Peer Reviewed

The Practitioner’s Guide to Neurologic Causes of Canine Anisocoria

When evaluating patients with asymmetric pupils, the practitioner needs to determine whether one or both pupils are abnormal in size and localize the lesion responsible for anisocoria.

December 18, 2015 |

Issue: January/February 2016

Anisocoria is defined as pupil asymmetry, and may be seen with ocular or neurologic dysfunction (Figure 1).¹ When 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:

Determine whether one or both pupils are abnormal in size
Localize the lesion responsible for anisocoria.

Miosis refers to smaller than normal pupil size, while mydriasis refers to larger than normal pupil size.

Neuroanatomy

Visual Pathway

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.¹

FIGURE 2. Visual pathway from the ventral aspect of the brain. The blue and orange lines represent visual fields from each eye. White arrow—optic chiasm. Black arrows—lateral geniculate nuclei in the thalamus.

Parasympathetic Function: Pupil Constriction

The parasympathetic pathway to the eye (Figure 3) is a short, 2-neuron pathway that originates in the midbrain. The paired parasympathetic nuclei of cranial nerve III (PSN CN 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.

FIGURE 3. The parasympathetic pathway. The first order neuron is depicted in orange and the second order neuron is depicted in yellow.

The parasympathetic pathway is best assessed using the pupillary light reflex (PLR):

When a bright light enters the eye, a proportion of crossed CN II fibers enter the pretectal nucleus in the midbrain to synapse with neurons which, in turn, synapse with efferent parasympathetic fibers in PSN 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 lesser in the opposite eye.

Sympathetic Function: Pupil Dilation

The opposing system is the sympathetic system, which is responsible for pupillary dilation. The sympathetic pathway (Figure 4) is a 3-neuron pathway that takes a longer course to the eye compared with the parasympathetic system.

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, 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.

FIGURE 4. The sympathetic pathway. The first order neuron is depicted in purple, the second order neuron is depicted in blue, and the third order neuron is depicted in teal.

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.

In a patient with left eye miosis, left thoracic limb paresis, and absent to decreased left thoracic limb reflexes and postural reactions, the practitioner would localize the lesion to a site that would affect all these structures simultaneously. In this patient, loss of left brachial plexus function and sympathetic innervation to the left eye could be explained by a single lesion at the brachial plexus, which is located between spinal cord segments C6 and T2.

Lesion Localization

Neurologic Examination

The neurologic examination allows the practitioner to localize the lesion to the visual, sympathetic, or parasympathetic pathways (Table 1). Since each neurologic test has a sensory (afferent) and a motor (efferent) component, the examiner must determine which component is affected.

Table 2 lists the tests most commonly used to evaluate the neuro-ophthalmic system. A detailed description of how to perform each test can be found elsewhere.²

Pharmacologic Testing

Pharmacologic testing can aid the practitioner with lesion localization within the sympathetic or parasympathetic system.

Physostigmine (0.5%) is a cholinesterase inhibitor and, thus, requires an intact postganglionic neuron to induce miosis.

Topical administration differentiates between preganglionic and postganglionic parasympathetic lesions.
When administered to both eyes, if a preganglionic lesion is present, it causes (relative to the normal eye) rapid pupil constriction.
In a normal eye, administration of physostigmine (and pilocarpine, below) causes slow or delayed constriction of the pupil.

Dilute pilocarpine (0.2%−1% solution) is a parasympathomimetic alkaloid that may be used to differentiate iris atrophy and other lesions of the iris, such as posterior synechia, from a lesion of the parasympathetic system.

Animals with ocular parasympathetic dysfunction have rapid pupillary constriction following the administration of pilocarpine.
Those with iris atrophy demonstrate only partial constriction.
Posterior synechia results in no or minimal response to pilocarpine, but the patient should also have a dyscoric pupil and other signs of past or current inflammation.
Pilocarpine can cause mild uveitis, resulting in blepharospasm, redness, and aqueous flare for up to 24 hours after administration.

