Study design

This systematic review was conducted in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) 2020 guidelines [13]. The review protocol defined the research question, eligibility criteria, search strategy, study selection procedures, data extraction, and risk-of-bias assessment prior to the literature search. The study protocol was registered in the OSF Registries on April 27, 2026 (https://osf.io/eqxmz/overview; DOI: https://doi.org/10.17605/OSF.IO/EQXMZ). The PRISMA 2020 checklist is provided in the Supplementary Materials (S1 Checklist).

Although registration occurred after completion of the initial literature search, the review was conducted according to a predefined protocol and no major deviations were introduced during study selection, data extraction, or methodological appraisal. Nevertheless, retrospective registration represents a methodological limitation, as it provides less transparency than prospective registration and may reduce the ability to fully verify that review methods were established before the review process commenced.

The research question for this systematic review was: “What is the association between deafness and the development of mathematical skills in deaf and hard-of-hearing (DHH) children and adolescents?”.

Inclusion criteria and eligibility

The study selection process followed the PICOS framework (Population, Exposure, Comparator, Outcome, Study design), in accordance with PRISMA 2020 recommendations [13]. Eligible studies met the following criteria:

1. Population: Deaf or hard-of-hearing (DHH) children under the age of 18.
2. Exposure: Presence of congenital or acquired hearing loss.
3. Comparator: Hearing peers, with or without specific learning disorders, when available; studies without a comparison group were also eligible if they included direct assessment of mathematical skills in DHH participants.
4. Outcome: Direct assessment of mathematical skills, including numeracy, arithmetic performance, and domain-specific mathematical abilities, as well as related cognitive processes (e.g., working memory, numerical representation, and problem-solving abilities) when directly linked to mathematical performance.
5. Study design: Observational studies, including cross-sectional, cohort, and case-control designs. Both quantitative and qualitative observational studies were considered eligible, provided they examined mathematical skills or related processes in DHH children.

The following study designs were excluded: randomized controlled trials, clinical trials, intervention studies, study protocols, meta-analyses and narrative reviews, case reports, conference papers, letters, commentaries, theses, and posters. These study designs were excluded because the aim of this review was to examine the association between hearing status and mathematical performance in naturalistic conditions. Such designs involve structured interventions that may influence participants’ performance and therefore limit comparability when assessing baseline mathematical competencies.

These inclusion criteria were intentionally defined to ensure methodological consistency across studies, prioritizing observational designs and direct assessments of mathematical skills. Although this approach may have limited the number of eligible studies, it was adopted to enhance the internal validity and interpretability of the findings.

Search strategy

To identify studies examining mathematical skills in deaf and hard-of-hearing (DHH) children, a comprehensive literature search was conducted in the following electronic databases: PubMed (National Center for Biotechnology Information, Bethesda, MD, USA), Scopus, and Web of Science.

The search was performed between March 31 and April 3, 2026, and included all studies published up to the search date. Search strategies combined controlled vocabulary (e.g., MeSH terms) and free-text keywords related to deafness, hearing loss, mathematical skills, numeracy, and arithmetic performance, following PRISMA 2020 recommendations [13]. The search strategy was adapted for each database according to its specific indexing system and search interface. Only studies published in English were considered. Filters related to observational study designs and publication types were applied where supported by the database interfaces, in accordance with the predefined eligibility criteria. In particular, non-primary research articles, including reviews, conference papers, study protocols, book chapters, and other non-eligible publication types, were excluded during the initial screening phase. In addition, the reference lists of included studies and relevant articles were systematically screened to identify further eligible records, as a complementary strategy to minimize the risk of missing relevant studies.

The complete search strings used for each database are reported in Table 1 and represent the full search strategies applied.

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Table 1. Database, search strings and number of articles found.

https://doi.org/10.1371/journal.pone.0358324.t001

Study selection and data extraction

Two authors with at least five years of experience in systematic review methodology (A.T., C.L.) independently screened the titles and abstracts retrieved through the search strategy, focusing on studies examining the relationship between hearing loss and mathematical skills in students. During the first phase, both reviewers independently assessed titles and abstracts and selected potentially relevant records using an intentionally inclusive approach and subsequently evaluated the full texts of all eligible articles to determine their suitability for inclusion. Any disagreements were first discussed and resolved through consensus between the two reviewers. When consensus could not be reached, a third reviewer with expertise in biostatistics (N.S.) was consulted. For the study authored by members of the review team (Laria et al., 2025), screening, data extraction, and methodological quality assessment were conducted by reviewers who were not involved in the original study to ensure objectivity.

