Calculation of relationship among accommodation and convergence
Introduction
Convergence is driven by third cranial nerve. Convergence is connected with accommodation and miosis. According the theoretical calculation convergence on 1 meter is equal to pupillary distance in centimeters. Calculation is derived from definition of 1 prismatic diopter which is defined as light beam 1 cm deviation on 1 m.
Goals
- Measure pupillary distance on far
- Calculate theoretical size of convergence on given distance
- Calculate theoretical value of accommodation convergence to accommodation
Equipment
Digital or manual PD meter, writing equipment, calculator.
Methods
Measure pupillary distance on far
Choose one method for distance PD measurement – direct method, infinity method or digital PD meter and measure distance PD
Calculate theoretical size of convergence on given distance
Calculate needed convergence (K) on distance 1 meter and standard reading distance (a = 40 cm).
Calculate theoretical value of accommodation convergence to accommodation
For calculation we need to know the size of the convergence K1 in accommodation 1 D (1 meter) and size of the convergence K2 in accommodation A = 2 D (distance a = 0.5 m).
Results
Measure pupillary distance on far
PDP = ………………. cm
Calculate theoretical size of convergence on given distance
K = ………………cm.m-1 [pD]
Calculate theoretical value of accommodation convergence to accommodation
AC/A = …………….. pD/D
Discussion
Cooperation of the accommodation and convergence is important for appropriate function of accommodative process. In case of inadequate values there are used compensation mechanisms, which is for example fusion vergence. Ocular muscles working with low energy, so in practice real convergence values are lower than theoretical. Particular values of accommodation and convergence established the line of direct proportionality which is called Donder’s line.
Conclusion, notes, comments
Who will have higher needs on convergence? Patient with lower or higher PD? Patient working on 1 meter or 30 cm?
Do the spectacles influence convergence of the patient?
What is the average values for AC/A?