A cover-uncover test may reveal a slight or latent muscle imbalance not otherwise seen (see p. 182).

Now assess the extraocular movements, looking for:

■ The normal conjugate movements of the eyes in each direction, or any See Table 5-10, Deviations of the deviation from normal Eyes (p. 182).

■ Nystagmus, a fine rhythmic oscillation of the eyes. A few beats of nystagmus on extreme lateral gaze are within normal limits. If you see it, bring your finger in to within the field of binocular vision and look again.

Sustained nystagmus within the binocular field of gaze is seen in a variety of neurologic conditions. See Table 16-9, Nystagmus (pp. 610-611).

■ A lid lag as the eyes move from above downward.

Lid lag of hyperthyroidism

To make these observations, ask the patient to follow your finger or pencil as you sweep through the six cardinal directions of gaze. Making a wide H in the air, lead the patient's gaze (1) to the patient's extreme right, (2) to the right and upward, and (3) down on the right; then (4) without pausing in the middle, to the extreme left, (5) to the left and upward, and (6) down on the left. Pause during upward and lateral gaze to detect nystagmus. Move your finger or pencil at a comfortable distance from the patient. Because middle-aged or older people may have difficulty focusing on near objects, make this distance greater for them than for young people. Some patients move their heads to follow your finger. If necessary, hold the head in the proper midline position.

In paralysis of the CN VI, illustrated below, the eyes are conjugate in right lateral gaze but not in left lateral gaze (left infranuclear ophthalmoplegia)

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