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Ocular System

Internuclear ophthalmoplegia (INO)

Ocular movement disorder caused by lesion of medial longitudinal fasciculus (MLF), characterized by impaired adduction of the ipsilateral eye with nystagmus of the abducting eye.

Ocular movement disorder caused by lesion of medial longitudinal fasciculus (MLF), characterized by impaired adduction of the ipsilateral eye with nystagmus of the abducting eye.


Aetiology

Brainstem infarctions (⅓ cases):

Usually unilateral and seen in older individuals

Demyelinating disorders (⅓ cases):

Mostly bilateral and seen in young adults and adolescents.
  • Multiple sclerosis (MS) (23% cases)

Other causes:

  • Trauma
  • Tentorial herniation
  • Infections (HIV, syphilis, cysticercosis, herpes zoster, tuberculoma)
  • Tumours (medulloblastoma, glioma, lymphoma, metastases)
  • Vasculitis (systemic lupus erythematosus (SLE), Sjogren syndrome)
  • Iatrogenic injury
  • Brainstem hemorrhage

Pathophysiology

Brain structures involving the horizontal movement in eyeballs:

  • Paramedian pontine reticular formation (PPRF)
  • Abducens nucleus
  • Medial longitudinal fasciculus (MLF)

Disorders of horizontal movement in eyeballs:

Resulting from the damage to the above brain structures
  1. Horizontal gaze palsy
  2. Internuclear ophthalmoplegia (INO)
  3. One-and-a-half syndrome

Medial longitudinal fasciculus (MLF):

Paired white matter tract passing close to the midline, through the brainstem lying ventral to the cerebral aqueduct in the midbrain and the fourth ventricle in the pons and medulla that serves as the final common pathway for different types of conjugate eye movements like saccades, smooth pursuit, vestibulocochlear reflex, and forms a communication between all the ocular motor nuclei
  • Lesion of the medial longitudinal fasciculus can produce impaired horizontal conjugate eye movements by blocking the connection between contralateral sixth nerve nucleus and the ipsilateral third nerve nucleus, thus affecting horizontal gaze
Neurologic pathway for horizontal conjugate eye movement: Illustration of a left internuclear ophthalmoplegia. A lesion in the left medial longitudinal fasciculus prevents excitatory signals from the right abducens nucleus to reach the left oculomotor nucleus, thus inducing an adduction deficit of the left medial rectus. | Wu, Y.‐T., Cafiero‐Chin, M. and Marques, C. (2015), Wall‐eyed bilateral internuclear ophthalmoplegia. Clin Exp Optom, 98: 25-30. doi:10.1111/cxo.12200

Clinical features

  • Total/partial failure of adduction ipsilateral eye
  • Monocular nystagmus of abducting eye
Photo of patient attempting to look to his left. Note inability to adduct right eye. | Bamford, R., & Singh-Ranger, G. (2012). Unilateral internuclear ophthalmoplegia after minor head injury. The western journal of emergency medicine, 13(1), 123–124. https://doi.org/10.5811/westjem.2011.8.6595

Diagnosis

MRI:

Diagnostic imaging of choice for MLF lesions in patients with INO
Magnetic resonance imaging of the brain showing frontal lobe contusions. | Bamford, R., & Singh-Ranger, G. (2012). Unilateral internuclear ophthalmoplegia after minor head injury. The western journal of emergency medicine, 13(1), 123–124. https://doi.org/10.5811/westjem.2011.8.6595

Management

Treatment of underlying cause

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