Near Addition Lenses for Myopic Children
Study Overview
• The study aimed to compare two methods for determining personalized near addition lens values for myopic children:
1. Sheard’s Criterion for binocular comfort.
2. Null Lag of Accommodation for accommodative accuracy.
• Participants: 53 myopic children aged 8-11 years with myopia between -1.0D to -3.0D.
Key Concepts
• Accommodative Lag: A common issue in myopic children where the eye focuses light behind the retina during near work, leading to hyperopic defocus and axial elongation.
• Phoria (Ph): Binocular alignment, with focus on esophoria (inward drift) and exophoria (outward drift) affecting lens effectiveness.
• Sheard’s Criterion: Fusional vergence should be at least twice the phoria for comfortable binocular vision.
• FA/Ph Ratio: Ratio of fusional amplitude to phoria used to assess binocular balance.
Findings on Near Addition Lenses
• Lag of Accommodation decreased linearly with increasing addition lens power:
• From 1.33D (no addition) to 0.50D with +2.00D, and to -0.12D with +3.00D.
• Near addition lenses caused an exophoric shift, especially with higher additions.
• Adaptation Effect: After 6 minutes of near work, a small but significant increase in accommodative lag and exophoric shift was noted.
Comparison of Addition Values
• Sheard’s Criterion: Optimal addition value averaged +2.16D ± 0.79D.
• Null Lag of Accommodation: Optimal addition value averaged +2.83D ± 0.44D.
• Addition values based on Sheard’s Criterion provided better binocular balance, especially in children with exophoria.
Clinical Implications
• Children with Esophoria: Benefit more from higher addition values as it balances accommodation and binocular vision.
• Children with Exophoria/Orthophoria: Lower addition values based on Sheard’s Criterion are more suitable to avoid increased exophoria.
• Addition lenses designed using Sheard’s criterion induced only a minor accommodative lag (0.38D ± 0.42D), providing better comfort.
Management Recommendations for School Myopia
• Use personalized near addition lenses instead of standard +2.00D PALs for effective myopia control.
• Monitor phoria status and accommodative lag to determine the best addition value.
• Low-dose atropine or plus lenses for near work may be considered for children with persistent high accommodative lag.
• Encourage outdoor activities to naturally slow myopia progression.
Additional Observations
• Near work adaptation to addition lenses occurs within the first 3–6 minutes but has minimal long-term impact on lens effectiveness.
• Binocular balance must be prioritized to ensure lens comfort and effectiveness in slowing myopia progression.
These insights highlight the importance of customizing near addition prescriptions for children to manage myopia progression effectively while maintaining comfortable binocular vision.
Detailed Insights on Study Methods and Results
Study Methods
• Participants: 53 myopic children aged 8–11 years (mean age: 9.4 ± 0.95 years) with myopia between -1.0D to -3.0D and a progression of at least 0.5D in the past year.
• Phoria Classification:
• Esophoria: 20 children (2.0 to 15.5 prism diopters [pd])
• Exophoria: 12 children (-2.0 to -5.5 pd)
• Orthophoria: 21 children (-1.5 to +1.5 pd)
• Testing Conditions:
• Measurements taken at 33 cm using addition lenses: 0D, +1.00D, +1.50D, +2.00D, +2.50D, +3.00D.
• Adaptation effects assessed after 6 minutes of near work with each lens.
• Accommodative response measured with a Shin Nippon open-field autorefractor.
Detailed Results
1. Adaptation Effect on Accommodation and Phoria:
• After 6 minutes of near work, a small but statistically significant increase in accommodative lag and a slight exophoric shift were observed.
• However, these changes were not clinically significant.
2. FA/Ph Ratio Patterns Observed:
• Orthophoria/Exophoria: FA/Ph ratio decreased with increasing addition power.
• Large Esophoria: FA/Ph ratio increased as addition power increased.
• Low Esophoria: FA/Ph ratio initially increased, then decreased with higher addition values.
3. FA/Ph vs. Null Lag Comparison:
• Addition values based on Sheard’s Criterion averaged +2.16D ± 0.79D.
• Addition values for null accommodative lag averaged +2.83D ± 0.44D.
• 75.5% of subjects using null lag values had an FA/Ph ratio below 2.0, indicating poor binocular balance.
4. Exophoric Shift with Higher Additions:
• Increasing addition power induced a linear shift toward exophoria, more noticeable in children already having exophoria.
• This exophoric shift could reduce the effectiveness of positive-lens treatment for exophoric children.
5. Impact of Prism Correction:
• Combining addition lenses with base-in prisms (BI) for esophoric children significantly improved accommodation and phoria balance.
• Example: +2.25D addition combined with 3 pd BI in each eye effectively minimized both accommodation lag and addition-induced exophoria.
Clinical Applications and Recommendations
Practical Implications for Myopia Control
1. Customized Lens Prescription:
• Sheard’s Criterion-based additions are more effective for balancing binocular vision in myopic children, especially those with exophoria or orthophoria.
2. Avoid Overprescription of High Additions:
• For highly esophoric children, too high an addition power may cause discomfort. In such cases, addition values are limited to +3.50D.
3. Importance of Adaptation:
• Short-term adaptation (~6 minutes) slightly affects accommodative lag and phoria but does not significantly impact clinical outcomes.
4. Need for Regular Monitoring:
• Regular follow-ups are necessary to monitor accommodative lag and phoria to adjust the near addition power appropriately.
5. Use of Prismatic Bifocals:
• Incorporating base-in prisms in bifocal lenses could provide better myopia control, especially for esophoric children.
Comparison with Previous Studies
1. Comparison with COMET Trial:
• The Correction of Myopia Evaluation Trial (COMET) used a standard +2.00D addition in PALs, which had mixed results.
• The current study suggests that personalized near additions are more effective than fixed additions for controlling myopia.
2. Alignment with Cheng et al. and Jiang et al.:
• Cheng et al.: Used +2.25D addition with 6 pd BI prisms for optimal results.
• Jiang et al.: Found +1.28D as the optimal near addition for young adults, but this study reports a higher requirement (+2.16D) in children.
Limitations and Considerations
1. Adaptation Time Limit:
• Only 6 minutes of adaptation was tested; longer adaptation might reveal more significant changes.
2. Testing Position:
• All tests were conducted in the primary gaze position. Testing in a downward gaze (reading posture) could yield more accurate phoria measurements.
3. Accommodation Testing Order:
• Measurements were taken from highest to lowest addition values, potentially introducing adaptation bias.
Final Clinical Insights
• Personalized near addition lenses based on Sheard’s Criterion are more effective for achieving binocular balance than those based solely on null accommodative lag.
• Managing school myopia requires a holistic approach that includes:
• Customized lens prescriptions.
• Prism incorporation where necessary.
• Regular monitoring and adjustments.
• Encouragement of outdoor activities and reduced near work strain.
• Ongoing clinical trials (like PACT) aim to confirm the long-term benefits of personalized near addition lenses in myopia control.