FunDamentals
A sustained misalignment of the eyes that is too small to disrupt foveal fusion?
Fixation Disparity
These are two primary clinical conditions that preclude a patient from being tested for fixation disparity
strabismus and amblyopia
the minimum amount of prism (or sphere) needed to eliminate fixation disparity
Associated Phoria
On an FD curve plot, the Y-axis intercept represents this value.
habitual fixation disparity (FD when no prism is present)
This common complaint of eye strain is a frequent indicator that FD is causing binocular vision dysfunction.
asthenopia
This type of fixation disparity stimulates convergence or positive fusional vergence (PFV).
Crossed (Exo)
To perform FD testing, a patient must possess this type of vision to ensure no suppression is occurring.
Single Binocular Vision
Unlike Associated Phoria, this type of phoria is measured while the eyes are dissociated using tests like the Cover Test or Maddox Rod
Dissociated Phoria
This curve type, found in 60% of patients, features a low, symmetric slope on both sides of the Y-intercept.
Type 1 (Normal)
This is the most common reason fusional vergence becomes overloaded, leading to a large fixation disparity.
weak accommodative vergence
This is the range, in minutes of arc, for a normal fixation disparity.
6 arc minutes exo to 4 arc minutes eso
This near-point testing card uses an arrow to help determine the magnitude and direction of the deviation.
Wesson Card (or Borish Card)
This is the amount of time you should have a patient wear a prism to check for prism adaptation before finalizing a prescription
15 minutes
A Type 1 curve is more likely to be symptomatic if the slope is greater than this value.
1’/Δ
When prescribing from a curve, clinicians should look for this area where vergence is the most stable.
flat region of the curve
If a target moves outside of this specific area, the patient will experience diplopia (double vision)
Panum’s Area
A fixation disparity test target must include these three elements to be valid.
binocularly-seen fusion lock, a portion seen only by the OD, and a portion seen only by the OS
Associated phoria testing is particularly useful for prescribing prism for this direction because the vergence mechanism is simpler and has less adaptation.
vertical prism
This curve type represents unstable binocularity and is often seen in patients with a history of treated strabismus.
Type 4 curve
Patients with a Type 2 (Eso) curve generally benefit from this type of lens addition.
plus add lenses
Because the center of a test target is straight ahead, the perceived deviation of a line is always this in relation to the actual turn of the eye.
opposite
This device is noted for being prone to bias because all the potential answers/targets are lined up together.
Saladin Card
When comparing measurements, Associated Phoria tends to be higher than Dissociated Phoria for this type of deviation.
Esos
This specific point on the FD curve represents the Associated Phoria.
X-axis intercept
While Type 2 and Type 3 curves may require prism or lenses, patients with a Type 1 curve usually respond well to this treatment.
Vision Therapy (VT)