Ophthalmic anatomy and diagnostics (Proceedings)

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).

Tear drainage occurs via the nasolacrimal drainage system. In the dog and cat, tears are spread lateral to medial across the corneal surface to drain from through the dorsal and ventral nasolacrimal punctae, into the dorsal and ventral canaliculi, which connect at a rudimentary lacrimal sac within the lacrimal bone, and then continue as the nasolacrimal duct to exit at the nasal puncta. Many have a second opening at the mouth. Integrity of the nasolacrimal system may be tested by placing fluorescein stain on the corneal surface and evaluating for its presence at the nasal and/or oral opening (Jones test). Positive results of this test are diagnostic, but negative results are not always accurate. Nasolacrimal flushing, with culture and cytology, may be useful to evaluate for NLD obstruction, as may contrast dacryocystorhinography.

The cornea and sclera make up the outer fibrous coat of the eye, which provides support and structure to the globe. The anteriormost cornea acts as a windshield to the eye, lets light in, and focuses it on the retina. It is the primary focusing structure of the eye. The cornea and sclera are structured similarly, but the cornea is clear due to its relative dehydration, avascularity, and regular collagen arrangement. It is made up of a thin outer epithelium, central thick stroma, and single cell layer-thick inner endothelium. The basement membrane of the endothelium is termed Descemet's memrane and is located between the endothelium and the stroma. The inner endothelium and outer epithelium are both hydrophobic. As a result, these layers both repel fluorescein stain, while the corneal stroma absorbs it. Thus, both normal corneas and those with descemetoceles are fluorescein negative, though the difference in the two should be grossly evident. Corneal ulceration occurs whenever the epithelium is absent and these lesions are fluoresecin negative. Another stain, Rose Bengal, may be useful in diagnosing ulcers in certain circumstances, such as herpesviral ulcerative keratitis.

The clear spherical biconvex lens is the second most important focusing structure of the eye. It is clear for similar reasons as the cornea. It is held in position by peripherally located lens zonules. Gradual lens zonular breakdown occurs in dogs that develop primary lens luxation. This is diagnosed by ocular exam using a slit-lamp biomicroscope or transilluminator and magnification. Surgical lens removal is the recommended treatment for this condition and causes animals to become hyperopic (far-sighted), but not blind. Lens fibers are slowly, but continuously added to the outside of the lens throughout life, resulting in gradual compression of the nuclear lens, and the development of visible nuclear lenticular sclerosis at middle age, around 7 years of age in the dog. Pharmacologic dilation facilitates this diagnosis.

The uvea consists of the iris and ciliary body anteriorly and the choroid posteriorly. It is highly vascularized and pigmented. This is the middle coat of the eye, and is sensitive to systemic inflammatory processes. Anterior veitis is commonly diagnosed through the identification of aqueous flare in the anterior chamber by the use of a focal light source and identification of lowered intraocular pressure by applanation tonometry. Uveitis may dictate a minimum database and serologic testing for systemic infectious diseases. The pupil is the opening in the iris, which acts to monitor light input. Its shape varies with species. Pupillary light responses may be helpful for diagnosis of ocular and neurologic diseases. The ciliary body acts to produce the liquid aqueous humor which fills the anterior and posterior chambers of the eye. It is the target of many drugs and procedures used in the treatment of glaucoma. The choroid lines the retina and contributes much to its blood supply.

he majority of aqueous produced by the ciliary body flows from the posterior chamber though the pupil into the anterior chamber and out through the iridocorneal angle located where the iris and cornea join. Congenital or acquired ICA obstruction may result in elevation in intraocular pressure as measured by tonometry. Goniocopy may be performed to evaluate the drainage angle to look for abnormalities like goniodysgenesis.

Vitreous fills the posterior segment of the eye. It helps hold the lens and retina in place and should be clear to allow light through. It suffers fortunately fairly few diseases. Exceptions include congenital opacities, degeneration, and inflammation secondary to uveal inflammatory diseases. If direct examination is insufficient, ocular ultrasound may help evaluate this region. Vitreal aspirate is another possibly useful diagnostic technique, with results sent for cytology and/or culture.

The retina is responsible for converting the light energy that enters the eye into chemical energy to pass to the brain via the optic nerve for interpretation. Photoreceptors ("rods" and "cones") absorb the photons of light, and the energy passes through bipolar cells to retinal ganglion cells. The RGC axons converge at the optic nerve. The two optic nerves extend caudally and meet at the optic chiasm. Many fibers cross or decussate, while others stay ipsilaterally. The occipital cortex is responsible for vision interpretation. Retinal position can be evaluated directly or by ocular ultrasound. Retinal function can be evaluated by electroretinogram. Vision loss in absence of significant gross ocular disease (cataract, PRA, glaucoma, etc.) and in presence of normal ERG, suggests a central source of vision loss. The optic nerve is the only cranial nerve which can be directly evaluated (by ophthalmoscopy). An enlarged optic nerve may indicate inflammatory CNS disease or elevated intracranial pressure. Brain tumor or other space-occupying lesion such as GME may cause vision loss if the optic nerve, chiasm, tracts, or visual cortex are involved. All these conditions are usually diagnosed through MRI and/or CSF tap.