Abstract
Purpose:
Convergence insufficiency (CI) is characterized by abnormal vergence eye movement frequently accompanied by abnormal accommodation and subjective symptoms, such as headache, blurred vision, and diplopia. CI is treated with vergence and accommodation exercises that are integrated so that the relative contributions of vergence and accommodation exercises to the outcome are concealed. The purpose of the present study was to determine the individual contributions of vergence and accommodation exercises for the treatment of CI in school children.
Methods:
In a prospective crossover study 44 children aged 9 to 13 years with CI were randomized to perform either vergence exercises followed by accommodation exercises each for 6 weeks or the 2 treatment regimes in the reverse order. The outcome measures were recovery from CI and the parameters vergence facility, positive fusional vergence, near point of convergence, monocular amplitude, and facility of accommodation.
Results:
After the first 6-week period, full recovery from CI was significantly more frequent in the group commencing vergence exercises than in the group commencing monocular accommodation exercises (p = 0.01), whereas there was no significant difference between these proportions after the second 6-week period (p = 0.45). Vergence facility and positive fusional vergence improved significantly more after the period with vergence exercises than after the accommodation exercises, whereas there was no significant difference between the effects of the two types of exercises on the other studied parameters.
Conclusions:
Vergence treatment induces a faster recovery of CI than accommodation treatment in school children. This may be used to improve compliance and success rate of the treatment.
Convergence insufficiency (CI) is a disturbance in binocular vision with a prevalence of 3% to 6% in school children
1,2 leading to symptoms such as headache, asthenopia, blurring, diplopia, and a possible negative impact on academic performance.
3 The clinical examination shows receded near point of convergence, reduced positive fusional vergence, and larger exodeviation at near than at distance.
4 Several studies have shown that school children diagnosed with CI have abnormal accommodation,
5,6 which is reflected in clinical guidelines for the treatment of CI that include exercises for both monocular accommodation and vergence, the latter including exercises for antisuppression and awareness of diplopia.
7 The effect of exercises aimed at improving vergence is assessed by measuring positive fusional vergence, vergence facility, and near point of convergence, whereas the effect on accommodation is assessed by measuring amplitude of accommodation and accommodation facility.
8 It has been shown that these measures of vergence and accommodation develop independently of each other in the age group of 7 to 15 year olds.
9,10 This suggests that clinical interventions can be performed separately on the individual elements characterizing CI, thereby possibly reducing the duration of orthoptic treatment required to obtain normal binocular vision.
Therefore, the present prospective randomized crossover study was performed with two groups of school children aged 9 to 13 years with CI randomized to perform either vergence exercises followed by accommodation exercises for each 6 weeks or the 2 treatment regimes in the reverse order.
By January 2016, the 6 public schools in the municipality of Randers, Denmark located nearest to the Danish College of Optometry and Visual Science (DCOVS) were contacted by e-mail and offered a preliminary examination for CI of children aged 9 to 13 years. Four of the schools representing 755 children returned a positive answer and passed the offer to the parents. None of the families declined participation, and a primary examination consisting of a measurement of the near point of convergence (NPC), positive fusional vergence at 40 cm (PFV), and unilateral cover test was performed in all children at a designated time during school hours. This revealed 69 children with signs of CI (NPC > 10 cm and PFV < 20 prism diopters [pd]) who were offered an extended examination at DCOVS to confirm the diagnosis. Among these, 62 children (89.9%) turned up for the study.
The baseline examination consisted of the following
11:
- 1. Monocular and binocular high-contrast Early Treatment Diabetic Retinopathy study (EDTRS) visual acuity was measured at 6 m (Topcon CC-100XP) and at 40 cm (Precision Vision, Cat. No. 2107).
- 2. Cycloplegic (cyclopentolate, 1%) and noncycloplegic autorefraction was measured using a Topcon TRK-2P Kerato-refractometer. Cycloplegic refraction was used to rule out uncorrected ametropia.
- 3. Unilateral and alternating prism cover test was performed at 6 m and 40 cm while the child viewed a 6/9 (0.2 logMAR) and 0.4/0.6 (0.2 logMAR) letter, respectively.
