INTRODUCTION

The palatal expansion technique is a therapeutic approach that has as its main objective the correction of a transverse discrepancy in the upper arch, through the expansion of the sutures.^1,2 Clinically, the indications for this procedure are associated with maxillary atresia, posterior crossbite, dental crowding, or nasal stenosis.^1,3

Although its main purpose is to correct a narrow upper arch, adjacent areas such as the mandible, nasal cavity, pharyngeal structure, temporomandibular joint, middle ear, and pterygoid process of the sphenoid bone^3,4 change after this procedure. Rapid maxillary expansion (RME) and semirapid maxillary expansion (SRME) have also been linked to improvements for patients with impaired breathing, enuresis, and hearing loss, especially in growing children who have maxillary constriction.^5–7

The first reports describing an association between improvements for patients with hearing loss and RME are from the 1960s⁸ and early 1980s.⁶ The investigation of hearing improvements after palatal expansion, as well as the magnitude and stability of this process, shows an important aspect of its direct impact on the quality of life of the juvenile patient. Improvements in hearing can occur after RME, especially in cases of conductive hearing loss derived from changes in the middle ear and Eustachian tube. Moreover, resolution may improve school performance and quality of life,⁹ an important additional beneficial effect in pediatric patients with maxillary constriction who have such injury.

Maxillary expansion is an effective technique in the treatment of transverse problems.^1,2 However, doubts remain about its additional effects and usage as adjuvant therapy in patients with hearing loss who request maxillary expansion.

This systematic review aimed to evaluate whether the palatal expansion technique can improve hearing loss in children and adolescents with preexisting hypoacusis. The type of rapid expansion (rapid or semirapid), stability, and magnitude of changes reported were also evaluated.

MATERIALS AND METHODS

This systematic review was registered at PROSPERO database (http://www.crd.york.ac.uk/PROSPERO) under registration code CRD42015030188 and performed according to Preferred Reporting Items for Systematic Review and Meta-Analysis (PRISMA) guidelines.¹⁰

Search Strategy and Study Selection

Searches were conducted in the following electronic databases, without language restriction, until May 2017: PubMed, Scopus, Web of Science, The Cochrane Library, LILACS, OpenGrey, and Google Scholar. All publications presented in the databases contained a combination of controlled predefined MeSH and free terms related to hearing loss and RME/SRME used with Boolean operators (or, and) to combine searches. The previously defined terms were adapted to the rules of syntax of each bibliographic database.

After consultation of the databases, duplicated results were removed from the combination of the results obtained from all surveyed sources. Additional citations were sought from the analysis of the reference list of all previously selected articles. The searches were conducted by two examiners (Dr Fagundes and Dr Rabello) and checked by a third examiner (Dr Mello) in cases of disagreement.

After the removal of duplicates, the titles and abstracts that did not fit the established eligibility criteria were excluded. The resulting articles were evaluated and judged by their full texts. The process of the search strategy is shown in Figure 1.

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Data Extraction and Risk of Bias

All relevant citations were saved in a bibliographic reference manager (EndNote, x7 version, Thomson Reuters) and, at first, titles and abstracts were analyzed according to the inclusion and exclusion criteria. The selected studies were evaluated by full text, and a final selection was performed.

The quality assessment and risk of bias (RoB) of the included studies were performed following the ROBINS-I tool (Risk of Bias in Non-randomized Studies-of Interventions).¹¹ This checklist included three main domains of bias: preintervention, at intervention, and postintervention. The RoB was judged for each domain and to overall evaluation as low, moderate, serious, critical, or no information for all included studies (Table 1).

Table 1. BIAS and Domains Considered in Risk of Bias (RoB) Evaluation According to the ROBINS-I Tool¹¹

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The results extracted from the selected articles were qualitatively evaluated. A quantitative evaluation of the studies seemed inappropriate given the methodological heterogeneity of the selected articles.

