Page 1290 - Problem-Based Feline Medicine

1282 PART 15 CAT WITH EYE PROBLEMS

● Diseases of the retina. Clinical signs
● Diseases of the central nervous system.
Typical history is of acute onset of vision loss in an old
Less frequent causes of abnormal size or response are: cat, or acute onset of bilaterally (usually) dilated pupils
● Neuropathy of the parasympathetic fibers within that are non-responsive or poorly responsive to light.
cranial nerve III, which supply the pupillary con-
Fundoscopic findings range from retinal hemorrhage
strictor muscle, or disease confined to the iris
and localized subretinal exudates, to extensive bul-
involving the constrictor muscle.
lous retinal detachment.
● Injury to the sympathetic fibers supplying the iris
dilator, most commonly trauma in the cranial thoracic Other signs suggestive of hypertension such as a
or cervical region, disease of the middle ear or disease bounding cardiac apex beat may be evident.
confined to the iris affecting the dilator muscle.
Other signs suggestive of renal and/or thyroid disease
● Hypertensive retinopathy, fungal or Toxoplasma
such as progressive weight loss, polydypsia, polyuria
infection causing chorioretinitis or anterior
or azotemic breath may be detected from the history
uveitis, feline infectious peritonitis (FIP) and
or physical examination.
retinal degeneration from inherited or nutri-
tional (taurine deficiency) causes are the most
common intra-ocular causes of abnormal pupil Diagnosis
size, shape or response.
Diagnosis is initially based on the characteristic clini-
cal signs.
DISEASES CAUSING SIGNS Confirmation of the diagnosis is based on a workup
OF ABNORMAL PUPIL SIZE, SHAPE involving:
OR RESPONSE ● Blood pressure measurement.
● Clinical pathology suggestive of renal and/or thy-
roid disease.
HYPERTENSIVE RETINOPATHY*** ● Other procedures such as imaging of the thyroid
with radio-isotopes or cardiac ultrasound.
Classical signs
● Acute-onset vision loss in an old cat. Treatment
● Acute onset, usually bilateral, dilated
Amlodipine (Norvasc, Pfizer) is a calcium channel
pupils non-responsive or poorly
responsive to light. blocker which has a greater effect on vascular smooth
muscle cells than on cardiac muscle cells. Dose is
1
approx 0.625 mg/cat/day, i.e. ⁄4 of a 2.5 mg tablet
1
(USA) or ⁄8 of a 5 mg tablet (Australia) q 24 h.
Pathogenesis
If hypertension is associated with hypertrophic car-
Hypertension causes vasoconstriction of pre-capil- diomyopathy then calcium channel blockers such as
lary retinal arterioles. Hypoxia of the arteriole Diltiazem may be used. For more detail see main ref-
causes smooth muscle necrosis, eventually leading erence on page 162 for details (The Cat With
to vascular dilatation and leakage of serum and Tachycardia, Bradycardia or An Irregular Rhythm).
blood.
b-blockers such as propranolol (Inderal) can also be
1
Leakage from vessels causes serous retinal detach- used in the management of the cardiac disease at ⁄4 of
ment, subretinal, intraretinal or preretinal hemorrhage. a 10 mg tablet q 8 h.
Hypertension is most commonly the result of hyper- Hyperthyroidism, when present, should be treated by
thyroidism, renal disease or is idiopathic. surgery, radioactive iodine (131-I) or anti-thyroid drugs