DISEASES OF THE ORBIT AND GLOBE - DOC

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					Ellen Bjerkås
NVH




DISEASES OF THE GLOBE AND ORBIT


Orbital disease


When discussing orbital diseases, both bulbar, orbital as well as periorbital structures must
be considered. Diseases of the orbit can broadly be divided into conditions causing
enophthalmia and those causing exophthalmia.




Exophthalmia
Most orbital diseases cause exophthalmia. One should note that many breeds of dogs have
constitutional exophthalmia, determined by the breed standard. Examples are pug, Boston
terrier, bulldog, pekinese. These dogs are predisposed to several eye diseases, like keratitis
and proptosis of the globe.


Proptosis (prolapse of the globe) can occur in dogs of all ages and breeds after injury, but is
most frequent in brachycephalic dogs. The globe is displaced forwards with concurrent
entrapment of the eyelids behind the globe. In the most predisposed breeds, pulling the skin
of the neck may be enough to cause the globe to proptose. Treatment aims at repositioning
the globe and regain normal conditions. When a dog with a proptosed globe is brought to the
clinic, the extraocular structures around the globe is most often so swollen that immediate
reposition is not possible. If the eye is severely injured, it should be taken out. Important
prognostic factors are the number of ruptured extraocular muscles, (can be seen as red
spots on the sclera), other lacerated structures, intraocular haemorrhage and missing indirect
pupillary light reflex from the proptosed to the normal eye.


Before reposition, the eye and surrounding tissues are moistened with saline. It is usually
necessary to perform a lateral canthotomy, i.e. making an incision in the lateral canthus to
enlarge the eyelid opening. The eyelids should be pulled forwards over the globe either with
holding sutures or with forceps if possible. Protection of the cornea is essential during
reposition.
After reposition the canthotomy is closed and the eyelids closed with temporary sutures,
leaving a little medial opening for topical medication of the eye. The eyelid sutures should not
penetrate the conjunctiva, as this will cause corneal irritation, and they should be secured
dorsally and ventrally with either small buttons or a piece of silastic tube to prevent damage
from the sutures. The sutures should be left for one to three weeks, depending of the actual
case. If in doubt, even if the chances of maintaining vision are limited, the eye should be
repositioned with the risk of possible later enucleation. An important complication is damage
to the innervation of the eyelids, preventing closure. Permanent shortening of the eyelids
should be considered in brachycephalic dogs. Medical treatment is dependent on clinical
signs. Infection should be treated with antibiotics, and intraocular structures evaluated for
uveitis treatment.




Exophthalmia may be due to enlargement of the globe itself or by space-occupying
processes in the orbit.
The most common causes of exophthalmia are acquired and include injuries, orbital cellulitis,
salivary retention cysts, neoplasia, foreign bodies and abscesses. The most common cause
of retrobulbar inflammation is a foreign body, either penetrating through the conjunctiva or
behind the last molar in the oral cavity. Myositis with swelling of the temporal muscles may
cause exophthalmia, with concurrent pain in the acute stage. German shepherds and
Norwegian elkhounds are among the predisposed breeds. In the chronic stage secondary
fibrosis of the affected muscles may cause enophthalmia and protrusion of the third eyelid.
Extraocular muscle myositis has mainly been described in young Golden retrievers, but may
occasionally be seen in dogs of other breeds as well. Typically, affected dogs present with
acute bilateral and symmetrical exophthalmia. Ultrasound of the retrobulbar area reveals the
swollen muscles.


It may be difficult to differentiate clinically the cause of exophthalmia. Thorough general
examination including inspection of the oral cavity, ocular examination and blood analysis is
necessary. In addition, imaging techniques including X-ray and ultrasound, or CT, MRI scans
when available, give important information. Fine needle aspirates from the retrobulbar area,
either percutaneous or via the mouth, are of diagnostic importance.


The direction of gaze may aid in diagnosis. Swelling caused by a retrobulbar process often
causes strabismus concurrent with the exophthalmia, while extraocular myositis leads to
(bilateral) exophthalmia with no strabismus. Presence or absence of pain is also an important
factor, although most space-occupying lesions will cause pain if their size is significant.
Retrobulbar processes
Retrobulbar process is the term for tissue pushing the orbit forwards. Important factors to
determine the cause of a retrobulbar process are rate of progression, whether the condition
is painful or not, and an evaluation of the animal’s general condition. The age of the animal
should also be considered. Malformations are primarily seen in young animals, while tumours
are found in older. Infection or inflammation in the retrobulbar area causes pain, especially
when chewing, and, consequently, anorexia. A thorough general examination including
inspection of the oral cavity, ocular examination and blood analysis is necessary to establish
an etiologic diagnosis. Exophthalmos caused by orbital cellulitis (diffuse inflammation of
orbital tissue), a retrobulbar abscess of tumour may be difficult to differentiate clinically. X-ray
or ultrasound, as well as fine needle aspiration from the retrobulbar area, either
percutaneously or via the mouth are of diagnostic importance.
       A special condition, extraocular myositis, may occur in the Golden retriever, causing
bilateral exophthalmos. Orbital cellulitis and retrobulbar abscesses are treated with systemic
antibiotics and drainage. Tumours are treated medically and/or surgically if possible.
Extraocular myositis is treated with long-term systemic steroids.


