Ophthalmology is the study of medical and surgical diseases of the eye. Intense study is required to understand the mechanism of action of the eye, its disease processes, and the methods to treat these disease processes. The eye is not an isolated organ; it interacts with the body and mind. There are no systemic diseases that do not have ocular effects. Some of the most common diseases (e.g., hypertension, diabetes) can have devastating effects on the eye. Blindness from diabetes (and trauma) is a major debilitating result that has extensive health care consequences. Such diseases as acquired immunodeficiency syndrome (AIDS) often have ocular involvement that requires surgery. However, the two most common ocular disease processes that affect health care in the United States are cataracts and glaucoma.

Everyone who lives long enough will eventually have cataracts. In a society in which people wish to remain active (e.g., driving, reading), cataract surgery is one of the most commonly performed surgeries. It requires tremendous health care expenditures every year. Fortunately, the high success rate of cataract surgery allows patients to return to normal activities.

Another common ocular disease is glaucoma, which affects up to 2% of the general population. It requires long-term treatment with medication or surgery. This disease also has a heavy impact on the public health care dollar because most patients with glaucoma are eligible for Medicare.

This chapter describes surgical diseases of the eye according to the subspecialties in ophthalmology: the cornea; the anterior chamber; the lens; the retina and vitreous; the nasolacrimal system, eyelids, and orbit; and the extraocular muscles. A short section at the end describes common ocular disorders. Figures 9-1 and 9-2 show the anatomy of the eye.

Figure 9-1   Orbital cavity, dissected from the front. CN, cranial nerve.

Figure 9-2   Cross-section of the eye.

DISEASES OF THE CORNEA

Anatomy and Physiology

The cornea is a clear, avascular structure that provides structural integrity to the anterior segment of the eye. It is a major refractive component. Forming approximately one sixth of the surface of the eye, the cornea is similar in structure to neighboring sclera. Corneal collagen is more uniformly oriented, however, and the cornea itself is dehydrated. These two factors contribute to the maintenance of corneal clarity. The cornea and the overlying tear film provide two thirds of the refractive component of the eye. The lens accounts for the rest.

The precorneal tear film, which is crucial to the health of the eye, is produced by the lacrimal gland and specialized glands in the conjunctiva and eyelids. The tear film has three layers: (a) the superficial layer, produced by the meibomian glands, is oily and helps prevent evaporation; (b) the middle layer, produced by the lacrimal gland, is watery and comprises the bulk of tear film; and (c) the innermost layer, produced by goblet cells, is mucoid and contributes to even spreading of the tear film. Deficiencies in any of these layers can lead to optical disturbances and compromise the health of the eye.

The cornea has five layers. In anterior to posterior order, these are the epithelium, Bowman’s membrane, the stroma, Descemet’s membrane, and the endothelium. The epithelium is a nonkeratinized, stratified squamous cell layer that forms a smooth surface over the cornea. It can be damaged easily by minor trauma that causes corneal abrasions. If the epithelium basement membrane is damaged, adhesion of the epithelium is compromised and recurrent abrasions may result. Because the cornea is supplied by numerous nerve endings, these abrasions can be extremely painful. Supporting the epithelium is Bowman’s membrane, which consists of randomly dispersed collagen that is firmly anchored into the corneal stroma. Although epithelium regenerates without scarring, Bowman’s membrane does not. The next layer, stroma, accounts for 90% of normal corneal thickness. Its three main constituents (collagen-producing fibroblasts, collagen lamellae, and mucopolysaccharide ground substance) combine to produce a tough, elastic protective coat. Posterior to the corneal stroma is Descemet’s membrane, the basement membrane for the fifth layer, endothelium. Corneal endothelium, derived from neuroectoderm, is a functionally complex monolayer of hexagonal cells that do not regenerate. The main function of this layer is to keep the cornea in a partially dehydrated state and therefore to preserve its clarity. It accomplishes this function by maintaining a tight barrier between the corneal stroma and aqueous humor and by pumping water out of the corneal stroma. The cell density of this layer decreases with age. Although remaining cells enlarge to accommodate lower cell counts, corneal clarity is compromised when the endothelial cell count falls to 400 to 700 cells/mm². As the cell count falls, corneal stromal edema increases, followed by corneal epithelial edema. These developments lead to loss of vision and breakdown of the corneal epithelium.

