoroscopy. The
four-stage sequence model divides swallowing into
the oral preparatory, oral propulsive, pharyngeal, andesophageal stages. In the oral preparatory stage, abolus is collected in the oral cavity and placed in aswallow-ready position. The isthmus of fauces isclosed by the tongue and soft palate. The isthmus offauces opens and the bolus is propelled from the oralcavity to the pharynx in the oral propulsive stage. The
tip and sides of the tongue contact the palate and serveas anchors to move the bolus from the anterior toposterior using a squeezing mechanism. The
pharyngeal stage usually occurs immediately after thebolus reaches the pharynx and the swallowing reflex isinitiated. The swallowing reflex is a complicatedmovement involving many nerves and muscles in theoral cavity, tongue, pharynx, and larynx. The hyoidbone moves anteriorly and superiorly, the larynx
closes, the upper esophageal sphincter opens, and thebolus moves from the pharynx into the esophagus. The
esophageal stage starts once the bolus comes into theesophagus. Esophageal peristalsis is activated and thebolus finally reaches the stomach. This four-stage
sequence model explains drinking well.
Recently, swallow with chewing was evaluated andit was found that chew-swallow could not be explainedby the four-stage sequence model. Therefore, theprocess model has advocated chew-swallow (eating)[4, 5]. In the process model, the bolus moves from thetongue surface to the postcanine region by the pullingback movement of the tongue (stage I transport). Then,the bolus is continuously chewed in the oral cavity andsoftened by moistening until the food bolus reaches
optimal cohesiveness and is ready for swallowing(processing). Simultaneously, the bolus is actively
driven by the tongue squeezing and transported to themiddle pharynx (stage II transport). The characteristicJpn J Compr Rehabil Sci Vol 11, 2020

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Kagaya H: Differences between drinking and eating from the viewpoint of dysphagia rehabilitation

Figure 1. Physiological models of swallowing.
Application to dysphagia rehabilitationof the process model is that food processing and stage
II transport overlap substantially in time. The
pharyngeal stage, which occurs from the initiation ofswallowing reflex, and the esophageal stage, whichstarts after the bolus comes into the esophagus, aresimilar between the four-stage sequence model and theprocess model; the key difference is the timing ofinitiation of the pharyngeal stage. The swallowing
reflex starts subsequently after the bolus reaches thepharynx in the four-stage sequence model (drinking),
but does not occur immediately after the bolus istransported to the middle pharynx in the process model(eating). Moreover, an isolated pharyngeal swallow(IPS) which consisted of pharyngeal and esophageal
stages (two stages) without prior intraoral transport by
the tongue was found in both drinking and eating [6].
IPS is considered to be a protective swallow to prevent
aspiration when the bolus reaches the pharynxunexpectedly. We distinguish drinking and eating
unconsciously, but the physiological phenomenon is
not the same.

The differences in this physiological phenomenon
greatly affect dysphagia rehabilitation. In general,
aspiration becomes less likely to occur with thickener,
while a larger amount of bolus leads to aspiration.
How about the presence or absence of mastication?
The level of difficulty for aspiration in each bolus usedin videofluoroscopic examination of swallowing (VF)was reported [7, 8] (Table 1). Liquid and solid food arelikely to be mixed in the oral cavity or we often eat
mixed food like miso soup during an ordinary meal. Atwo-phase mixture (4 g of corned beef hash and 5 mLof thin liquid) mimics this state. The lowest risk ofaspiration is with 4 mL of thick liquid, while 30 g ofliquid from a cup is the most likely to be aspirated. In
Table 1, corned beef hash and a two-phase mixture
account for chew-swallow and mixed food is likely to
be aspirated. Therefore, aspiration may be overlooked
if we check only liquid drinking. The bolus comes into
the pharynx before the swallowing reflex in chewswallow by stage II transport, and in particular, theliquid part of the bolus reaches even the hypopharynx
before the swallow onset with a mixture, resulting inTable
1. Level of difficulty for aspiration with each bolus.
4 mL ofthickliquid
4 mL of thick liquid○ less likely to aspirate● likely to aspirate

8 g ofcornedbeef hash

4 mL ofthin
liquid

10 mL ofthin
liquidOne cupswallow

Twophasemixture

30 g ofliquidfrom a cup

○○○

○○○○

○

○

○

○

○

○

○

○

○

○

○

○

○

8 g of corned beef hash●

4 mL of thin liquid●●

10 mL of thin liquid

●

●

●

One cup swallow

●

●

●

●

Two-phase mixture

●

●

●

●

●

30 g of liquid from a cup

●

●

●

●

●

○
●

Two-phase mixture = 4 g of corned beef hash and 5 mL of thin liquid.
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Kagaya H: Differences between drinking and eating from the viewpoint of dysphagia rehabilitation 51an increased risk of aspiration [9]. A bolus that does
not require mastication is safer for patients with a highrisk of aspiration.
The stage transition duration (STD) is defined asfrom the moment at which the bolus passes the lower
border of the ramus of the mandible to the time atwhich the maximal excursion of the hyoid is initiated[10]. An STD of more than 1.0 s is assessed as delayed
swallowing reflex and prolonged STD values areassociated with an increased risk of aspiration.
However, it was recently found that delayed STD waswithin normal limits in chew-swallow and did notcause aspiration. Moreover, STD differs even in
normal individuals between with and without
mastication [11, 12]. We have to interpret STD
carefully in chew-swallow.
Conclusions
The concept of chew-swallow has brought about aparadigm shift in dysphagia rehabilitation. Dentures inold people and personal habit affect the presence orabsence, and number of times, of mastication. Whenselecting meals for patients with dysphagia, we must
check the state of mastication. We need to recognizethat drinking and eating are different in dysphagia
rehabilitation.

References

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