Rare ‘See-Saw’ Eye Movements Reveal How the Brainstem Steers Our Gaze

Neurologists in Lima have documented one of the rarest forms of involuntary eye movement ever described, and in doing so they have mapped out, with unusual precision, how a small cluster of cells in the brainstem keeps our eyes working in perfect tandem. In a study published in the Journal of Neurology, a team from the Instituto Nacional de Ciencias Neurológicas reports the case of a 30-year-old man whose eyes oscillated in a bizarre, alternating pattern known as hemi-see-saw nystagmus, while his palate trembled rhythmically in synchrony with a second oscillation. The paper pairs this striking clinical picture with a systematic review of every comparable case the authors could find across seven research databases, producing the largest synthesis yet of this enigmatic disorder.

To understand why the case matters, it helps to picture what the patient’s eyes were actually doing. In hemi-see-saw nystagmus, one eye rises while it rotates toward the nose, and at the same moment the other eye drops while rotating outward, like two children on opposite ends of a see-saw moving in counterphase. The movement typically follows a jerk pattern, with a slow drift in one direction punctuated by rapid corrective snaps. Classic see-saw nystagmus, in which both eyes move together in this rocking fashion, has been recognized for decades and is usually linked to lesions around the pituitary gland or the diencephalon. The hemi form, affecting the eyes in a dissociated, half-and-half manner, is far less common and points to damage on one side of the brainstem.

The anatomical culprits identified by the researchers are two structures long known to specialists but obscure to almost everyone else: the interstitial nucleus of Cajal, a tiny hub in the midbrain that helps hold the eyes in vertical and torsional positions, and the medial longitudinal fasciculus, a thick cable of nerve fibers that shuttles signals between the nuclei controlling each eye. Both structures participate in the ocular tilt reaction, the reflex that keeps the visual world level when the head tilts. When a stroke or other injury knocks out one side of this circuitry, the brain loses its ability to balance vertical and torsional eye position between the two eyes, and the see-saw oscillation emerges as the system hunts for an equilibrium it can no longer compute.

The systematic review assembled by Gutierrez-Arratia and colleagues identified 18 published reports containing 57 cases of hemi-see-saw nystagmus or closely related dissociated vertical-torsional nystagmus, including their own index patient. The pooled data reveal a clear demographic and etiological profile. The average patient was 56 years old, roughly two-thirds were men, and the overwhelming majority of cases, just over 75 percent, were caused by ischemic stroke, with hemorrhagic stroke accounting for another 14 percent. A scattering of rare conditions made up the remainder: Chiari I malformation, demyelinating disease, cavernous malformations, a hereditary ataxia called ataxia with oculomotor apraxia type 2, Joubert syndrome, and brainstem encephalitis each contributed a single case.

Perhaps the most consequential finding concerns the waveform of the oscillation. Jerk nystagmus dominated the dataset, appearing in more than 82 percent of cases, while a smooth, pendular waveform showed up in only 17.5 percent. Crucially, every pendular case occurred in patients who also had oculopalatal tremor or its imaging signature, hypertrophic olivary degeneration. This association is not coincidental. Oculopalatal tremor arises after a brainstem or cerebellar injury disrupts the connections to the inferior olive, a structure in the medulla that then undergoes a peculiar transsynaptic degeneration, becoming enlarged and abnormally rhythmic. The damaged olive begins firing in slow, synchronized oscillations that propagate through the cerebellum and brainstem, driving rhythmic movements of the palate and, in some patients, pendular drifting of the eyes.

Among the jerk cases, the review pinpointed the pontine tegmentum, the floor of the brainstem’s upper half, as the most frequent lesion site, involved in nearly 64 percent of patients. Every single pontine case also showed internuclear ophthalmoplegia, the telltale disconnection syndrome produced by damage to the medial longitudinal fasciculus, in which one eye fails to adduct when the patient looks toward the opposite side. The ocular tilt reaction was present in 90 percent of these patients. Prognosis tracked the underlying mechanism with striking consistency: complete resolution of the nystagmus was reported predominantly in cases associated with internuclear ophthalmoplegia, whereas persistent, non-improving oscillations were confined to cases tied to oculopalatal tremor, with one remarkable exception of spontaneous resolution after twenty years.

