Both the fixation disparity and strabismus are deviations under conditions that allow fusion, but the fixation disparity is so small that the images in the 2 eyes, although not exactly on the foveas



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  1. What is the difference between a fixation disparity and a strabismus?

Both the fixation disparity and strabismus are deviations under conditions that allow fusion, but the fixation disparity is so small that the images in the 2 eyes, although not exactly on the foveas, are within Panum’s fusional areas so the person sees single; in strabismus the image for the turned eye is outside of Panum’s fusional area and without suppression the person sees double.

  1. Explain how a person could have an eso dissociated phoria and an eso fixation disparity.

With an eso dissociated phoria the eyes want to turn in when fusion is broken. When the person is allowed to look with both eyes they use NEGATIVE FUSIONAL VERGENCE to turn their eyes out to fuse. If NFV does not correct for the entire eso dissociated phoria the person is left with a residual eso deviation when fused, which is the eso fixation disparity.

  1. How could a person have an exo dissociated phoria but an eso fixation disparity? Why is this uncommon?

With an exo dissociated phoria the eyes want to turn out when fusion is broken. When the person is allowed to look with both eyes they use POSITIVE FUSIONAL VERGENCE to turn their eyes in to fuse. Sometimes fusional vergence overcorrects for the phoria, and in this case the person with the exo phoria would have an eso fixation disparity. This is uncommon because we think of the visual system as not wasting energy by overcorrecting.

  1. How does base-out prism induce an exo fixation disparity?

Base out prism causes convergence. As you increase the amount of BO prism, at some value the eyes do not converge enough to exactly meet the prism demand (the images are not exactly on the foveas, but are still within Panum’s fusional areas, so you have created a fixation disparity). If you are ‘not converged enough’, you are ‘too diverged’, and that is an exo fixation disparity.

  1. When a person with a Type I forced vergence – fixation disparity function has a steep slope (greater than 1) should they have comfortable binocular vision? Why or why not?

A steep slope means that when just small increases in vergence demand are imposed on the vergence system (in the form of increasing prism), vergence does not change as much as the demand, leaving progressively larger fixation disparities. The vergence system is said to respond poorly to vergence stress, so no, binocular vision should not be comfortable.

  1. The forced vergence – fixation disparity function through –1.00 D lenses is shifted to the base-out side of the graph; explain why.

Minus lenses stimulate accommodation, and with accommodation you automatically get accommodative convergence. So you start out more converged before you even start adding prism. This shifts the entire curve to the more converged BO side of the graph.

  1. The forced vergence – fixation disparity function through +2.00D lenses is shifted to the base-in side of the graph. What ‘fixation disparity type’ would be helped by added plus power? Why?

Plus lenses cause relaxation of accommodation, and an automatic relaxation of accommodative convergence. So you start out more diverged before adding prism, which shifts the curve to the more diverged BI side of the graph. Type IIIs handle BO prism normally, but typically they cannot take very much BI before lose fusion. Adding plus power moves their Type III graph from the right to the left so that it is more centered, and this allows them to take more BI prism before they see double.

  1. If you know what the fixation disparity of your patient is, can you ‘calculate’ the associated phoria? Why or why not?

You cannot calculate the associated phoria from the fixation disparity because everyone’s forced vergence – fixation disparity curve that relates the associated phoria to the fixation disparity is different. You must measure each (except when the fixation disparity is zero arcmin, you need zero prism diopters to fix it).

  1. With which instrument did you measure both the fixation disparity & the associated phoria?

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