====== AC/A Ratio ======
Accommodative Convergence to Accommodation Ratio

===== Gradient Method =====
  * Ratio = Change in deviation / change in accommodation
  * Fixation distance is fixed.
  * Measure deviation with and without a modifying lens
  * The lens is changing the amount of accommodation
  * **Stimulate Accommodation**
    * Measure alignment fixating at 6 meters    * Remeasure with a -1.00 lens before both eyes
    * Calculate the difference between the two alignment measures to get the AC/A ratio
  * **Relax Accommodation**
    * Measure alignment fixating at 0.33 meters
    * Remeasure with a +3.00 lens over both eyes
    * Divide the difference in the two alignment measures by 3 to get the AC/A ratio
  * Results
    * Average: 3.7 
    * Range: 0.9 - 9.8
    * Low: 0 - 2.0
    * Normal: 2.5 - 5.0
    * High: > 5.0

===== Heterophoric Method =====
  * Assumes that convergence is wholly due to accommodation and ignores tonic convergence or proximal convergence (fusion) mechanisms
    * AC/A = Pupil Distance + (Dn - D0/ D)
      * Dn = alignment measurement at near
      * D0 = alignment measurement at distance
      * D = diopters of accommodation
  * Results
    * Average: 4.4
    * Range 2.7 - 7.7 

===== Slope Gradient Method =====
  * Measure the alignment at a set distance with the following lenses: +3.00, +1.00, -1.00, -3.00  
  * Graph values and determine slope 

===== Factors that Affect the AC/A ratio =====
  * Glasses/Bifocals
  * Anticholinesterases
  * Surgery
  * Time
  * Orthoptics