RESULTS

Study Selection and Characteristics

A total of 74 studies were identified from the searches, with exclusion of 34 duplicated results. The remaining 40 titles and abstracts were analyzed according to the inclusion and exclusion criteria, with the exclusion of 28 studies. The remaining studies (n = 12) were evaluated by full text, and three were excluded. Two of these were excluded because of a conflict with the PICO strategy: one study included adult patients in the sample,¹² and the other did not evaluate hearing levels.¹³ The third study was excluded because of overlapping samples of two articles,^14,15 and only the study by De Stefano et al.¹⁴ was selected. As a result, nine studies^7,14,16–22 were included in this review (Figure 1).

After the RoB evaluation, two studies^20,21 were ranked as low risk and seven^{7,14,16–19,22} as moderate risk. At the domains “Bias due to Confounding,” “Bias in Selecting Participants for the Study,” “Bias due to Deviations From Intended Intervention,” “Bias due to Missing Data,” and “Bias in Selecting Reported Result,” a low RoB was reported for all studies. A moderate RoB was observed in most of the studies to the domain “Bias in Measuring Outcome,” except to Micheletti et al.²¹ and Kiliç et al.,²⁰ which presented low RoB. The domain “Bias in Classifying Interventions” also reported moderate RoB to some studies. No article presented serious RoB in any domains of this tool (Table 2).

Table 2. Risk of Bias (RoB) of the Included Studies, According to the ROBINS-I Tool¹¹

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The characteristics of the selected studies regarding the qualitative analysis are shown in Table 3. Among the nine^7,14,16–22 articles included, all studies were prospective nonrandomized clinical trials.

Table 3. Summary of Characteristics and Results of the Included Studies^a

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Table 3. Extended

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DISCUSSION

An improvement in hearing levels was found in most studies after maxillary expansion and at the end of the retention or follow-up period, with improvements in hearing levels varying from approximately 2 to 19 dB. The evaluated studies were all nonrandomized and presented considerable variation in sample characteristics, treatment features, methods of hearing level evaluation, and follow-up period, presenting a risk of bias varying from low to moderate among the selected studies.

The main results analyzed were the hearing levels after maxillary expansion in children and adolescents with previous hearing loss. Seven^{7,14,17–20,22} studies described an improvement in hearing levels after maxillary expansion that lasted until the end of the appliance retention period or longer. This result could represent an additional positive effect of maxillary constriction treatment.

A relationship between maxillary constriction and hearing was previously proposed to be related to the Eustachian tubes, the middle ear, and mouth breathing.⁸ A correlation was shown between conductive hearing loss and this maxillary condition²³ in subjects between 7 to 40 years with a posterior crossbite and a high palatal vault. The mechanism linking maxillary expansion and hearing improvement may be related to soft tissue changes. The correction of the palatal anatomy can influence muscular function, with the stretching of the elevator and tensor veli palatine muscles, thus allowing for correct function of the tympanic membrane and the auditory system.^24,25 Moreover, there may be a possible association between maxillary expansion and the attenuation of infectious processes.²⁶

Controversially, Ceylan et al.¹⁶ was the only study included that described a complete reversion of hearing improvement after the retention period. In that study, the sample consisted of 11 female and 3 male participants, from 10.3 to 16.8 years of age, and clear information about the hearing problems of patients at the beginning was not presented. The authors associated this reversion with possible relapse in the RME procedure.

Evaluation of the stability of hearing improvement for a longer period was described by three of the studies, with follow-up periods of 10 months,²⁰ 1 year,^14,21 or 2 years.^7,18,19 In all of those studies, improvement in hearing levels was observed at the end of the retention period and became stable after a longer follow-up. In the studies with a 2-year follow-up, a transpalatal arch was used^7,18,19,27 for retention after removal of the maxillary expansion appliance.

Two different types of maxillary expansion were described in the articles: RME^{7,14,16–18,20–22} and SRME.¹⁹ Despite the different protocol of activation in SRME, all included studies reported the use of orthopedic appliances to achieve palatal suture opening, with consequent treatment of the posterior crossbite as a parameter of maxillary expansion success.

