Brachiocephalic Skull Conformation
Shih
Tzu has several genetic eye diseases. One of the most important
diseases in this breed is due to the brachiocephalic skull.
Brachiocephalic dogs have shortened noses and very prominent eyes due
to shallow orbits-- the bony socket that surrounds the eye. The shallow
bony orbit in the short-nosed dog causes the eyeball to be in a more
prominent position, giving the appearance of being larger than dogs of
similar sizes with longer noses.
Along with this skull conformation, there is the problem of
lagophthalmos an inability to properly
close the eyelid over the
cornea. Lagophthalmic dogs generally have an enlarged palpebral
fissure-- the space between the upper and lower eyelids.
Brachiocephalic dogs classically have their entire cornea exposed when
the eyes are open and generally, they have the white of the eyeball
exposed also. This conformation causes two clinically serious
consequences to the Shih Tzu.
First, they are more prone to exposure keratitis inflammation of the
cornea. In its most severe form, it is ulcerative. A deep ulcer can
cause the cornea to rupture. Chronic low-grade exposure keratitis leads
to central scarring of the cornea and pigmentary changes on the surface
of the cornea that can decrease vision.
The second big problem related to brachiocephalic skull conformation in
Shih Tzu is proptosis. When a globe moves forward
out of the orbit, as
happens with proptosis, the eyelids clamp behind the eyeball itself and
cause the venous blood from the eye not to be able to return. This
causes a lack of oxygen to the retina and can lead to blindness within
minutes. If a Shih Tzu has a proptosed globe, it is an emergency!
Veterinary care has to be sought immediately (within 20 minutes) in
order to attempt to save vision and save the eye.
Other problems found in the Shih Tzu include eyelash disease, which
consists of two conditions. The first condition is Distichiasis the
eyelashes abnormally located at the eyelid margin, which may cause
ocular irritation. Distichiasis may occur at any time in the dog's
life. The hereditary basis of the condition is not established.
Distichiasis can cause severe scarring of the cornea and/or ulcers that
could lead to blindness. The second condition is ectopia cilia. Ectopic
cilia is like Distichiasis in that ectopic cilia are eyelashes that
emerge through the eyelid conjunctiva. They most commonly occur in
the
upper central eyelid. These are found in younger dogs and can cause
significant pain as exhibited by squinting and facial wetness. Like
Distichiasis, corneal ulcers can occur.
Two conditions that involve the inside of the eye in the Shih Tzu
include hereditary cataracts or juvenile cataracts and progressive
renal atrophy (PRA). The breeding
recommendation for either one of
these conditions is NO. This is unlike the breeding recommendation in
the previous conditions that were described, where the breeding advice
is breeder option. A cataract is a lens opacity, which can affect one
or both eyes and may involve the lens partially or completely. In cases
where cataracts are complete and affect both eyes, blindness results.
Progressive retinal atrophy is a degenerative disease of the retinal
visual cells, which progresses to blindness. Usually seen in the young
adult animal, it starts out as night blindness, which progresses slowly
to complete blindness. PRA is recessively inherited in most breeds.
Other problems that the Shih Tzu has include dry eye, which is
technically called Keratoconjunctivitis sicca (see the
separate article
on dry eye). Dry eye is dryness of the cornea and
the conjunctiva. It is an abnormality of the
tear film that most
commonly is a deficiency of the water part of the tear film. The mucous
and fat layers of the tear film are also affected, but when the water
layer is not present, the mucous in the fat layer makes very hard, dry,
ropy debris, which is found on the eyelids. Dry eye leads to chronic
corneal irritation and this leads to ulcers or scarring which can cause
visual impairment. A less common defect found in the Shih Tzu is
retinal detachment. This is due to a vitreoretinal dysplasia. The
vitreous is the gel found in the back of the eye, and it is very
closely adhered to the retinas. Dysplastic means that it has not
developed normally which means that the abnormal interface between the
vitreous and the retina can cause the retina to come loose. This causes
blindness.
~Denise M. Lindley, DVM,
MS,
Diplomate, ACVO, Staff Opthamologist,
President, Animal Eye Consultants.
Reprinted from the ASTC Bulletin
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