- 4. Ocular motility test was performed to rule out paralytic or restrictive deviations.
- 5. NPC was measured in centimeters with a Royal Air Force (RAF) ruler and a nonaccommodative target (a vertical line), which was moved slowly toward the child's nose until diplopia was reported, or loss of fixation was observed.
- 6. PFV was measured at 40 cm while the child viewed a vertical row of 0.4/0.6 line (0.2 logMAR) letters and while the demand on fusional vergence was gradually increased by use of a prism bar until either diplopia was reported or eye movement toward the base of the prism observed. If the child perceived the letters as blurred prior to loss of fusion, the blur value was recorded.
- 7. Vergence facility (VF) was measured at 40 cm for 1 minute using a 12 pd base out and 3 pd base in flipper while the child viewed a vertical row of 0.4/0.6 line (0.2 logMAR) letters, and the number of cycles per minute was recorded.
- 8. Monocular accommodation amplitude (MAA) was measured twice using an RAF ruler by slowly moving a horizontal line consisting of 0.4/0.6 (0.2 logMAR) letters toward the child until the first sustained blur was reported. To increase precision, the result was measured and recorded in cm and subsequently converted to diopters during initial data analysis.
- 9. Monocular accommodation facility (MAF) was measured at 40 cm for 1 minute using a ± 2.00 accommodative flipper while the child viewed a vertical row of 0.4/0.6 (0.2 logMAR) letters, and the number of cycles per minute was recorded.
- 10. The frequency of symptoms related to reading and other types of close work was assessed with the Convergence Insufficiency Symptom Survey (CISS), which has been validated for children aged 9 to 18.12–14 The questionnaire consists of 15 items related to frequency of eye and visually related symptoms. Questions were read aloud to the child by the examiner. The child selected 1 of 5 possible answers scored on a scale from 0 (never) to 4 (always). The sum of scores was calculated (from 0 to 60); a higher score representing more frequent symptoms.
All measurements of visual acuity, vergence, and accommodation were performed with habitual correction, if any.
In 50 of the 62 (80.6%) children, the examination confirmed the presence of CI defined as: CISS score > 15, NPC ≥ 10 cm, exophoria at least 4 pd larger at near than at distance, and PFV ≤ 20 pd or failed Sheard's criterion (PFV less than twice the near phoria), and the diagnosis was confirmed at a re-examination 1 to 2 weeks later.
- • Visual acuity > 0.1 logMAR (6/7.5).
- • Clinically significant uncorrected ametropia (difference between cycloplegic refraction and habitual correction exceeding 0.5 D of myopia or 0.75 D of hypermetropia, astigmatism, or anisometropia).
- • Manifest comitant or incomitant strabismus.
- • Systemic diseases, such as diabetes, metabolic disorders, or ocular pathology, reported by the parents.
Four children were excluded, 2 because monocular visual acuity was lower than 0.1 (logMAR), 2 because of uncorrected hypermetropia ≥ 1 D. This left 46 children as potential participants in the study. Among these, the parents of 44 children consented to let their children participate. Recruitment was started by March 2016 and continued until October 2018 when all 44 children had been included. Two of the children were subsequently excluded due to violation of the treatment protocol and one due to dropout leaving 41 children to complete the study. The clinical data at baseline are shown in the
Table.
Table. Baseline Characteristics of the Participants in the Two Groups
Table. Baseline Characteristics of the Participants in the Two Groups
The calculation of sample size assumed a power of 80%, a
p value of 5% and was based on standard deviations and clinically significant differences in NPC, PFV, VF, MAA, and MAF reported by the Convergence Insufficiency Treatment Trials (CITT) study group
5,15,16 and resulted in a minimum requirement of 18 persons to each of the 2 treatment regimes.
The children were randomized to perform either 6 weeks of vergence exercises followed by 6 weeks of accommodation exercises or the reverse. Each of the two 6-week treatments consisted of three 2-week phases (
Fig. 1). Every 2 weeks, the children came to the clinic at DCOVS for evaluation and exercises for the next phase were demonstrated and practiced.
