Wiktionary
n. pupillary athetosis; spasmodic dilation and contraction of the pupil between the sphincter and dilator muscles.
Wikipedia
Hippus, also known as pupillary athetosis, is spasmodic, rhythmic, but regular dilating and contracting pupillary movements between the sphincter and dilator muscles. Pupillary hippus comes from the Greek hippos meaning horse, perhaps due to the rhythm of the contractions representing a galloping horse.
It is particularly noticeable when pupil function is tested with a light, but is independent of eye movements or changes in illumination. It is usually normal, however pathological hippus can occur.
Pathologic hippus, the phenomena of increased oscillation or their amplitude, is associated with aconite poisoning, altered mental status, trauma, cirrhosis, and renal disease; suggesting a common pathway of frontal lobe dysfunction. A retrospective study of 117 hospitalized patients with hippus noted an increased 30-day mortality when compared to controls and adjusted for other factors (odds ratio=4.1, p<0.001).
Usage examples of "hippus".
And how came it that in the midst of his annoyance with Hippus, the thought of his schoolfellow Anton suddenly flashed across him, and of the day when he had walked with him last?
What may have been the original of the term Hippa, and Hippus, will be matter of future disquisition.