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Chapter 21 Key words: disparity vergence, prism, lens, eso, exo Outline Part IV: Accuracy of Vergence (Fixation Disparity)
If we mainly relied upon fixation disparity to overcome the phoria we would have considerable eye strain. However eye strain is relieved with adaptable tonic vergence. The stress associated with disparity vergence is manifest as a small residual error of convergence known as fixation disparity. |
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Fixation disparity (FD) is the difference between the convergence angle under binocular viewing and the angle subtended by the target at the centers of rotation. It is a failure of the two visual axes to simultaneously intersect the object of regard during attempted binocular fixation. It is also referred to as a micro strabismus because the eyes are not binocularly aligned during attempted fusion. Retinal slip is another synonym. The main difference between a strabismus and FD is that the patient is not fusing with a strabismus but are with FD. Fixation disparity is the residual error of a partially corrected phoria. Usually a large phoria underlies a small fixation disparity. FD is like the tip of the iceberg above the surface of the water and the phoria is the whole iceberg. Indirect measures are made with devices that allow the clinician to readjust the alignment of nonius lines until they appear aligned to the patient. One such device is the Disparometer. It contains many possible offsets of the nonius lines that can be selected by rotating a wheel behind the instrument. The patient views the polarized nonius lines that are centered in a field of binocular print. The wheel is rotated until the lines appear vertically aligned. This is a very sensitive binocular vernier task. It is important to ask the patient to keep the letters clear at all times to prevent accommodative fluctuations from causing fluctuations of accommodative vergence. Phoria |
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The fixation disparity can be plotted as a function of added base-in prism on the left and base-out prism on the right. As expected, base-in induces eso FD and base-out induces exo FD. This produces a sigmoidal function of FD plotted as a function of prism value. There are three key points on the FD curve. (See Fig 21.2 above) These are the Y intercept that represents the habitual FD when no prism is worn, the X intercept which is the associated phoria or the amount of prism that reduces FD to zero and the slope of the function between the X and Y intercept which represents how adaptable the vergence system is to prism. It shows how well the tonic system is able to relieve the effort produced by the phasic disparity vergence system that is stimulated by prism. The relationship between the amount of FD and the balanced activity of tonic and phasic vergence is based on the differences in ability of phasic and tonic vergence to sustain responses. Phasic is fast but it is not sustained. Tonic is slow but it is easily sustained. When phasic vergence is active, the vergence response tends to slip or go into error which stimulates further effort to maintain the response. When the tonic response is present, vergence doesn't decay or slip and no extra effort is needed to maintain the response. When the tonic adaptation is incomplete, some fusional vergence is necessary, and fixation disparity acts as a stimulus to evoke the necessary fusional vergence needed to align the eyes precisely with the target of regard. FD is a reflection of the amount of fusional vergence that is active in the total vergence response. |
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These categories describe the degree of phasic and tonic activity of the vergence system for a given individual. The type I FD curve has symmetrical slopes for responses to base-in and base-out prism. In contrast, type II curves have steep slopes on the base-in side and shallow slopes on the base-out side. This indicates that there is a tonic prism adaptation response to the exophoria produced by base-out prism but that there is little tonic adaptation to the esophroia produced by the base-in prism and the phasic disparity vergence system is left to maintain eye alignment as best it can. It uses FD as an additional stimulus to help maintain its response. Type III curve is the mirror image of type II. Here the slope is steep on the base-out exo side and shallow on the base-in eso side. Interestingly these patients adapt readily to base-in prism, they diverge easily but they have trouble adapting to base-out prism and show lots of exophoria while wearing base-out. The type IV curve is anomalous. It has a very limited range due to the failure of vergence to respond to moderate amounts of prism. When large prism is placed before one of these patients, they become strabismic. In summary, we can think of the forced duction fixation disparity test as a provocative test similar to tests of interocular pressure after drinking water or tests of blood pressure and pulse rate after exercise. We want to know how the oculomotor system handles the stress of fusional disparity stimuli. If it copes well by adapting tonic vergence, there is little residual phoria, but if it is unable to fuse without extreme effort there is residual fixation disparity. We can estimate the underlying stress by measuring the associated phoria. This is a measure of the amount of fusional vergence yet to be adapted. The slope of the function between the x and y intercepts indicates how readily the vergence adapts to varying amounts of prism. If it cannot change its adapted state quickly, the slope will change steeply from one prism value to the next. However if the response is rapid and complete, the slope will be very flat. A flat slope is a sign of a highly adaptable system. A steep one is a sign of someone who needs to learn to adapt using orthoptics eye exercises. Lenses - Accommodative Vergence |
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