One eye that will not adduct. Two eyes that will not look to one side. One eye that can only abduct. Each of these patterns tells you where to look in the brainstem, and you can work it out at the bedside. This article shows the pathway for horizontal gaze once, then shows where the lesion is for each pattern.

Eye movement disorders fall into two groups. Supranuclear disorders sit on the path from the brain down to the nuclei of cranial nerves III, IV and VI. Peripheral disorders sit between the nucleus and the muscle. Here we stay above the nerves, with horizontal gaze only.

Read a horizontal gaze problem in this order
  1. Ask the patient to look right and left. Which eye fails, and in which direction?
  2. One eye fails to adduct: think of the MLF (medial longitudinal fasciculus) on that side (internuclear ophthalmoplegia).
  3. Both eyes fail to look to one side: think of the PPRF (the horizontal gaze center in the pons) on that side (horizontal gaze palsy).
  4. Only one eye can still move horizontally, and only outward: one-and-a-half syndrome.
  5. Check convergence every time: bring a target toward the nose and watch whether the eye that would not adduct now adducts.

The pathway for looking to the right

Follow the signal for a look to the right.

  1. It starts in the left cerebral cortex (the frontal eye field).
  2. It crosses to the right PPRF (paramedian pontine reticular formation), the horizontal gaze center in the pons.
  3. The right PPRF drives the right abducens nucleus, and the right lateral rectus contracts.
  4. The right abducens nucleus also sends a second set of neurons across the midline. They run up in the left MLF (medial longitudinal fasciculus) to the left oculomotor nucleus, and the left medial rectus contracts[1].

This is the base for everything below, so it is worth learning well. The side of the MLF is the part people mix up. One way to hold it:

  • The MLF is on the same side as the medial rectus that contracts.
  • The PPRF is on the same side as the lateral rectus that contracts, which is also the direction of gaze.
The pathway for looking to the right, viewed from above. The left cerebral cortex sends the signal to the right PPRF, then to the right VI nucleus and the right lateral rectus. Fibers from the right VI nucleus cross the midline, run up in the left MLF to the left III nucleus and the left medial rectus.
The pathway for a look to the right, viewed from above. The MLF that carries the signal is on the side of the medial rectus that contracts.

Now let us see what happens when each part is damaged.

Reading each lesion

Left MLF: internuclear ophthalmoplegia (INO)

On right gaze, the left eye does not adduct. On left gaze, everything is normal. So when a patient presents with acute double vision and an adduction deficit in one eye only, think of the MLF, for example a small infarct in the pons.

Two more signs complete the picture. The abducting right eye shows nystagmus on right gaze. And when the patient looks straight ahead, the eyes are aligned. The INO is named for the side of the adduction deficit, which is the side of the lesion[1].

The most useful test is convergence. In an isolated MLF lesion, the eye that would not adduct on side gaze adducts on convergence. The medial rectus and the third nerve are healthy, and convergence reaches the oculomotor nucleus by a different route. That test helps separate an INO from a medial rectus or third nerve problem. A large lesion can take convergence too[1].

Left MLF lesion on the pathway. The signal from the right VI nucleus stops in the left MLF, so the left medial rectus does not contract on right gaze.
Left MLF lesion. The left medial rectus gets no signal on right gaze.

A mild INO can be subtle. Ask the patient to look quickly from side to side: the adducting eye arrives late[2]. The common causes are demyelination (multiple sclerosis) and stroke[1].

Left internuclear ophthalmoplegia. On right gaze the right eye abducts with nystagmus but the left eye stays in the midline. Left gaze is normal. On convergence both eyes adduct.
Left INO. The left eye fails to adduct on right gaze, but adducts on convergence.

Myasthenia gravis can look like an INO. Ocular myasthenia gravis can produce an adduction deficit with nystagmus of the other eye: a pseudo-INO[2]. Look for ptosis, a deficit that changes through the day, and fatigue on sustained gaze before you call it a brainstem lesion.

Right PPRF: horizontal gaze palsy

On right gaze, neither eye moves: the right eye does not abduct and the left eye does not adduct. Left gaze is normal. Convergence is preserved here too.