Data extraction was performed independently by the same two reviewers using a predefined data extraction form. Extracted information included study characteristics, sample size, study design, assessment tools, and main outcomes. Any disagreements were resolved through discussion and, when necessary, by consultation with the third reviewer.

The extracted dataset underlying the review findings is available in the Supplementary Materials (file S1 Dataset).

Methodological quality assessment

Two reviewers with expertise in systematic review methodology and critical appraisal independently assessed the included studies using the JBI Critical Appraisal Checklists. A third reviewer, a biostatistician (N.S.), was consulted to resolve any discrepancies. The JBI tool provides design-specific checklists, which were applied according to study design: the Checklist for Analytical Cross-Sectional Studies, the Checklist for Case-Control Studies, and the Checklist for Qualitative Research [14]. Each checklist includes items evaluating key methodological domains such as clarity of inclusion criteria, validity and reliability of measurement tools, identification and management of confounding factors, appropriateness of statistical analyses, and completeness of outcome reporting.

A formal assessment of the overall certainty of evidence using the GRADE approach was not performed. This decision was made because the included studies were highly heterogeneous with respect to study design, participant characteristics, assessment instruments, mathematical domains evaluated, and outcome reporting. In addition, the limited number of studies available within individual mathematical domains and the absence of comparable quantitative outcome measures precluded a meaningful assessment of the certainty of evidence across outcomes. Therefore, the overall strength of the evidence was considered through narrative synthesis, taking into account methodological quality, consistency of findings, and limitations identified across the included studies.

JBI critical appraisal tools

The JBI Critical Appraisal Checklist for Analytical Cross Sectional Studies was applied to ten quantitative observational studies: Titus (1995) [15]; Zarfaty et al. (2004) [16]; Rodríguez Santos et al. (2014) [17]; Blatto Vallee et al. (2007) [18]; Lang and Pagliaro (2007) [19]; Ross and Hoemann (1975) [20]; Palma et al. (2010) [21]; Kritzer KL. (2009) [22]; Pagliaro CM (2013) [23]; and Swanwick R (2005) [24].

Notably, two distinct studies by Kritzer (2009) were included: one quantitative [22] and one qualitative [25], which were assessed using different JBI tools.

This checklist includes eight items assessing: (1) clarity of inclusion criteria for the sample, (2) detailed description of study participants and setting, (3) validity and reliability of exposure measurement, (4) use of objective and standardized criteria for outcome measurement, (5) identification of potential confounding factors, (6) description of strategies used to address confounding factors, (7) validity and reliability of outcome measurement, and (8) appropriateness of the statistical analyses employed.

The JBI Critical Appraisal Checklist for Case-Control Studies was applied to the study by Laria et al. (2025) [26].

This checklist comprises ten items assessing: (1) comparability of cases and controls, (2) appropriateness of matching or statistical adjustment, (3) use of the same criteria for identification of cases and controls, (4) validity and reliability of exposure measurement, (5) consistent measurement of exposure for cases and controls, (6) identification of confounding factors, (7) strategies to address confounding factors, (8) validity and reliability of outcome assessment, (9) appropriate statistical analysis, and (10) adequacy of follow-up or exposure period where applicable.

The JBI Critical Appraisal Checklist for Qualitative Research was used to evaluate the qualitative observational study by Kritzer (2009) [25].

This checklist comprises ten items examining: (1) congruity between the stated philosophical perspective and the research methodology, (2) congruity between the methodology and the research question or objectives, (3) congruity between the methodology and data collection methods, (4) congruity between the methodology and data analysis, (5) congruity between the methodology and interpretation of results, (6) positioning of the researcher culturally or theoretically, (7) consideration of the influence of the researcher on the research process, (8) adequate representation of participants’ voices, (9) adherence to ethical standards and evidence of ethical approval, and (10) coherence between the data analysis and the conclusions drawn.

Each item was rated as Yes, No, Unclear, or Not applicable. Methodological quality was interpreted through a qualitative assessment of individual JBI appraisal domains, including participant selection, measurement validity, management of confounding factors, and appropriateness of statistical analyses. Item-level ratings are reported in Table 3 to ensure transparency of the appraisal process.

