Anisocoria Part 1 Anisocoria is a difference in the size of the pupils. The pupil can be a normal or an abnormal shape. It is important to determine
Anisocoria Part 1
Anisocoria is a difference in the size of the pupils. The pupil can be a normal or an abnormal shape. It is important to determine which is the abnormal pupil and if there are any abnormalities in vision. Understanding the pupillary light response is essential when assessing patients with anisocoria.
The pupillary Light response (PLR) is divided into a sensory afferent pathway and a motor efferent pathway. Dogs and cats should always have a direct response (the eye that is being stimulated by light) and an indirect response (the contralateral eye). The direct response in dogs and cats is always stronger than the indirect response.
Afferent system – This is the sensory visual part of the pathway involving the retina, the ganglion cells, optic nerve, optic tract and pretectal nucleus. Lesions in the afferent pathway will cause vision deficits.
Efferent system - This is the motor part of the pathway that involves the EW – nucleus, oculomotor nerve, ciliary ganglion and the iris sphincter muscle. Lesions in the efferent pathway will result in a loss of pupil constriction.
Diagram of Pupillary light reflex
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Tips for assessing the PLR
1. Place the animal in a room with normal lighting
2. Compare the pupils from a distance with the patient looking straight towards you
3. Use a bright focal light for testing (a dull light will increase the difficulty in assessing which eye is normal)
4. Assess the direct and indirect response (having an assistant can help)
5. Determine which eye is visual either by menace testing, obstacle course or both
6. Always take a through history and perform a comprehensive neurological test
Miosis
Conditions that cause intraocular pain cause the iris to spasm. As a result miosis will occur. Corneal ulceration and anterior uveitis are such conditions.
Topical miotic agents such as pilocarpine, phospholine iodide, xalatan, timolol, carbochol, and demecarium bromide result in unilateral miosis.
Neurologic conditions causing unilateral miosis include Horner's syndrome and acute intracerebral diseases that produce compression of the midbrain tectum or a prosencephalic lesion.
Mydriasis Afferent mydriasis = a dilated pupil + loss of vision. Affected tissues include the retina, optic nerve or optic chiasm.
Retinal causes include chorioretinitis, retinal detachments, retinal haemorrhages, glaucoma, congenital ocular defects, etc.
Lesions of the optic nerve and chiasm include unilateral optic neuritis or neuropathy, neoplasia (meningioma, glioma, lymphosarcoma), congenital colobomas or optic nerve hypoplasia, certain infectious diseases (blastomycosis, cryptococcosus, toxoplasmosis) and granulomatous meningoencephalitis and trauma. Examples of trauma include globe proptosis, excessive globe traction during an enucleation, trauma during caudal tooth root extractions and probing or draining during retrobulbar disease investigation.
Large lesions of the optic chiasm will cause symmetrical, bilateral mydriasis and blindness.
Afferent mydriasis and other cranial nerve deficits may also be seen with cavernous sinus syndrome.
Efferent mydriasis = a dilated pupil due to disruption of the parasympathetic innervation of the iris sphincter. Vision is normal unless there is brain trauma present.
Unilateral lesions of the efferent branch of the oculomotor nerve (CN III) cause efferent mydriasis. Examples of such lesions include oculomotor nucleus or nerve contusion or compression, certain midbrain and cerebellar lesions, infectious or inflammatory diseases, neoplasms, or vascular abnormalities.
Occasionally retrobulbar diseases may cause efferent mydriasis, but they are much more likely to affect the optic nerve and produce afferent mydriasis.
Efferent lesions of the short-ciliary nerves cause a change in pupil shape in the cat, because different branches of the nerves innervate different aspects of the iris. Lesions of the temporal branch result in a hemi-dilated pupil shaped like a "D" in the OS and a "reverse D" in the OD. Lesions of the nasal branches of the nerve cause the exact opposite pupil shape to appear.
Idiopathic forms of efferent mydriasis also occur and are not usually accompanied by other neurologic deficits.
Iris sphincter disease - The most common cause is iris atrophy, a senile or degenerative change of the iris. Other causes include congenital defects such as aniridia and iris colobomas.
Drugs mydriatics - These include the topical parasympatholytic (atropine, tropicamide) and sympathomimetics (epinephrine, phenylephrine) agents.
Alternating anisocoria
Cats can develop a "spastic pupil" resulting in alternating anisocoria between eyes. The pupil may fail to dilate in the affected eye, or not dilate as well as the opposite eye. All affected cats have been positive for feline leukemia virus (FeLV), and their vision is usually normal. It is believed that this abnormality arises from a viral inflammation that affects either the sympathetic or parasympathetic innervation of the iris.
Other medical disorders - Many causes of anisocoria have their origin in a systemic illness. It is important to determine whether the anisocoria is due to an intrinsic eye problem, or associated with an underlying illness.
Distortion of pupil shape
Pupils may be unequal in size because one is abnormal in shape, or its mobility has been changed. Intrinsic ocular conditions that may alter pupil shape include congenital lesions such as iris colobomas and persistent pupillary membranes, uveal neoplasms or diffuse melanosis, posterior synechiae, anterior movement of the lens, clotted hyphema, the presence of iris cysts, etc.
