Influencing variables in the quality of life of children with cochlear implants: a systematic review

Accepted: March 03, 2020 Study conducted at the Faculdade de Odontologia de Bauru – FOB, Universidade de São Paulo – USP Bauru (SP), Brasil. 1 Faculdade de Odontologia de Bauru, Universidade de São Paulo – USP Bauru (SP), Brasil. Financial support: Coordenação de Aperfeiçoamento de Pessoal de Nível Superior Brasil (CAPES) Código de Financiamento 001 e Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) – Número de Processo: 2016/24216-0. Conflict of interests: nothing to declare. ABSTRACT


INTRODUCTION
Bilateral hearing losses of severe and / or profound degree occurring in childhood, especially in the pre-lingual phase, potentially restrict the communicative development with significant losses in the psychosocial, cognitive, emotional, academic areas, in addition to the quality of life of this population (1,2) .
The cochlear implant (CI) is considered an important technological resource that is highly effective in rehabilitation children with pre-lingual hearing loss who do not have benefits with the use of the hearing aid (HA) (2)(3)(4) . However, surgery and CI adaptation alone do not guarantee the full benefit of its users. Several variables can interfere with the performance and quality of life of implanted children, such as: etiology, age at surgery and CI activation, time of auditory sensory deprivation, preoperative auditory residue, the number of electrodes inserted in the cochlea, the time of daily use of the device, insertion in specialized speech therapy based on the aurioral approach and family involvement in the therapeutic process (4)(5)(6)(7)(8)(9)(10) .
This range of variability makes the implantation process complex, multidimensional and creates challenges in understanding the reasons why some children achieve better results in assessment, development, and quality of life tests when compared to the evolution of other children with the same criteria for the indication and adaptation of the CI (4,6,11,12) .
In order to guarantee better development results in children with hearing loss, studies suggest that the therapeutic process with electronic devices should be started early, associated with appropriate measures of habilitation with specialized speech therapy, augmented with intense planning of family guidance and counseling. This premise accompanied by other variables can influence the development of auditory skills, the process of acquisition and development of spoken communication, in addition to the insertion and participation of children in different environments, with better results on self-confidence, autonomy, well-being, happiness, schooling processes, socialization and the quality of life of these children and their families (4,6,10,11,(13)(14)(15) .

PURPOSE
The present study aimed to conduct a systematic review based on verifying how the quality of life of children with cochlear implants was evaluated in published studies and critically analyzing what were the influencing variables.

RESEARCH STRATEGY
The present systematic review was conducted according to the criteria guided by the Cochrane Handbook (16) library, based on the PRISMA method (Preferred Reporting Items for Systematic Reviews and Meta-Analyzes) (17,18) , which recommends the use of the following stage: (I) identification, (II) selection, (III) eligibility and (IV) inclusion.
The guiding question responsible for conducting search strategies aimed at studies with high scientific evidence was developed based on the research theme and was guided by the PICO strategy (Population, Intervention, Comparison, Outcome). Thus, the following question was established: What variables are considered to influence the quality of life of children with cochlear implants?
To search for the articles, the descriptors and synonyms listed in the indexing vocabulary of PubMed, Medical Subject Headings (MeSH terms) and in the library of Health Science Descriptors (DeCS) were selected, in the languages: Portuguese, English and Spanish (Chart 1). Table 1 presents the advanced search strategies used in the search for articles, organized based on combinations of descriptors and synonyms using the "AND" bullet operator, and the number of studies identified (stage I) in the bibliographic survey in each one of the selected and accessed electronic databases: Pubmed / MEDLINE, LILACS, Scopus, SciELO, Embase, EBSCO / CINAHL and Web of Science.

SELECTION CRITERIA
The selection of studies (stage II) was conducted by two researchers, independently and blindly, following the following  (19) for the speech therapy course (Chart 2). The following were excluded: 1) repeated studies; 2) book chapters; 3) studies with incomplete abstract or article; 4) studies that did not address the proposed theme; 5) studies with subjects over 18 years old; 6) studies carried out with children with Auditory Neuropathy Spectrum Disorder, hypoplasia of the auditory nerve, malformation of the external, middle or internal ear, unilateral deafness, children who are not oralized or with other impairments associated with hearing loss; 7) studies carried out with children not using a CI; 8) studies with children adapted only with simultaneous bilateral CI; 9) studies with the main objective of investigating quality of life and / or comparing groups (elderly, adults, users of other electronic devices, non-oral children, children with normal hearing), without the objective of investigating the influencing variables on the development of the group of children with CI; 10) studies with results acquired only from subjective impressions without objective data and standardized protocols.

Total
-1062 Chart 2. Levels of scientific evidence according to the criteria proposed by ASHA.

