Internuclear Ophthalmoplegia

From EyeWiki

Internuclear Ophthalmoplegia

Disease Entity


Internuclear ophthalmoplegia (INO) is an ocular movement disorder that presents as inability to perform conjugate lateral gaze and ophthalmoplegia due to damage to the interneuron between two nuclei of cranial nerves (CN) VI and CN III (internuclear). This interneuron is called the medial longitudinal fasciculus (MLF). The MLF can be damaged by any lesion (e.g., demyelinating, ischemic, neoplastic, inflammatory) in the pons or midbrain. The MLF is supplied by branches of the basilar artery and ischemia in the vertebrobasilar system can produce an ischemic INO.[1]

The MLF is a heavily myelinated nerve tract connecting the oculomotor nucleus (CN III) of the ipsilateral side with the paramedian pontine reticular formation (PPRF) and CN VI of the contralateral pons. Thus, demyelinating lesions in the midbrain or pons often produce a unilateral or bilateral INO in young patients.

The INO is characterized clinically by an ipsilesional adduction deficit (partial or complete) with a contralateral, dissociated, horizontal abducting saccade/nystagmus on attempted gaze to the contralesional side. Hering’s law of equal innervation has been hypothesized as a possible explanation for the dissociated contralateral horizontal gaze evoked nystagmus in the abducting eye. Increased innervation to the underacting adducting muscle would result in an enhanced stimulus to the contralateral abducting muscle.[2] There is slow adducting saccadic velocity in the affected side. A skew deviation with the ipsilateral hypertropic eye may be noted. Vertical gaze nystagmus may be noted on upgaze.

The INO can be unilateral or bilateral and may present with or without (neurologically isolated) other brainstem findings.


Associated Syndromes:


Wall-Eyed Bilateral Internuclear Ophthalmoplegia exists when there is bilateral damage to the MLF. This damage causes a primary position exotropia (eyes are looking at the opposite “wall.”, thus the possibly outdated term “wall eyed”). The most common etiology is infarction of the midbrain in older patients and demyelinating disease in young patients.[3] The exotropia (XT) is likely decompensation of fusional mechanisms and the XT is not present in every case of bilateral INO. Bilateral INO may also show vertical gaze evoked nystagmus on superior gaze.

See video: [1]


This is a less common variant of INO, similar to the WEBINO above. As in WEBINO, patients with a unilateral MLF lesion (monocular INO) have a primary position XT.[4]

One and one half syndrome:

This syndrome occurs when there is a lesion to the MLF and the PPRF or CN VI nucleus on the same side resulting in an INO in one eye and an ipsilateral horizontal gaze palsy[5].

Eight and a Half Syndrome:

This syndrome is characterized by having one-and-a-half syndrome and a facial fascicular nerve (CN VII) palsy. The fascicle of CN VII wraps around the nucleus of CN VI in the dorsal pons. There is conjugate horizontal gaze palsy on looking to one side followed by INO on looking to the opposite side, along with unilateral facial weakness. The close proximity of the PPRF, facial nerve nucleus and MLF located in the dorsal pons makes this syndrome much more likely. The lesion is most often vascular or demyelinating in the dorsal tegmentum of the caudal pons[6].

Half and Half Syndrome:

A syndrome that consists of an INO in one eye combined with an ipsilateral CN VI fascicular involvement with sparing of the sixth nerve nucleus. Thus, there is “half” of a horizontal gaze palsy (INO) plus an additional “half” (abduction deficit from CN VI fascicular palsy).

Posterior INO: (Lutz)

This syndrome is a rare ophthalmoplegia, either bilateral or unilateral that exhibits contralateral adducting eye (rather than abducting eye) nystagmus with abduction restriction on physical exam. It is the reverse of the typical INO, and although the lesion localization is not consistent, it likely is due to CN VI pre-nuclear input asymmetry[7].


A lesion in the MLF interrupts the neural communication to the CN III subnuclei that allows for conjugate horizontal gaze arising from the final common pathway for horizontal gaze (CN VI nuclei). An INO is a common presentation of multiple sclerosis (MS) in younger patients. In older people, stroke is a more common etiology. Less common causes for an INO include traumatic, neoplastic, inflammatory (e.g., sarcoid, Behçet disease, lupus), or infectious [e,g., cryptococcosis, Borrelia burgdorferi (Lyme disease)], nutritional and metabolic etiologies.[8],[9],[3] INO is rare in children but may result from neoplasms (e.g., medulloblastomas or pontine gliomas), hydrocephalus or similar etiologies to adults.[10]

Bilateral INO
INO pathway


Typically, the paramedian pontine reticular formation (PPRF) receives information from the higher cortical centers such as the frontal eye fields, occipital and parietal lobes and the superior colliculus. From the PPRF the signal travels to the ipsilateral nucleus for the abducens nerve (CN VI). The abducens nucleus would then send an excitatory signal through the MLF to the contralateral medial rectus muscle (CN III). The activation of the contralateral medial rectus and ipsilateral lateral rectus muscle produces horizontal conjugate eye movement. The side of the INO is named by the side of the adduction deficit, which is ipsilateral to the MLF lesion. Because of the close proximity of the the MLF to the midline, bilateral involvement may not be uncommon (BINO).


