ABSTRACT

Schirmer strips and conjunctival swabs are used in ophthalmology for the collection of tears and fluids. One of the biggest challenges during the COVID-19 pandemic has been accurate diagnosis and, in some cases, ocular manifestations are among the first symptoms. In this context, this study aimed to collect evidence to support the use of Schirmer strips and conjunctival swabs as a method of sample collection for viral analysis. A literature search was conducted following the Scoping Review protocol defined by The Joanna Briggs Institute. Studies were analyzed regarding virus research, collection methods, and sample analysis. The findings support that viruses can be detected on the ocular surface through analysis of Schirmer strips and conjunctival swabs. However, additional studies with larger samples and time data are necessary to confirm these conclusions.

Keywords: Antigen, surface/isolation & purification; Conjunctiva; Tears; Eye protein/analysis; Specimen handling; Polymerase chain reaction/methods; COVID-19; Eye manifestations

RESUMO

A fita de Schirmer e o swab conjunctival são utilizados na oftalmologia como métodos de coleta para lágrimas e fluidos. Durante a pandemia da COVID-19, um dos desafios foi o diagnóstico correto e se sabe que, em alguns casos, as manifestações oculares podem ser um dos primeiros sintomas. Nesse contexto, este estudo tem como objetivo levantar evidência que destaque o uso de fitas de Schirmer e de swabs conjuntivais como método de coleta para análise viral. Conduziu-se uma revisão de literatura seguindo o protocolo para Scoping Review definido pelo Joanna Briggs Institute. Os pesquisadores analisaram os estudos em busca do vírus pesquisado, os métodos de coleta e os métodos de análise. Vírus podem ser detectados na superfície ocular através da análise de fitas de Schirmer e de swabs conjuntivais, entretanto novos estudos com populações maiores e com definições claras de tempo são necessários para conclusões mais assertivas no tema.

Descritores: Antígeno de superfície/isolamento & purificação; Túnica conjuntiva; Lágrimas; Proteína do olho/análise; Manejo de espécimes; Reação em cadeia da polimerase/métodos; COVID-19; Manifestações oculares

INTRODUCTION

Viruses are intracellular parasites that are the smallest known infectious agents. Although the mechanisms of viral disease are still not completely clear, there are several factors that directly contribute to viral tropism, such as viral receptors in the host cell, the specific cell line, and physical barriers that enable and/or inhibit infections. Once inside a cell, the virus may damage or destroy it through direct cytopathic effects, host antiviral immune responses, and/or transformations of the infected cells⁽¹⁾.

The eye is a known site of viral infections, and viruses may appear in the intra- or extraocular space without visible systemic reverberation and affect multiple structures with variable manifestations^(2,3). Viral diagnosis is made based on clinical signs/symptoms and laboratory test results. Various laboratory tests are available with the specificity and sensitivity varying from one microorganism to another. Cell culture and analysis of genetic material from samples collected from blood, mucosa, or secretions are the main methods.

Based on clinical information and previous experience, the physician should decide between available diagnoses considering the patient’s singularities. For the collection of samples from the eye, the most commonly used method is conjunctival swabs, although Schirmer strips have also shown good results.

Current scenario

Ever since the World Health Organization (WHO) declared COVID-19 a global pandemic, the academic community has concentrated its efforts to treat the pandemic and help people return to normal life. The Chinese and ophthalmologist Li Wenliang, MD, was the first to report on the novel catastrophic virus SARS-CoV-2. It is now known that ocular manifestations can be one of the first symptoms of COVID-19 and, consequently, the eye may contribute to the understanding of COVID-19 pathophysiology^(4,5). Although several studies on ocular manifestations of SARS-CoV-2 have been conducted, the virus collection methods and associations with the ocular surface were not clearly stated.

Schirmer test

The idea of collecting tears as a clinical test was first introduced in 1900 by Köster⁽⁶⁾. The test consists of the placement of filter paper on all extensions of the conjunctival sac while the nasal mucosa ies stimulated to produce tearing caused by nasal irritation. The objective of the test is to exhaust tear production to evaluate the function of lacrimal glands. As this test can take up to 90 min to perform, it is not viable for daily medical practice⁽⁶⁾.

In 1903, Otto Schirmer, a German ophthalmologist, shortened the length of the paper strips and quantified tear production for 5 min using three methods, each of which analyzed a distinct tearing stimulation pathway: ocular and palpebral mucosa, nasal mucosa, and the retina^(6,7). In the last century, several modifications have been made to the Schirmer test. Nevertheless, this test remains important in the quantification and standardization of tear volume. Currently, a filter paper strip measuring 60-mm long and 5-mm wide is inserted at the temporal side of the conjunctival sac while the patient’s eyes are closed. The strips are removed after 5 min and the wet part is measured. A result greater than 15 mm is considered to be normal, but the value varies with medication use, age, and the presence of chronic diseases^(8,9).

