Convergence insufficiency (CI) has one or more of the following signs: 1) exophoria that is greater at near than at distance, 2) a remote near point of convergence (NPC), and 3) decreased positive fusional vergence (PFV) at near.^1-3

Some authors have defined CI using all three signs, while others use only one or two of these signs. The CITT Study Group is studying the condition in which all three signs are present. Members of our group have completed a preliminary study demonstrating the prevalence of CI based on this definition in a school-age population.

Review of the literature reveals considerable variability in the reported prevalence of CI ranging from 1.75 to 33%.^{2, 4-10} This variation may be accounted for by the different definitions used by the authors and by the different clinical samples studied. A more accurate estimate of the true prevalence of CI would be based on population-based studies. Four such studies ^11-14 narrow the range from 2.25% to 8.3% (Table 1-1). No supportive data could be found to indicate whether the prevalence of CI varies across ethnic and/or racial subgroups.

Common symptoms of CI include discomfort, eyestrain, headaches, blurred vision, diplopia, sleepiness, difficulty concentrating, movement of print, and loss of comprehension after short periods of time (Table 1-2). ^3,4,15-19 Most symptoms of CI are associated with reading or other close work. Therefore, CI may have a significant negative impact on quality of life, potentially interfering with school, work performance, and leisure activities. The CITT Study Group, therefore, decided that symptoms must be part of our definition of CI. However, none of the population-based studies included the presence of symptoms in their definitions of CI. Therefore, we developed a CI Symptom Survey which provides a symptom score that is used as a component in the diagnosis of CI and as the primary outcome measure for the CITT Pilot Study and for this full-scale CITT Study.

Although symptoms of CI are commonly reported in the literature, the CI Symptom Survey is the first standardized symptom tool for documenting the type and frequency of symptoms in CI subjects. Existing vision-specific quality of life measures have focused on visual acuity and the postoperative effects of surgery. Therefore, such measures are not appropriate for evaluating subjects with CI who have normal visual acuity and are expected to have difficulties with performing sustained visual activities within arm’s length.

The CITT study group, in preparation for its planning grant, performed a preliminary study to help address this issue. Borsting et. al.²⁰ gathered validity-related evidence on child and parent versions of the CI Symptom Survey. Symptoms were measured prospectively on 14 school-aged (8-13 years) children with CI and 14 children with normal binocular vision (controls). The two groups had similar characteristics for age, gender, and cognitive level; children with learning disabilities or attention deficit disorder were excluded from the study. The CI group scored significantly higher than the controls on both the child and parent surveys. Thus, CI appears to have a significant negative impact on children’s visual comfort.

The CI Symptom Survey was further refined and studied in the recently completed CITT Planning Grant.²¹ The new CI Symptom Survey form is illustrated in the Appendix located at the end of this chapter. This study demonstrated that the CI Symptom Survey has moderately good repeatability for individuals 9 to <18 years of age. The instrument also has the ability to differentiate between CI and normal binocular vision subjects.

Symptoms are usually the main reason patients with CI seek professional eye care. Thus, an important eligibility criterion for this trial will be that the CI patient must demonstrate a significant level of symptoms i.e., a CI Symptom Survey score of ≥16. Because reduction of symptoms is the primary goal of subjects who seek care, the score on the CI Symptom Survey will also be the primary outcome measure for our study.