Dilute phenylephrine (1%) can be used to test the sympathetic system.

When applied topically to both eyes, it should cause rapid (typically 20 minutes or less) dilation in an eye with a postganglionic sympathetic neuron dysfunction.
Administration in the normal eye has no effect.

Differential Diagnosis

Lesion localization permits the clinician to assemble an appropriately ranked list of potential causes (Table 3). Common differential diagnoses are discussed below using the DAMNITV schema.

Iris Atrophy

Iris atrophy—thinning of the iris stroma, especially at the pupillary margin where the iris sphincter muscle is located—is a common finding in older dogs. This condition may result in anisocoria and/or PLR abnormalities if dysfunction of the sphincter muscle is notably asymmetric (Figure 5).

FIGURE 5. A 9-year-old, castrated male dachshund exhibiting anisocoria secondary to iris atrophy of the right eye.

Diagnosis is made by careful examination of the pupillary margin.

Iris atrophy appears as scalloping along the edge of the pupil and/or thinning of the tissue, which allows light reflected from the tapetum to pass through the translucent, atrophied areas.
In very early iris atrophy, loss of the iris stroma may expose the posterior pigmented epithelium of the iris, resulting in the darker areas in the pupillary margin, which are actually iris atrophy.

A direct PLR will be slow or absent, while the consensual (indirect) reflex in the contralateral eye should be normal.

Neoplasia

Neoplasia may occur anywhere throughout the central or peripheral nervous system.

Intracranial Neoplasia
The most common intracranial neoplasm in dogs and cats is meningioma. Anisocoria may result from disruption of either the afferent or efferent pathways of pupil innervation. Meningioma routinely occurs in the CN II, cerebrum, brainstem, and spinal cord. Glioma, lymphoma, peripheral nerve sheath tumors, and cranial thoracic masses may cause anisocoria but are less common.

Iris & Ciliary Body Neoplasia
Neoplasia of the iris and ciliary body can also cause anisocoria, through prostaglandin-mediated uveitis (miosis), secondary glaucoma (mydriasis), synechia formation (miosis, mydriasis, or dyscoria), or mass infiltration of the iris with subsequent physical obstruction of the pupil.

Idiopathic

Sympathetic Dysfunction
Approximately 50% of dogs with sympathetic dysfunction (commonly termed Horner’s syndrome) are diagnosed with idiopathic dysfunction.³ The lesion may be located in the second or third order sympathetic neuron based on pharmacologic testing.^4-6 No treatment is indicated for these animals and many spontaneously recover.⁶

Dysautonomia
Dysautonomia is an idiopathic disease that affects both branches of the autonomic system. Risk factors for development of dysautonomia include young to middle age, medium to large breed dogs, and living in rural housing.⁷

Many affected animals are visual, but demonstrate mydriasis with absent PLR, along with other systemic signs of autonomic dysfunction. Dysautonomia is not treatable; therefore, humane euthanasia is usually recommended due to poor quality of life.

Infectious/Inflammatory

Otitis Media
Otitis media accounts for 1% to 4% of the cases of sympathetic dysfunction in dogs and cats.^3,8 While management of otitis is important, miosis often persists.

Uveitis
Uveitis (intraocular inflammation) can also cause anisocoria. Although this can occur as a result of iridal swelling, more commonly, prostaglandins active within the eye during acute inflammation act directly on the iris sphincter muscle to cause miosis. Thus, a unilateral or asymmetric uveitis can lead to anisocoria.⁹

Additional signs of uveitis include blepharospasm, episcleral hyperemia, deep corneal vascularization, diffuse corneal edema, aqueous flare, swollen and/or hyperemic iris, and decreased intraocular pressure. These should be assessed before a neurologic cause of anisocoria is investigated.

Posterior Synechia
Posterior synechia—adherence of the iris to the lens—can result in a fixed, nonmobile pupil that can either be relatively miotic, mydriatic, or abnormally shaped.

