• Test the function of the third, fourth, and sixth cranial nerves together.
• Have the patient seated, facing you.
• Put your left hand on top of the patient's head so it will not move when he is moving his eyes from side to side.
We test saccadic movements (command) by asking the patient to look right, left, up, and down and pursuit movements by having him follow an object. Test both. Some diseases affect one type of movement but not the other.
1. Hold a pen 0.5 m in front of the patient's nose. If your pen is any closer, you are making his eyes converge at the same time the other movements are being examined.
2. When examining lateral eye movements, hold the pen vertically; when testing up-and-down movements, hold the pen horizontally. If you test lateral eye movements by holding the pen horizontally, the patient will have to separate the two images by the length of the pen in order to see two pens. If you hold the test pen vertically, he will need only to separate the two images by the width of the pen in order to see two.
3. Move your pen from side to side, asking the patient to follow it with his eyes. The area of lateral binocular vision is limited by the bridge of the nose and how far the eyes are sunk into the head. From Figure 4-1 it is clear that double vision can occur only when an object is between C and B. There cannot be binocular double vision lateral to C or B.
4. Test the vertical movements once with the patient's eyes turned to the right halfway between looking straight ahead and an extreme right lateral gaze, and once with the eyes to the left, halfway between looking straight ahead and an extreme left lateral gaze.
5. Test the lateral movements to the right and left with the eyes starting in the primary position. Stay in the binocular field (Figure 4-1).
In the analysis of any diplopia, put the eyes in the position that causes the greatest separation of the two images.
For example, let us assume that the separation is greatest on looking up and to the left.
The false image is the nonmacular, less distinct, image. It is always the outside image; that is, if the diplopia is lateral with two images side by side, the image farther away from the midline is the false one. If the diplopia is vertical with two images one on top of the other and maximum separation occurs when the patient looks up, the false image is the one closer to the ceiling. With a vertical diplopia and maximum separation when looking down, the false image is closer to the floor. This is true whether the ocular palsy is divergent or convergent.
The next step is to cover either eye. Ask the patient which image disappears. Give him several chances while you slowly cover and uncover one of his eyes and he holds his gaze so that the two images are maximally separated. If he tells you the near or inside image always disappears when you cover his right eye, then the outside image must vanish when you cover his left eye. The false image is coming from the left eye, and the left eye is thus the paretic one.
Determine what muscle moves the eye in this particular direction and what is the nerve supply of that muscle.
Maximal separation is up and to the left, and the false image is from the left eye. The elevator of the left eye when the eye is abducted is the superior rectus, the nerve supply of which is the third cranial nerve. Therefore, the cause of diplopia is a partial left third nerve palsy.
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