At the end of each instruction session, the participants were required to demonstrate the procedure. Families were given written instructions including contact information in case of uncertainties about the procedures. A logbook was delivered in order to enforce home adherence, and all equipment necessary for homebased exercises was provided by DCOVS.
The examination protocol was repeated at all instruction sessions, except for CISS that was only assessed at week 6 when the treatment regime was changed and after completion of treatment at week 12.
Each instruction session lasted approximately 45 minutes, including the evaluation of progress and instruction. The treatment program required that the child spent 20 minutes per day, 5 days per week performing exercises at home. This corresponded to a total of 25 hours during the 12-week treatment program. All exercises were performed as suggested by the CITT study group.
8
The primary outcome measure was the proportion of patients with full recovery at weeks 6 and 12 (i.e. CISS score below 16, NPC below 10 cm, and PFV more than 20 pd). Improvement was defined as a CISS score of less than 16 or a 10-point decrease combined with at least one of the following: normalized NPC, an improvement in NPC of more than 4 cm, normal PFV, or an increase in PFV of more than 10 pd. Children who did not meet criteria for either full recovery or improvement were considered nonresponders.
Secondary outcome measures were changes in CISS score and vergence and accommodation parameters.
Data entry, validation, and analysis were performed in Stata SE version 16 (College Station, TX, USA). The baseline parameters in the two groups were compared using t-test for continuous variables and χ2 test for proportions. There were no significant differences between the proportion of boys and girls between the two treatment groups, and no significant differences in any of the variables between the two genders. Hence, the data was not stratified according to sex. The proportion of participants with full recovery was stratified by treatment group and calculated at the change of treatment regime (week 6) and when treatment was completed (week 12) and was analyzed using logistic regression with adjustment for baseline measures that significantly differed between treatment groups and therefore adjusted p values were reported.
Changes from baseline at each examination and between treatment regimens for NPC, PFV, VF, MAA, MAF, and CISS were analyzed using 2-way repeated measures ANOVA and Greenhouse-Geisser correction was applied to adjust for sphericity.
17 Comparison of baseline measures between children with and without full recovery showed no significant differences (
p > 0.3 for all comparisons), hence the 2-way repeated measures ANOVA was not adjusted for the baseline level. Post-tests to assess change from one examination to the following examination and between treatment regimens at each time of examination were performed using paired and unpaired
t-tests, respectively.
To the authors’ knowledge this is the first study to evaluate and compare the effects of vergence and monocular accommodation exercises in children with CI. The overall success rate of 71% after 12 weeks was comparable to previous studies in which vergence and monocular accommodation exercises had been combined for the entire treatment period.
7 Among the participants with partial recovery and among nonresponders, the improvement of objective measures (82.4%) was better than that of subjective measures (52.9%; data not shown) suggesting that acquired oculomotor skills may have required conscious efforts with a consequent induction of fatigue.
18,19 This suggests that an extension of the treatment period beyond 12 weeks might have been beneficial for a minority of the children. In spite of this, one participant showed no effect of the treatment. Whether the lack of response in this one subject reflects a special disease entity remains to be elucidated. The study was conducted for 12 weeks and therefore the long-term effect of the instituted treatment is unknown. The results of other studies support that the effect of treating CI with vergence and monocular accommodation exercises is sustained after 1 year.
20 Furthermore, neuroimaging has shown increased activity in cortical areas involved in oculomotor control 1 year after successful orthoptic treatment of CI
21 and reduced activity in cortical areas involved in attention as an indication of a reduced need for conscious effort once oculomotor skills have been automatized.
22 In order to test these hypotheses, follow-up examinations are planned for the participants of the study.
Our findings suggest that parameters related to monocular accommodation were improved by exercises aimed at treating vergence abnormalities and vice versa. This might be explained as a consequence of both types of exercises stimulating both vergence and accommodation, although the underlying mechanism may differ. Thus, vergence exercises stimulate both systems directly whereas monocular accommodation exercises stimulate accommodation directly and vergence eye movement indirectly in the occluded eye but without feedback to the vergence system.