Overall methodological appraisal categories (e.g., low or moderate methodological concerns) were derived through qualitative consideration of the methodological strengths and limitations identified across JBI domains, rather than through numerical scoring or predefined thresholds. Studies were classified as presenting low methodological concerns when no substantial methodological limitations were identified across key methodological domains and when any remaining limitations were unlikely to materially affect interpretation of the findings. Studies presenting one or more relevant methodological limitations in these domains were classified as having moderate methodological concerns. Particular attention was given to limitations related to confounding control, validity and reliability of outcome measures, and completeness of methodological reporting, as these factors were considered most likely to influence the interpretation of study findings and are directly related to the internal validity and interpretability of observational research. This qualitative categorization was introduced as an interpretative framework to facilitate a consistent and transparent synthesis of methodological quality across studies and does not reflect an official scoring system defined by the Joanna Briggs Institute.

Given the inclusion of both qualitative and quantitative studies, different JBI appraisal tools were applied according to study design. Although these tools assess distinct methodological aspects, methodological judgments were derived through qualitative appraisal of key methodological domains to facilitate a consistent interpretation across studies.

Methodological quality assessment

The methodological quality of the twelve included studies was assessed using the Joanna Briggs Institute (JBI) Critical Appraisal Tools, applying the checklist most appropriate to each study design. Specifically, the JBI Analytical Cross-Sectional Checklist (8 items) was used for cross-sectional and comparative observational studies, the JBI Case-Control Checklist (10 items) for the case-control study, and the JBI Checklist for Qualitative Research (10 items) for the qualitative study.

For studies assessed with the JBI Analytical Cross-Sectional Checklist, the reported items were: I1, inclusion criteria clearly defined; I2, study subjects and setting described in detail; I3, exposure measured in a valid and reliable way; I4, objective and standard criteria used for measurement of the condition; I5, confounding factors identified; I6, strategies to deal with confounding factors stated; I7, outcomes measured in a valid and reliable way; and I8, appropriate statistical analysis used.

For the JBI Case-Control Checklist, the reported items were: I1, groups comparable other than the presence of disease/exposure; I2, cases and controls appropriately matched; I3, same criteria used for identification of cases and controls; I4, exposure measured in a standard, valid, and reliable way; I5, exposure measured in the same way for cases and controls; I6, confounding factors identified; I7, strategies to address confounding factors stated; I8, outcomes assessed in a standard, valid, and reliable way; I9, exposure period sufficiently long to be meaningful; and I10, appropriate statistical analysis used.

For the JBI Checklist for Qualitative Research, the reported items were: I1, congruity between the stated philosophical perspective and the research methodology; I2, congruity between the research methodology and the research question or objectives; I3, congruity between the research methodology and the methods used to collect data; I4, congruity between the research methodology and the representation and analysis of data; I5, congruity between the research methodology and the interpretation of results; I6, statement locating the researcher culturally or theoretically; I7, consideration of the influence of the researcher on the research and vice versa; I8, adequate representation of participants and their voices; I9, ethical approval or evidence of ethical considerations; and I10, conclusions supported by the analysis and interpretation of the data.

The appraisal results are presented in Table 3, which reports the individual checklist items and their ratings (Yes, No, or Unclear) for each study, together with methodological notes that support interpretation of the appraisal findings.

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Table 3. Methodological appraisal of included studies using the JBI Critical Appraisal Tools. Individual checklist items are reported as Yes (Y), No (N), or Unclear (U).

https://doi.org/10.1371/journal.pone.0358324.t003

Overall, the methodological rigor of the included studies was heterogeneous. Most studies fulfilled several applicable JBI criteria, particularly those related to the clear description of participants and study settings, the use of clearly defined outcome measures, and the application of appropriate statistical analyses. However, methodological limitations were identified across the body of evidence.

The most frequently observed concerns included small sample sizes, limited consideration or management of potential confounding factors, and incomplete reporting of methodological procedures. In several studies, confounding variables were insufficiently addressed or not explicitly controlled for, thereby reducing confidence in the interpretation of group differences. Small sample sizes were particularly common among studies involving deaf and hard-of-hearing participants and may have limited statistical power and generalizability.

Additional limitations included incomplete descriptions of participant selection procedures, insufficient information regarding the validity or reliability of some assessment measures, and restricted reporting of methodological details. Furthermore, studies published in earlier decades reflected reporting standards that were less comprehensive than those expected in contemporary research, making the assessment of some methodological domains more challenging.

