Internuclear Ophthalmoplegias

Internuclear Ophthalmoplegias

Neuroanatomy Overview

Final common pathway

Conjugate horizontal eye movements (simplified / Left frontal eye field)

The left frontal eye field drives the eyes to the right.

It projects to the right abducens nucleus.
Right abducens nucleus drives the right eye to the right.
Right abducens nucleus projects up the left MLF to the left oculomotor nucleus.
Left oculomotor nucleus drives the left eye to the right.

Additional Circuitry:

Right abducens motor neurons innervate the ipsilateral lateral rectus muscle (LR), which drives the ipsilateral eye to the right (it abducts it).
Right abducens interneurons send fibers up the contralateral MLF to the oculomotor nucleus to innervate the medial rectus, which drives the eye to the right (it adducts the eye).

Internuclear Ophthalmoplegias

Common Pathologies

Note that the side of the lesion is purely for educational purposes - neither side is more prone to injury than the other

Right abducens nucleus lesion

What happens in a right abducens nucleus lesion?

Injury to the abducens motoneurons causes loss of ipsilateral eye abduction.
Injury to the abducens interneurons causes loss of contralateral eye adduction.
In a complete abducens nuclear injury, there is loss of gaze to the side of the lesion: in this example, neither eye can deviate to the right.

MLF injury

What happens when the MLF is injured, as commonly occurs from demyelinating plaques in multiple sclerosis?

Here, we study a left MLF syndrome

The right abducens motor neurons innervate the ipsilateral lateral rectus muscle (LR">Internuclear ophthalmoplegia, which drives the ipsilateral eye to the right (it abducts it).
Its interneurons send fibers up the contralateral MLF but it is blocked (injured) – we're studying a left MLF lesion. So the left eye cannot adduct.
In an internuclear ophthalmoplegia, the unaffected eye abducts but the ipsilateral eye is unable to adduct.

For reasons not show here, the unaffected eye is not totally unaffected, it actually has horizontal nystagmus upon abduction, presumably because of the divergence that occurs from the left eye adduction failure.

Bilateral MLF injuries

What happens when both of the MLF tracts are injured?

Bilateral internuclear ophthalmoplegia.

Pathologic processes that cross midline, such as demyelinating plaques, hemorrhages, or

tumors, can cause this form of injury because the medial longitudinal fasciculus tracts run close together in the midline of the brainstem.

Abducens motor neurons innervate the ipsilateral lateral rectus muscle (LR) for each eye, which drives them outward (in abduction).
The abducens interneurons send fibers up the contralateral MLF on each side, but each side is blocked.
Neither eye can adduct: the right eye can't turn horizontally to the left and the left eye can't turn horizontally to the right.

Right abducens and neighboring MLF

What happens when the right abducens nucleus is injured along with the neighboring MLF?

One-and-a-half syndrome.
Injury to the right abducens motoneurons causes loss of ipsilateral eye

abduction.

The left abducens interneurons send fibers that are blocked along the damaged MLF, so the right eye can't adduct.
The opposite abducens motor neurons innervate the ipsilateral lateral rectus muscle (LR), which drives the ipsilateral eye outward (it abducts it).
Injury to the right abducens interneurons causes loss of innervation to the L MLF and thus loss of left eye adduction.
When both the right abducens nucleus and the adjacent right medial longitudinal fasciculus are injured, the right eye is unable to move in the horizontal plane and the only intact movement is left eye abduction (and it has nystagmus from the right medial longitudinal fasciculus injury); thus, one-and-a-half of the two complete eye movements are impaired, so the injury pattern is called one-and-a-half syndrome.

Internuclear Ophthalmoplegias

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