Bibliometric analyses provide relevant information to serve as a basis for understanding the evolution and state of the art of a given field of research. Citation metrics, first introduced by Eugene Garfield in 1955,1 have been used to highlight the contribution of investigators, research groups, institutions and countries to each discipline, and to identify the main topics of research interest. Two of the most widespread citation metrics are the impact factor (IF) ,2 commonly employed to categorize and rank scientific journals, and the h-index.3 The h-index was developed by Jorge E Hirsch in 2005 to provide a qualitative rather than quantitative appraisal of the research impact of individual investigators, although it may also be applied to institutions, countries or even whole fields of research. The h-index of an author is defined as the maximum value of h, such that the author has published h papers that have each been cited at least h times.3 Thus, an author with an h-index of 12 has published at least 12 papers which have received 12 cites each or more.

In the discipline of optometry and vision sciences, bibliometric analysis gained momentum with a series of papers by Efron, Brennan and Nichols published between 2011 and 2013,4-6 followed by papers by Cardona and Sanz in 2014 and 2015, the latter exploring the citation parameters and topics of research in the contact lens field.7,8 A recent proliferation of publications has evidenced a renewed interest in citation metrics in areas of research such as contact lenses,9,10 visual therapy,11 refractive error,12 vision and sports,13 meibomian gland and dry eye,14,15 orthokeratology16,17 and scleral lenses,18,19 amongst others.

Of note, in a seminal paper published in 2021, Efron and co-workers identified the top 200 leading optometric researchers worldwide.20 These authors maintain and curate a version of the Global Optometrist Top 200 Research Ranking (available at https://optomrankings.com/), with live updates every 24 h.21 As of July 2021, the ranking provided by Efron and co-workers identifies seven Spanish optometrists in the top 200, of whom Robert Montés-Micó has the highest h-index (47, with a global ranking of 16/200) and David Piñero, with 320 papers since 2003, is the most prolific author (h-index 37, global ranking of 52/200). Overall, Spain as a country occupies the fifth position in number of optometrists in the top 200 and country h-index, after countries with a longer and well-established research tradition in optometry and vision sciences such as USA, Australia, UK and Canada.

Difficulties may arise when comparing the scientific outcome of different countries, given the particular characteristics of each country in terms of number and antiquity of research institutions, types of research and tenure-track career paths, public and private research funding, degree of co-operation and relationship between ocular health and vision care specialists, overall social consideration and implementation of optometry as a health profession, or even main language used for scientific communication. It was therefore the aim of the present study to conduct a bibliometric analysis of the optometry and vision sciences related publications authored by optometrist researchers from Spain in order to identify the most prolific authors, the institutions where they pertain, their topics of research interest and their preferred target journals.

The Scopus search engine (Elsevier, www.scopus.com) was employed for this analysis. The study was conducted between the months of May and July 2021 and aimed at identifying authors with a Scopus h-index > 10. To generate an initial list of authors, the Scopus search engine was queried with the following search terms:

TITLE-ABS-KEY(optometr* OR "contact lens*" OR refracti* OR *ocular) AND (LIMIT-TO (SRCTYPE,"j")) AND (LIMIT-TO (AFFILCOUNTRY,"Spain")) AND (LIMIT-TO (SUBJAREA,"MEDI") OR LIMIT-TO (SUBJAREA,"HEAL"))

This search strategy returned a total of 8621 documents. In order to identify researchers with an h-index > 10, the author list of these documents was retrieved, and authors with a number of publications of 20 or more were selected (159 authors). Although this number was arbitrarily chosen, it was considered that with less than 20 articles it was highly improbable for an author to reach an h-index > 10. Next, the author profile provided by Scopus of each of the selected researchers was accessed to manually collect the following information: h-index, number of published articles, number of citations, number of different co-authors, affiliation and ORCID identity (if available). In this regard, Scopus links institutions to authors according to the information provided by their most recent publication.

As the aim of this study was to identify Spanish optometrist researchers working in Spanish institutions, secondary internet searches based on author names discovered during the primary search were conducted to exclude authors not working in Spain, not of Spanish nationality and not optometrists. Authors with multiple degrees (e.g., optometrist and physicist; optometrist and biologist; optometrist and ophthalmologist; optometrist and pharmacologist) were not excluded from the analysis as long as their main body of publications were related to the visual sciences.

In addition, to discover potential authors missed by the primary Scopus search, the list of the top contributing co-authors of each investigator was manually reviewed. Similarly, once the most relevant institutions were identified by the outcome of the primary search, the online directory provided by the website of each institution was consulted, whereupon all investigators not included in the primary search outcome were submitted to a further Scopus analysis to determine their h-index and scientific outcome. Finally, authors with h-index > 10 were ranked in descending order of h-index values and, when necessary, ties were broken by referring to the total number of published papers and number of citations.