The cornea metabolizes glucose primarily through glycolysis. It receives 90% of this substrate from aqueous humor. Oxygen is delivered to the corneal epithelium from the tear film and the environment. Corneal tissue posterior to and including the corneal stroma receives oxygen from the aqueous humor. Given its proximity to glucose-rich aqueous humor and oxygen-rich air, the cornea functions well even though it is avascular. The tear film provides added immunologic protection in the form of enzymes and immunoglobulins.

Pathophysiology

The cornea can be damaged by trauma, infection, and metabolic imbalances. Additionally, the cornea can require treatment for structural defects or to replace diseased tissue.

Clinical Presentation and Evaluation

Examination of the cornea involves observation of the tissue and measurements of its curvature. The slitlamp is the most important instrument for evaluating the cornea because it allows creation of an optical cross section that permits direct visualization of any pathology. Examination is facilitated by the use of a topical anesthetic. By adjusting the light beam and the incident angle of observation, different areas within the cornea can be highlighted. Fluorescein stain is instilled to detect areas of corneal abrasion. In these areas, the cornea glows strongly under a cobalt blue light. Even a simple penlight can provide useful information, however. An irregular corneal surface can be detected by an irregular light reflex from any point source of light.

The curvature of the cornea, and therefore its refractive ability, is measured with a variety of instruments. The most common quantifiable method uses a keratometer, a device that measures the radius of curvature of the central cornea. These measurements are important in guiding refractive procedures and predicting appropriate intraocular lens (IOL) power. Newer, more expensive machines provide a topographic analysis of the corneal surface, much the same way that topographic maps represent the earth’s surface. These are helpful for refractive procedures. The quality of the refracting surface of the cornea can be assessed with the keratometer and a keratoscope. A keratoscope projects a Placido’s disc (concentric rings that show surface topography, analogous to elevation lines on topographic maps). These measurements are helpful in postoperative care, especially in suture removal.

The endothelial cell layer of the cornea is visualized and photographed with specular microscopy, a technique that uses an optical device that allows direct observation of the corneal endothelium. This technique provides information about the quality and quantity of the endothelial cells that are responsible for maintaining the relatively dehydrated state of the cornea, thus ensuring its clarity.

Treatment

Much corneal surgery arises from diseases that do not respond to nonsurgical treatment. Examples include keratoconus (corneal abnormality in which the cornea becomes misshapen into a conical shape) initially treated with a contact lens; infectious keratitis (inflammation of the cornea) treated with intensive fortified antibiotics; and herpes simplex keratitis treated with antivirals. Other cornea problems, such as aphakic bullous keratopathy (a breakdown in the cornea that leads to large defects in the epithelium) with diffuse corneal edema, are surgical diseases from the outset. This discussion involves only the diseases of the cornea that are treated primarily with corneal surgery. However, many medical interventions typically precede the necessary surgery.

Corneal surgery is performed for three main reasons: (a) to produce a clear visual pathway (e.g., cornea transplant for a corneal scar); (b) to maintain the integrity of the globe (e.g., lamellar cornea transplant for a perforation); and (c) to alter the inherent corneal structure to produce refractive or functional changes (e.g., radial keratotomy to reduce myopia). Much of the recent success with corneal surgery is the result of such technologic advances as the operating microscope, finer and better suture materials, improved instrumentation, and better tissue preservation.

For patients to have good vision after surgery, it is important to maintain a smooth corneal surface and clear corneal substance. Care is taken to approximate wounds carefully, without torque, to ensure that sutures are placed at equal depths within tissue, and to place an ideal tension on the suture. Various devices are used to help determine the correct tissue tension, although most surgeons find experience and practice the best aids.

Corneal Transplantation

Corneal transplantation is the most commonly performed tissue transplant in the United States (approximately 30,000 procedures/year). The success of this procedure has been facilitated by several factors (e.g., improved means of tissue preservation, the nationwide network of eye banks) that have increased the availability of corneal tissue. Because the cornea is avascular, aggressive immunosuppression therapy (required by most transplant patients) is not needed. Corneal transplantation involves replacing the full-thickness cornea (penetrating keratoplasty; Fig. 9-3) or the partial-thickness cornea (lamellar keratoplasty; Fig. 9-4). The location of the corneal opacity determines the procedure. For full-thickness corneal lesions, only a full-thickness penetrating keratoplasty will suffice. For anteriorly placed corneal lesions, a lamellar keratoplasty can be performed. Often a full-thickness procedure is performed even though a partial-thickness procedure would do, because in most patients who undergo the partial-thickness procedure, scarring causes the development of a fine haze that interferes with vision (Fig. 9-5).

