Diplopia, or double vision, is a common complaint. Diplopia results from a faulty alignment of the eyes. Normally, when the eyes fixate on an object, the object is seen clearly, despite the fact that the two retinal images are not exactly superimposed. These slightly different images, however, are fused by the brain;it is this fusion that produces binocular vision, or the perception of depth. When the eyes are misaligned, the two images fall on different parts of the retinas, only one falling normally on the fovea. The field of vision of the deviated eye is different, so that its image is not projected on its fovea;therefore, this second image is different and not superimposable. The patient may close one eye to relieve this distressing situation. A compensatory head posture may be used by the patient to relieve the double vision (see Fig. 10-146). Elevation or depression of the patient's chin is used to overcome a vertical deviation. Tilting of the head is often used to counteract the torsional and vertical deviation. Suggested questions for the patient with diplopia are listed in Chapter 21, The Nervous System.
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