The Internuclear Ophthalmoplegias

Abstract

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].