Prolapse of orbital fat
Prolapse of orbital fat may occur both in horses, dogs and cats. There seems to be a certain
breed disposition in the Persian cat, otherwise animals of any breed may be affected. A soft
swelling is seen in the conjunctiva between the globe and the eyelid. In the horse the fat
usually protrudes at the base of the third eyelid. Fine needle biopsy is important for
establishing a diagnosis. Prolapsed orbital fat may be removed surgically.


Enlarged globe – buphthalmos
It may not always be easy to determine if exophthalmos is caused by a retrobulbar process
or if the globe itself is enlarged. Comparison with the other eye is important. In buphthalmos
caused by glaucoma the intraocular pressure is usually markedly elevated. Intraocular
pressure may also be slightly elevated in retrobulbar disease due to venous compression.




Enophthalmos
This describes an eye retracted into the orbit, with secondary protrusion of the third eyelid.
The condition may occur either due to pathological small eyes or other factors causing the
globe to loose retro- or peribulbar support and sink back into the orbit. Enophthalmia is most
commonly seen related to painful conditions in or around the eye when the globe is pulled
into the orbit by the retractor bulbi muscle, or in connection with generalised malaise and
dehydration. Other causes of enophthalmia include malformation, neurologic disease and
end-stage globe.


Anophthalmia or microphthalmia both cause a misalignment between eye and orbit.
Anophthalmia, where the eye is actually missing is rather uncommon. Microphthalmia, on the
other hand, is not uncommon in the dog. Microphthalmia is a familiar condition in several
breeds of dogs, but may occur spontaneously as a malformation in all breeds. Examples of
dog breeds especially predisposed are the dobermann, Australian shepherd and West
Highland white terrier. Moderate microphthalmia is frequently overlooked by both owners and
examining veterinarians. Collie and Shetland sheepdog normally have small eyes, and it may
be difficult to distinguish between microphthalmia and normal eyes of small sizes in this
breed. In microphthalmia, vision may be normal, reduced or absent. Approximately 75% of
microphthalmic eyes develop cataracts, often progressing to complete cataract in 1-3 years,
and other abnormalities, including persistent pupillary membrane and retinal dysplasia may
also be present. One or more staphylomas (scleral hernias) may also be present in
microphthalmic eyes. Large staphyloma can be seen as a subconjunctival bulging. In
addition, microphthalmic eyes often have concurrent retinal dysplasia


From a distance it may be difficult to distinguish between a microphthalmic eye and a
phthisical eye, i.e. an end-stage globe after severe trauma or inflammation. However, the
clinical history and the results of more thorough examination enable the correct diagnosis
and prognosis.


Horner's syndrome
The disease is characterized by enophthalmos caused by lack of tone in the extraocular
muscles with subsequent protrusion of the third eyelid. The pupil is miotic (small) due to
diminished stimulation of the dilator muscle of the iris, and the eyelids are drooping because
of paralysis of the small eyelid muscles. In addition, horses will sweat on the neck on the
affected side. The condition is caused by injury of the sympathetic innervation passing via
the spinal cord to the last cervical - first thoracic vertebrae. Here the nerve leaves the spinal
cord and goes along the neck, under the ear and to the eye and extraocular region. The ear
should be thoroughly inspected for otitis. Tumours in the cranial thoracic region or along the
neck may also cause affection of the nerve. Medical diagnosis to determine the site of lesion
can be performed by applying 10% phenylepinephrine topically to both eyes and noting the
time taken for the pupils to dilate. The pupil of a normal eye and an eye affected by Horner's
syndrome due to a cerebral lesion will dilate in 60-90 minutes. If a lesion in the spinal cord
causes the Horner’s syndrome, the pupil dilates in about 45 minutes and in the case of a
peripheral lesion, the dilation occurs in about 20 minutes. The increased sensitivity to
phenylephrine in peripheral lesions is due to so-called “denervation hypersensitivity”, the iris
dilator muscle being hypersensitive to catecholamines after the muscle has been
denervated.


So-called “idiopathic Horner’s syndrome” where no underlying disease is found on
examination may be seen in all breeds of dogs, however, with the Golden retriever being
especially exposed. These cases usually heal within about three months with only supportive
therapy being indicated.