The index case that anchors the report illustrates how these mechanisms can stack on top of one another in a single patient. The 30-year-old man harbored an arteriovenous malformation, a tangle of abnormal vessels, in the pontomedullary junction of his brainstem. His examination revealed pendular hemi-see-saw nystagmus, a right-sided one-and-a-half syndrome in which horizontal eye movements were almost completely abolished in both directions for one eye and partially for the other, gaze-evoked nystagmus, an ocular tilt reaction, an incomplete Horner syndrome affecting pupil size and eyelid position, and the rhythmic palatal movements of oculopalatal tremor. The constellation demonstrates how a single vascular lesion can simultaneously sever the pathways for conjugate gaze and ignite the oscillating inferior olive, producing both the jerk-like dissociated vertical-torsional movements and the superimposed pendular oscillation.

The authors argue that this pendular phenotype points to a distinct brainstem-cerebellar mechanism rather than a simple variant of the usual jerk pattern. In the classic model, hemi-see-saw nystagmus reflects an imbalance in the neural integrators that hold eccentric eye positions, with the interstitial nucleus of Cajal on one side effectively taken offline. Animal work supports this framework: inactivating tissue just below the interstitial nucleus of Cajal in monkeys reliably induces the hemi-see-saw pattern. But when the inferior olive starts to oscillate, its rhythmic output through the cerebellar circuits imposes a smooth, pendular tempo on the eye movements, overriding or blending with the integrator imbalance. The Lima team suggests that recognizing this dual mechanism could refine how clinicians interpret eye movement recordings and predict which patients might recover.

Treatment implications remain modest but real. Recovery in stroke-related cases appears to depend on the integrity of the pathways involved, with internuclear ophthalmoplegia-associated oscillations showing the best outcomes, presumably because compensatory circuits can gradually recalibrate the vertical-torsional balance. Oculopalatal tremor-related oscillations, by contrast, tend to persist, consistent with the self-sustaining nature of the olivary rhythm. The literature contains isolated therapeutic attempts, including a reported improvement of hemi-see-saw nystagmus with the muscle-relaxant baclofen and experimental damping of pendular nystagmus with vibration applied to the head, but no large treatment trials exist for a condition this rare. The authors emphasize that larger series with detailed anatomical imaging and oculographic recordings will be needed to clarify the structure-function correlations and to establish reliable prognostic markers.

For a general audience, the study is a reminder of how much diagnostic information is encoded in the way our eyes move. Eye movements are among the most precisely controlled motor outputs of the nervous system, coordinated by interconnected nuclei distributed from the midbrain to the medulla, and different patterns of misbehavior localize lesions with almost surgical accuracy. A neurologist who recognizes a hemi-see-saw pattern can infer not only that the brainstem is damaged but which side, which level, and which specific fiber tracts, often before advanced imaging confirms the finding. By cataloging 57 cases and separating the jerk and pendular phenotypes, the Peruvian team has converted a scattering of anecdotal reports into a coherent clinical map, one that should help clinicians worldwide read these extraordinary oscillations for what they are: the visible signature of specific circuits failing in specific, predictable ways.

Subject of Research: Hemi-see-saw nystagmus and oculopalatal tremor in brainstem lesions

Article Title: Hemi-see-saw nystagmus with superimposed pendular oculopalatal tremor: case report and systematic review

Article References: Hemi-see-saw nystagmus with superimposed pendular oculopalatal tremor: case report and systematic review. (n.d.). https://doi.org/10.1007/s00415-026-14213-1

Image Credits: AI Generated

DOI: 10.1007/s00415-026-14213-1

Keywords: hemi-see-saw nystagmus, oculopalatal tremor, nystagmus, brainstem, stroke, interstitial nucleus of Cajal, medial longitudinal fasciculus, inferior olive, hypertrophic olivary degeneration, systematic review, neuro-ophthalmology, ocular tilt reaction