Before the study outset, the authors of the CISS gave us their consent to develop a Spanish version of their instrument. The CISS questionnaire consists of 15 items. In reply to each question, the subject indicates the frequency of each symptom on a Likert scale, with scores ranging from 0 to 4: never (0), infrequently (1), sometimes (2), often (3), or always (4). The scores of every item are added to determine the final score, which ranges from 0 (least symptomatic) to 60 (most symptomatic). The recommended cut-off is ≥21 for adults
2 and ≥16 for children 9 to 18 years of age.
1 The study was conducted in two stages. The questionnaire was first translated and adapted to Spanish (May 2016 to April 2017), and then the validity and repeatability of the Spanish version were assessed (May 2017 to January 2018).
Because the CISS assesses near-vision-related symptoms and not only convergence insufficiency symptoms, we enrolled subjects from the general population from four different institutions in Madrid, Spain: a primary school (CEIP Vargas Llosa, Madrid), a secondary school (IES Juan Rodriguez Villanueva), a university faculty (Optics and Optometry Faculty of the Universidad Complutense de Madrid), and a technology company (DXC). In addition, over the period from January 2019 to March 2019, we performed a psychometric analysis of the English CISS on subjects from the personal network of one of our researchers (CP-G) in Swindon, Wiltshire, UK. Subjects 9 to 30 years of age were recruited by convenience. Participants received no compensation for their cooperation. Exclusion criteria were mother tongue different from the questionnaire language, prior visual surgery (not refractive), active visual or neurologic disease, any medication that could affect vision, or any kind of disability preventing the subject from reading or understanding the instrument's questions. Out of 665 subjects enrolled, 449 (mean age, 18.62 years; range, 9–30 years; female, 54.95%) completed the Spanish version of the CISS (CISSVE), and 216 (mean age, 15.81 years; range, 12–20 years; female, 37.61%) completed the original CISS.
The study was approved by the Research Ethics Committee of the Hospital Clínico San Carlos (Madrid, Spain), and its protocol adhered to the tenets of the Declaration of Helsinki. All participants gave their written informed consent prior to participation. For participants younger than 18 years, consent was obtained from a parent or guardian. All children older than 12 years also provided their consent before any testing was done. Other than responses to the questionnaires, no other clinical data were collected in any subject.