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Jerky and irregular vertical eye oscillations observed in patients with progressive supranuclear palsy (PSP) have temporal features that resemble those of head oscillations, according to a study published in the Journal of the Neurological Sciences.
PSP is a neurodegenerative disorder with clinical characteristics, including early postural instability, axial rigidity, bulbar and supranuclear gaze palsy. Previous research has found evidence of abnormal eye movements in this patient population, such as abnormal size and velocities of rapid gaze-shifts, the researchers explained. Squarewave jerks are common and large in patients with PSP. Jerky eye oscillations accompanied by slow drifts and rapid corrections, like nystagmus, are characteristic of degenerative cerebellar disorders.
The objective of the current study was to assess gaze-holding in 6 patients with PSP who showed vertical jerky eye oscillations and synchronously measure head oscillations.
Researchers measured the eye movements in 6 patients (age range, 66-84 years; 4 women) who had a clinical diagnosis of PSP. The patients had an average time from disease onset of 6.1 ± 3.92 years. Horizontal and vertical eye oscillations were measured with a limbus tracker.
The researchers noted that the patients’ eyes were moving upward at a relatively slow speed, although there were rapid corrective downward movements, which is consistent with vertical nystagmus, and more specifically, the downbeat nystagmus. The oscillations were not observed during the clinical examination, especially when the patient’s head was passively stabilized.
The head movements were then simultaneously measured with a gyroscope synchronized with eye movement signals and built into an eye tracking device. Head oscillations were present in the velocity trace when the head was not tightly restrained, and their cycle width was similar to that of eye oscillations.
Each patient had an increased oscillation intensity between 1 and 2 Hz, and the frequency range of increased intensity of eye oscillations correlated with that of head oscillations. The deficits were found in all patients.
“These results suggest that eye oscillations and head oscillations occur in the same frequency range,” the researchers stated. “The results provide a proof of principle that jerky eye oscillations in PSP are secondary to subtle amplitude jerky head oscillations.”
The findings are consistent with the possibility that the eye oscillations are secondary to a normal vestibulo-ocular reflex that occurs owing to head oscillations, or a “pseudonystagmus,” they noted.
The researchers added that the results emphasize the importance of stable gaze holding during ocular motor assessment, especially with instrumented measures.
“Inadequate head stabilization can lead to transmission of oscillatory movement disorder leading to physiological vestibular-ocular reflex that appears like oscillations,” they concluded. “Such false diagnosis of oscillations not only leads to incorrect diagnoses, but can lead to inaccurate therapeutic measures.”
Reference
Ticku H, Fotedar N, Juncos J, Factor SA, Shaikh AG. Pseudonystagmus in progressive supranuclear palsy. J Neurol Sci. Published online January 17, 2022. doi: https://doi.org/10.1016/j.jns.2022.120157
