Page 1289 - Problem-Based Feline Medicine

62 – THE CAT WITH ABNORMAL PUPIL SIZE, SHAPE OR RESPONSE 1281

TRAUMA
● Dyscoria associated with iris synechia or prolapse into a corneal wound* (p 1286)
Pear- or tear-shaped pupil, with adhesion of the acute angle of the tear to the lens capsule or to a
traumatic defect in the cornea.
● Feline hemidilated pupil syndrome* (p 1287)
D shape (left eye) or reverse D (right eye) shaped pupil, which becomes accentuated on stimula-
tion of the pupil with bright light.

the indirect response in the opposite eye is regarded
INTRODUCTION
as normal in cats. Only static anisocoria (i.e. where
This chapter refers only to diseases within the globe one eye is not being stimulated by a light source) is
causing an abnormal pupil size, shape or response. regarded as abnormal.

For other disease processes occurring remotely from Disease processes therefore either involve the retina,
the eye see main reference on page 1282 for details afferent and efferent nerve pathways or iris and are most
(The Cat With Anisocoria or Abnormally Dilated or often degenerative, hypertensive, infectious or toxic
Constricted Pupils). in nature.
For infectious conditions affecting the iris and therefore
WHERE?
pupil size or response see main reference on page 1295
for details (The Cat With Abnormal Iris Appearance). Abnormalities in pupil size or response can originate from
lesions or dysfunction in any of the following regions:
MECHANISM? ● Retina, optic nerve, optic chiasm, optic tracts

Disease processes affecting pupil size or response can or midbrain which form the afferent arm for
generally be divided into: reflexes which regulate pupillary responses, i.e.
● Those which affect afferent pathways, i.e. going they detect the amount of light getting into the sys-
towards the central nervous system (CNS) from the tem and transmit information about the light levels
retina. to the midbrain.
● Those which affect efferent pathways, i.e. going ● Parasympathetic nervous system. The efferent arm
away from the CNS towards the effector organ, in this for pupillary constriction consists of the parasympa-
case the dilator and constrictor muscles of the iris. thetic nucleus of the oculomotor nerve, parasympa-
● Those which affect the effector organ, the iris thetic fibers in cranial nerve III (oculomotor nerve),
(these diseases are largely covered in The Cat With ciliary ganglion and long ciliary nerves.
Changed Iris Appearance). ● Sympathetic nervous system. The efferent arm for
pupillary dilation consists of the thalamus, cervical
The term anisocoria refers to a difference in pupil size spinal cord long tracts, T1–T3 spinal nerves, pre-
between the two eyes of the same animal. ganglionic fibers in the cervical region, cranial cer-

The term dyscoria refers to an abnormality in shape. vical ganglion and postganglionic fibers especially
as they course through the middle ear.
The terms mydriasis and miosis refers to the states of ● Iris. Diseases which affect pupil size or response
pupil dilation and pupil constriction, respectively. can affect either the dilator or constrictor muscles.

To demonstrate complete normality of pupil function,
the pupil must be shown to dilate in a dark environment
and to constrict after stimulation by bright light. WHAT?
Dynamic contraction anisocoria, where the direct The most common causes of abnormalities of pupil size
response of an eye to light stimulation is greater than or response are: