Chapter 8
Sclera

Anatomy and Physiology

The sclera makes up most of the fibrous tunic of the eye (the cornea makes up the remainder). It is thickest posteriorly around the optic nerve; a sieve-like opening (lamina or area cribrosa) is present here allowing the optic nerve to exit the globe. In most animals, the sclera is thinner at the equator than near the limbus. There are three fairly distinct divisions of the sclera: episclera (fairly loose connective tissue beneath conjunctiva), sclera proper (dense, white fibrous tissue), and lamina fusca (innermost zone made up of many elastic fibers).

The sclera extends anteriorly beyond the base of the iris to various distances (less than 4 mm), so that the drainage angle is obscured (usually can only be seen with special lens by technique of gonioscopy).

The bird, and some reptiles and amphibians have bony plates (ossicles) in their sclera anteriorly and cartilaginous plates posteriorly.

Congenital Abnormalities

Melanosis or nevus

Melanin deposits may be present in the stroma and can be extensive. They may extend into the cornea. Their surface is smooth. Although they may be slightly elevated, the overlying conjunctiva and episcleral tissues are not involved. They may appear as geographic zones or firm nodules; they may be stationary, slowly grow, or stay stationary for years only to slowly grow in later life.

These deposits rarely cause difficulty for the patient, and are not known to be directly or consistently associated with neoplastic transformation. They are best left alone. If they enlarge sufficiently to cause a problem, they carefully should be excised and examined histologically.

Coloboma

Coloboma means a defect or notching. Seen in collies and related breeds (Shetland sheepdog, Australian shepherd) as part of their ocular maldevelopment syndrome, or in other animals as isolated lesions.

Types:

Typical coloboma - lies in the fetal fissure - this is a region which extends ventrally from the optic disk to the cornea - would be considered the 6 o'clock area.

Atypical coloboma - occurs in a region other than the fetal fissure.

Generally is impractical for most colobomas.

Ectasia

Ectasia in ophthalmology means a thinning of sclera or cornea, usually with bulging.

This is seen in collies and related breeds (Shetland sheepdog, Australian shepherd) as part of their ocular maldevelopment syndrome, or in other animals as an isolated lesion.

Although scleral ectasia itself may not be harmful, its significance lies in its association with multiple ocular defects such as in the collie or Australian shepherd dog.

Treatment is not available.

Acquired Abnormalities of the Sclera

Ectasia

Ectasia of the sclera can also occur as an acquired lesion in situations where there is destruction secondary to inflammation or trauma (spontaneous or after surgery).

Treatment is aimed at the underlying cause primarily. Surgery to reinforce or replace the ectatic site are also possibilities to consider, but should be left to the discretion of someone expert in the field.

Episcleritis

Cause

Generally unknown. Usually noninfectious; may be a hypersensitivity reaction. Usually in adult dogs or cats. It can be classified into anterior and posterior forms.

Anterior episcleritis

May be diffuse or nodular, unilateral or bilateral (Paulsen, et al.).

Diffuse episcleritis: Episcleral and conjunctival blood vessels are engorged.

Perilimbal cornea becomes opaque gray-white and is vascularized.

Although intraocular pressure may be lower than normal, there is no significant change in the iris or anterior chamber that would signify iritis.

There may be a serous ocular discharge which may become mucopurulent if secondary bacterial conjunctivitis is present.

Nodular episcleritis: Some of the signs seen in the diffuse form may be present, but usually to much less degree.

The hallmark is the presence of a small, firm nodule in the sclera. These nodules usually are near the limbus and may slowly enlarge.

Treatment: Subconjunctival injections of triamcinolone may help. Will probably have to be repeated several times.

Strong topical steroids are sometimes helpful (dexamethasone is best).

Systemic steroids have also been advocated by some, but I believe this is too drastic and rarely indicated.

Prognosis: Diffuse form may continually recur, but can be controlled and is not blinding.

Posterior episcleritis

Not commonly seen.

Signs: Enophthalmia and protrusion of third eyelid due to contraction of retractor bulbi muscle due to pain from inflammation of posterior episcleral region.

Ocular motility may be reduced or absent.

The anterior ocular segment may be normal or slightly reddened.