After the list of optometrists with an h-index > 10 was completed and refined, the top 10 most highly cited articles of each author were retrieved. Each article was then accessed to collect the name of the journal where it was published and the key word list provided by the author of that article. In those articles not containing specific key word information, the title and abstract were reviewed to determine the main topic or topics of research. The resulting compilation of target journals and key words was employed to determine the top 10 journals of choice for Spanish optometrist researchers and the 10 topics reflecting the highest research interest.

The primary and secondary search strategies revealed a total of 32 authors with an h-index > 10 (Table 1). The author with the highest h-index was Robert Montés-Micó, from the Universitat de València, with an h-index of 47 (year of first publication 1997), followed by David Piñero, from the Universitat d'Alacant, with an h-index of 33 (year of first publication 2003) and by Teresa Ferrer-Blasco, from the Universitat de València, with an h-index of 28 (year of first publication 2000). These same investigators, but in a different order, were the most prolific authors, with 311 publications authored by David Piñero (attracting 4765 citations), 304 by Robert Montés-Micó (7717 citations) and 146 by Teresa Ferrer-Blasco (2758 citations). David Piñero had shared his publications with the largest number of different, unique co-authors (416), followed by Marc Biarnés (298) and César Villa-Collar (242). Overall, 5 (15.6%) authors had an h-index between 20 and 29 and 2 authors (6.2%) had an h-index ≥ 30. Almost all authors were registered to ORCID.

A total of 14 (43.7%) investigators with an h-index > 10 were female. The highest-ranking female on the list was Teresa Ferrer-Blasco, from the Universitat de València, followed by Assumpta Peral, from the Universidad Complutense de Madrid, with an h-index of 21 (39 papers, 1118 citations, 55 co-authors) and Eva Yebra-Pimentel, from the Universidad de Santiago de Compostela, with an h-index of 19 (82 papers, 1167 citations, 91 co-authors).

The top 10 most frequently selected key words, as determined by their occurrence in the 10 most highly cited documents of each author, are summarized in Table 2. The main topics of interest of Spanish researchers were topography (6.5%), cornea (6.2%) and contact lens (5.0%), followed by contrast sensitivity (4.8%) and myopia (4.7%). The most highly cited articles and the most recent articles of the top 10 Spanish optometrist researchers are listed in Table 3.

The top 10 target journals for the most highly cited papers are shown in Table 4, which also provides information on their IF and website, as well as the number of top 10 highest cited articles published there. The Journal of Cataract and Refractive Surgery (IF 2020 = 3.351; 18/62 in the category of Ophthalmology of the Journal Citation Reports, Science Citation Index Expanded) was the journal of choice for 61 (19.1%) of the top 10 highest cited papers of Spanish optometrist researchers with h-index > 10, followed by Optometry and Vision Science (IF 2020 = 1.973; 44/62), with 33 (10.3%) articles, and Journal of Refractive Surgery (IF 2020 = 3.573; 15/62), with 31 articles (9.7%). The journal with the highest IF was American Journal of Ophthalmology (IF 2020 = 5.258; 6/62), which published 10 articles (3.1%) of the top 10 most highly cited papers of all authors under consideration. Overall, 82 articles (25.6%) were published in journals located at the first quartile (Q1) of the Ophthalmology category of the Journal Citation Reports and 154 articles (48.1%) in journals located at the second quartile (Q2). Only 3 (0.9%) of the top 10 highest cited papers of all authors were published in Journal of Optometry, which in 2021 was first included in the Emerging Sources Citation Index (ESCI) of the Journal Citation Reports, within the category Ophthalmology, with a Journal Citation Indicator (JCI) of 0.77 and with a total of 566 citations.

Finally, the top 5 institutions to which the identified authors were affiliated are summarized in Table 5. With only 3 exceptions, all authors were affiliated to universities. The Universidad Complutense de Madrid hosted the largest number of authors (6, 18.7%), followed by the Universitat de València (5, 15.6%) and the Universidad de Valladolid (4, 12.5%).

A bibliometric analysis of publications authored by Spanish optometrist researchers provided evidence to support the notion that Spain is rapidly becoming a country of reference in the field of research in optometry and vision sciences, in the tradition of countries with more established optometric research profiles such as USA, Canada, UK and Australia.

Primary and secondary search strategies identified 32 Spanish optometrists with an h-index > 10, mostly affiliated to public universities. Of these authors, 7 had an h-index ≥ 20, representing a 3.5% of the top 200 researchers worldwide, headlined by the Australian researcher Konrad Pesudovs, with an h-index of 70.