Figure 9-3   Penetrating keratoplasty.

Figure 9-4   Lamellar keratoplasty.

Figure 9-5   Slitlamp photo of lamellar keratoplasty. Residual opacification is seen in the host bed.

Indications

Corneal transplantation is used to treat pseudophakic and aphakic bullous keratopathy, keratoconus, corneal dystrophy, and infectious keratitis. Indications for penetrating keratoplasty (replacing full-thickness cornea) are listed in Table 9-1.

Pseudophakic bullous keratopathy, or permanent corneal clouding after IOL implant surgery, is the most common indication for corneal transplantation in the United States. All cataract surgery causes some loss of endothelial cells. In some patients, this loss brings the patient under the critical number of remaining cells required to maintain corneal clarity. Depending on the degree of endothelial damage, the cornea may become cloudy immediately after the surgery. More characteristically, this clouding occurs months to years after the surgery. Aphakic bullous keratopathy is corneal opacification after cataract surgery without an IOL implant.

IOL implants after cataract surgery became popular in the late 1970s. Early lens design led to a high incidence of corneal edema and problems (e.g., glaucoma, hyphema [blood in the anterior chamber of the eye]) that appeared years later. Sometimes these lenses can be replaced with better-designed implants before the cornea becomes permanently hazy. Once the cornea becomes opacified, visual rehabilitation requires a penetrating keratoplasty combined with IOL exchange. Although the incidence of pseudophakic bullous keratopathy is less than 1%, the sheer number of cataract surgeries performed in the United States annually makes this keratopathy the most common indication for corneal transplantation. As advances in cataract surgery produce less trauma to the eye, the incidence of pseudophakic bullous keratopathy is expected to decrease.

Patients with pseudophakic bullous keratopathy first notice diminution in vision as the cornea imbibes fluid. During the early stages of corneal edema, the patient has hazy, blurred vision that is worse on awakening and gradually clears. With the lids closed during sleep, fluid cannot evaporate from the corneal surface. After the eyes are open, the imbibed fluid evaporates and the corneal haze decreases. Hypertonic saline drops and ointment promote corneal deturgescence, as does the use of a hair dryer on a low-heat setting held at arm’s length and directed toward the eyes. As the cornea becomes more edematous, small blisters form in the corneal epithelium. These blisters tend to break down and cause severe ocular pain and irritation.

Penetrating keratoplasty alleviates discomfort from corneal epithelial breakdown and improves visual acuity. For patients who are poor candidates for corneal transplant, other procedures that can be tried include bandage contact lenses (extended-wear lenses that can remain in the eye), corneal surface scarification (with multiple fine-needle punctures or a diathermy probe), and conjunctival flaps. For a flap, a thin layer of healthy, intact conjunctiva is mobilized and pulled down over the corneal surface as a “hood” flap and sutured into place (Figs. 9-6 and 9-7). A conjunctival flap can also be used to treat corneal infections that are unresponsive to antimicrobial therapy. Applying the flap over the infected cornea apparently stimulates the body’s immune system to eradicate infection. If there is good visual potential, penetrating keratoplasty may be performed later, through the flap, after the inflamed corneal stroma becomes scarified.

Figure 9-6   Conjunctival flap.

Figure 9-7   Well-healed conjunctival flap.

Keratoconus is progressive dystrophy of the cornea in which the central cornea becomes thinner than normal, with a subsequent forward bulge. The bulge takes on a conoid shape that causes visual distortion and decreased visual acuity. It is usually bilateral, although one eye may be much more involved than the other. Most cases are sporadic, but 10% are hereditary. Keratoconus has a higher incidence in such systemic disorders as Down’s syndrome, atopy (type I allergic reaction), and Marfan’s syndrome. Eye rubbing may play a role in the development of keratoconus, and there is ongoing debate over whether contact lens wear may be a cause.

