13th February 2025

Internuclear ophthalmoplegia

Internuclear ophthalmoplegia (INO) is an ocular movement disorder caused by pathology of the medial longitudinal fasciculus (MLF) within the brainstem. It results in impaired adduction of the ipsilateral eye and nystagmus during abduction of the contralateral eye (Figure 6).

Neuroimaging in the form of an MRI brainstem is useful to identify a lesion. The underlying causes are:

• ⅓:  infarction
• ⅓: demyelinating disorder, such as multiple sclerosis
• ⅓: other causes, e.g. trauma, infections, vasculitis

Treatment depends on the underlying cause. Approximately half of all cases of INO will resolve within 1 year.

Important differential diagnoses to consider include myasthenia gravis and sixth nerve palsy.

Image Source: Wikimedia Commons (2021). eyeball-4303894, Licensed under a creative commons licence. Available for free at: https://thenounproject.com/icon/eyeball-4303894/ (Accessed: 18 January 2024).