Prism adaptation changes over time as the eye absorbs the correction. Prism adaptation is present in patients and the mechanism is related to changes in the tonic position of the vergence system. The vergence system has both fast and slow components and prism adaptation is different at both distance and near. Patients should be assessed before being prescribed prism corrections.

1. Prism Adaptation ∆ correction changes with time It appears that ∆ is absorbed Is it worth giving ∆? Who can be prescribed ∆? How can we check who will benefit?

2. Tonic Vergence Maddox 1893 Alpern 1946 Ellerbrock 1950 Carter 1963 Henson and North 1980

3. Measuring Adaptation “ Flashed” Maddox Rod Measure baseline phoria Insert ∆ Measure new “phoria” Remove ∆ Measure new “phoria”

4. Vertical ∆ Adaptation

5. Vertical ∆ Adaptation

6. Distance Horizontal ∆ Adaptation

7. Distance Horizontal ∆ Adaptation

8. Near Horizontal ∆ Adaptation

9. Near Horizontal ∆ Adaptation

10. Slope Relates to Adaptation

11. Adaptation

12. Rate of Adaptation

13. Amplitude of Adaptation

14. Amplitude of Adaptation

15. Symptomatic Patients Patients with BV problems have abnormal adaptation Abnormality greatest at distance producing symptoms Horizontal & vertical systems are functionally independent Poor correlation between phoria, FD, fusional reserves and adaptation

16. Effect of Age: Convergent

17. Effect of Age: Divergent

18. Effect of Age: Convergent

19. Effect of Age: Divergent

20. Clinical Assessment Place ∆ before eye Allow binocular viewing Leave patient for 3 minutes Measure phoria Compare to baseline measure of phoria

21. Tonic Vergence Tonic position starting point for all vergence responses Additional demand induces shift in starting point Fusional effort in each direction altered To restore relative balance the tonic position shifts

22. Model of Adaptation Fast fusional vergence: responds to retinal disparity Slow fusional vergence: driven by output of the fast system

23. Computer Simulation

24. Model (Schor 1980)

25. Summary Prism adaptation present in patients Mechanism of adaptation related to tonic position Vergence system has fast and slow components Adaptation different at Distance and Near Assess patients prior to giving ∆