The disease tends to progress through adolescence and stabilize in young adulthood, although worsening may occur at any age. Most patients achieve excellent vision with rigid gas-permeable contact lenses. These lenses ameliorate associated irregular astigmatism (a refractive error in which the defect is not the same in all meridians). Surgery is indicated in patients who cannot use contact lenses because of steep or irregularly shaped corneas or in those who have central corneal scarring.

Penetrating keratoplasty is the procedure of choice for keratoconus, with a success rate of 80% to 90%. Some surgeons achieve good results with epikeratophakia (a procedure in which a graft is placed on the cornea) to flatten the corneas of patients who have small, central cones; have little scarring; and cannot tolerate contact lenses. This procedure uses a donor cornea that is frozen before being lathed commercially into a tissue contact lens. Epikeratophakia is advantageous because the globe does not have to be entered (reducing the risk of endophthalmitis). Immunologic rejection of the lens has never been reported. Visual results tend to be poorer than with penetrating keratoplasty, however. Occasionally, a patient’s cornea has thinned to such a degree that penetrating keratoplasty is not possible: too little tissue is left on which to sew the donor graft. In these patients, lamellar keratoplasty is performed to “bulk up” the recipient cornea. After healing occurs, penetrating keratoplasty can be performed.

Some patients experience acute rupture of Descemet’s membrane (if the membrane becomes thinned because of the keratoconus), with sudden inflow of aqueous humor into the corneal stroma. This condition, known as acute hydrops, is usually accompanied by acute pain and decreased vision. When the hydrops heals, the cornea often flattens. If the scarring does not involve the visual axis, patients who previously could not wear a rigid contact lens can now be fitted with one. However, most patients with acute hydrops ultimately need penetrating keratoplasty, which is best deferred until all of the corneal edema resolves.

Corneal dystrophies are inherited conditions that cause variable findings according to the anatomic layer affected. They are bilateral and largely autosomal dominant, with variable penetrance.

Superficial corneal dystrophies include map-dot-fingerprint dystrophy (Cogan’s microcystic dystrophy). Abnormalities in the corneal epithelium cause changes in the epithelium basement membrane that manifest as fine map, dot, or fingerprint lines on slitlamp examination of the cornea. Patients with this condition may be asymptomatic, but spontaneous episodes of recurrent corneal erosion can occur. Conservative treatment includes topical lubricants, hypertonic saline drops or ointment, and bandage contact lenses. If these measures do not help the epithelium adhere to its underlying basement membrane, paradoxically, debridement to the epithelium in the involved area may help achieve stability. Anterior stromal puncture is gaining acceptance. This procedure uses a fine needle to make small puncture marks just beyond Bowman’s membrane, into the corneal stroma. These small scars allow the overlying epithelium to achieve better adhesion to its basement membrane. Even when the punctures are made within the visual axis, visual deficits are rare, although patients may complain of changes in the quality of their vision.

Many corneal stromal dystrophies occur, but the three classic ones are granular, lattice, and macular. Granular dystrophy is characterized by deposits of hyaline material into the corneal stroma. These deposits produce a “bread crumb” appearance, with intervening clear areas of cornea. This condition progresses slowly and leads to a gradual worsening of vision. Symptoms often do not start until midlife. Treatment with penetrating keratoplasty offers a good prognosis. Lattice dystrophy results from amyloid deposits within the corneal stroma. These deposits give the appearance of fine, refractile lines that form a web. This pattern is often best seen in retroillumination (slitlamp technique). Recurrent erosions are common and may lead to stromal haze and decreased vision. The treatment of choice is penetrating keratoplasty. Recurrences of lattice dystrophy in the corneal graft are common, but may take years to develop (Fig. 9-8). Macular dystrophy is usually more severe than granular or lattice dystrophy. Unlike the other types, macular dystrophy has an autosomal recessive inheritance pattern. Mucopolysaccharide deposits tend to spread diffusely through the corneal stroma, leading to decreased vision at an early age. These deposits reach the periphery, with no clear cornea between them. Penetrating keratoplasty is performed to improve vision.