Symptoms of INO may vary in severity. Symptoms range from blurry vision, horizontal diplopia, difficulty in tracking high speed objects, or dizziness on lateral gaze. A good ocular examination is often all that is required to diagnose of INO. The patient may also complain of headache or other deficits due to the involvement of the brainstem. Vertical oblique diplopia related to associated skew deviation may also be another symptom.


The diagnosis is made clinically with testing of the eye’s ability to perform conjugate movements. In one review of INO, some of the most common causes included: 1) Infarction - 38%- Unilateral INO in 87% of cases; 2) Multiple sclerosis in 34%-Unilateral in 27%; and 3) Unusual causes included trauma, tentorial herniation, infection, tumor, iatrogenic injury, hemorrhage, vasculitis.[11]

Imaging tests such as a CT scan or MRI brain (stroke protocol and with contrast for demyelination) are ordered after a diagnosis is made to discover where the damage is located so that the physician can then assess which route to take based on what is causing the damage. If the etiology is inflammation or infection, high dose corticosteroids can help. If the etiology is MS and not relapsing-remitting, then treatment becomes harder. In general MRI is superior to CT scan for evaluation of INO. MRI should involve fine overlapping cuts to ensure detection of lesions which may be very small and may otherwise be missed.

Differential Diagnosis

The differential diagnoses include

  • Myasthenia gravis- This may present with pseudo-INO, but the vertical nystagmus on upgaze is not seen.
  • Third nerve palsy
  • Chiari malformation


Dalfampridine, a potassium channel blocker prescribed for gait impairment was used in in a case series and the authors reported improvement in saccades and ocular motility in patients with INO secondary to demyelination in Multiple sclerosis. Improved neuronal conduction along MLF has been discussed as a possible explanation for this effect seen[12]


The prognosis of most patients with an INO is good and most cases improve with time but the final outcome depends in part on treatment of the underlying etiology. Ischemic and demyelinating INO typically recover. The resolution may take 1 day to 1 year.[1] Rapid recovery is usually seen in the absence of other neurological signs including facial palsy, ataxia, vertigo, pyramidal tract dysfunction, sensory symptoms, and dysarthria.[13] Patients with WEBINO or WEMINO may benefit from patching, prism, or strabismus surgery to correct any residual primary position symptomatic deviation (XT) that does not recover.[14]


  1. 1.0 1.1 Kim, Jong S. "Internuclear ophthalmoplegia as an isolated or predominant symptom of brainstem infarction." Neurology 62.9 (2004): 1491-1496.
  2. Zee DS, Hain TC, Carl JR. Abduction nystagmus in internuclear ophthalmoplegia. Ann Neurol. 1987 Apr;21(4):383-8.
  3. 3.0 3.1 Shinoda, Koji, et al. "Wall-eyed bilateral internuclear ophthalmoplegia (WEBINO) syndrome in a patient with neuromyelitis optica spectrum disorder and anti-aquaporin-4 antibody." Multiple Sclerosis Journal 17.7 (2011): 885-887
  4. Jeon, Sang Beom, et al. "Wall-eyed monocular internuclear ophthalmoplegia (WEMINO) with contraversive ocular tilt reaction." Journal of Clinical Neurology 1.1 (2005): 101-103. 
  5. Menon, Vimla, et al. "Isolated “one and a half syndrome” with brainstem tuberculoma." The Indian Journal of Pediatrics 71.5 (2004): 469-471. 
  6. Bocos-Portillo, Jone, et al. "Eight-and-a-half syndrome." JAMA Neurol 72.7 (2015): 830
  7. Bijvank, JA Nij, et al. "A rare cause for visual symptoms in multiple sclerosis: posterior internuclear ophthalmoplegia of Lutz, a historical misnomer." Journal of neurology 264.3 (2017): 600-602. 
  8. Cogan, David G. "Internuclear ophthalmoplegia, typical and atypical." Archives of Ophthalmology 84.5 (1970): 583-589. 
  9. Takeshige, Haruka, et al. "Pathways linked to internuclear ophthalmoplegia on diffusion-tensor imaging in a case with midbrain infarction." Journal of Stroke and Cerebrovascular Diseases 25.11 (2016): 2575-2579
  10. Chen, Ko-Ting, Tzu-Kang Lin, and Tsung-Che Hsieh. "Isolated Internuclear Ophthalmoplegia After Massive Supratentorial Epidural Hematoma: A Case Report and Review of the Literature." World neurosurgery 100 (2017): 712-e5. 
  11. Keane JR1. Internuclear ophthalmoplegia: unusual causes in 114 of 410 patients. Arch Neurol. 2005 May;62(5):714-7. 
  12. Serra, Alessandro et al. “Improvement of Internuclear Ophthalmoparesis in Multiple Sclerosis with Dalfampridine.” Neurology 83.2 (2014): 192–194. PMC. Web. 9 Sept. 2018 
  13. Eggenberger E, Golnik K, Lee A, et al. Prognosis of ischemic internuclear ophthalmoplegia. Ophthalmology. 2002;109(9):1676-1678. doi:10.1016/s0161-6420(02)01118-1
  14. Murthy, Ramesh, et al. "Botulinum toxin in the management of internuclear ophthalmoplegia." Journal of American Association for Pediatric Ophthalmology and Strabismus 11.5 (2007): 456-459. 
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