Levels of evidence Study types 1a
Systematic review or high-quality meta-analysis of randomized controlled trials 1b High quality randomized controlled trials 2a Systematic review or high-quality meta-analysis of non-randomized controlled trials 2b High-quality, nonrandomized controlled trials 3a Systematic review of cohort studies 3b Individual cohort studies or low quality randomized controlled trials 4 Clinical outcome studies 5a Systematic review of case control study 5b Individual control case study 6 Case series 7 Expert opinion without explicit critical evaluation Source: ASHA (19) . and abstracts, the texts that left doubts about the eligibility criteria were read in full. Figure 1 represents the details of the selection process for publications included in the systematic literature review.

RESULTS
From the rigorous reading of the 34 studies chosen to read in full in the inclusion stage (IV), eight articles were classified as containing the necessary answers to the research question.
The details of the characteristics of the articles included in the systematic review of the literature in relation to the authors, title, level of scientific evidence, case series and age group, objectives, protocols used and variables evaluated, occurred through a standardized protocol sheet used in order to facilitate analyzing the data and retrieving the details relevant to the research ( Table 2).
The eight selected articles were published between 2009 and 2016 and classified as evidence level 4, with the exception of the inclusion of a systematic review with evidence level 2a. In clinical research focused on the field of audiology, there are few studies that present the recommended designs for a systematic review (levels 1 and 2). The series of systematic reviews ranged from ten to 259 children, of both sexes, aged between 18 months and 18 years of age. With regard to methodological quality, all works used validated tools to study the quality of life of children with cochlear implants and the influencing variables.  According to the World Health Organization (WHO) (20) , quality of life concerns "the individual's perception of his insertion in life in the context of the culture and value systems in which he lives and in relation to his goals, expectations, standards and concerns". This definition indicates the complex relationship between the multifactorial aspects that involve physical, mental, psychological, emotional and spiritual wellbeing, social, family and affective relationships, health, school and education, housing, past experiences, current and future of individuals in the face of cultural contexts and ethical and moral values, and other circumstances of life (15) .
In children with cochlear implants, in addition to the dimensions discussed above, the study of quality of life also encompasses a range of aspects related to self-esteem, self-image, and selfconfidence built upon the diagnosis of hearing impairment, satisfaction with the use of a CI, independence, self-sufficiency, self-care, mobility, pain, the use of cell phones and hearing aid devices, preferences, auditory perception and understanding of speech, communication, and also, the attitude of others towards all these factors (25,26) . In the present systematic review, we note the investigation of aspects of the physical, psychological, emotional and social quality of life of children with cochlear implants, preferentially influenced by demographic variables, auditory skills and language skills.
Demographic variables influencing quality of life were identified in four studies (23)(24)(25)28) . In the analysis between sex, socioeconomic level, chronological age, age at hearing aid adaptation, age at CI activation, hearing age (time of using electronic devices), and quality of life performed in the first study (27) , the variable age at the use of the first amplification (hearing aid) revealed a statistically significant negative correlation with quality of life, reinforcing that the lower the age at the hearing aid adaptation, the greater the quality of life. Taking into account that, the children who received the hearing aid earlier also had the longest time of use of the device, the highest hearing ages correlated in a statistically significant positive way with the highest studied quality of life indexes.
Considering that in Brazil the possibility of newborn hearing screening (NHS) and early diagnosis of hearing loss are ensured by Law No. 12,303 / 2010 (29) and Ordinance No. 587/2004 (30) that institutes the National Policy for Hearing Health Care, this result reinforces the importance of the NHS programs to be solidly articulated with the intervention programs and speech therapy that precede cochlear implant surgery, which will allow these children to use the amplification early and continuously, culminating in a better quality of life over time.
The early intervention with a positive impact on quality of life was also demonstrated in a systematic review study (25) in which the results of the selected articles demonstrated that children who underwent CI surgery at an earlier age had statistically significant negative correlations with quality of life. Children with older hearing age and older chronological age obtained statistically significant positive correlations related to quality of life.  On the other hand, studies carried out comparative analyzes between demographic factors: age at activation, chronological age, hearing age and quality of life (24) ; and the auditory age and quality of life of children with a CI 23 , and found no statistically significant correlations. However, in the last study (23) when comparing the groups organized according to the hearing age (G1: children with hearing age greater than 24 months, and G2: children with hearing age less than 24 months), the statistically significant difference between the groups confirmed the positive interference of time and daily use of CI on the specific domain of communication in the investigation of the assessment of quality of life in the group of children with older hearing age.
Of the eight studies selected, the auditory skills variable was examined and compared with the quality of life in seven studies (21)(22)(23)(25)(26)(27)(28) . It is natural that this variable is widely studied in view of the fact that the development of auditory skills is pointed out in the specialized literature as the main prerequisite for the acquisition of spoken language (3) , which in turn impacts on quality of life (23,24,26) . The studies investigated in a systematic literature review (25) revealed that the early development of hearing skills has statistically significant relationships with quality of life assessments. In the same vein, researchers (26) assessed the development of the hearing skills of children with a CI and verified the presence of a statistically significant correlation with the communication domain in the assessment of quality of life.
