Pertinent ophthalmic anatomy for veterinarians in private practice is reviewed from the outside in, as are related diagnostic
tests and pertinent diseases. In order, the orbit, eyelids, third eyelid, tear film, nasolacrimal drainage system, cornea
and sclera, lens, uveal tract (iris, ciliary body, choroid), iridocorneal angle and aqueous dynamics, vitreous, retina, optic
nerve, and visual cortex are reviewed.
The orbit is the area surrounding and protecting the eye itself. A major component is bone- parts of the lacrimal, zygomatic, frontal,
sphenoid, palatine, and maxillary bones in the dog. Horses and cattle have closed orbits. Cats and dogs have open orbits,
which have a supraorbital ligament closing the lateral portion of the orbit. The latter have an incomplete orbital floor,
such that retrobulbar disease, including retrobulbar abscess/cellulitis and orbital neoplasia, may be identified within the
mouth. The position of the orbits affects globe position, and varies by both species and breed. As a rule, carnivores have
rostrally set globes for improved binocular vision, while herbivores have laterally positioned globes for improved peripheral
vision. The zygomatic salivary gland is located ventrally, the lacrimal gland dorsolaterally, and the third eyelid gland
ventromedially within the orbit. Fascia, fat, extraocular muscles, and the globe itself fill the remaining space within the
orbit. Advance imaging techniques, such as magnetic resonance imaging (MRI) and Computed Tomography (CT), are ideal for evaluation
of the orbit. Orbital ultrasound can also be useful. Fine needle aspirate, biopsy, and cultures are important adjunctive
diagnostic techniques performed to help further diagnose the condition.
The eyelids also serve a protective purpose, as well as producing components of the tear film and spreading the tear film over the corneal
surface. The outer layer of the eyelid is skin; followed more centrally by muscle (orbicularis oculi), tarsal plate (minimal
in dogs) and meibomian glands; and lined internally by conjunctiva. The eyelid skin can be affected by the same conditions
as other skin- including allergic, bacterial, parasitic, and fungal disease and tumors- and the same diagnostic techniques
apply, including cultures, skin scrapes, DTM assays, and biopsies. The meibomian glands are common sources of benign eyelid
tumors in dogs and usually excisional biopsy with histopathology is performed. The meibomian glands and conjunctiva produce
the oily and mucoid portions of the tear film, respectively.
The third eyelid consists of a T-shaped piece of cartilage surrounded by conjunctiva and having a tear-producing gland at its base. It is
located ventromedially in the common veterinary species and provides adjunctive protection to the globe. It is passively
prolapsed whenever enophthalmia is present. This is therefore commonly a sign of ocular pain, as most veterinary species
have the ability to retract their globes via their retractor bulbi muscles when as a reaction to ocular surface pain like
corneal ulceration. Neurologically induced enophthalmia from sympathetic denervation (Horner's syndrome) is an alternative
cause. Foreign bodies may become lodged behind the third eyelid resulting in irritation and direct corneal ulceration. Atraumatic
forceps may be used to evert and examine the area behind the third eyelid, usually in the awake state. The third eyelid gland
may become prolapsed in some breeds of dog due to congenitally poor ligamentous attachments. The gland is responsible for
about 40% of aqueous tear production, so replacement of the gland is strongly recommended.
The tear film consists of an inner mucinous layer integrally attached to the cornea in normal individuals, a thick central aqueous layer,
and an outer oily layer. As previously discussed, the meibomian glands produce the outer oily layer, and the conjunctival
goblet cells produe the inner mucinous layer. The aqueous layer is produced by the lacrimal gland (60%) and the third eyelid
gland (40%). Poor tear production by the latter two glands results in aqueous tear deficiency or Keratoconjunctivitis Sicca
(KCS), which is usually diagnosed by Schirmer Tear Test I. Signs similar to KCS may occur with qualitative tear film deficiency,
due to lack of one or more of the other components of the tear film, and is disgnosed by evaluating tear film break-up time
(TBUT).
