Heidi Barnes Heller, DVM, Diplomate ACVIM (Neurology), and Ellison Bentley, DVM, Diplomate ACVO
University of Wisconsin–Madison
Anisocoria is defined as pupil asymmetry, and may be seen with ocular or neurologic dysfunction (Figure 1).1When anisocoria is caused by neurologic disease, unequal pupil size may result from malfunction of the sympathetic, parasympathetic, or visual systems.
FIGURE 1. Representation of a dog with anisocoria.
When evaluating patients with asymmetric pupils, the practitioner needs to:
Miosis refers to smaller than normal pupil size, while mydriasis refers to larger than normal pupil size.
- Determine whether one or both pupils are abnormal in size
- Localize the lesion responsible for anisocoria.
The visual pathway (Figure 2) is composed of the retina, optic nerve (also known as cranial nerve II), lateral geniculate nuclei (LGN) in the thalamus, and occipital cortex in the cerebrum.
When light enters the eye, it activates the retinal photoreceptors. This information travels along cranial nerve (CN) II via the optic chiasm to the optic tracts and then the LGN. Optic radiations relay the visual information from the LGN to the occipital cortex. In dogs, about 75% of optic nerve fibers cross to the opposite cerebral cortex at the optic chiasm.