Lesions of the sixth nerve result in a paralysis of the abducens muscle and a resultant weakness of lateral or outward movement leading to a crossing of the visual axes. The affected eye deviates medially, that is, in the direction of the opposing muscle. Diplopia is experienced as horizontal separation that is greatest when viewing in the direction of the sixth nerve palsy and in the distance (Fig 13-7A). With incomplete sixth nerve palsies, turning the head toward the side of the paretic muscle overcomes the diplopia.
Many causes of combined ocular motor palsies, which are discussed in a later section, are listed in Table 13-3 and are illustrated in Fig. 13-7 and in the following text.
The Analysis of Diplopia
A common cause of binocular diplopia (i.e., seeing a single object as double) is an acquired paralysis or paresis of one or more extraocular muscles. The signs of the oculomotor palsies, as described previously, can manifest in various degrees of completeness. With complete palsies, the affected muscle can often be surmised from the resting dysconjugate positions of the globes. With incomplete paresis, noting the relative positions of the corneal light reflections and having the patient perform common versional movements will usually disclose the faulty muscle(s) as the eyes are turned into the field of action of the paretic muscle. The muscle weakness may be so slight, however, that no strabismus or defect in ocular movement is obvious, yet the patient experiences diplopia. It is then necessary to use the patient’s report of the relative positions of the images of the two eyes. Certain precautions should be taken in testing: one is cognizance of the absence of diplopia when the visual axes are widely separated and, the object or light used for testing should not be obscured by the patient’s nose.
Two rules are applied sequentially to identify the affected ocular muscle in the analysis of diplopia: 1.	The direction in which the images are maximally separated indicates the action of the pair of muscles at fault. For example, if the greatest horizontal separation is in looking to the right, either the right abductor (lateral rectus) or the left adductor (medial rectus) muscle is weak; if maximal when gazing to the left, the left lateral rectus and right medial rectus are implicated (Fig. 13-6A and B). As a corollary, if the separation is mainly horizontal, the paresis will be found in one of the horizontally acting recti (a small vertical disparity should be disregarded); if the separation is mainly vertical, the paresis will be found in the vertically acting muscles, and a small horizontal deviation should be disregarded.
2.	The second step in analysis identifies which of the two implicated muscles is responsible for the diplopia. The image projected farther from the center is attributable to the eye with the paretic muscle.
The simplest maneuver for the analysis of diplopia consists of asking the patient to follow an object or light into the six cardinal positions of gaze. When the position of maximal separation of images is identified, one eye is covered and the patient is asked to identify which image disappears. The red-glass test is an enhancement of this technique. A red glass is placed in front of the patient’s right eye (the choice of the right eye is arbitrary, but if the test is always done in the same way, interpretation is simplified). The patient is then asked to look at a flashlight (held at a distance of 1 m), to turn the eyes sequentially to the six cardinal points in the visual fields, and to indicate the positions of the red and white images and the relative distances between them. The positions of the two images are plotted as the patient indicates them to the examiner (i.e., from the patient’s perspective; Fig. 13-7). This allows the identification of both the field of maximal separation and the eye responsible for the eccentric image. If the white image on right lateral gaze is to the right of the red (i.e., the image from the left eye is projected outward), then the left medial rectus muscle is weak.