Learning disorders may occur due to multifactorial reasons, among which are auditory information processing alterations,1,2 attention deficit, interpersonal relationship difficulties, behavioral disorders, cognitive deficits, disadvantaged socioeconomic background,3 family history of learning difficulties and disabilities,4 as well as others, such as mouth breathing – which can compromise learning.5

When breathing is performed only through the mouth, it can be considered a pathological adaptation resulting from difficulty of breathing through the nose,6 and it results in the inspiration of a drier, unfiltered air, at a colder or warmer temperature than the expected, which ultimately overwhelms the tonsils and the larynx and can cause chronic inflammation. If such pathological adaptation occurs over a long period, it can result in tonsillar hypertrophy and subsequently, varying degrees of upper airway obstruction. Thus, there will be resistance to gas flow, permanent increase in energy expenditure, and adaptations that are structural (high-arched palate and dental malocclusion) and functional (orofacial muscle flaccidity, dysphonia, and sleep apnea, for instance), that can impair the quality of sleep, mood, behavior, and school performance,7 although there is no significant scientific evidence to support the association between the altered breathing pattern and learning difficulties.

Considering the high prevalence of mouth breathing in childhood8 and the possibility of its impact on learning, this study was designed in order to verify, through a systematic review of the literature, whether this breathing pattern influences children's learning process.

The methodological approach used in this review follows, including article search strategy and eligibility criteria, the data collection phase, and analysis. This systematic review was carried out following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines.9

Search strategy and eligibility criteria

This systematic review study was conducted with no restrictions regarding the year and language of publication. The P.I.C.O strategy was used, considering schoolchildren who were mouth breathers aged 7–11 years (P=patient), assessed regarding the aspects related to learning (I=intervention) and compared with schoolchildren who were nasal breathers (C=intervention comparison or control), aiming at verifying the possibility of learning disability in those with altered breathing pattern (O=outcome), using the following guiding question: “Is the mouth-breathing child more likely to have learning disabilities when compared to nasal breathers?”.

The study design is explained in Fig. 1, with the study eligibility criteria. For the studies considered preliminarily eligible, the full text was obtained and assessed in order to verify whether they met all the inclusion criteria. The following inclusion criteria were used: observational studies (controlled, cross-sectional, prospective, or retrospective longitudinal studies) on the subject, with schoolchildren aged between 7 and 11 years.

Figure 1.

Search strategy flowchart and article selection.

(0.56MB).

The exclusion criteria were: ambiguous results, poor presentation of results, study duplication based on the database search, review studies, communications, case reports, scientific meeting abstracts, monographs, comments, or editorials. Studies regarding syndromic patients and those with intellectual disabilities were also excluded, as well as registries not directly related to the final outcome of this study (Fig. 1).

The keywords were selected in DeCS (VHL Health Sciences Descriptors) and MeSH (PubMed), in order to identify relevant studies in the PubMed, SciELO, LILACS, and Scopus electronic databases. The controlled descriptors were “learning” and “mouth breathing,” “aprendizagem” and “respiração bucal.” Boolean operators (OR and AND) were used for descriptor combination. This research was carried out on July 15th, 2015. The gray literature was identified by searching in Google Scholar, by consulting the first hundred records of each combination.

School failures occur for different reasons; according to the last census, conducted by Instituto Nacional de Estudos e Pesquisas Educacionais Anísio Teixeira23 (INEP, BRASIL, 2013), in 2013, 6.1% of students fail to pass on to the following grade, with one of the reasons being the presence of learning disabilities. Literature has described several factors for its emergence, such as hearing,1,2 attention, interpersonal relationships, behavior and cognition disabilities, socioeconomic status,3 family history,4 as well as mouth breathing.5

This respiratory pattern is considered a pathological adaptation6, which can affect the quality of sleep, mood, behavior, and school performance7; however, there are few studies that show such interrelation. Moreover, the prevalence of mouth breathing is considered high in childhood8 and, thus, the present systematic review was carried out.

As shown by the sample composition, little has been investigated on the subject (2.8% of 357 studies), demonstrating the need for further studies in this area.

Half of the studies in the sample used a control group,13,17,19,20,22 and it is noteworthy that Kajihara and Nishimura19 compared their results with the control group of another study, although they belonged to the same research group. Thus, it is suggested that studies on the subject with control groups be performed to attain greater result reliability. Additionally, for evidence-based practice, that is, for a professional decision to be made based on the obtained scientific results, according to Muir Gray24 it is ideal when such studies show high evidence strength, which are usually controlled and randomized studies; such randomized controlled studies were not identified in our sample.

As for the nosological diagnosis confirming the etiology of mouth breathing, half of the researchers we studied15,17,19,20,22 submitted their sample groups to otorhinolaryngological assessment, demonstrating obstructive mouth breathing, although with different etiologies. It should also be noted that Abreu et al.13 stated that the study group underwent medical assessment, but did not mention which specialty performed such assessment or what procedures were performed.

