Page 856 - Clinical Small Animal Internal Medicine

824 Section 8 Neurologic Disease

positive with C‐type virus particles present within the
VetBooks.ir ciliary nerves or ganglia.
Unilateral miosis in dogs and cats can be secondary to
anterior uveitis, Horner’s syndrome (discussed later), or
pharmacologic agents such as pilocarpine or topical
prostaglandin analogs. Anterior uveitis is differentiated
from Horner’s syndrome based on concurrent ophthal­
mic clinical signs including conjunctival hyperemia,
episcleral congestion, corneal edema, blepharospasm,
aqueous flare, keratic precipitates, and ocular hypotony.

Internal and External
Ophthalmoplegia

Unilateral mydriasis may be secondary to an afferent or
efferent defect. Afferent defects involve the retina, optic
nerve, or optic tract. Efferent defects involve the oculo­
motor nucleus, oculomotor nerve, or iris sphincter mus­
cle. Vision and PLR testing are vital in distinguishing
Figure 77.1 Idiopathic optic neuritis in a dog. Note the optic disc afferent from efferent defects. Animals with afferent
is edematous and hyperemic with peripapillary retinal lesions are nonvisual, have an absent direct PLR, and an
detachment. absent consensual (affected to nonaffected eye) PLR.
Animals with efferent lesions are visual, have an absent
Anisocoria direct PLR, and a normal consensual (affected to nonaf­
fected eye) PLR. The presence of a normal consensual
Pupil diameter is regulated by autonomic control of the PLR from the nonaffected eye to the affected eye rules
iris sphincter and iris dilator muscles. Anisocoria is out an efferent arm lesion.
defined as unequal pupils. Physiologic anisocoria may Efferent lesions may involve the oculomotor nucleus,
occur with heterochromia iridis or age‐related iris oculomotor nerve, or iris sphincter muscle. Peripheral
atrophy with the affected pupil dilated. When not physi­ parasympathetic fibers along the medial aspect of the
ologic, determination of which pupil is abnormal can oculomotor nerve are responsible for pupillary constric­
be challenging. Examination of the patient in bright, tion. The oculomotor nerve also innervates extraocular
ambient, and dim light conditions facilitates diagnosis. muscles (superior, medial, and inferior rectus, inferior
An abnormal mydriatic pupil will not constrict com­ oblique) and the levator palpebrae superioris muscle.
pletely in bright light compared to the normal pupil. Lesions affecting just the peripheral aspect of the nerve
In contrast, an abnormal miotic pupil will not dilate will result in mydriasis only, termed internal ophthalmo-
completely in dim light compared to the normal pupil. plegia (Figure 77.2). Lesions affecting the entire nerve
Unilateral mydriasis in dogs and cats can have primary will result in lateral strabismus and ptosis, adding the
ocular or neurogenic causes. Ocular causes include iris diagnosis of external ophthalmoplegia. External ophthal­
atrophy, glaucoma, end‐stage retinal degeneration, optic moplegia is defined as paralysis of one or more of the
nerve atrophy, and pharmacologic dilation with agents extraocular muscles and may only involve the oculomo­
such as tropicamide or atropine. Neurogenic causes tor nerve or may also involve the trochlear and abducens
include a lesion in the parasympathetic nerve, oculomo­ nerves.
tor nerve, or brainstem. Due to the unique innervation of When evaluating mydriasis, it is important to distin­
the cat sphincter muscle by two short ciliary nerves con­ guish ophthalmoplegia from a pupillomotor defect.
trolling either side of the iris, parasympathetic denerva­ Pupillomotor defects are usually secondary to iris atro­
tion may result in hemiplegia and a “D” shaped or reverse phy or previous topical anticholinergic medication
“D” shaped pupil. Iris sphincter hemiplegia has been administration. Animals with iris atrophy may have a
associated with trauma and iris stromal infiltration by slight direct PLR or may have a characteristic scalloped
lymphoma. Mydriasis alternating between eyes is second­ appearance to the pupillary margin. When these signs
ary to feline spastic pupil syndrome. Cats affected with are absent, pharmacologic testing is useful in differenti­
feline spastic pupil syndrome tend to be feline leukemia ating a pupillomotor defect from ophthalmoplegia.