In today's society, in which the emphasis on vision is associated with tasks requiring near vision, the visual system may be unable to perform this type of activity efficiently, leading to visual discomfort, fatigue or asthenopia and impaired visual performance.1 In many cases, the cause is an abnormality in any of the accommodative and/or vergence systems, which can lead to the development of what are termed accommodative and non-strabismic binocular dysfunctions.2 Accommodative and vergence dysfunctions can interfere with a child's academic progress or a person's ability to function efficiently in the course of his or her work. Children may abandon a task due to their inability to maintain adequate accommodation and/or vergence in the plane of fixation.1 In addition, those who perform extended periods of close vision work, such as reading or the prolonged use of computers, are more likely to report the symptoms and signs associated with these vision disorders.3,4 Nevertheless, the symptoms associated with prolonged near vision work can be reduced with the correct treatment to improve accommodative and vergence function.4,5

These dysfunctions are commonly encountered in clinical practice6 and present a variety of associated symptoms, including blurred vision, difficulty in focusing at different distances, headache and ocular pain, among others.7–10 In general, all of these symptoms are categorised under the generic name of asthenopia. However, the symptoms that the patient perceives may differ depending on the type of causative disorder2; it would therefore be reasonable to conclude that there are different types of asthenopia.11 In fact, one of the problems that clinicians face when diagnosing these dysfunctions is how to determine which symptoms are associated with each disorder and how to quantify their frequency and severity.12

The aim of this study is to determine by means of a scoping review the most common symptoms associated with accommodative and non-strabismic binocular dysfunctions described in the scientific literature published between 1988 and 2012. A further aim is to determine the manner in which subjects’ symptoms are obtained in order to quantify their frequency and severity. We elected to study a long period of time in this scoping review so as not to omit any possible relevant information on these anomalies.

We conducted a scoping review through an exhaustive search in health science databases for research published between 1988 and 2012. The search was performed in January 2013 using the Medline database (via PubMed), CINAHL, PsycINFO and FRANCIS.

We designed two search strategies. The first strategy was based on the use of free-text terms related to accommodative and non-strabismic binocular dysfunctions, searching all database fields. The search equation included boolean operators, truncated symbols and wildcard characters specific to the selected databases. The second search strategy combined the use of controlled MeSH terms and free-text terms related to questionnaires, asthenopia, visual symptoms and visual discomfort. This second strategy was only implemented in Medline. Table 1 summarises the search equations employed in the two strategies.

The inclusion criteria consisted of research published in English which examined any aspect of the symptoms associated with accommodative and non-strabismic binocular dysfunctions, regardless of the type of population studied, from children to adults. We wanted to obtain any type of studies in which authors described symptoms of patients with these anomalies obtained by means of questionnaires, case histories, or both. We excluded articles on strabismic binocular anomalies, papers on ophthalmic examination tests or eye diseases and non-original publications such as letters to the editor, editorials, theoretical reviews and conference proceedings. Published reports of a unique clinical case were also excluded.

The search identified 657 articles eligible for review. These were analysed in accordance with the established inclusion and exclusion criteria, leading to a final selection of 56 studies7–9,13–65 that reported some aspect of the symptomatology of accommodative and binocular dysfunctions. Two authors independently performed the data extraction (S.C.B. and P.C.M.) so that when there were inconsistencies, they were resolved by consensus. Reference lists from all identified studies were also examined.

Once the articles had been selected, all the terms used in each of the 56 studies to refer to the symptoms associated with the dysfunctions were extracted. Since relatively similar questions were often used to obtain the subjects’ symptoms, once these different ways of asking about the symptoms had been compiled they were grouped into categories that contained or referred to the same symptom.

Table 2 shows the methodological characteristics of the 56 studies analysed. Besides author and year, the table also gives sample characteristics, type of dysfunction analysed and how each author obtained the symptoms referred to in each study. In the 35 studies8,9,13–16,18–20,22–29,32–35,37,41–49,51,55,56,59 which used questionnaires to analyse symptoms, a total of 11 different questionnaires9,24,41,42,48,49,51,55,56,59,66 were employed by the different authors.

Table 3 shows the characteristics of the 11 questionnaires found in the 56 studies, indicating the authors who developed each questionnaire, which authors used each of them, the type of dysfunction to which they refer, the target population on which they were used, the number of items they contain and how many of these items are associated with far or near vision. It also shows the characteristics of each individual questionnaire, specifying the number of questions asked and how they are calibrated, what scoring system is used and whether the questionnaire has been psychometrically validated.