Posterior dystrophies include Fuchs’ endothelial dystrophy. This common autosomal dominant disorder is characterized by the early development of endothelial dysfunction associated with decreased endothelial cell counts. This disorder is most commonly found in postmenopausal women. Slitlamp examination shows endothelial excrescences (corneal guttae), often associated with pigment. The amount of corneal edema depends on the severity of the disease (Fig. 9-9). As the disease progresses, epithelial breakdown may occur, eventually producing subepithelial scarring. Symptoms of blurred vision and irritation usually do not occur until the fifth or sixth decade. In the early stages, topical hypertonic saline may be used to dehydrate the cornea, and bandage contact lenses may be inserted to neutralize corneal astigmatism. In later stages, corneal transplantation is the only alternative to improve visual acuity.

Figure 9-8   Recurrent lattice dystrophy in a penetrating keratoplasty.

Figure 9-9   Corneal edema secondary to Fuchs’ dystrophy.

Infectious keratitis is usually caused by bacteria, rarely by fungi or parasites. Infectious corneal infiltrates are associated with contact lenses, especially extended-wear soft contact lenses. Symptoms include pain, redness, purulent discharge, decreased vision, and photophobia. After appropriate cultures and smears are obtained by scraping the involved cornea under topical anesthesia, treatment is with broad-spectrum, fortified topical antibiotics. Fortified antibiotics are more concentrated than the usual ones and are specially formulated by a pharmacist. Typical antibiotics used are fortified vancomycin or gentamicin (50 mg/mL hourly), but these could change depending on patient sensitivities or the availability of new antibiotics. Most such “corneal ulcers” are successfully treated with medication. If a patient does not respond to initial treatment, cultures are repeated to look for more unusual organisms. Occasionally, corneal biopsy is needed to identify the microbial agent. For patients who do not respond to medical therapy, corneal transplantation is required to excise the infected tissue.

The corneal transplantation technique is modified slightly because of the presence of infectious organisms. The size of the graft is determined by the size of the corneal infiltrate. If the corneal infiltrate is not totally excised, the residual organisms will reinfect the graft and possibly the inside of the eye. After the infected cornea is excised, the surgical tray is replaced with a new sterile tray of instruments. These instruments are used on the donor cornea to finish the surgery. The host corneal tissue button is sent not only to microbiology but also to pathology to check for organisms at the wound edge, just as one would look for evidence of tumor cells at the edge of a tumor resection. Fortified antibiotics are used topically after surgery, and the surgeon may use intraocular or intravenous antibiotics. Although preservation of a clear graft is desirable, the overriding concern is elimination of the infectious process. Consequently, topical steroids that suppress the immune system are used with caution, if at all.

The most common conditions that require cornea transplantation because of infection are fungal or amebic infectious keratitis and severe bacterial keratitis that has led to corneal perforation. If an infected cornea perforates, then cyanoacrylate tissue adhesive can be used to seal the perforation and allow the underlying tissue to heal after the acute infectious process is eradicated.

Sterile corneal ulceration may occur in association with such connective tissue disorders as rheumatoid arthritis, systemic lupus erythematosus, and Wegener’s granulomatosis. The sterile infiltrates usually occur in the corneal periphery. They often respond to topical or anti-inflammatory therapy. If they perforate, cyanoacrylate tissue adhesive can be used. A bandage contact lens is placed over the cornea to protect against mechanical irritation from lid movement that can dislodge the adhesive. If successfully applied, the glue allows the corneal stroma to fill in beneath it. The glue usually spontaneously dislodges in a few months. Then the bandage contact lens is removed. The use of tissue adhesive is limited by the size of the corneal perforation. A perforation that is larger than 2 to 3 mm cannot be sealed with glue and requires a “patch” graft. This type of graft involves plugging the hole in the cornea with a piece of donor corneal tissue.

Technique

Donor Tissue. A cornea donor may be any deceased person who is younger than 75 years of age and has no transmissible disease or history of eye disease. Cornea tissue is refused from donors who test positive for human immunodeficiency virus, those who have had previous intraocular surgery, and those with hepatitis, rabies, sepsis, Jakob-Creutzfeldt disease, or glaucoma. The quality of donor tissue depends on the age of the donor, the time between death and tissue harvest, and the time spent in processing and preservation. Although younger donors and rapid harvest, processing, and preservation are preferred, many older donors can have excellent corneal quality as documented by endothelial cell counts. Both the eye bank and the surgeon assess the quality of the tissue. Tissue typing is not performed.