A lesion of the right abducens nucleus gives the same picture when the patient tries to look right, not a sixth nerve palsy. The nucleus drives both the right lateral rectus and the crossing fibers to the left medial rectus, so losing it takes out the whole gaze to that side[2].

Right PPRF lesion on the pathway. Nothing reaches the right VI nucleus, so neither the right lateral rectus nor the left medial rectus contracts on right gaze.
Right PPRF lesion. The signal for right gaze stops before the VI nucleus.
Right PPRF lesion. On right gaze neither eye moves. Left gaze and convergence are normal.
Right horizontal gaze palsy. Neither eye moves to the right, but convergence is normal.

One-and-a-half syndrome

Now suppose a larger lesion takes out the right PPRF and the right MLF together.

  • On right gaze, neither eye moves (the gaze palsy).
  • On left gaze, the left eye abducts, but the right eye does not adduct (the right INO).

Of the two eyes’ four horizontal movements, only one is left: abduction of the left eye, often with nystagmus[1]. Gaze to the right is lost in both eyes (the "one"), and gaze to the left is lost in the right eye (the "half"), hence the name one-and-a-half syndrome. Convergence is usually still possible. The common causes are multiple sclerosis and brainstem stroke, followed by tumors[2].

Right one-and-a-half syndrome on the pathway. One lesion covers the right PPRF, the right VI nucleus and the right MLF.
Right one-and-a-half syndrome. The lesion takes out the right PPRF and VI nucleus and the right MLF.
Right one-and-a-half syndrome. On right gaze neither eye moves. On left gaze only the left eye abducts. Convergence is normal.
Right one-and-a-half syndrome. The only horizontal movement left is abduction of the left eye.

Left cerebral cortex

This one is a bonus. A large lesion of the left hemisphere, such as a left middle cerebral artery infarct, also stops the patient looking to the right, like a right PPRF lesion. Because the pull to the right is gone, the eyes rest toward the left, toward the side of the lesion. This is the conjugate deviation that comes with a large stroke.

Left cortex lesion on the pathway. The signal for right gaze never leaves the left hemisphere, so nothing reaches the right PPRF.
Left cortex lesion. The signal for right gaze never starts.

A seizure does the opposite. A left hemisphere seizure overstimulates the pathway, so the eyes turn to the right, away from the lesion[1].

The same logic works in the pons: when the right PPRF is lost, the eyes tend to rest toward the left, away from the lesion.

Whenever a patient with a horizontal gaze problem walks in, check convergence. If an eye that will not adduct to the side adducts on convergence, its medial rectus and third nerve work, and the problem is above them.

Summary

  • Learn the pathway for one direction: cortex, opposite PPRF, abducens nucleus, then crossing fibers in the MLF to the opposite medial rectus.
  • One eye that will not adduct, with nystagmus of the abducting eye, is an INO on the side of the eye that will not adduct.
  • Both eyes that will not look to one side point to the PPRF or abducens nucleus on that side.
  • One-and-a-half syndrome leaves only abduction of the eye on the other side from the lesion.
  • Convergence is usually preserved in all of them. Rule out myasthenia before you call it an INO.

Figure credits

The pathway diagrams are my own drawings. The eye-position drawings were made with an image generator, using as a style model the figure from the 53rd National Examination for Certified Orthoptists (Japan), afternoon session, question 66 (booklet No. 4), published by the Ministry of Health, Labour and Welfare. The labels on the eye-position drawings are mine.

References

[1] Salmon JF. Kanski’s Clinical Ophthalmology: A Systematic Approach. 10th ed. Elsevier; 2025. Chapter 19, Neuro-ophthalmology: supranuclear disorders of ocular motility; abnormalities of horizontal gaze.

[2] Yanoff M, Duker JS, eds. Ophthalmology. 5th ed. Elsevier; 2019. Chapters 9.14, Disorders of supranuclear control of ocular motility, and 9.15, Nuclear and fascicular disorders of eye movement.

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