Test-retest Reliability of Two Measurements of the Visual System
Condition: Reproducibility of Results
Study type: Observational

Summary: 
      Binocular vision tests are guided by the principle that uniform motion and focus of the eyes
      may be trained, but exercising the simultaneous movement of the pupils is necessary and
      important in achieving optimal and lasting results (Horwood & Toor, 2014). These tests have
      been in use for over 70 years and represent the foundation of therapy for visual deficiencies
      such as the inability of the eyes to move together. Despite their frequent use, no one has
      ever formally evaluated how good these tests are.

      One such evaluation method is reliability. A reliable test is one that will give the same
      result when repeated on multiple occasions (i.e. test-retest reliability) or by different
      people (inter-rater reliability). An unreliable test gives widely varying results when
      repeated, which would make changes in a patient's test results difficult to interpret.
      Therefore, this study aims to determine the test-retest reliability of seven binocular vision
      tests across two independent measurements. The investigators will measure 20 healthy people
      ages 18 to 35 years for each of these tests on two separate occasions, one week apart. Since
      these tests have been in use for many years, we are expecting the two measurements to be
      within acceptable range of each other.

      The tests being investigated measure: 3D vision (i.e. gross stereoscopic acuity), uniform
      movement of the eyes (i.e. "motor punctum proximum"), ability of the eyes to adapt to a
      target moving towards and/or away from the eyes (i.e. binocular fusion with convergence and
      divergence), quick movements of the eyes (i.e. saccadic movements), natural alignment of the
      eyes (i.e. anatomic oculomotor deviation), and double vision as a target is moved towards the
      eyes (i.e. convergence fusional proximum).

      The results of each test will be analyzed individually. If the tests are perfectly reliable,
      we would expect the results of the first test to equal the results of the second test for
      each participant. To examine the test-retest reliability the difference between the first and
      second test for each individual and across all individuals' scores summed together will be
      assessed.
    

Detailed description: 
      Binocular vision tests are guided by the principle that uniform motion and focus of the eyes
      may be trained, but exercising the simultaneous movement of the pupils is necessary and
      important in achieving optimal and lasting results (Horwood & Toor, 2014). These tests have
      been in use for over 70 years and represent the foundation of treatment for convergence
      insufficiency and other visual dysfunctions. Despite their frequent use, these tests have yet
      to be formally evaluated for their validity or reliability in healthy adults.

      One such method of evaluation is reliability. A reliable test is one that will provide the
      same results when repeated on multiple occasions (i.e. test-retest reliability), by different
      people (i.e. inter-rater reliability), or the same person (i.e. intra-rater reliability). An
      unreliable test yields widely varying results when repeated, which would make changes in a
      patient's test results difficult to interpret. In this study, test-retest reliability will be
      estimated using a single clinician (equivalent to intra-rater reliability) as it examines the
      degree to which a test is able to produce consistent results or whether it is dependent on
      the situation or the state of the individual being tested (Rousson, Gasser, & Seifert, 2002).
      In future studies, inter-rater reliability will be examined which is largely dependent on and
      concerned with adequate training of raters as opposed to the properties of the test itself.

      Furthermore, for health measures - such as the binocular vision tests - to be useful in
      clinically guiding patient management, they must be valid and sensitive to the system of
      interest (i.e. visual system). We are currently unable to quantify the validity of these
      binocular vision tests as this domain lacks a "gold-standard" (Hachana et al., 2013), thus,
      test-retest reliability must be determined first (Broglio et al., 2007). Therefore, this
      study aims to determine the test-retest reliability of seven binocular vision tests across
      two independent measurements. Twenty healthy participants aged 18 to 35 years will be
      measured on two separate occasions, one week apart. Since these tests have been in use for
      many decades, it is hypothesized that the two measurements will be within acceptable range of
      each other.

      The binocular vision tests undergoing examination differ from optometry tests in that they
      use more advanced equipment which can measure very small deviations in several domains of the
      visual system. These seven binocular vision tests measure various elements of the visual
      system and will be described in detail below:

      Gross stereoscopic acuity: (range 0-15 arc seconds) Our binocular vision allows us to see in
      three dimensions (3D), or more simply, to see depth. In this test, seated participants
      wearing 3D glasses are shown images. Inability to see depth or 3D will cause images to appear
      as points instead of objects. The objects are presented in different stages, with each stage
      requiring them to discriminate different levels of depth perception. The test is scored in
      optical units, with a range of 0 to 15 arc-seconds. The maximum score corresponds to the
      level where the last object was identified.

      Near point of convergence and near point of convergence - break: (cm) When an object is
      moving towards our eyes, they symmetrically converge in order to maintain focus. However,
      there is a point at which our eyes no longer symmetrically converge (point of convergence).
      This test measures the distance (cm) between the bridge of the nose and point of convergence
      in seated participants as an object is moved closer to the head.