The donor cornea and a thin rim of surrounding sclera are preserved in solutions that contain nutritional, preservative, and antibiotic solutions. Although the risk of transplanting contaminated tissue is rare, cultures of the donor corneal rims are taken at the time of preservation and also at the time of surgery. Currently, donor tissue may be preserved for 5 to 7 days after death. Cryopreservation allows corneal preservation for many years, but is neither practical nor economical.

Full-Thickness Transplant. Corneal transplantation may be performed with an eyelid block and either local retrobulbar injection or general anesthesia. A lid speculum is placed in the conjunctival sac, avoiding pressure on the globe itself. Usually, a small stainless steel ring (Flieringa’s ring) is sewn to the sclera to support the globe when the eye is open. Because the eye is an elastic structure, it tends to collapse when the cornea is removed, especially in children.

A corneal trephine (circular blade) of appropriate size is centered on the cornea, and a partial-thickness (80%) cut is made in the host. The donor cornea is prepared with a separate trephine. The endothelial side of the corneal button is up. To decrease the likelihood of postoperative complications and to simplify closure of the eye, this cut is approximately 0.5 mm larger in diameter than the host trephination. After the donor cornea is punched, it is placed on a protected Teflon block and covered by its preservative solution. Great care is taken to avoid trauma to the donor cornea that could cause endothelial cell loss and lead to graft failure.

After the donor cornea is prepared, the host eye is entered with a sharp blade. The host button is cut out with curved corneal scissors, and further intraocular surgery is performed as needed. Such surgery might include removal and replacement of an IOL, placement of a secondary IOL in a previously aphakic eye, removal of prolapsed vitreous humor from the anterior chamber of the eye, resection of scar tissue, or cataract extraction and primary IOL implant.

Once these other procedures are completed, the donor cornea is brought to the operative field and sewn into place. Various suturing methods are used, depending on the surgeon’s preference (Figs. 9-10 and 9-11). The ultimate goal is to achieve a perfectly replaced cornea with no induced astigmatism. After the cornea is sewn to the recipient bed and the anterior chamber is reformed with balanced salt solution, the wound is inspected to make sure it is secure. The supporting ring is removed, antibiotics and steroids are administered if necessary, and a pressure patch with a shield is applied.

Figure 9-10   Clear penetrating keratoplasty with intact interrupted sutures.

Figure 9-11   Clear penetrating keratoplasty with intact running sutures. A peripheral iridectomy (hole in the iris to prevent glaucoma) is seen at the 2 o’clock position.

Postoperative management of penetrating keratoplasty includes topical steroids, topical antibiotics, and topical lubricants. If intraocular pressure is elevated, topical or systemic antiglaucoma medications are used. Because topical antibiotics are potentially toxic to the epithelium, they are gradually withdrawn over a period of several weeks. Topical steroids are used for several months to reduce intraocular inflammation and prevent immunologic graft rejection.

Visual rehabilitation after penetrating keratoplasty takes much longer than that after other anterior segment surgical procedures. Avascular, clear corneal wounds heal much more slowly than limbal (vascular area between the cornea and the sclera) wounds. Complete visual recovery may take 1 year or longer. If patients obtain good visual acuity with a stable refraction and not too much astigmatism, sutures are best left in place. Removing them can cause large astigmatic errors. It is important to warn the patient about the potential for erosion and exposure of disintegrating sutures, which can be a source of irritation and infection. All patients must be told to seek prompt ophthalmic evaluation if they experience pain, redness, irritation, or discharge that suggests suture exposure.

Intraoperative complications include damage to adjacent anterior segment structures (e.g., iris, lens). Bleeding from the iris or anterior chamber angle may occur during removal of a poorly designed or poorly positioned IOL. The most feared intraoperative complication is expulsive hemorrhage, the forceful expulsion of the intraocular contents after acute choroidal hemorrhage. This rare complication is more common in elderly patients, those with glaucoma, and those with previous eye surgery. The most important priority is closure of the eye. Secondary surgical intervention can be performed several days later, after the ocular status stabilizes.

