Three advanced electronic searches for titles were carried out during September 2021 in the following databases: PubMed, LiLACS and SciELO, using the following key terms and Boolean operators. Table 1 shows the searches.

TABLE 1 TABLE 1 Title Searched Carried out in PubMed, LiLACS y SciELO Source: the authors.

As inclusion criteria, articles with primary data, in English or Spanish language, referring to swallowing and chewing disorders in adults with Down syndrome, that is, older than 18 years of age, were considered. Studies published between 2000 and 2021 were included. Bibliographic or systematic reviews were excluded.

The titles and abstracts of all the articles initially identified were read and then screened considering the inclusion and exclusion criteria, as well as the purpose, methodology, population assessed, and findings of each study. The resulting pool of titles, excluding those whose topic was not related to our research question, were read in full. Once we obtained the final sample, we consolidated and collected the data, and classified the articles according to themes, to address each of the aspects that served to respond the research question.

Figure 1 presents a flow chart with the steps to identify and select studies. The initial search generated 82 titles and, after removing 11 duplicates, 71 articles were obtained to evaluate in the screening stage. Subsequently, the titles and abstracts of the studies found were read and 55 articles were eliminated, as they were not related to the subject of the review. Therefore, 16 were considered for full reading. Finally, the sample for the Review consisted of 11 articles (Table 2).

FIGURE 1 FIGURE 1 Flowchart of the identification and selection process Source: the authors.

TABLE 2 TABLE 2 Main Characteristics of the Studies Selected

TABLE 2 (cont.)

The 11 selected studies were conducted in seven countries: Japan, Hashimoto, et al. (9); Singapore, Smith et al. (10); Spain, Zárate, et al. (11); France, Allison, et al. (12), Hennequin, et al. (18) Mazille, et al. (19), and Hennequin, et al. (13); UK, Thacker, et al. (14); US, Chenbhanich, et al. (15); and Brazil, Pinheiro, et al. (17), and Bermudez, et al. (16). We selected Studies with the following types of research designs: quasi-experimental, Hashimoto, et al. 2014 (9), and Hennequin, et al. (13); observational, Smith, et al. 2014 (10), Allison, et al. (12), and Hennequin, et al. (18); clinical evaluation, Zárate, 2001(11); questionnaire, Thacker, et al. (14); retrospective, Chenbhanich, et al. (15), and Bermúdez, et al. (16); longitudinal study, qualitative, applied, field research, Pinheiro, et al. (17); randomized controlled trial with placebo, Mazille, et al. (19). The Studies were published between 2001 and 2019. All the articles focused in clinical, anatomical, and functional characteristics during swallowing and chewing in people with Down syndrome.

Down syndrome is an autosomal chromosomal anomaly that induces selective hypotonia and poor neuromotor control, which can affect facial growth. The lips, masticatory muscles, larynx, and tongue muscles are hypotonic. The latter cause an abnormally large tongue appearance, which is favored by weakness and a low and anterior tongue position in the mouth. Usually, they present a scarce development of the middle third, with a mandible of normal development, for which a discrepancy in the intermaxillary relationship can be observed. Such a discrepancy affects the necessary occlusion for the stabilization of the mandible and the hyoid bone, in functions such as swallowing and chewing. They tend to compensate for the lack of interarch contacts by adopting a mandibular protrusive position to increase them, which is facilitated by the instability of the temporomandibular joint (that characterizes them, due to hyperlaxity of the ligaments of said joint) (13). It has also been possible to demonstrate a rudimentary sucking-swallowing feeding pattern and chewing with a rotating pattern, with depressed and ineffective food control. The lack of the mature pattern, moreover, causes chewing to occur for short periods (little or none). This aspect is also linked to the description of gastrointestinal problems aggravated by the lack of chewing.

Mazille et al. (19) proposed to assess the impact on chewing patterns of an orthotic device designed to increase the number of interarch contacts in a group of young adults with Down syndrome. They confirmed that chewing disturbances in adults with this syndrome are related to disorganized masticatory muscle activity. In addition, they noted that the use of an occlusal appliance increased chewing time, reduced chewing frequency, and increased the number of interarch occlusal contacts. However, they could not conclude whether the use of such a device improved chewing, recommending carrying out a study that would measure the comminution of the food bolus (19).