      Positive fusional vergence: (diopters, prism convergence units) This test measures how well
      someone can adapt to challenges in focusing light on the retina. There are two almost
      identical tests. One test occurs with an object placed at 3m from the seated participant, and
      the other with an object at 30cm from the seated participant. Light from an image is passed
      through a prism. This is analogous to moving the image further away from the body. In
      response, the eyes must diverge (separate) to focus on the object, just as they would if the
      image actually moved away from the body. Different prisms are used to create increasing
      challenges. The score for these tests is simply the maximum amount of prism convergence
      (dioptres, noted on the prism, as one would note diopters on eye glasses) that the seated
      participant can accommodate at 3m and at 30cm.

      Negative fusional vergence: (diopters, prism convergence units) This is the same test as (3),
      except that the prisms diverge the light and the participant has to converge their eyes to
      maintain focus. The score for these tests is simply the maximum amount of prism divergence
      (diopters, noted on the prism, as one would note diopters on glasses) that the seated
      participant can accommodate at 3m, and at 30cm.

      Saccadic movements or oculomotor capacity: (Score = bad, medium, good) A light appears on the
      screen and the participant move their eyes to fix on the object. While they eyes adjust, they
      will temporarily cover small distances until they achieve a fixed focus. These are called
      saccadic movements. Lights appear and disappear, in different locations on the screen, at a
      rate of 100 per minute, lasting 2 minutes. The test result is scored by the evaluator based
      on a global impression over the entire 2 minutes, with 3 separate sub scores on an ordinal
      scale for quality (bad, medium, good), for synchronization (bad, medium, good) and saccadic
      correction (many corrections, few corrections, no corrections). The three sub scores are
      combined into an overall score according to our industry partner's (Apexk) proprietary
      algorithm.

      Anatomic oculomotor deviation: (diopters, prism convergence units) This test measures the
      eyes' natural deviation (heterophoria) and also allows the detection of strabismus. In
      strabismus, anatomic deviation is evident and the person's dominant eye is looking at you,
      but the "lazy/deviated" eye is not. In heterophoria anatomic deviation is not visible to the
      naked eye and the deviation has to be triggered by covering in sequence, one eye at a time,
      to trigger the deviation. There are two identical tests: one occurs with an object placed at
      3m from the seated participant (far vision), and the other with an object at 30cm from the
      seated participant (near vision). In this test, seated participants focus on an object. These
      movements can be seen by the clinician. The clinician covers/uncovers eyes to trigger
      movements and uses a prism to cancel the movement. The prism that achieves this cancellation
      is the measure of anatomic deviation. The rating of the prism that achieves this cancellation
      is considered the score for this test, with one score for the object placed at 3m and another
      score for the object placed at 30cm. Participants with strabismus are excluded in our study
      because strabismus is a contra-indication to post-concussion visual training, which is part
      of our larger study and thus, patients with strabismus do not represent our target
      population.

      Convergence fusional proximum: (diopters, prism convergence units) This test is similar to
      (2) above. When an object is moving closer to our head, the eyes symmetrically converge. When
      the object is moved beyond the participant's ability to converge, the participant will start
      to see two images (double vision). This test measures the distance between the bridge of the
      nose and point where double vision (cm) occurs in seated participants as an object is moved
      closer.

      In addition, demographic information relevant to our study will be collected in order to
      appropriately describe the population and explore if these factors modify the test-retest
      correlations. Variables such as age, sex, highest level of education achieved (i.e. secondary
      school, CEGEP, university), the use of corrective lenses for vision problems, occupation, and
      any relevant past medical history (i.e. migraines, vision problems, medication, etc) will be
      included.

      The main objective of this study is to evaluate the agreement and consistency between
      measurements of the visual system taken at two different times to determine the test-retest
      reliability of the seven binocular vision tests. To do so, the results of each test will be
      analyzed individually. If the tests are perfectly reliable, it is expected that the value of
      the first test is equal to the value of the second test for each participant across the range
      of values for all individuals. Reliability will be assessed using the intra-class coefficient
      (ICC) which measures the between-group variance divided by the total variance (sum of
      between- and within-group variance). The difference between the first and second test for
      each participant is called the within-group (each participant represents a group) variance.
      The difference across individuals is the between-group variance. If the within-group variance
      is 0 (first and second measures are identical), then the ICC = 1 (between-group variance /
      (between-group variance + 0)). In addition to the ICC, limits of agreement will be estimated
      using the paired data and illustrated via Bland-Altman plot (Bland & Altman, 1986).

      As the binocular vision tests we are examining have applicability in many health conditions
      (e.g. concussion, Alzheimer's, etc.), this study has the potential to contribute to the
      improved management of symptoms related to the visual system.
    

Observational model: Other
Time perspective: Prospective
Sampling method: Non-Probability Sample
Study population: 
        The target population is healthy adults aged 18 to 35 years. This study is being conducted
        as part of a larger project examining the correlation of the binocular vision tests with
        symptoms of concussions; therefore, this population has been selected to match that target
        population of young adults with concussion symptoms to ensure applicability of findings to
        the patient population in the larger study. Participants who have conditions or are taking
        treatments that could affect the results of the binocular vision tests will be excluded.
      
Keywords: test-retest reliability; Binocular vision tests