Postoperative complications include rejection, endophthalmitis, persistent epithelial defects, infectious keratitis, elevated intraocular pressure, retinal detachment, and epithelial downgrowth. The incidence of rejection is approximately 10% to 20% for patients with a good prognosis (e.g., corneal scars, keratoconus, certain corneal dystrophies). Most rejection episodes occur 3 to 12 months after surgery. The incidence is higher in younger patients, presumably because of their more active immune systems. If caught early, rejection episodes can often be aborted by the intensive use of steroids, topically in the form of drops or ointment, subconjunctivally by injection, or orally. Patients who undergo penetrating keratoplasty are instructed to seek prompt ophthalmic evaluation at the first signs of rejection (e.g., conjunctival injection, pain, irritation, light sensitivity, decreased vision). In its early stages, rejection often is not noticed by patients, but it is observed during a routine follow-up visit. Permanently rejected corneas become cloudy. They may be retransplanted, although with each subsequent transplant, the chance of rejection increases.

The prognosis for corneal transplants depends primarily on the underlying disease. Favorable prognostic factors include the lack of preoperative inflammation, avascularity of the diseased cornea, normal intraocular pressure, and healthy ocular surface and adnexa. Poor indicators include active inflammation, surface disorders (e.g., dry eye syndrome, lid abnormalities, corneal vascularity, glaucoma, chemical burns). The results of penetrating keratoplasty in children are not as good as in adults because of such factors as associated congenital ocular abnormalities, difficulty with follow-up, and amblyopia (poor or decreased vision in an anatomically normal eye). Some disorders (e.g., certain corneal dystrophies, herpes simplex keratitis) tend to recur in corneal transplants. All of the stromal dystrophies recur in transplants, although it may take years for such recurrence to become apparent. The recurrence may differ from the characteristic appearance of the primary dystrophy.

The prognosis for a clear corneal transplant is fairly good, but in some cases, astigmatism and anisometropia (significant difference in refractive error between the eye that has had surgery and the one that has not) often preclude full visual rehabilitation. Contact lenses are helpful in neutralizing the astigmatism or anisometropia. Unfortunately, some patients cannot tolerate contact lenses (e.g., some elderly patients have difficulty handling or tolerating the lenses). Astigmatic keratorefractive surgery (discussed later) is sometimes required to correct excessive astigmatic or anisometropic refractive errors. Visual results after penetrating keratoplasty depend not only on the clarity and regularity of the transplanted corneal button, but also on the healthy function of the retina and optic nerve.

Partial-Thickness Transplant. Lamellar corneal transplant is useful for patients with anterior corneal disease (e.g., those who have scarring or inherited dystrophies). This procedure obviates the need to enter the eye, thereby virtually eliminating the risk of endophthalmitis. In addition, the recipient corneal endothelium is left intact, thereby eliminating the risk of immunologic endothelial rejection. The procedure also provides a patch graft in patients with acute corneal perforations that require immediate surgical closure.

The surgical procedure of lamellar keratoplasty is technically more demanding than that of penetrating keratoplasty. A corneal trephine is used to make a partial-thickness incision into the host cornea. Then a lamellar dissection is carried across the outlying trephination. The lamellar dissection is deep enough to remove the anterior stromal opacification while leaving a posterior bed of clear corneal stroma for the corneal donor. Donor tissue also requires precise lamellar dissection and is best performed from an intact donor globe.

The postoperative management is similar to that for penetrating keratoplasty, although fewer topical steroids are used. The healing of lamellar grafts tends to be quicker than that of penetrating keratoplasty. The major drawback of lamellar keratoplasty is the difficulty in achieving a smooth lamellar bed. Consequently, there is an increased potential for interface scarring that may restrict vision. However, in select patients with anterior corneal stromal opacification, lamellar transplant rather than penetrating keratoplasty is the treatment of choice.

Recently, new procedures have been developed to replace the damaged endothelial tissue without requiring a full corneal transplant. The technique is referred to as deep lamellar endothelial keratoplasty (DLEK). It involves surgically removing the diseased endothelium and replacing it with donor endothelium. This can be done through a small incision and does not require sutures. This procedure reduces astigmatism and other structural problems associated with a full-thickness corneal transplant.

Refractive Surgery

Refractive corneal surgery involves changing the refractive power of the eye by surgically modifying the shape of the cornea. A variety of procedures are used, but only the most common are discussed here.