An analysis of the different symptoms named in the 56 studies revealed that up to 267 different expressions were used to ask about symptoms, both in the case histories and in the 11 questionnaires related to these dysfunctions. Of these, 162 expressions appeared in publications reporting the use of questionnaires while the remaining 105 were employed in publications which did not use a questionnaire.

Due to this high number of different ways to name the symptoms (267), it was decided to group terms which described the same symptom by category. The aim was to obtain categories of symptoms in order to summarise those which actually referred to the same concept and thus avoid duplication of information. Table 4 shows an example of this phenomenon as regards the category of “double vision”. The 15 different ways used to name the symptom of double vision were grouped into the same category, called “diplopia”. Table 4 also shows the different ways employed to ask about double vision, the different studies that used them and whether they were compiled using a questionnaire.

Following a qualitative process, each of 267 expressions used to ask about symptoms were reviewed and designated to a category that embodied the meaning of the symptom. Thus, the 267 different forms were grouped into 34 categories of symptoms. Table 5 shows the 34 categories of symptoms established, and specifies how many different forms refer to each category and the number of studies that report on this symptom. For each category, it also indicates the number of studies which referred to children, adults or both, accommodative dysfunction, binocular dysfunction or both, and whether each category is associated with far vision, near vision or both, according to the number of authors who specified this.

Tables 6 and 7 show the symptoms associated with each particular binocular and accommodative dysfunction. There have only been included the anomalies for which the authors described symptoms particularly associated with each dysfunction. In both of them there is also information about the number of studies which refer to each symptom and if the category has been named by a particular questionnaire or by means of the case histories with their number of studies.

The results of this scoping review show the disparity of symptoms associated with accommodative and non-strabismic binocular dysfunctions, and that these are fundamentally related to binocular dysfunctions and mainly associated with close vision tasks. There is no consensus concerning which symptoms should be considered in the diagnosis of each of these anomalies. The review also revealed that there are no specific questionnaires for most of the accommodative and binocular dysfunctions except for convergence insufficiency (CI).

CI is the dysfunction which has received most research attention; 50% of the studies analysed concerned this binocular anomaly. According to the scientific literature, CI is one of the most common anomalies of binocular vision, with prevalence values in the clinical population that range between 2.25% and 33%.6 It is also one of the binocular vision anomalies that has received most research attention, not only in studies related to its diagnosis8,46,48,52,55 but also in several recent clinical trials concerning this disorder.34,43–45 It is therefore logical that it should be the dysfunction on which most information is available about its symptomatology. In fact, the scientific literature shows that the remaining accommodative and non-strabismic binocular dysfunctions have been studied to a lesser extent.5,6,10

When analysing symptoms related to different populations, the review shows that most of the studies are related to children (29 reports), with 12 studies about adults and 15 articles related to both populations. It should be noticed that symptoms reported by adults and children could be different between them, not only when considering the case history but using a particular questionnaire. Moreover, a child's response to a certain question could be different if it was administered by a parent versus the examiner. However, the review shows that authors have not differentiated questions for case histories nor for the questionnaires reported by adults and children.

As regards the way in which symptoms are obtained, we observed that 21 studies7,17,21,30,31,36,38–40,50,52–54,57,58,60–65 analysed symptoms using patients’ descriptions of their case histories or on the basis of questions posed by the person conducting the examination. Of the 11 questionnaires used, the CI-specific CISS V-15 questionnaire was the most frequently employed, having been used in 21 studies8,13–16,19,20,23,25–27,29,32–35,43–46,48, followed by the 19-item College of Optometrists in Vision Development Quality of Life (QOVD-QOL) questionnaire18,22,42,47 (developed for visual abnormalities in general) and the Conlon survey28,32,37 developed for visual disorders in general and the Academic Behaviour Survey13,24,35 (for CI). The remaining questionnaires were used once only.9,41,49,51,55,56,59 Consequently, although in general more studies were based on the use of questionnaires (35 studies), most of them used the CISS V-15 and thus there were actually a higher percentage of articles in which subjects’ symptoms were obtained from their case histories.

As regards psychometric validation, only three of the questionnaires used had been validated, the CISS V-15,48 CIRS symptom questionnaire parent version55 and the Conlon survey.66 The CISS V-15,48 is a CI-specific questionnaire that has proven useful in differentiating subjects with CI from those with normal binocular vision and has been widely used to develop the diagnosis of this disorder in both child and adult populations, and to monitor the effectiveness of different treatments in the various clinical trials conducted to date. However, since it has only been validated for CI, its use cannot be generalised to other dysfunctions unless validating for other anomalies.

In the same way the CIRS symptom questionnaire parent version55 has been proved to be a valid instrument for differentiating children with CI from those with normal binocular vision, although it has only been used on one occasion.