Perilo et al.21 sent a Questionnaire for Assessment of Respiratory Characteristics, consisting of 22 closed questions with yes/no answers to the parents/guardians. However, the authors’ study, due to the absence of the otorhinolaryngological evaluation to confirm the etiology of the altered breathing pattern, did not mention the number of affirmative questions that should have been marked in order to consider the subject a possible mouth breather. Such bias was minimized by observing labial sealing for 5min, observed by two evaluators during a distractor task. Thus, they did not divide the sample into mouth and nasal breathers, analyzing the respiratory characteristics of the sample and comparing them with the cognitive-linguistic skills.

The assessed studies that comprised the sample used non-probabilistic samples, chosen intentionally or by convenience, that can result in bias in data interpretation as it depends on the researcher's appraisal. Despite the abovementioned fact, Kuroishi et al.22 carried out a sample calculation study to determine the sample size that would provide a more representative population and more accurate results.25

Although school failure is a complex analysis variable since other aspects can influence its occurrence, it is inferred that students with learning disorders have a greater chance for failure. Therefore, it would be worthwhile for studies to show whether the sample consisted of students that did or did not fail school. The research by Fensterseifer et al.20 established a minimum of two years, and the study by Vera et al.16 reported complaints (by most of the family members of mouth breathers) of learning difficulties, while 39% had failed (from one to three times), more often in elementary school. Other studies14,15,17–19,21,22 did not mention school failure, and in the study by Abreu et al.,13 the students were required to be at the appropriate school year for age, i.e., those students who failed were not included in the study, which may be considered a weakness.

Another important factor for analysis is the place where the selected studioes occurred and the time period during which the studies were performed, since climate change must be taken into account.26 Most studies in our sample were carried out by Brazilian researchers,13,15–22 which justifies our interest in the subject. Only four studies (40%)13,15,17,20 indicated the period (in months or years) during which the sample selection was obtained.

Researchers27 have emphasized the impact of climate change on upper airway (UA) disorders in children younger than 13 years in the metropolitan region of São Paulo, in the months that correspond to the start of winter. They added that the peak of respiratory disease morbidity occurs in May, possibly due to thermoregulation problems in subjects adapted to the milder climate/weather of April, explaining that people with thermoregulation problems are more sensitive to respiratory and cardiovascular diseases, especially the pediatric population.

There is an increase in hospital consultations27 during this period and, consequently, a higher probability of school absenteeism, which can impair academic performance, depending on the chronicity of the condition.

In Brazil, respiratory diseases accounted for 5% of the years of life lost due to premature death,28 emphasizing the importance of public health measures to reduce this percentage.

Another analysis variable concerns the impact of urbanization on the living conditions and health of the Brazilian population. It can be observed that of the ten selected studies, half were performed in capital cities13,15,17,20,21 and the other half14,16,18,19,22 in well-developed cities. In large Brazilian cities, according to Maricato,29 there has been an increase in ecological social inequality, a situation in which there is an uncontrolled growth of cities, with population increase in the outskirt regions. Usually, the individuals living in the outskirts of Brazilian cities have a low socioeconomic status (as demonstrated by research)30 and, thus, are more vulnerable to factors that can compromise quality of life and health.

In addition to the aforementioned facts, it can be observed that school failure is higher in boys than in girls, predominantly among African-descendants and/or those from low-income families.30 This study sample was mostly comprised of boys, confirming what literature has shown, both with respect to school failure and the possibility of the presence of a comorbidity factor, such as ADHD.16

Thus, it is possible to conclude that the understanding of the determinants of learning and its failure are multifactorial and complex; moreover, there are few validated tools to investigate learning disorders, and that creates difficulty when comparing the procedures. Only two studies22 used validated tools, another21 used a published protocol that was adapted to Brazilian students, and another19 applied an evaluation test that was used in a different non-validated study; the remaining studies13–16,18,20 did not use assessment protocols with the students, but instead used school failure records or questionnaires applied to family members or teachers, increasing the analysis bias in such studies.

The School Achievement Test (Teste de Desempenho Escolar – TDE), developed by Lilian Stein,31 was partially used in one study (10%).22 The study compared the experimental group with a control group and the subjects were matched for age and educational level, with part of the sample showing mouth breathing. It was verified that such students had lower school performance than the control group in reading comprehension, arithmetic, and working memory tasks (except for numbers).

Regarding the assessment of mathematical skills, two19,22 of the studies that applied tests found results indicating difficulties in mouth breathers, while the other one did not.21 Reading comprehension was considered worse in mouth breathers,22 as well as writing skills in subjects with respiratory disorder characteristics, when compared with those without such disorders.21

Therefore, in general, most researchers reported learning difficulties in subjects with abnormal breathing pattern.

Thus, studies involving mouth breathing and learning require further investigation, since the number of studies for analysis was small and showed diverse evaluation procedures. Controlled, interdisciplinary studies, with the inclusion of standardized assessments, using validated tools and uniform sample groups must be performed so that a systematic meta-analysis review study can be carried out and, thus, more scientific evidence will become available, both for clinical practice and for the implementation of public health and education policies.

There is evidence that the breathing pattern can influence the learning process. This systematic review showed that mouth breathers are more likely to have learning difficulties than nasal breathers. Further studies are needed in order to increase the scientific evidence for clinical practice, and for the implementation of public health and education policies.

The authors declare no conflicts of interest.