Page 1191 - Problem-Based Feline Medicine

57 – THE BLIND CAT OR CAT WITH RETINAL DISEASE 1183

● Nystagmus may be present.
HEREDITARY RETINAL DEGENERATION
IN ABYSSINIAN CATS ● Loss of vision occurs rapidly, and mydriasis, hyper-
reflective tapetum and attenuated retinal blood
vessels are evident.
Classical signs
● There is loss of pupillary light reflexes in advanced
● Progressive loss of vision occurs in young cases.
Abyssinian cats.
Rod–cone degeneration.
● Loss of pupil light reflexes in advanced
● Retinal degeneration does not begin until 1.5–2
cases.
years of age, and blindness follows over the next
● Tapetal hyper-reflectivity and retinal
2–4 years. The typical history is of older cats that
vessel attenuation.
go slowly blind.
● Relatively rare disease.
● The retina on either side of the optic disc begins to
develop a subtle gray discoloration. A more dif-
Pathogenesis fuse grayness of the tapetal area follows. The tapetal
area then becomes hyper-reflective and the retinal
Two forms of hereditary retinal degeneration have been
blood vessels attenuate.
described in Abyssinian cats.
● The non-tapetal fundus may show pale areas mixed
● Rod–cone dysplasia.
with heavily pigmented areas.
– Inheritance is autosomal dominant.
● There is loss of pupillary light reflexes in advanced
– Rods and cones degenerate at the same rate.
cases.
– Degeneration begins centrally, with progressive
● Electroretinography (ERG) shows a progressive
loss of the photoreceptor layer.
reduction of the B-wave amplitude which correlates
– The photoreceptor inner segments remain rudi-
to the loss of rhodopsin.
mentary, and the outer segments fail to elongate.
● Rod–cone degeneration.
– Inheritance is autosomal recessive.
Diagnosis
– Rods degenerate rapidly, but cones are spared
until the late stage of the disease. Initially, diagnosis is made on the breed and age of the
– Degeneration begins in the mid-peripheral cat, and the clinical presentation of vision loss. Pupils
regions of the retina. may become dilated, and the tapetum will be hyper-
– Histological changes begin at approximately reflective in the advanced stage of the disease.
35 days post-natally, with immature rod outer
Histopathology will help to confirm the disease if cats
segment discs. The cones appear normal at this
can be sacrificed.
stage. The rod outer segments begin to form
vacuoles, and the disc material clumps with sub- ERG with reduced B-wave amplitude is useful in the
sequent drop out of the cells. Two to three years early stages of the disease.
later, the cones start to degenerate and drop out.
Rhodopsin levels are reduced in affected cats.
Cats may also have plasma lipid abnormalities. Differential diagnosis
Taurine deficiency will present with loss of papillary
Clinical signs
light reflexes, hyper-reflective tapetum and retinal
Inherited retinal degeneration is a rare disease causing vessel attenuation. This may affect cats of all ages and
blindness in young (≤ 4 years) Abyssinian cats. It all breeds. Dietary history and plasma taurine levels
occurs very rarely in other breeds of cats. should help to differentiate this disease.

Rod–cone dysplasia. Hypertensive retinopathy usually occurs in older cats.
● Retinal degeneration begins at 4 weeks of age, and There is always hemorrhage present around retinal
cats are blind by 1 year of age. The typical history blood vessels, or there is focal areas of hemorrhage
is of a young cat that goes rapidly blind. present in the vitreous. There will be varying degrees of