Conjunctival swabs

Conjunctival swabs are the most commonly used method for microbiological analysis because they allow the collection of cells and materials spread in the conjunctival sac instead of exclusively tears. One exception is in-office rapid antigen tests for adenovirus (AdV) because they are faster and present a good sensitivity and specificity of 89% and 94%, respectively⁽¹⁰⁾. The method of collection is fast and simple: a swab with a cotton tip is gently passed using rotational movements on the conjunctival sac⁽¹¹⁾. Topical anesthesia can be used to make the procedure more comfortable for the patient, since there is no significant difference in the final results when samples are analyzed by polymerase chain reaction (PCR) methods⁽¹²⁾. Proxymetacaine 0.5% is recommended for ocular surface anesthesia since it shows the fewest bactericidal effects among commercially available eye drops⁽¹³⁾. It is also recommended to use sterile swabs, since calcium-containing swabs can inhibit polymerase activity⁽¹⁴⁾.

In this context, this study aims to collect and evaluate scientific evidence of the use of Schirmer strips and conjunctival swabs as a method of virus collection on the ocular surface.

METHODS

A literature review was conducted according to The Joanna Briggs Institute (JBI) recommendations for scoping reviews⁽¹⁵⁾. All searches and publication access were completed in June 2020 and there was no restriction on article publication date. The guiding research question “Is it possible to detect a virus on the ocular surface with the Schirmer test and/or conjunctival swab?” was formed for the selection and search of the studies. This was built through the Population, Concept, and Context strategy. In this way, “P” was defined as adult patients (>18 years old), “C” as the Schirmer test and conjunctival swab, and the last “C” as all viruses.

For the literature search, the following descriptors, synonyms, and keywords were used: “adult patients,” “Schirmer test,” “conjunctival swab,” and “virus.” The Boolean operators AND, NOT, and OR were used between descriptors. The controlled descriptors were “adult patient(s),” “Schirmer test,” “conjunctival swab(s),” and “virus.” The uncontrolled descriptors were “adult(s)” OR “patient(s),” “Schirmer strip(s),” and “ocular virus” OR “viral infection.” The search was performed using the databases PubMed, Web of Science, and Blibioteca Virtual em Saúde (BVS). Included articles were only those written in English, published in indexed sources, and with quantitative or qualitative approaches, primary studies, and reviews.

Insightful reading of the title, abstract, and keywords was performed to select the articles according to the previously established inclusion and exclusion criteria. When the title, abstract, and keywords were insufficient, the full text was also analyzed. All articles were called studies, enumerated in chronological order, and evaluated by three different researchers. Recommendations by JBI were adapted for the study singularities and used for data extraction. This article followed the Preferred Reporting Items for Systematic reviews and Meta-Analyses extension for Scoping Reviews (PRISMA-ScR) guidelines and checklist developed under EQUADOR (Enhancing the QUAlity and Transparency Of health Research) network guidance⁽¹⁶⁾.

RESULTS

Following the database search, 418 potential studies were identified. After reading the title, abstract, and keywords, 79 studies were selected. Of these, 27 articles were excluded because they were duplicate results. The full texts of the 52 remaining articles were read, and 16 were excluded for not answering the guiding question. Using the described methodology, the literature search identified 36 articles that met all criteria. This process is shown in figure 1 and the included studies are presented in table 1.

Figure 1. Summary of the systematic searches and processing of identified studies.

The three researchers analyzed all identified studies about virus research, collection methods, and sample analysis. Most of them were experimental or observational studies developed in the United Kingdom (7/36). Studies on the topic began in 1997, but 55.6% were only published in the years 2011-2021.

Most studies were on AdV^(17-29) and Hesper Simplex Virus (HSV)^{(20,24,27,30-38)}, followed by SARS-CoV-2^(38-46). Varicella-Zoster virus (VZV) ^(30-32), cytomegalovirus (CMV)^(31,32,47), Epstein-Barr virus (EBV)^(31,32), Ebola (EBO)^(48,49), human papillomavirus (HPV)⁽⁵⁰⁾, and Zika virus⁽⁵¹⁾ have also been identified on the ocular surface. Enterovirus (EV), coxsackievirus A24 variant (CA24v), and SARS-CoV were not positive on ocular samples collected using conjunctival swabs or Schirmer strips^(19,21,52).

The preferred collection method was conjunctival swab alone in 29 out of 36 articles; six studies used Schirmer strips and only one study compared both collection methods⁽²⁴⁾. One study was a narrative review about SARS-CoV-2⁽⁴⁴⁾. The majority of the samples were analyzed by PCR or reverse transcriptase-PCR (RT-PCR), with the exception of studies number 1⁽¹⁷⁾ and 17⁽³³⁾ that used immunochromatography (IC) and enzyme immunoassay (EIA) and immunoblot analysis, respectively. Details of the methods and findings of the included studies are provided in table 2.