For this reason, in a study carried out in 2015, researchers evaluated the effect of the increase in the number of interocclusal contacts on the granulometry of the food bolus, using kinematic parameters of mastication and the frequency of food refusal in a group of young adults with Down syndrome. An improvement in food acceptance, chewing frequency, and bolus granulometry was evidenced after 8 weeks of using an appliance designed to increase occlusal contacts. Using the number of pairs of posterior contacts, the atomization of food during chewing was increased, additionally favoring sensory input, which improved motor coordination, due to the stimulation of periodontal and pulpal mechanoreceptors (13). Likewise, within the findings obtained in relation to chewing, Hennequin et al. (13) found out that by increasing the number of occlusal contacts through a plate, people with Down syndrome tended to reject less food. They felt subconsciously more confident in proper spraying, making food particles small enough, partly replacing the lack of occlusal contacts with a better distribution of interarch contacts, and thus stabilizing the mandible during swallowing. That allowed a better breathing-swallowing coordination, reducing the risk factor associated with aspiration of food. The authors concluded that it is essential to optimize interarch dental contacts when performing oral rehabilitation in people with Down syndrome and suggest that dental condition be analyzed when assessing feeding or swallowing behaviors (13).

In an investigation carried out by Allison et al. (12), different chewing indicators were analyzed in people with normal development and people with Down syndrome, through a video recording, in order not to alter the context or the behavior of the people with Down syndrome evaluated. The variables studied were chewing time (number of seconds between the time the food was placed in the mouth and swallowed), number of chewing cycles (number of closed chewing actions during chewing time) and number of open chewing cycles (chewing taken with the mouth open during the chewing time). When comparing the groups, it was possible to observe, on the one hand, that none of the participants rejected soft foods, but when increasing the hardness of the foods, the percentage of adults with Down syndrome who rejected solid foods increased, in this case of carrot (59.1 %) and, on the other hand, that the chewing time was longer than in the control group for semi-solid consistency (sausage), but shorter for puree. It was also possible to observe in people with neurotypical development that there was a positive correlation between masticatory time and the number of cycles with the mouth closed, and that the number of rejections had no correlation with the hardness of the food. The authors concluded that both the chewing time, the number of closed and open chewing cycles, and moments of food refusal are valid variables to consider in people with Down syndrome, and the observations made through video recordings are useful to analyze these processes.

Subsequently, Hennequin et al. (18) assessed the differences in chewing indicators in a group of adults with Down syndrome and a control group of people without this condition, when eating different types of food. For this, they used video recordings, as previously proposed by Allison et al. (12). They found differences in masticatory function indicators between both groups, which could be observed in most of the ingested foods. Although the chewing frequency was lower in the group with Down syndrome, both the chewing time and the number of chewing cycles did not always show differences between the two groups. When considering open masticatory cycles, these were reported more frequently among people with Down syndrome. On the other hand, the rejection of certain foods was observed in the group with Down syndrome, but not in the control group. Another of the findings reported is that people with Down syndrome can demonstrate adaptive behavior in the face of functional disabilities, increasing or decreasing the number of chewing cycles depending on the type of food. In addition, since there was a rejection of foods with harder consistencies, it could be assumed that people with Down syndrome adjust their intake according to the texture of each food.

On the other hand, Hennequin et al. showed that people with Down syndrome developed at least three strategies when eating: Those foods that were difficult to chew were swallowed before forming a food bolus of appropriate granulometry. Others were chewed for a long time and/or with more cycles. Finally, food refusal was considered as a third strategy. A consequence possibly associated with masticatory dysfunction may be a nutritional deficiency, since a person with Down syndrome may lack the nutrients present in foods rejected for their texture or consistency (e.g., raw fruits and vegetables) and, at the same time, they prefer more tender foods, such as those rich in carbohydrates, which could promote obesity. Therefore, the authors concluded that people with Down syndrome demonstrated a systematic reduction in chewing frequency when eating different foods, highlighting the importance of educating parents and professionals about the chewing difficulties of this population (18).

Considering that the generalized muscular hypotonia present in people with Down syndrome favors masticatory disorders, Pinheiro et al. (17) carried out a study measure the effects of functional electrical stimulation (FES) on the orofacial musculature and on various functions, including chewing, in persons with Down syndrome. The results indicated that FES therapy favored lip closure, improved the appearance of the cheeks, favored the lateralization of the tongue to both the right and left sides, produced a favorable change in the respiratory mode, improved swallowing and, after the FES, the participants were able to make an unforced lip closure. It also reduced the tension of the facial muscles during swallowing. In relation to the masticatory function, cutting and crushing were aspects that improved significantly, showing that the cut was made by the incisors and the crushing presented an alternate bilateral pattern. The authors concluded that FES in conjunction with masticatory training significantly stimulated the orofacial musculature, improving chewing, breathing, and swallowing functions in people with Down syndrome.