Other evidence (27) points to the influence of hearing skills on quality of life after using a CI, without statistically significant differences between the groups of children using unilateral a CI and users of sequential bilateral CI. However, it reveals statistically significant better results for the adapted group with bilateral CI in tests of discrimination, location, movement tracking and speech perception in noise when compared to the group with the unilateral CI.
The researchers' attention also turned to a refined analysis of hearing skills after using an image-guided cochlear implant programming technique, called Image-Guided Cochlear Implant Programming -IGCIP (22) . Such strategy aims at the detailed analysis of the Computed Tomography images of patients with a CI in order to estimate the exact position of the electrodes in the cochlea in relation to the auditory nerve to identify the electrodes with high levels of overlap and stimulation, and to disable them when required. This technique allows the creation of more personalized CI maps, promotes the improvement of stimulation and, consequently, the auditory perception of speech. The results of the study (22) showed a statistically significant increase in the quality of life of children with a CI due to the improvement in the auditory perception of speech through the use of the referred programming technique. Although it is a technology not used in all cochlear implant centers, such results are encouraging to enable technological advances in the routines for monitoring implanted children, whether in the public or private system.
Considering that the results of the cochlear implant in children are multidimensional and multifactorial, some variables can be strongly manifested in the quality of life of some children, and not in others. An example of this was observed in studies (21,23,28) in which no statistically significant correlations were found between auditory speech perception and quality of life. Even so, in one of the aforementioned studies (28) when investigating the specific ability to perceive vocal emotion in non-linguistic sounds, a statistically significant correlation was observed with the highest quality of life scores.
Spoken language is also pointed out in the specialized literature as one of the possible variables that impact the quality of life of children with cochlear implants. In the present systematic review, the correlation between spoken language skills and the quality of life of children with a CI was described by three studies (25,26,28) . The first authors (26) examined the development of lexical and speech skills and found statistically significant correlations between these skills and the domains of quality of life. The studies identified in a systematic review (25) agreed with these findings and pointed out that the early development of language and communication skills correlates in a statistically significant way with quality of life. On the other hand, researchers (28) who investigated verbal cognitive performance and language skills found no statistically significant correlations with quality of life.
There is diversity in the literature regarding the variables that influence and predict the quality of life of children with a CI. In the present study, the heterogeneity of ages in CI surgery, the auditory ages and the chronological ages of the evaluated population, as well as the use of different instruments for investigating hearing, language and quality of life skills, may justify the limitation in the consensus between the results obtained.
Another possible justification may be related to the fact that, in the pediatric and child population, assessments of quality of life commonly occur through the parents' view, either due to the scarcity of standardized, specific and sensitive instruments to the children's perception, or due to the difficulty of capturing precise answers in this audience. In the present study, six (21)(22)(23)(24)26,27) of the identified articles assessed children's quality of life from the parents' perspective, and only two (25,28) took into account the children's self-perception. Considering the scope and subjectivity of the concept of quality of life, it is natural that, in some cases, parents' perceptions when compared to those of CI users themselves, present divergences. Despite the scarce results in the literature, it is believed that, when compared and / or combined with parents' perceptions, children's views on their own quality of life may contribute to the expansion of the quality of life research scenario and its influencing variables.
Taking into account the complexity in which the therapeutic process aimed at children with cochlear implant involves, the fact that there is no consensus among the selected studies does not minimize the value of the results, which demonstrated the impact of objective clinical measures on subjective development and the quality of life of children with cochlear implants since the clinical variables analyzed related to early implantation, the use of the electronic device, the best hearing and language skills correlated with the increase in the quality of life of this population. It is understood that the complexity of the CI indication, implantation and follow-up process hinders the expected balance between the clinical variables and quality of life. Furthermore, despite the positive results described in the literature, it is not possible to say that the control of these factors will guarantee the best auditory, language and quality of life results. Finally, it is noteworthy that no studies were found to investigate the correlation between the quality of life of children with a CI and the variables: family permeability, parental education, and socioeconomic aspects, factors that deserve attention and justify the performance of new research in the area.

CONCLUSION
The influencing factors that correlated with the quality of life of children with cochlear implants in the selected studies were early implantation, the use of the electronic device, auditory skills and language skills.
The present systematic review adds to the previously published bibliographic findings on the topic, identifies additional texts and suggests the relevance of further research in the area. It is hoped that the present study will help to understand the importance of investing in the influencing variables in the development and quality of life of children with a CI, resulting in practical investments in the pre and post-surgical clinical speech therapy routine of these children.