The instrument selected for RoB assessment showed a moderate RoB in eight studies evaluated and reported a low risk in two studies^20,21 on the question researched (Table 2). The problems regarding the domains “Bias in Measuring Outcomes” and “Bias in Classifying Interventions” were highlighted. The domain “Bias in Classifying Interventions” was marked as moderate for three articles because of the lack of data regarding hearing problems.^16,18,19 The domain “Bias in Measuring Outcomes” was considered moderate in seven studies because of the absence of a control group.^{7,14,16–19,22}

Seven^{7,14,16–19,22} of the evaluated studies did not include an untreated control group, and none of the studies were randomized. This fault was also cited in other systematic reviews that evaluated additional effects of RME,^23–25 especially regarding long-term evaluations.²⁵ In all of the studies, the steps of palatal expansion were compared: before expansion, after expansion, and/or after the retention period for each subject involved.

In addition, in the studies with a control group, the difference between hearing levels was compared between the RME and control groups. Characteristics of the control groups differed between those two studies. Micheletti et al.²¹ described a control group with maxillary constriction and absence of hearing loss who underwent RME and were observed for the same period as the RME group with hearing loss, and Kiliç et al.²⁰ reported a control group observed for the same period of the case group, without RME or hearing loss.

The isolation of the effects resulting from the expansion and hearing loss in patients with maxillary constriction and hearing loss from the comparison with an untreated group should be an ideal scenario for the accomplishment of this type of study.²⁸ Therefore, more robust studies are needed to clarify the anatomical changes resulting from the expansion of maxilla, which may guide the possible therapeutics of this procedure in other aspects, as for the case of hearing loss.

The similar designs among the studies selected for this systematic review might be related with their origin since five^7,16,18–20 of the nine studies included were from the same department of the same university. Even though an independent sample was described in each study, this kind of conformity can induce confounding effects and increase the RoB in sample selection.²⁹

In addition, poor information was provided about the audiological condition of the subjects. Two studies reported otitis,^14,17,22 and other papers described conductive hearing loss/middle ear dysfunction.^7,16,18–21 In this context, four studies^7,16,18,19 classified the level of hearing impairment before RME intervention but showed no differences in the final hearing assessments. In addition, the type of ear dysfunction described among the studies showed no relationship with the changes in hearing observed. Moreover, most studies lacked a statement on the blinding assessment,^16,18,19,22 and this was probably related to the main topic of this study. Perhaps the blinding of examiners or patients was not feasible because of the uncontrolled nature of the studies and the use of maxillary expansion appliances.

Considering the methods of hearing evaluation, most of them were concentrated on audiometric examinations, with measurements of threshold hearing levels and the air-bone gap. The pure-tone audiogram is considered the gold standard for the assessment of hearing loss,^30,31 aside from measuring the level of recognition of pure-tone sounds at different frequencies.²⁸ The audiometric test was performed in all studies, along with other examinations such as tympanograms,^{14,18,19,21,22} video-otoscopy,²² and impedance testing¹⁷ to evaluate the intrinsic factors associated with hearing loss. Tympanometry evaluates the transmission and pressure of the middle ear and helps to assess changes in the tympanic membrane.^32,33 All of the included studies conducted this evaluation, but statistical analysis was carried out in only two of them.^18,19 The results showed an increase in middle ear volume^14,18,19 and the stabilization of normal pressure levels^21,22 of the tympanic membrane after a qualitative analysis.

Finally, maxillary expansion is a safe treatment to correct maxillary constriction in growing children and adolescents.³ The effects of this treatment in children and adolescents with conductive hearing impairments may contribute to an increase in quality of life.¹⁶ However, there are few previous studies on this theme in the literature, mostly case reports and literature reviews. This systematic review showed that the existing prospective studies exhibited qualitative pitfalls, which suggests the need for further primary studies focused on the additional effects of palatal expansion.

CONCLUSIONS

• The evidence available indicated that there is an improvement in hearing loss after maxillary expansion in children and adolescents with hearing impairments.

• However, more well-conducted studies are necessary to ensure a more reliable conclusion.