Module 30 
 

Home

Catalog

Contact us
 

 

Get Post-Test

 
     
 

Module 30:

More EOM Related Skills:
 

Section 2:

 Fusional Vergence Measurements
   
   
  Fusional vergence measurements measure the patient's binocular ability to maintain fusion under "stress".  The stress is supplied by increasing prism power that moves the retinal images slowly away from the foveas.  The eye muscles then have to work harder to move the images back to the foveas and maintain fusion.  Increasing prism power is applied until the eye muscles "give up" and fusion is broken.  These measurements can be useful in the evaluation of patients whose complaints may arise from poor fusional ability, such as "convergence insufficiency".

The ability to bring the eyes together (convergence) is called "positive fusional vergence" (PFV).

The ability to move the eyes apart (divergence) is called "negative fusional vergence" (NFV).

This test uses the Risley prisms in the phoroptor.

  • Both eyes are un-occluded with the distance or near manifest refractions in place.
  • The target is an isolated letter on the Snellen chart on the line above the line of best visual acuity of the poorer seeing eye.
  • The Risley prisms are placed before each eye, set up for horizontal power ("0"s at 90 degrees).  See the section below on the Risley prisms.
  • Instruct the patient to tell you if the letter becomes blurry, and when the letter becomes double.
  • Slowly move the prism dials simultaneously inward to increase base-in prism until blurring occurs.  Note the prism power (e.g. 2 D OD and 2 D OS = 4 D total) and continue adding prism power.  When the images break into two, note the prism power again.
  • From this point, slowly reduce the prism power before each eye until the image is single again (recovery).
  • Record the total (OD plus OS) prism power for each position: blur, break, and recovery.
  • Base-in prism tests NFV.  Use base-out prism to test PFV.  Test at distance and at near (40 cm).  The chart on the phoroptor near vision rod can be used for testing at near.
   
 

The Risley Prism

 

The Risley prism (RP) is mounted on the same arm as the cross-cylinder (CC) on the AO type phoroptor.

 

 

It swings into place over the lens opening.  There is one on each side.  They can be used individually, or in combination.

 

 

The prism can be rotated so that the base direction  can be aligned in any direction.  There are click stops for vertical (zero at 180 degrees) and horizontal (zero at 90 degrees) base orientations.  

 

 

Prism power is increased and decreased with a wheel that is opposite the zero mark on the prism.  A small arrow points to the prism power that is dialed in.  In the horizontal orientation (pictured above), moving the arrow temporally dials in base-out prism, and moving the arrow nasally dials in base-in prism.  The photo above demonstrates 3 D base out prism over the left eye.

Back to main text

 
  Back to top                   Get Post-Test