One study evaluated tongue pressure generation in adults with Down syndrome as a measure of tongue function during swallowing. The measurement parameters (duration, maximum magnitude, and integrated value of tongue pressure) were different between participants with Down syndrome and a control group. Hashimoto et al. (9) reported that the length and width of the palate in the participants with Down syndrome was significantly smaller than in the controls and is correlated with the lingual pressure obtained. Likewise, they suggested that a shorter palatal length favors the positioning of the tongue in the anterior sector of the maxilla, affecting the exertion of tongue pressure against the back middle part of the palate. In this way, by contacting the tip of the tongue with the lingual surface of the anterior upper teeth, an anterior seal of the oral cavity is produced, which compensates the underdeveloped maxilla with a decreased palatal length in people with Down syndrome. This study concluded that swallowing difficulties could be caused by the presence of a narrow and short palate, which would alter tongue control, activity, and movement in people with Down syndrome (9).

Smith et al. (10) evaluated oral feeding among adults with Down syndrome. Regarding eating and liquid drinking behaviors, they found difficulties in handling the utensils to transfer food and, on some occasions, it was observed that 30.4 % of individuals positioned the plate near their mouth and put their tongue on the edge of the plate, bringing it tongue closer with the spoon. 30.4 % of participants chewed the water and the spoon. 8.7 % added water and mixed it with the food before eating, while 21.7 % spit the food on the plate or on the table. Support and guidance can be beneficial for the normalization of feeding, when feeding; however, the support must be indirect since self-feeding must be promoted. It was further suggested that chewing liquids or chewing on an empty spoon might be associated with cognitive decline. In the oral phase, alterations were found in lip closure, tongue movements, and chewing and 17.4 % of people with Down syndrome, who swallowed multiple times for each bolus. Regarding the pharyngeal phase, the authors could only speculate on the issues observed, given the difficulty to exploring this phase by observation, which indicates alterations in this stage were detected more commonly in participants who ate a soft and wet diet. In addition, they reported 10 % of individuals reported changes in voice quality after feeding. On the other hand, they reported cough in more than 50 % of the participants, this being a possible sign of aspiration.

In the esophageal phase, an association between some esophageal motor disorders, particularly achalasia (a primary esophageal motor disorder whose etiology is unknown), and infectious, autoimmune, or genetic factors was found (11). Esophageal dysmotility (a motor disorder of the esophagus that affects the peristalsis of the esophageal body present in cases of Down syndrome) was also observed. The symptoms of dysmotility are not evident, but they include difficulty to swallowing, heartburn, regurgitation, and non-cardiac chest pain (11). Another study reported a prevalence of 5 % congenital gastrointestinal malformations, including esophageal atresia, and 7.2 % gastroesophageal reflux disease (GERD), whose symptoms include vomiting, regurgitation, aspiration pneumonia, and chronic cough (16).

Dysphagia is a significant problem in people with intellectual disabilities, as it can affect the health, safety, and general well-being of the person. A study that sought to know the most common causes of hospitalization in adults with Down syndrome found that 29.6 % was due to aspiration pneumonia, with the most common comorbidities (70.3 %) being GERD and dysphagia, requiring multiple admissions. Because of this, 16.2 % the patients required feeding via a gastrostomy tube, which was also associated with hospital readmissions (15).

Thacker et al. (14) conducted a study to investigate potential risk factors associated with history of choking among adults with intellectual disabilities, including those with Down syndrome. The latter were at higher risk of choking than adults with other disabilities. This may be because persons with Down syndrome have poor tongue coordination, favoring the uncontrolled entry of food into the pharynx that, when combined with a delay in triggering the swallowing reflex, increases the risk of aspiration (14).

Smith et al. (10) suggest that the anatomical differences and hypotonicity associated with Down syndrome, combined with sensory, cognitive, or behavioral disorders can cause difficulties in oral feeding, affecting health, safety, and social integration. Next, Tables 3 and 4 present the most relevant findings in chewing and swallowing functions, compared to the characteristics in neurotypical people.

TABLE 3 TABLE 3 Comparative Chart of Masticatory Function Source: the authors.

TABLE 4 TABLE 4 Comparative Chart of Swallowing Function Source: the authors.