Topics of research interest of Spanish optometrists were widespread and revealed common over-arching themes similar to those described by Efron and co-authors,20 thus suggesting that optometry research in Spain is fully integrated within the international community of investigators. Indeed, many Spanish researchers have an extensive network of co-authors from international institutions: for instance, Marc Biarnés, from the Barcelona Macula Foundation, has 298 co-authors, of whom more than half are not Spanish.

It must be acknowledged that differences between Scopus and other search engines such as Web of Science (Clarivate) or Google Scholar (Google) may yield slightly different values of h-index and thus, may introduce changes to the list of authors, both in their overall rank and in their actual inclusion in the list of top authors, particularly in the lower tiers. For example, one of the main reasons for choosing Scopus was that this search engine defines Optometry as a stand-alone category, whereas Web of Science only contemplates the category Ophthalmology, also implementing a more limiting criterion for journal inclusion, based on IF, than Scopus, which includes a wider range of Optometry journals, probably providing a more accurate representation of the output of optometrist researchers. Similarly, to determine h-index, Google Scholar also considers citations on sources different from journals, including books, doctoral theses, and even some websites. In addition, a different primary search equation may have retrieved other authors, although it may be assumed that the combination of primary and secondary search strategies would have identified similar investigators to those listed in the present study. Also, it must be noted that Scopus only considers citations accrued from 1970 onwards, although this limitation may be safely ignored in this study.

The results of this citation analysis revealed many young researchers with h-index values of or near 10, who at the time of publication may have already achieved an h-index of 11. Given the progressive difficulty to reach high h-index values, established authors are probably more stable in their rank than those in the lower tiers. The aim of this study was to portray the state of the art at the time of writing (July 2021), and results would benefit from a periodically updating system such as that offered in www.optomrankings.com21. Besides, although particular care was taken to complement the primary search outcome with a manual secondary search based on co-authors and institutions, the authors of this analysis would like to apologize for any unwanted omission and error in their results.

Another aspect to consider is affiliation. Scopus determines affiliation according to the information provided by authors in their last publication. It may be the case of recent changes in institution in which undue credit is given to the new host institution in detriment of the one where the author had conducted his or her main body of research. A similar concern may arise about the non-inclusion of Spanish authors currently working abroad who, until now, had undertaken their research in Spanish institutions.

It is interesting to note that of the top 10 journals of choice for Spanish investigators, 4 of them are optometry or contact lens journals: Optometry and Vision Science (American Academy of Optometry), Ophthalmic & Physiological Optics (British College of Optometrists), Contact Lens and Anterior Eye (British Contact Lens Association) and Clinical Experimental Optometry (Optometry Australia, the New Zealand Association of Optometrists, and the Hong Kong Society of Professional Optometrists). None of these are currently located in the first quartile of the Journal Citation Reports, which is an important consideration when deciding target journals, as journal rank of authored publications has direct implications in the selection processes and tenure track progress of investigators. It may be that the topic of research outweighs journal rank when selecting target journals for those studies more closely related to optometry, contact lenses and vision. Although only 3 of the 320 most highly cited papers were published in Journal of Optometry, the recent inclusion of this open access journal in the Emerging Sources Citation Index (ESCI) of the Journal Citation Reports will probably encourage more top researchers to submit their work to this journal.

The h-index was selected for this analysis. Although this index has been described as superior to other citation metrics to assess the publication output of individual investigators, it is nevertheless not free of limitations. Some of these limitations were highlighted by Efron and co-workers in their investigation.20 For instance, the h-index may favor authors with a longer career, as longer careers may result in more publications and more time for these publications to reach the required number of citations. Similarly, a large number of co-authors from different institutions may lead to increased citation and h-index inflation, an overestimation that has led some authors to suggest the fractional allocation of citations amongst co-authors (h-frac) .22

Also, different subdisciplines may have different scope and impact: a recent bibliometric analysis of different fields of research within the ophthalmic literature demonstrated that dry eye, contact lens and refractive error publications have different metrics in terms of subdiscipline h-index, time span of publication, citations and even percentage of uncited papers.23 Finally, even though optometrists with more than one degree were only included in the analysis if their main body of research was in the optometry field, working on multiple fields may expand the network of co-authors and research interests to other areas with superior metrics, probably leading to an overestimation of the actual contribution of these investigators to optometry research.

In conclusion, the citation analysis of the research production of Spanish optometrists has demonstrated the excellent health of optometry research conducted in Spanish institutions, with a growing number of prolific authors publishing in high impact journals and making important contributions to the field. Although citation metric parameters are not without limitations, this type of analysis is useful to determine the impact and scope of a field of research and may be of interest to new and experienced researchers, librarians, advisory bodies, optometry associations and general public.

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.