Although vergence and accommodation loops can be opened by occlusion and nonaccommodative targets,
23 the interaction between the two systems cannot be completely cancelled out due to their neurological coupling, hence it is less likely that exercises targeting each of the systems separately can be designed.
The finding that vergence exercises induced a more rapid recovery of positive fusional vergence and vergence facility may be due to the fact that these parameters require an adaptive response to recalibrate the interaction between vergence and accommodation implying that adaptive ability is essential for efficient improvement of the vergence system.
24,25 The positive effect of monocular accommodation exercises on vergence parameters may be explained by a more rapid and precise accommodation, which improves performance during vergence assessment. The similar recovery rate observed for NPC with both types of exercises could be due to similar stimulus demands to vergence and accommodation during assessment of NPC.
All monocular accommodation parameters improved independently of the treatment regime. Although monocular accommodation exercises are a direct treatment of accommodation dysfunction, the effect of vergence exercises on accommodation parameters could be explained by the fact that vergence exercises stimulate both vergence and accommodation to maintain single and clear vision. Similarly, the identical improvement of subjective symptoms resulting from the two treatment regimens suggests that parameters that can be measured objectively must normalize before subjective symptoms disappear. Also of relevance, there was no difference in the effects of the two treatment regimens at the end of the 12-week treatment period. This study corroborates previous findings
5,6 that a vast majority of children with CI have accommodation insufficiency, which according to clinical guidelines should be treated with a combination of vergence and accommodation exercises.
8 The similar rate of recovery of monocular accommodation parameters in the two treatment groups imply that vergence exercises and monocular accommodation exercises are equally efficient at treating accommodation insufficiency. However, this requires further investigation.
The treatment phases in the present study lasted 6 weeks, and since the effect on the studied parameters had not leveled out at that time, it cannot be excluded that a continuation of each of the interventions alone might have resulted in an effect similar to that obtained by the two treatment regimens applied in succession. An evaluation of the effects of the interventions with a longer follow-up should be the subject of a future study. The study has shown that vergence exercises are a more efficient approach to treating CI in school children in the short term, both by the higher proportion of patients achieving recovery and by the faster onset of the effect.
The finding that vergence exercises accelerate improvement for children with convergence insufficiency has the potential to improve compliance with treatment and thereby long-term outcome.
The child was seated at a table with an Aperture Ruler (Bernell Corporation, catalog no. BC1050BK), which consists of a ruler-like apparatus, a plastic slide with a single aperture, and 12 cards with varying convergence demands. The child was asked to look through the aperture and to fuse the two pictures into a single and clear image, report perceived depth in the eccentric rings, and that the two suppression control objects were seen. Once all these tasks could be achieved, the convergence demand was increased by proceeding to the next card.
The child alternated focus between two 0.4 logMAR letters charts at 4 m and 40 cm, respectively, reading aloud the first line at the distance chart, the second on the near chart, and so on.
After 1 week, the level of difficulty was increased by changing the letter size to 0.3 logMAR and decreasing the viewing distance of the near chart to 20 cm.
The child viewed a 0.4 logMAR letter chart at 4 m through a −3.00 D lens held at 30 cm vertex distance and when clarity was obtained the accommodative demand was gradually increased by slowly moving the lens toward the eye until clarity could no longer be obtained. The child then moved the lens back and forth attempting to reduce the vertex distance. The procedure was repeated while the child viewed 0.4 logMAR letters at 40 cm through a +1.00 D lens.
After 1 week of home exercises, the level of difficulty was increased by changing the letter size to 0.3 logMAR and the lenses to −6.00 D and +2.50 D, respectively.
The child was provided with 4 lenses with the power of, respectively, −2.00 D, −1.00 D, +1.00 D, and +2.00 D labeled with random letters. The child was instructed to look at 0.2 logMAR letters at 40 cm through each of the lenses, obtain clarity, and to sort the lenses from weakest to strongest.