DISCUSSION

Our literature search found that viruses can be identified on the ocular surface through analysis of conjunctival swabs or Schirmer strips. However, most studies have focused on AdV, HSV, and SARS-CoV-2, and other viruses have not been thoroughly investigated.

Studies focused on the detection of AdV and HSV were linearly developed over the last 20 years and randomly conducted all over the world. In contrast, in 2020, scientists exponentially published studies on SARS-CoV-2 because the virus was first seen in the last days of 2019. During the first few months of 2020, the WHO declared it a pandemic, and the world’s research efforts were directed to overcoming this pandemic.

In contrast to SARS-CoV⁽⁵²⁾, SARS-CoV-2 RNA was found by RT-PCR in conjunctival swabs^(38-46). However, samples were collected from a few patients and only a low and varying percentage presented positive results and/or ocular symptoms⁽⁴⁴⁾. Most of these studies were developed in China^{(38,39,41,45)}, mainly because Chian was the first epicenter of the virus. In some cases, ocular manifestations are one of the first symptoms of COVID-19. As a result, some researchers believe that the eye may contribute to the understanding of COVID-19 pathophysiology^(4,5). Recent studies have proved the presence of angiotensin-converting enzyme 2 (ACE2) and transmembrane serine protease 2 (TMPSS2) in the conjunctiva, limbus, and cornea, with prominent staining in the superficial epithelium surface, which are key factors for SARS-CoV-2 infection in human cells⁽⁵³⁾. These results support the necessity of ocular protection in preventing the spread of viruses.

Research on viral screening is, in most cases, correlated with external ocular symptoms with the purpose of solving clinical doubts about pathogenic agents and identifying methods for fast and accurate viral diagnosis. Studies on VZV, CMV, and EBV were mostly combined with HSV to determine coinfection and differential diagnosis^(30-32).

In contrast, only one study correlated viral detection on the ocular surface with intraocular symptoms. This study was published in 2000 and aimed to verify the efficacy of intravenous ganciclovir treatment in immunocompromised acquired immunodeficiency syndrome (AIDS) patients with CMV retinitis. The results showed high clinical relevance for confirming and differentiating diagnoses of CMV retinitis when ophthalmoscopic findings were determined by PCR methods of conjunctival swab samples⁽⁴⁷⁾. No other studies on this topic were identified by this review, likely because the incidence of CMV retinitis in the AIDS population significantly decreased with the introduction of effective antiretroviral therapy and early accurate diagnosis^(54-57).

The most commonly used method for sample collection was conjunctival swabs. This method also collects conjunctival cells, while Schirmer strips only allow the collection of tears since fluids pass to the filter paper because of gravity, viscosity, and capillary flow dynamics -the same physical processes that explain how liquids impregnate porous materials differently^(58,59). Consequently, the collected samples represent two different materials: (1) tears, cells, and fluids dispersed in the conjunctival sac and tears and (2) substances dissolved in it.

In the identified studies, samples were typically analyzed by PCR or RT-PCR depending on the viral genetic material of DNA or RNA, respectively. PCR methods are widely used because they allow for the replication and detection of low loads of viral DNA/RNA⁽⁶⁰⁾. The point-of-care test was also compared with PCR effectiveness, sensitivity, and specificity were high; however, this was only explored for AdV⁽²⁸⁾, likely due to epidemiological factors related to uncontrolled and fast spread of AdV conjunctivitis⁽⁶¹⁾.

This review included 36 articles from diverse countries and time periods, which suggests worldwide interest in the detection of viruses on the ocular surface in the last decades. The findings of this review may contribute to future research by clarifying key concepts to support the design of future research on ocular viruses.

Lastly, this review is subject to several limitations. First, only three databases were consulted. New articles on the theme are constantly being published, and so relevant studies may have been missed. Moreover, the analyzed studies typically involved a small number of patients and lacked clear definitions of collection time and viral persistence since disease onset. Additional studies with larger populations and time data are necessary to develop more definitive conclusions on this issue.

In conclusion, viruses can be detected through the analysis of samples collected by Schirmer strips and conjunctival swabs. Prior studies were generally conducted to understand viral infection, to develop accurate diagnostic methods, and to follow patients’ responses to treatment.

ACKNOWLEDGEMENTS

This study was supported by São Paulo Research Foundation (FAPESP), Grant #2020/07365-8.

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Submitted for publication: March 24, 2021.
Accepted for publication: August 1, 2021.

Funding: This study received no specific financial support

Disclosure of potential conflicts of interest: None of the authors have any potential conflicts of interest to disclose