Despite these limitations, many studies demonstrated important methodological strengths, including clearly defined participant groups, standardized assessment procedures, appropriate comparative designs, and transparent reporting of analytical methods. The detailed presentation of individual JBI appraisal items in Table 3 allows transparent evaluation of the methodological strengths and limitations of each study and facilitates interpretation of findings within the context of the overall evidence base.

No studies were excluded on the basis of methodological appraisal alone. Instead, the results of the quality assessment were considered during the interpretation and synthesis of findings, allowing potential sources of bias and methodological limitations to be taken into account when evaluating the overall body of evidence.

Table 3 summarizes the methodological appraisal of the included studies, including the JBI appraisal tool applied, item-level ratings for each checklist domain, and the overall methodological appraisal assigned through qualitative evaluation of study strengths and limitations.

As reported in Table 3, the methodological quality of the included studies was generally characterized by moderate methodological concerns. Most studies demonstrated methodological strengths in several JBI appraisal domains, particularly in the description of participants and study settings, the definition of hearing status, the use of consistent outcome assessment procedures, and the application of appropriate statistical analyses.

Across the included studies, the most frequently identified methodological limitation concerned the management of potential confounding factors. Although many authors acknowledged variables that could influence mathematical performance, explicit strategies to control or adjust for these factors were often absent. In addition, several studies provided limited information regarding the validity and reliability of the assessment tools used, resulting in unclear ratings for this domain.

Two studies, conducted by Rodríguez-Santos et al. [17] and Lang et al. [19], were judged to present low methodological concerns, reflecting clear participant selection criteria, appropriate consideration of potential confounding variables, consistent outcome measurement procedures, and robust statistical analyses.

The remaining ten studies were classified as presenting moderate methodological concerns. These concerns were mainly related to limited control of confounding variables, incomplete reporting of measurement validity or reliability, and small sample sizes, which may reduce the generalizability of findings.

No studies were classified as presenting high methodological concerns. Accordingly, no studies were excluded on the basis of methodological quality. Instead, the results of the methodological appraisal were considered during the interpretation and synthesis of the evidence, allowing potential sources of bias and study limitations to be taken into account when evaluating the overall findings of the review.

Limitations

This review has some limitations that should be considered when interpreting the findings. First, although the review aimed to identify the available studies in the literature addressing mathematical competencies in deaf and hard of hearing (DHH) children under the age of 18, the overall number of eligible studies was limited. This primarily reflects the current state of the evidence base, although the use of strict eligibility criteria may also have contributed to the limited number of included studies. Nevertheless, it highlights the need for further research in this population.

Second, the included studies showed substantial heterogeneity in terms of sample characteristics, assessment tools, mathematical domains examined, and reporting practices. This variability limited direct comparability across studies and precluded the possibility of conducting a quantitative synthesis. In particular, mathematical performance was assessed using a wide range of non-comparable instruments (e.g., standardized tests, task-based experimental measures, and qualitative assessments), and effect size data were not consistently reported across studies. In addition, the use of strict inclusion criteria may have led to the exclusion of potentially relevant studies, although this choice was made to ensure greater methodological homogeneity and interpretability of the findings.

Furthermore, several studies were characterized by small sample sizes and incomplete reporting of relevant confounding variables, such as linguistic background, socioeconomic status, and educational context. These factors may have contributed to variability in the reported outcomes and constrained the generalizability of individual findings. Given the observational nature of the included studies, the findings should be interpreted as evidence of associations rather than causal relationships. Therefore, it is not possible to determine whether hearing status, language accessibility, educational context, or other related factors directly influence mathematical performance in DHH children and adolescents. Finally, some older studies did not fully meet contemporary standards of methodological transparency, which influenced quality ratings but also reflects historical differences in research practices.

The search strategy was conducted using PubMed, Scopus, and Web of Science, which provide broad multidisciplinary coverage of biomedical, clinical, and educational research. However, additional databases specifically focused on education and psychology, such as ERIC and PsycINFO, were not searched. Consequently, some potentially relevant studies may not have been identified and this should be considered when interpreting the completeness of the evidence base. In addition, the search strategy did not include grey literature sources, which may have limited the identification of some potentially relevant studies.

Despite these limitations, this review provides a structured overview of the available evidence and identifies consistent patterns that can inform future research and educational practice. The methodological appraisal indicated that most included studies presented moderate methodological concerns, particularly regarding the management of confounding factors and the reporting of measurement validity and reliability. These limitations should be considered when interpreting the overall strength of the evidence.