Clinical Vignette 1—Initial Presentation
A 9-year-old female/spayed, domestic shorthair cat presents with the left eye appearing smaller to the owner for 1 week. Ophthalmic examination of both eyes is normal with the exception of anisocoria (left pupil smaller than the right), smaller palpebral fissure, and partially prolapsed nictitans in the left eye. Menace reflex is positive in both eyes and direct and consensual pupillary light reflexes (PLRs) are positive; however, movement of the left pupil is less due to the initially small pupil size.
Clinical Vignette 2—Initial Presentation
A 10-year-old male/neutered miniature poodle presents for annual vaccinations and wellness examination. Ophthalmic examination reveals anisocoria with the right pupil larger than the left. Menace reflex is positive in both eyes. In the right eye there is a negative direct PLR but positive consensual reflex to the left eye; in the left eye there is a positive direct PLR but a negative consensual reflex to the right eye. The remaining ophthalmic examination is unremarkable.
Problem Definition and Recognition
Anisocoria is defined as unequal pupil size. It is often the first abnormality noted by an owner or detected on a routine physical examination. Anisocoria can occur with primary ophthalmic disease defined as anterior to and including the retina; or it can occur with primary neurologic disease defined as posterior to and including the optic nerve. Ophthalmic and neurologic etiologies overlap for the optic nerve. Careful examination can narrow the list of possible etiologies for anisocoria and help the clinician diagnose potentially blinding or even life-threatening disease.
Pathophysiology
To work through a case of anisocoria requires an understanding of the PLR pathway, the visual pathway, and sympathetic innervation to the eye (see Fig. 47-1).
The afferent arm of the PLR and visual pathway is composed of the retina, optic nerve, chiasm, and a portion of the optic tract. Both pathways are the same until just before the lateral geniculate body of the thalamus where sensory fibers for the PLR leave the optic tracts to activate the parasympathetic centers of the oculomotor nerve. The parasympathetic fibers that run with the oculomotor nerve and the iris constrictor muscle make up the efferent arm of the PLR. Meanwhile, the afferent arm of the visual pathway continues beyond the lateral geniculate body to the optic radiation and occipital (visual) cortex. Because the efferent arm of the PLR breaks away from the visual pathway fibers long before the end of the visual pathway (occipital cortex), the PLR pathway is independent of vision.
FIGURE 47-1. Diagram illustrating the visual pathway, pupillary light reflex pathway, and the sympathetic nerve pathway (with permission for use from Dr. T. Mark Neer, Oklahoma State University, Center for Veterinary Health Sciences).
Although there is thought to be some crossover, in general the parasympathetic pathway is responsible for pupillary constriction and the sympathetic pathway is responsible for pupillary dilation. Therefore, a lesion along the parasympathetic pathway will result in a dilated (mydriatic) pupil and a lesion along the sympathetic pathway will result in a constricted (miotic) pupil. Anisocoria due to an abnormal mydriatic pupil can be due to a lesion in the afferent or efferent PLR pathway. Lesions of the afferent pathway (retina, optic nerve, chiasm) will also affect vision. The pupil will be mydriatic because there is no afferent signal telling the pupil to constrict. Lesions of the efferent pathway will not affect vision and include a disease or condition affecting the mobility of the iris/pupil to constrict or a lesion along the parasympathetic pathway. Interruption of the sympathetic pathway or a disease/condition affecting the iris itself can cause miotic pupils.
Diagnostic Plan
Ophthalmologic Examination
A systematic approach should be used when examining a patient with anisocoria. Begin with a thorough history including any medications put in the eye. Topical atropine 1% effects can last up to 2 weeks in the normal eye. Then a complete ophthalmic examination is required including vision assessment (menace reflex and ability to track cotton balls), PLRs (direct and consensual), assessment of related cranial nerves (III, IV, V, VI, VII) by observing globe position and conjugate eyeball movements, ability to retract the globe (corneal reflex), palpebral reflex, and intraocular assessment. The ophthalmologic etiologies of anisocoria are listed in Table 47-1.
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