12.1 Overview 157
■Possible presence of AHP, strabismus, and refractive errors
■Decreased amplitude of nystagmus in null point
■Dampening of nystagmus on convergence
■The intensity of nystagmus increases with fixation, decreases with sleep or inattention
12.1.2Manifest Latent Nystagmus (MLN)
MLN is most commonly associated with infantile or childhood onset esotropia as well as ambylopia. MLN is defined as jerk nystagmus that develops at an early age and increases with monocular viewing, triggered by occlusion of one eye. Previously latent nystagmus was distinguished from MLN where no nystagmus was detected when both eyes were open. However, it has been shown that in cases clinically diagnosed as “latent nystagmus,” nystagmus is seen on eye movement recordings even when both eyes are open. Hence MLN/latent nystagmus is considered as a single entity (MLN).
Characteristically, the amplitude of MLN decreases in adduction and increases in abduction, with the fast phase of the nystagmus beating toward the side of the fixating eye or open eye. MLN has a distinctive slow phase with an exponentially decreasing or linear velocity in all positions of gaze as shown in Fig. 12.4. As nystagmus decreases in adduction in patients with MLN, they frequently develop an AHP toward the side of the fixating eye when the fellow
eye is occluded. The AHP changes to the other side in an alternating monocular occlusion, which helps in the diagnosis of MLN. If patients with MLN have alternating fixation the head turn can change spontaneously, depending on which eye is fixing. Figure 12.5e, f shows an alternating AHP to the right and left in one of our patients who had fusional maldevelopment syndrome with latent nystagmus confirmed on eye movement recordings. The patient has exotropia and is freely alternating. He is always keeping the fixing eye in adduction and therefore his head posture is alternating with a turn to the right with the right eye fixing and left with the left eye fixing. When one eye was patched his head turn was unidirectional in the direction of the open eye. The cause of MLN appears to be due to disruption of binocular vision during visual development, especially when the motion sensitive areas of the middle temporal and medial superior temporal cortex do not develop binocular function.
Patients can have a combination of congenital and latent nystagmus. According to Dell’Osso [2], 80% of nystagmus is congenital nystagmus, 15% is MLN, and 5% is a combination of both forms.
12.1.2.1Clinical Characteristics of Manifest Latent Nystagmus (MLN)
■Onset in infancy
■Nystagmus is horizontal and conjugate
■Associated with strabismus and amblyopia
Right Eye
Left Eye
LEFT EYE |
RIGHT EYE |
LEFT EYE |
BOTH EYES |
COVERED |
COVERED |
COVERED |
UNCOVERED |
RIGHT BEATING
R
10º
0.5 sec
L
Fig. 12.4 Original horizontal eye movement recordings of both eyes of a patient with manifest latent nystagmus (MLN) and exotropia during an alternating cover test. Eye movements to the right are represented by an upward deflection, and eye movements to the left by a downward deflection. The fast phase is always beating toward the open eye (to the right with the left eye covered and to the left with the right eye covered). When both eyes are open the direction of the fast phase is toward the dominant left eye. The velocity of the slow phase is decelerating or linear. Arrows indicate blinks