Meanwhile, the Conlon survey,66 validated by RASCH analysis, was developed to analyse the symptoms associated with any type of visual anomaly, including accommodative and binocular dysfunctions, and has shown to be a reliable and valid measure of visual discomfort for adults.

Despite the existence of these three validated questionnaires, our results indicate that there is a lack of specific questionnaires for each of the existing accommodative and binocular dysfunctions. The 11 questionnaires identified were used for different dysfunctions, although the vast majority of them (7) were employed for CI.9,24,48,49,51,55,56 The Russell59 and Sterner41 questionnaires were used to analyse accommodative insufficiency (AI), while the two remaining questionnaires42,66 were employed for accommodative and binocular dysfunctions in general.

The existence of 267 different ways of naming the symptoms that can be grouped into 34 categories indicates a wide disparity in forms of referring to the same symptom. A clear example is the case of the symptom of “blurred vision”, for which 34 different ways of referring to it were found.

An analysis of the 34 symptom categories shows that most of these categories were related to children although the great majority of these symptoms have also been used for adult populations and for the combination of children and adults. This finding is related to the fact that most of the studies are focused in children population. However it is worth noting that there are several symptoms (“head or book tilt”, “inability to estimate distance accurately”, “be distracted” and “extraordinary reading or writing posture”) which have been only reported in children population. The same happens when considering symptoms as “red eye”, “watery eyes”, “dry or gritty eyes” and “eye turn noticed” which have only been reported in adults population, although certainly there are only few studies which have reported them. These findings show that authors do not seem to differentiate symptoms for children and adults.

In addition to that, the vast majority of these categories were associated with binocular dysfunctions compared to symptom categories related to accommodative dysfunctions. In general, it should be noted that symptoms overlap between accommodative and binocular anomalies. No category was associated exclusively with accommodative dysfunctions, although two symptoms in a high number of studies (“feel sleepy” and “pulling eyes”) were only related to binocular anomalies. These findings confirm that it is difficult to disjoin accommodation from vergence system. Due to the link between both systems, a deficiency in one system could cause an abnormality in the other so that symptoms would overlap. For that reason, it should be difficult to determine a very specific question which could only be related to either an accommodative deficiency or a vergence one.

Similarly, it is worth noting that the vast majority of authors associated the symptoms with near vision rather than with far vision. Some categories were associated with both distances, and there were even cases where the authors did not specify the distance associated with the symptoms. No category was exclusively associated with far vision. This finding is clearly related to the fact that CI is the most frequently studied dysfunction67,68 and its symptoms are mainly associated with near vision; thus most categories are related to near vision. It is possible that if other dysfunctions had been analysed, the findings in this respect might have indicated the existence of other symptoms more specific to far vision. Nevertheless, the present analysis indicates that there is a lack of specific questionnaires related to dysfunctions affecting far vision.

Of the symptoms cited most frequently, the category of “headache” appeared in almost all of the 56 articles analysed. The other frequently mentioned symptoms are all mainly related to binocular dysfunctions and are particularly closely associated with near vision. It is also evident that few symptoms were specific to each entity and many overlap. This information may influence clinical management as it is difficult to associate a particular dysfunction with a particular symptom.

When each particular accommodative and binocular dysfunction was considered separately, certain singularities were observed with respect to some symptoms, for example when referring to diplopia in accommodative dysfunctions such as accommodative insufficiency or accommodative excess. CI was associated with the vast majority of symptoms identified; 26 of 34 categories were associated with this dysfunction. This finding highlights the importance given in the scientific literature to CI over and above other dysfunctions. The symptoms most commonly associated with CI are precisely those symptoms which coincide with the questions in the CISS V-15.

The other dysfunctions also had different categories of associated symptoms. However, only a limited number of authors referred to each category for dysfunctions such as convergence excess, accommodative insufficiency, accommodative infacility, accommodative excess or basic exophoria.

In summary, the results of this scoping review demonstrate that gaps exist in our current knowledge. There is a wide range of symptoms related to accommodative and non-strabismic binocular dysfunctions reported in the scientific literature. There is no consensus concerning which symptoms should be considered in the diagnosis of each of these anomalies observing that few symptoms were specific to each entity and many overlap. According to the questionnaires, only the use of three validated symptom questionnaires were reported in the scientific literature, and two of them were specific for CI. However, no specific questionnaires were found for being for the remaining accommodative and binocular dysfunctions when there is the suspicion of an accommodative or binocular anomaly. This coverts the task of identifying the type of symptoms and their frequency and severity in something extremely difficult in spite of being an important aspect for diagnostic purposes. So, future studies should be done in this sense. Further questionnaires might be developed to address symptoms related to accommodative and nonstrabismic binocular dysfunctions other than CI.