Web-Vet TM Neurology Specialists
Abnormalities of Vision, Pupil Size & Eye Position
Anisocoria
ANISOCORIA
Anisocoria is pupil asymmetry and may be seen with ocular or neurologic dysfunction. When anisocoria is caused by neurologic disease, unequal pupil size may result from dysfunction of the sympathetic, parasympathetic, or visual systems.
When evaluating patients with asymmetric pupils, the aim is to:
Determine which pupil is abnormal in size
Localize the lesion responsible for anisocoria.
Miosis refers to a smaller than normal pupil, while mydriasis refers to a larger than normal pupil.
Neuroanatomy
Visual Pathway
The visual pathway is composed of the retina, optic nerve (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.
Parasympathetic Function: Pupil Constriction
The parasympathetic pathway to the eye is a short, 2-neuron pathway originating in the midbrain. The paired parasympathetic nuclei of cranial nerve III, along with the somatic nerves from the oculomotor nerve (CN III), send fibers—called first-order neurons, or preganglionic fibers—to the eye. After synapsing in the ciliary ganglion, the short, postganglionic fibers course to the iris sphincter muscle and cause pupil constriction.
The parasympathetic pathway is best assessed using the pupillary light reflex (PLR):
When a bright light enters the eye, a proportion of CN II fibers enter the pretectal nucleus in the midbrain to synapse with neurons which, in turn, synapse with efferent parasympathetic fibers in CN III.
These parasympathetic fibers transmit this information to the eye, resulting in pupillary constriction.
A relay between the paired PSN CN III in the midbrain results in indirect (or consensual) PLR.
Clinically, this can be observed when a bright light is shone in one eye, and the opposite eye also constricts. The degree of constriction is slightly less in the opposite eye.
Sympathetic Function: Pupil Dilation
The opposing system is the sympathetic system, which is responsible for pupillary dilation. The sympathetic pathway is a 3-neuron pathway from the hypothalamus to the eye.
Sympathetic function originates in the hypothalamus of the brain and courses as the first-order neuron through the brainstem and cervical spinal cord to thoracic spinal cord segments T1 to T3.
The second-order neuron exits the spinal cord between T1 and T3, courses cranially through the thoracic cavity as the vago-sympathetic trunk, out the thoracic inlet, and along the jugular groove to the cranial cervical ganglion.
The third-order neuron exits the cranial cervical ganglion, runs through the middle ear, then alongside the ophthalmic branch of the trigeminal nerve (CN V), and ends in the periorbital muscles, third eyelid, and iris dilator muscles.
Dysfunction anywhere along this pathway results in a comparatively miotic pupil, which results from failure of the iris to dilate in reduced ambient light, typically accompanied by enophthalmos, ptosis, and third eyelid protrusion over the globe - this collection of signs is Horner syndrome.
Lesion Localization
Neurologic Examination
The neurologic examination allows us to localize the lesion responsible for the anisocoria to the visual, sympathetic, or parasympathetic pathways. Since each neurologic test has a sensory (afferent) and a motor (efferent) component, the examiner must determine which component is affected.