In the context of eye movement control, an “internuclear” lesion is one that disrupts the medial longitudinal fasciculus (MLF), a bundle of fibers that connects the CN VI nucleus on one side of the pons to the medial rectus subnucleus (of CN III) on the contralateral side of the midbrain (see Chapter 1, Fig 1-27). This type of lesion produces an internuclear ophthalmoplegia (INO).
The cardinal sign of a unilateral INO is slowed adducting saccadic velocity in 1 eye, usually associated with abducting nystagmus of the fellow eye. The eye with the slowed adduction may have a full or limited range of adduction (Video 7-1; Fig 7-5). Convergence may be spared or disrupted. A skew deviation, often with hyperdeviation ipsilateral to the lesion, may be present. The INO is named for the side with slowed adduction (ie, a right INO refers to slowed adduction of the right eye secondary to a MLF lesion on the right side of the brainstem). Patients with INO may report horizontal diplopia; they may also experience vertical-oblique diplopia due to an associated skew deviation, episodic diplopia related to head–eye movements if the lesion is partial, or difficulty tracking fast-moving objects (eg, when playing sports) because of the mismatch in saccadic velocity between the eyes.
Left internuclear ophthalmoplegia.
Courtesy of M. Tariq Bhatti, MD.
Access all Section 5 videos at www.aao.org/bcscvideo_section05.
A bilateral INO classically produces bilateral adduction lag, and bilateral abducting nystagmus. It can also cause vertical, gaze-evoked nystagmus that is most noticeable in upgaze. This nystagmus is due to disruption of vertical vestibular pursuit and gaze-holding pathways, which ascend from the vestibular nuclei through the MLF. A large-angle exotropia may occur in bilateral INO (ie, “wall-eyed” bilateral INO, or WEBINO, syndrome) and is often caused by a midbrain lesion near the CN III nuclei.
Figure 7-5 Bilateral internuclear ophthalmoplegia (INO) in a 53-year-old man with diplopia on lateral gazes. A, Horizontal gaze in either direction results in full abduction of the ipsilateral eye but virtually no adduction of the contralateral eye. Alignment in primary gaze (center panel) is nearly orthotropic. B, Axial fluid-attenuated inversion recovery (FLAIR) magnetic resonance imaging (MRI) brain scan showing edema (bright signal indicated by arrows) in the area of the medial longitudinal fasciculus bilaterally at the level of the upper midbrain (left) and pons (right).
(Courtesy of Prem S. Subramanian, MD, PhD.)
The 2 most common causes of INO are demyelination and stroke. In adolescents and younger adults, INO is typically caused by demyelination. In older adults, microvascular disease is the most common cause. Other causes include infection, neoplasm, trauma, and progressive supranuclear palsy (PSP). Myasthenia gravis can produce pseudo-INO, which usually lacks the vertical gaze–evoked nystagmus of a true INO and is often accompanied by myasthenic eyelid signs. The adduction paresis of myasthenic pseudo-INO may resolve transiently after intravenous administration of edrophonium and typically responds to appropriate systemic therapy (see Chapter 14).
Amezcua L, Morrow MJ, Jirawuthiworavong GV. Multiple sclerosis: review of eye movement disorders and update of disease-modifying therapies. Curr Opin Ophthalmol. 2015;26(6): 534–539.
Frohman TC, Galetta S, Fox R, et al. Pearls & Oy-sters: the medial longitudinal fasciculus in ocular motor physiology. Neurology. 2008;70(17):e57–e67.
McGettrick P, Eustace P. The W.E.B.I.N.O. syndrome. Neuro-Ophthalmology. 1985;5:109–115.
Mills DA, Frohman TC, Davis SL, et al. Break in binocular fusion during head turning in MS patients with INO. Neurology. 2008;71(6):458–460.
Figure 7-6 One-and-a-half syndrome. This 15-year-old patient had a brainstem glioma that caused a gaze palsy to the left (right photograph) and a left INO (evident here as incomplete adduction of the left eye on gaze to the right; left photograph). The only intact horizontal eye movement was abduction of the right eye.
(Courtesy of Steven A. Newman, MD.)
Excerpted from BCSC 2020-2021 series: Section 5 - Neuro-Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.