Internuclear Ophthalmoplegia and Gaze Palsies
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Direct answer
Internuclear ophthalmoplegia is impaired adduction of one eye on lateral gaze, caused by a lesion of the medial longitudinal fasciculus, with nystagmus in the abducting eye. It is most often due to multiple sclerosis in young adults and to brainstem stroke in older people. Convergence is usually preserved in internuclear ophthalmoplegia, which helps separate it from a third nerve palsy.
What you must remember
- Medial longitudinal fasciculus: Links the abducens nucleus to the contralateral medial rectus subnucleus of the oculomotor nucleus.
- Unilateral internuclear ophthalmoplegia: Failure of ipsilateral adduction on lateral gaze, with abducting nystagmus of the opposite eye.
- Bilateral internuclear ophthalmoplegia: Both eyes fail to adduct on lateral gaze, classically in demyelination of young adults.
- Preserved convergence: Convergence uses a separate midbrain pathway, so it is spared in internuclear ophthalmoplegia.
- One-and-a-half syndrome: A horizontal gaze palsy to one side plus internuclear ophthalmoplegia on the same side, leaving only abduction of the other eye.
- Parinaud syndrome: Upgaze palsy from a dorsal midbrain lesion, often with convergence-retraction nystagmus and light-near dissociation.
- Doll's eyes reflex: Preserved in supranuclear lesions but lost in brainstem nuclear and internuclear lesions.
Gaze palsies classified by the level of the lesion
Gaze palsies are classified by the level of the lesion, which the pattern of eye movements reveals. The table lists the commonest patterns from pontine to midbrain.
| Lesion | Eye findings | Typical cause |
|---|---|---|
| Unilateral internuclear ophthalmoplegia | Ipsilateral adduction failure, contralateral abducting nystagmus | Multiple sclerosis, brainstem stroke |
| Bilateral internuclear ophthalmoplegia | Both eyes fail to adduct on lateral gaze | Multiple sclerosis in young adults |
| One-and-a-half syndrome | Gaze palsy to one side plus internuclear ophthalmoplegia on that side | Pontine stroke, demyelination, tumour |
| Parinaud syndrome | Upgaze palsy, convergence-retraction nystagmus | Pineal tumour, hydrocephalus, midbrain lesions |
A preserved doll's eye reflex points to a supranuclear cause, while its loss indicates the brainstem itself. Recognising the pattern quickly narrows the diagnosis to demyelination, stroke or a midbrain mass.
The same eye pattern means different things by age. Bilateral internuclear ophthalmoplegia in a young adult points towards demyelination, whereas the same finding in an older patient with vascular risk factors points towards brainstem infarction.
Where the exam tries to catch you
Examiners separate internuclear lesions from third nerve palsy by the convergence and pupil findings.
- Trap: Labelling failed adduction with preserved convergence as a third nerve palsy Correct: internuclear ophthalmoplegia spares convergence and the pupils, whereas a third nerve palsy usually involves both.
- Trap: Calling a one-and-a-half syndrome a sixth nerve palsy Correct: it combines a horizontal gaze palsy with internuclear ophthalmoplegia on the same side, so only abduction of the other eye remains.
- Trap: Forgetting that the doll's eye reflex separates supranuclear from nuclear palsies Correct: reflex eye movements are preserved in supranuclear lesions and lost in brainstem nuclear lesions.
Frequently asked questions
Which anatomical structure is damaged in internuclear ophthalmoplegia?
Internuclear ophthalmoplegia results from damage to the medial longitudinal fasciculus, the tract that links the abducens nucleus to the contralateral medial rectus subnucleus. Its fibres coordinate conjugate horizontal movements, so the adducting eye lags behind the abducting eye. The lesion is typically in the pons or midbrain.
Why is convergence preserved in internuclear ophthalmoplegia?
Convergence is generated by a separate midbrain pathway that does not use the medial longitudinal fasciculus. Internuclear ophthalmoplegia therefore spares convergence even though adduction fails during lateral gaze. This preserved convergence is a key discriminator from third nerve palsy.
What is a one-and-a-half syndrome?
One-and-a-half syndrome is a horizontal gaze palsy to one side combined with an internuclear ophthalmoplegia on the same side. It is usually caused by a lesion of the pontine paramedian reticular formation and the adjacent medial longitudinal fasciculus. Only abduction of the contralateral eye remains, which explains the name.
Why is multiple sclerosis a common cause of bilateral internuclear ophthalmoplegia?
Multiple sclerosis commonly produces demyelinating plaques in the medial longitudinal fasciculus, which is a typical site for lesions. Bilateral internuclear ophthalmoplegia in a young adult is therefore strongly suggestive of demyelination. Brainstem stroke is the usual cause in older patients.