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Internuclear ophthalmoplegia (INO) is a specific type of horizontal gaze disorder caused by a lesion in the Medial longitudinal fasciculus (MLF) — a key brainstem tract that coordinates conjugate eye movements.

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INO results in a failure of adduction in the ipsilateral eye during horizontal gaze and abduction nystagmus in the contralateral eye.

Relevant Anatomy

Medial longitudinal fasciculus (MLF):

MLF is a paired white matter tract located in the dorsal brainstem (midbrain and pons).
It connects the abducens nucleus (CN VI) of one side to the oculomotor nucleus (CN III) of the contralateral side.
Coordinates horizontal conjugate gaze, particularly adduction of the medial rectus during lateral gaze.

Pathophysiology

A lesion in the MLF interrupts the signal from the contralateral abducens interneurons to the ipsilateral medial rectus subnucleus of the oculomotor nerve.
Result: During lateral gaze:
- Ipsilateral eye cannot adduct.
- Contralateral eye abducts but shows nystagmus (due to disconjugate gaze and cerebellar compensation).

Clinical features

Feature	Description
Adduction deficit	Affected eye (ipsilateral to MLF lesion) fails to adduct
Abduction nystagmus	Seen in the contralateral eye during attempted horizontal gaze
Preserved convergence	Often intact (because the medial rectus is innervated by a separate pathway not using the MLF)
Diplopia	Common during lateral gaze
Unilateral or bilateral	Unilateral INO often due to stroke; bilateral INO suggests multiple sclerosis

Causes of INO

Etiology	Notes
Multiple sclerosis	Most common cause in young adults; typically bilateral INO
Brainstem stroke	Most common cause in older patients; usually unilateral
Brainstem tumors	Glioma, metastasis
Infections	e.g., neurocysticercosis, Listeria rhombencephalitis
Wernicke encephalopathy	Rare but possible
Trauma	Diffuse axonal injury may involve the MLF

Relevant Anatomy

Pathophysiology

Clinical features

Radiology

MRI Brain (Axial T2/FLAIR, DWI)