Ophthalmoscopic examination may reveal choroidal inflammation if there has been extension into this tissue.

Treatment: Use systemic steroids at high concentrations. May have to use retrobulbar steroids if systemic steroids cannot be used.

Prognosis: Must remain guarded because the cause is obscure and recurrence may be frequent. The eye may have to be enucleated because of uncontrollable pain (as evidenced by continuing enophthalmia, blepharospasm). The eye may become blind if the optic nerve becomes inflamed and damaged.

Ocular nodular fasciitis

This is a benign nodular lesion of connective tissue occurring in sclera, cornea, or third eyelid. It is rare, but has been seen in adult dogs and cats. These nodules may grow slowly or rapidly and usually are located near the limbus. They do not cause the same degree of inflammatory response seen in anterior episcleritis. Their cause is unknown.

Unlike anterior episcleritis, these nodules generally do not respond to steroid therapy, although you probably should try medical therapy initially to rule this out. Definitive treatment requires surgical excision followed by topical steroids to control inflammation.

Some of these nodules easily can be removed, but many diffusely infiltrate the sclera at their base and cannot be removed completely. If some of the mass must be left, this usually is of no concern and recurrence is not a problem.

If the cornea is involved, superficial keratectomy will be necessary.

Trauma

Episcleral hemorrhage

See section on subconjunctival hemorrhage.

Lacerations and perforations

Horse: The perilimbal sclera apparently is weak in the horse and severe blows to the eye cause rupture here. Ciliary body and iris usually prolapse through the wound and there is marked hemorrhage into the eye. Sometimes the lens may protrude through the wound.

If the wound is clean or occurred within the last 6 hours, it can be flushed with saline and the uveal tissue reposited followed by suturing of the sclera. (The technique is similar to any intraocular surgery.) If the wound is dirty, old, or the uveal tissue is necrotic, the protruding uveal tissue should be excised before closing the wound. In most cases, there may be little chance of saving vision and in many the eye will become phthisical despite adequate repair.

Other animals: Automobile accidents, fights, or other injuries account for scleral trauma. The tear in the sclera can be anywhere and may even be posterior, beyond view; in these latter cases, the eye may not appear particularly traumatized, but will be very soft; the latter finding should alert you to the probability of a perforation not visible to you.

Treatment is similar to that for the horse and the prognosis is similar. A posterior perforation carries a poor prognosis for saving the globe. If you suspect this, refer the patient to a specialist.

Prolapse of orbital fat in cattle

Retrobulbar fat occasionally can dissect anteriorly between the sclera and Tenon's capsule to prolapse into the anterior episcleral space.

Causes: Congenital - may be seen in young animals who apparently are normal. Fat may extend close to the limbus.

Eye movements during examination may result in temporary prolapse of fat.

Space occupying lesions in the orbit.

Treatment: No treatment usually indicated because vision is not affected and the patient apparently is not uncomfortable. A thorough exam should be done, however, to rule out more serious orbital disease. If there is excessive prolapse, the fat could be removed followed by close suturing of the conjunctiva to the sclera; nevertheless may re-occur.

Neoplasia

Primary neoplasms of the sclera are rare.

One neoplasm, the origin of which is not clear but is believed to be episcleral or scleral tissue, is the epibulbar melanoma (Martin). This neoplasm almost always is benign in behavior even though it may grow sufficiently large to cause exposure problems. Clinically, it may mimic a staphyloma, but is firm and not indentable. Histologically, it often is more like a melanocytoma than a melanoma. Surgical excision is usually curative.

Secondary invasion of the sclera from other tissues is more common.

If the mass arises from the uveal tract, it may break through the sclera giving the impression of a mass arising from the sclera. Careful intraocular examination may reveal the true origin of the mass. Neoplasms arising from conjunctiva, eyelids, or orbit also may appear as scleral in origin.

Treatment depends on the nature of the neoplasm and the degree of ocular involvement (Harling, et al.; Martin). Those arising from the uveal tract usually require enucleation; those in the orbit require exenteration; more superficial ones may be excised and some may respond to radiation therapy. Generally, the neoplasm should be handled as it would elsewhere in the body.