Immune-Mediated or Infectious Disease
Immune-mediated or infectious disease can affect any portion of the central nervous system (CNS), causing visual, sympathetic, or parasympathetic dysfunction in any combination.

The majority of dogs with immune-mediated CNS disease require immunosuppressive therapy to control their clinical signs; this, however, requires that infectious and neoplastic causes are first eliminated.^10,11

Infectious agents implicated as causes of CNS disease include viruses, bacteria, fungi, protozoa, and, rarely, rickettsial infections. Treatment and prognosis varies depending on the cause of the infection.

Glaucoma
Glaucoma (increased intraocular pressure) can cause a unilateral mydriasis. The mechanism by which mydriasis occurs in glaucoma is not entirely clear, but likely involves iris or optic nerve dysfunction associated with elevated intraocular pressure. Other signs may include blepharospasm, episcleral hyperemia, deep corneal vascularization, and diffuse corneal edema.

Trauma

Brachial Plexus Avulsion
Brachial plexus avulsion is a common cause of damage to the second order sympathetic neurons between T1 and T3. Brachial plexus injury commonly occurs after vehicular trauma and may result in partial or complete loss of the affected nerve roots. Amputation of the affected limb may be indicated if recovery is not likely; however, ocular sympathetic dysfunction does not resolve with amputation of the limb.

Head Trauma
Head trauma is a common cause of parasympathetic dysfunction due to compression of the midbrain at the level of the PSN CN III. Compression occurs secondary to hemorrhage or increased intracranial pressure, resulting in herniation of the brain.

Elevated intracranial pressure, with damage to the cerebrum or diencephalon, results in miosis due to disinhibition of the parasympathetic fibers and, possibly, damage to the sympathetic fibers. As brainstem compression occurs, miosis progresses to mydriasis with absent PLR.

Prognosis is guarded when miosis is detected, and grave after mydriasis, with absent PLR, is identified.¹ Treatment to decrease intracranial pressure should be started as soon as possible.

Jugular Venipuncture
Jugular venipuncture can cause anisocoria due to iatrogenic damage to the second order sympathetic neuron as it courses through the jugular groove, deep to the jugular vein. Excessive probing for the vein may result in damage to this nerve. Care should be exercised when attempting jugular venipuncture, performing deep ear flushes, and using choke chains to minimize risk of trauma to the surrounding structures.

Vascular

Disruption of Blood Flow
Disruption of blood flow to any part of the autonomic or visual system may result in compromised function. The cause is unknown in almost 50% of dogs with cerebrovascular disease, therefore, treatment is supportive.¹² Treatment should be directed at the underlying cause in dogs for which a diagnosis is obtained.

Fibrocartilagenous Embolism
Fibrocartilagenous embolism (FCE) is a common cause of spinal cord dysfunction in dogs. Miosis may occur following a FCE in the cervicothoracic spine due to interruption of the cervical sympathetic fibers.

In Summary

Anisocoria may occur secondary to disease of the eye, optic nerve, and central and autonomic nervous system. Lesion localization is critical to developing an appropriate differential diagnosis list. This can often be effectively conducted using knowledge of the neuroanatomical pathways, along with standard cranial nerve testing.

Pharmacologic testing may help further isolate sites of dysfunction within the autonomic system. Further diagnostic testing should aim for an etiologic diagnosis and can be chosen based upon neuroanatomical localization. Treatment should always be directed at the underlying cause of anisocoria.

CN = cranial nerve; CNS = central nervous system; FCE = fibrocartilagenous embolism; LGN = lateral geniculate nuclei; PLR = pupillary light reflex; PSN CN III = parasympathetic nuclei of cranial nerve III

Figures 1–4 Illustrations courtesy Pamela Boutilier, DVM, MVSc, Diplomate ACVIM (Small Animal Internal Medicine), SAA.