 Periodic alternating sandwich are another variety of horizontal nystagmus. In this disorder, note how the nystagmus are beating to the right, slow down, and then shift direction. Now they'll start beating to the left. This cycle can repeat itself indefinitely as long as forward gaze is maintained. Occasionally, alternating positions of gaze will be seen to maintain the eyes in their relative null position. This disorder is, again, localizing and has approximately the same localizing value as downbeat nystagmus. Rarely, as in this patient's case, they can be seen on a congenital basis. Note how the nystagmus maintain horizontal direction in all positions of gaze, including up gaze. This is also seen, if you recall, in congenital nystagmus.