Radial keratotomy, the most common refractive surgery performed, is used to correct myopia. Partial-thickness radial cuts are made in the cornea, sparing the visual axis (Figs. 9-12 and 9-13). This pattern of incisions tends to flatten the central cornea, thereby reducing the myopic refractive error. The procedure is usually performed under topical anesthesia. The amount of correction can be modified by varying the size of the spared central optical zone, the number of radial incisions, and the depth of the incisions. The predictability of radial keratotomy depends on the degree of myopia being treated. Patients with low to moderate myopia have an 80% to 90% chance of obtaining an uncorrected visual acuity of 20/40 or better. The chances of obtaining better visual acuity decrease with increasing myopia.

Figure 9-12   Radial keratotomy.

Figure 9-13   Eight-incision radial keratotomy.

Surgical complications are rare, although many patients note glare and fluctuating visual acuity in the early postoperative period. These problems tend to resolve with time. Repeat procedures are sometimes required. If the result still does not offer adequate uncorrected visual acuity, the patient must return to wearing spectacles or contact lenses. Questions have been raised about the stability of radial keratotomy. In some patients, the effect of surgery gradually increases over a period of several years. There is also concern that deep radial incisions may weaken the structural integrity of the cornea, increasing its susceptibility to traumatic rupture.

Other refractive procedures include corneal relaxing incisions, compression sutures, wedge resection, epikeratophakia (application of a donor cornea), and keratomileusis (grinding a new curvature on the cornea). Patients with congenital astigmatism or residual postoperative astigmatism usually can be managed with glasses or contact lenses. If astigmatism is excessive, surgical alternatives include relaxing incisions, compression sutures, and wedge resection. Relaxing incisions centered on the steep axis of the corneal astigmatism cause flattening of the cornea in that meridian, thereby reducing the amount of astigmatism. Corneal compression sutures and wedge resections are used to steepen a flat corneal meridian. Visual rehabilitation after wedge resection is much longer than with relaxing incisions. The effect of corneal compression sutures is variable and may not be permanent. The predictability of astigmatic surgery is less than ideal, but in patients with excessive postoperative astigmatism, there is nothing else to offer for visual rehabilitation.

Epikeratophakia and keratomileusis are examples of refractive procedures that use lamellar keratoplasty. Epikeratophakia is used primarily to correct aphakia in patients who cannot tolerate contact lenses and are not good candidates for a secondary IOL. The procedure involves suturing a commercially prepared lenticula of corneal tissue on top of the recipient cornea after the corneal epithelium is removed. Recovery is fast and the procedure is easily repeatable if necessary. Keratomileusis involves shaving off a thin anterior section of the cornea, freezing it, lathing it to the desired power and shape, thawing it, and resuturing it into its original site. This procedure is reserved for highly myopic patients whose vision cannot be corrected otherwise and who do not want to wear glasses or contact lenses. This procedure requires expensive equipment, the use of which can be difficult to master. Major drawbacks include poor predictability, irreversibility, and the potential for surgically induced irregular corneal astigmatism. One of the most exciting developments in corneal surgery is the introduction of the Excimer laser, which vaporizes tissue with a high degree of precision. Although clinical trials are still technically underway, the laser is greatly in demand for sculpting the anterior cornea to produce refractive change. If the encouraging preliminary results hold up over the long term, this laser could revolutionize the practice of ophthalmology.

Pterygium Excision

A pterygium is a plaque-like extension of fibrovascular tissue onto the superficial cornea. Pterygia characteristically originate nasally from pinguecula (accumulation of connective tissue that thickens the conjunctiva) and grow onto the adjacent corneal surface. Their cause is unknown, but their incidence seems to correlate with exposure to ultraviolet light.

Pterygia are commonly restricted to the peripheral cornea, where they do not interfere with visual acuity. Some cause localized irritation that can be controlled with topical lubricants and vasoconstrictors. However, some pterygia extend toward the visual axis and cause corneal astigmatism and visual distortion. If a pterygium grows into the visual axis, it may interfere with visual acuity by causing opacification and obscuration of the central cornea. Surgical excision is recommended when vision begins to be distorted before the pterygium encroaches on the visual axis.

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