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When it is dark a
cat’s pupils dilate to allow more light to reach the retina to provide
better vision. When a cat is nervous his pupils will also dilate.
These normal functions of the eye are the result of the nerves in the
eye communicating information to the brain. The message is sent
through the nerves to the iris which dilates or constricts the pupil.
When there are
disruptions in this pathway the pupil will not respond normally to
light. Mydriasis is the excessive dilation of the pupil due to
disease, trauma or medication. Ophthalmic and neurologic examinations
along with specific testing are needed to find the cause of this
abnormality. Blood pressure, intraocular pressure and feline leukemia
testing are among the first diagnostics needed. The prognosis varies
from excellent to grave and treatment is based on the specific
diagnosis. Below are some of the causes of mydriasis.
Mydriasis with
vision defects
This occurs when
there is damage to the retina, optic nerve, optic chiasm or the optic
tract. If only one eye is damaged the dilation may be mild in the
affected eye. If both eyes are involved the dilation of the pupil is
much more noticeable.
1.
Hypertension, intraocular hemorrhage (most
common)
2.
Infectious disease
(Toxoplasmosis, Panleukopenia, FIP, FeLV, FIV, Histoplasmosis,
Cryptococcosis)
3.
Glaucoma
4.
Neoplasia- such as lymphosarcoma
5.
Progressive retinal atrophy
6.
Toxic insult to the retina producing retinal degeneration (example-enrofloxacin)
7.
Trauma
8.
Central retinal degeneration- inadequate taurine levels
Mydriasis with
normal vision
1.
Medications
a.
Atropine,
tropicamide
b.
Anesthetic agents
(example-ketamine)
c.
Opioids (buprenorphine)
dilates pupils in cats but not dogs
2.
Trauma or tumors
involving the brain or nerves
3.
Dysautonomia-
generalized disease of autonomic nerves
4.
Hepatic
encephalopathy
Dilated pupils are an
important clinical sign of disease and investigation into the cause is
essential. Many times the patient is suffering from a systemic disease
and this is the first clue that the patient needs attention.
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