Episcleritis
Episcleritis is an immune mediated reaction of the vascular tissue overlying the sclera. The
condition may be diffuse, affecting parts of or the whole episclera. A nodular episcleritis has
been described primarily in the collie, but is seen in other breeds as well. Episcleritis may be
unilateral, but most often there are bilateral inflammatory signs. Cornea is often concurrently
affected with peripheral vessel growth and peripheral corneal dystrophy. Fine-needle
aspirates of nodules show inflammatory cells. The condition is treated with anti-inflammatory
medications.




Ocular tumours
The most common primary tumour is the melanoma, originating either from the iris or ciliary
body. Most often the tumours are pigmented, but cases of unpigmented ocular melanomas
may occur. In the dog the melanomas are usually well demarcated and may grow for several
years before causing clinical problems. The melanomas are usually not malignant, do rarely
spread through the sclera and will rarely metastasise. Small tumours may be successfully
treated with surgery or diode laser. When the tumour causes secondary inflammation, the
eye should be surgically removed.


Melanomas in the cats, however, are far more malignant. Tumour tissue can infiltrate the
sclera and surrounding structures and may metastasise, usually to the liver. Iris melanomas
in the cat present as brown, velvety elevations on the iris. Tumour tissue can be difficult to
distinguish from benignant pigmentation, but will grow to infiltrate more of the iris. Extirpation
of the globe may be indicated, as the melanomas are malignant, however, it may be difficult
the optimal time for enucleation.
Melanomas may also originate from the choroid and from limbus, the junction between
cornea and sclera. Choroidal melanomas are suspected to occur as a familial trait in the
Labrador retriever. Limbal melanomas are primarily seen in the German shepherd, but may
occur also in other breeds as pigmented well-demarcated elevations. Small limbal
melanomas can be surgically removed fairly easily, but larger require substitution of parts of
the globe wall by new tissue, for instance with Biosist®.


Medulloepithelioma is a type of tumour infrequently diagnosed in the dog. The tumour
primarily infiltrates the ciliary body and may extend forwards and backwards in the eye.


Post-traumatic sarcoma occurs in the cat following serious intraocular damage. Tumour
growth is usually seen long time after the initial injury. The tumour is suspected to originate
from lens epithelium.


Metastases to the eye of tumours elsewhere in the body occur primarily in malignant
lymphoma in the dog and lymphosarcoma in the cat. The iris functions as a lymph node in
the eye and may be heavily infiltrated by tumour cells. Sometimes ocular signs are
diagnosed before the animal shows signs of generalised illness. This should remind that
general examination should always be included in the work-up of an ocular condition.
Metastases of other tumour types occasionally occur to the eye, primarily in mammary
adenocarcinoma in the bitch.


Enucleation
Enucleation can be performed either trans-palpebral or trans-conjunctival. In the first case
the eyelids are sutured together before removal of globe, extraocular tissue and eyelids. This
technique is used in endophthalmitis where there is a risk of pus leaking into the orbit.


In trans-conjunctival enucleation the globe is removed together with the third eyelid while
most of the extraocular tissue is left in the orbit. The conjunctiva is severed and the globe is
dissected out. To minimise bleeding, extraocular muscles are cut as close to the globe as
possible. A pair of forceps is placed over the optic nerve and large blood vessels before the
eye is removed. Large vessels are ligated and deeper tissues are sutured with absorbable
material. The third eyelid and tear glands are removed, as well as the eyelid canthi and the
caruncle. The skin is closed with simple interrupted sutures. This method gives a
cosmetically better result. Extreme care should be taken to obtain sterile conditions, as the
nerve sheath around the optic nerve is in direct connection with the meninges. Especially in
cats the distance between the globe and optic chiasm is short and one should be careful not
to pull the optic nerve when removing the eye, as this may cause injury to the nerve to the
other eye.


Enucleation in the bird is more complicated because the eye is large and surrounded by a
relatively narrow orbital ring. To be able to remove the eye, it may be necessary to open the
orbit from the lateral side by removing some of the bone.


An alternative to enucleation is to put in an intraocular prosthesis, a black silicone ball. The
prosthesis comes in different sizes, and the size of the actual prosthesis is determined by
measuring the diameter of the contralateral eye. The eye is opened dorsally with an incision
parallel to and 4-5mm behind the limbus after the Tenons capsule has been dissected as a
flap in the surgical area. All intraocular tissue is carefully removed without damaging the
corneal endothelium. The prosthesis is placed into the eye with a special instrument and the
wound is closed in two layers. The cornea will gradually be infiltrated by pigment.
Postoperative care includes topical and systemic antibiotics, as well as systemic non-
steroidal anti-inflammatory drugs.

				
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