Internuclear Ophthalmoplegia & Gaze Palsies

Overview

Conjugate horizontal eye movement depends on a compact circuit. The abducens (CN VI) nucleus in the pons houses two populations of neurons: motor neurons that abduct the ipsilateral eye, and interneurons that cross the midline and ascend through the medial longitudinal fasciculus (MLF) to the contralateral oculomotor (CN III) subnucleus supplying the medial rectus. A single pontine gaze center, the abducens nucleus, is therefore enough to drive both eyes together during a horizontal saccade: it fires the ipsilateral lateral rectus directly and the contralateral medial rectus indirectly, through the MLF.

Because the circuit is so compact, a small brainstem lesion produces a specific, reproducible deficit. Internuclear ophthalmoplegia, one-and-a-half syndrome, and the conjugate gaze palsies below are variations on interrupting different pieces of that same pathway: the paramedian pontine reticular formation (PPRF), the abducens nucleus itself, or the MLF. That is what makes each one such a reliable localizing sign on exam.

Internuclear Ophthalmoplegia

Internuclear ophthalmoplegia (INO) results from a lesion of the MLF itself, interrupting the crossed projection from the abducens nucleus to the contralateral medial rectus subnucleus. The hallmark finding is impaired adduction of the eye on the side of the lesion during contralateral conjugate gaze, often with abducting nystagmus of the opposite (fellow) eye. In a right INO, looking left produces a right eye that adducts poorly or not at all, while the left eye abducts fully but with nystagmus. Convergence is typically preserved, since the near-response pathway to the medial rectus doesn't route through the MLF. That dissociation, failed adduction on lateral gaze but intact adduction on convergence, is a useful bedside confirmation that the deficit is truly internuclear rather than a primary CN III or medial rectus problem.

The lesion is named for the side of the impaired adduction, not the direction of gaze: a right INO means the right eye fails to adduct, regardless of which direction is tested. Bilateral INO in a young patient is a classic presentation of multiple sclerosis, since demyelinating plaques favor the periventricular white matter the MLF runs through. A unilateral INO in an older patient is more often small-vessel ischemic stroke.

One-and-a-Half Syndrome

One-and-a-half syndrome combines two lesions in the same pontine tegmentum: a PPRF or abducens nucleus lesion, producing a complete horizontal gaze palsy toward the side of the lesion, plus an MLF lesion on the same side, producing an INO on attempted gaze to the opposite side. The ipsilateral eye cannot move horizontally at all, neither abducting nor adducting, while the contralateral eye can only abduct (with nystagmus) and cannot adduct past midline. The "one" refers to the complete gaze palsy in one direction; the "half" refers to the INO, since only abduction of the fellow eye survives. Convergence is again typically spared, which confirms the deficit sits at or proximal to the internuclear level rather than in the medial rectus itself.

Because both structures sit close together in the dorsal pontine tegmentum, one-and-a-half syndrome is a classic localizer for a small paramedian pontine lesion. When it appears in a young patient with no vascular risk factors, demyelinating disease should be suspected ahead of ischemic stroke.

Horizontal & Vertical Gaze Palsies

A horizontal conjugate gaze palsy results from a lesion of the PPRF or abducens nucleus itself, rather than the MLF, and unlike INO it affects both eyes equally. Neither eye can move past midline toward the side of the lesion, because the shared pontine gaze center driving that whole saccade is gone. This is the key exam distinction from INO, which by definition always spares one eye's movement, the abducting eye.

Vertical gaze palsies localize rostrally, to the midbrain, rather than the pons. The rostral interstitial nucleus of the MLF and the posterior commissure coordinate vertical conjugate gaze. A dorsal midbrain lesion compressing this region, classically a pinealoma or other pineal region mass, produces Parinaud syndrome: upgaze palsy, convergence-retraction nystagmus on attempted upgaze, light-near dissociation of the pupils, and eyelid retraction (Collier's sign).

Skew Deviation

Skew deviation is a vertical misalignment of the eyes caused by disruption of supranuclear otolithic (utricular) input to the ocular motor nuclei, rather than a cranial nerve palsy or restrictive process. It arises from posterior fossa or brainstem lesions affecting the graviceptive pathways that run from the vestibular nuclei up through the MLF and interstitial nucleus of Cajal toward the midbrain. Because it isn't tied to a single cranial nerve, skew deviation can be hard to distinguish from a fourth or third nerve palsy on cover testing alone. Localizing it correctly usually needs the broader clinical picture: other brainstem signs, and the pattern of vertical deviation across different gaze positions.

Skew deviation is also one of the three components of the HINTS exam (head impulse, nystagmus, test of skew) used in the acutely dizzy patient. Its presence, detected by alternate cover testing, points toward a central otolithic disruption and raises concern for posterior circulation stroke rather than a peripheral vestibular cause. See the companion pages on central and peripheral/vestibular nystagmus for the full HINTS framework.

Diagnosis & Localization

All of these syndromes are, at their core, bedside exam diagnoses. The pattern of what does and doesn't move, and whether convergence is spared, localizes the lesion before any imaging is obtained. MRI of the brainstem is the confirmatory study of choice once a syndrome is recognized clinically, both to identify the underlying lesion (demyelinating plaque, small-vessel infarct, mass) and because these findings can be the presenting sign of a first demyelinating event or an evolving stroke that needs urgent management.

Related Topics

Continue exploring Neuro-ophthalmology: