6533b828fe1ef96bd12881f1

RESEARCH PRODUCT

The Internuclear Ophthalmoplegias

subject

description

Internuclear ophthalmoplegia (INO), which is caused by an ipsilateral medial longitudinal fasciculus (MLF) lesion, is characterized by adduction paresis of lateral gaze, usually with spared convergence [1–4]. In the opposite eye, abduction nystagmus and hypermetric abduction saccades are the main clinical and electro-oculographic abnormalities [1, 5, 6]. The origin of both is still debated. Abduction nystagmus has been explained by (a) an additional horizontal gaze paresis [7]; (b) vergence mechanisms aimed at alignment of the visual axes [8]; (c) interruption of descending excitatory projections from oculomotor nucleus internuclear neurons to contralateral abducens nucleus motoneurons [9]; (d) impaired inhibition of the medial rectus muscle of the abducting eye [10, 11]; or (e) adaptation to contralateral medial rectus undershoot [12]. Hypermetric abduction saccades reflect an increased phasic innervation [13, 14] thought to (a) account for the medial rectus paresis of the contralateral eye [12, 15] or (b) overcome an increased traction of the ipsilateral medial rectus due to deficient inhibition of the tonic resting activity during abduction movements [11]. Slowing of abduction saccades is another frequent additional feature in INO [11, 16–19] that has been attributed to various conditions such as (a) horizontal gaze paresis [16–18], (b) impaired medial rectus inhibition [11, 19], or (c) intranuclear intrapontine sixth nerve lesions [20].