hippus

The term "hippus" refers to an oscillation in the size of the pupil, which is the aperture in the iris that controls the amount of light entering the eye. This involuntary fluctuation can occur even when light conditions are stable.1

The pupil's size is regulated by muscles within the iris. Constriction is managed by the sphincter muscle, while dilation is controlled by radially oriented fibers. These muscles are innervated by the oculomotor and sympathetic nerves, respectively.1

Pupillary responses are complex and can be triggered by various stimuli. The light reflex causes pupils to constrict in bright light and dilate in darkness. Additionally, pupils constrict when focusing on near objects (the near reflex), which helps to decrease aberrations and increase depth of focus. Strong emotional stimuli, such as fear or pain, can also cause pupillary dilation.1