The complaint of double vision suggests that the separate eyes are not both fixed on the point of regard. The eye that is "off line" sees the object of regard but it appears displaced. This failure of the eyes to work together is because of malfunction of one or a group of eye muscles or the neurological mechanisms that control them.
From the clinical point of view, it is convenient to divide the eye muscles into horizontal and vertical groups. The horizontal muscles, the medial and lateral recti, are easy to understand because their actions are in one plane and they simply adduct (turn in) or abduct (turn out) the globe. The vertical recti are best considered as having primary and secondary actions. It is important to realise that the action of the vertical recti changes with the position of the globe. For example, when the eye is abducted the superior rectus elevates the globe, but when the eye is adducted the superior rectus rotates the eye inwards round an anterior-posterior axis (intorts). In a similar manner, the inferior oblique elevates the adducted eye and extorts the abducted eye (Figure 22.8). In order to test the action of the superior oblique muscle, one must first ask the patient to adduct the eye and test for depression in adduction. That is to say, a superior oblique palsy prevents the eye from looking down when it is turned in. The main line of action of the vertical recti is seen when
the eye is abducted and that of the obliques is seen when the eye is adducted.
Examination of a patient with double vision entails first of all testing the gross eye movements in the cardinal positions of gaze and then noting the degree of separation of the images in these various positions. The Hess chart is one of several ingenious methods of recording the abnormal eye movements. The principle is to place a green filter before one eye and a red filter before the other and to ask the patient to look at a screen on which are placed a number of small illuminated white dots. The patient is then asked to localise the dots with a pointer. The amount of false localisation can then be measured in all positions of gaze. This technique is invaluable when assessing the recovery of an ocular muscle palsy.
Young children adapt to double vision rapidly by suppressing the image from one eye, and under the age of eight years the suppression can lead to permanent amblyopia if the situation is not relieved. In adults, the double vision may persist and be disabling for months or even years if not treated by incorporating prisms into the spectacles or by muscle surgery.
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