Corneal Disorders

Corneal Disorders

CORNEAL DYSTROPHIES

Corneal dystrophies are inherited as autosomal dominant traits and manifest when the person is about 20 years of age. They are characterized by deposits in the corneal layers. 

Decreased vision is caused by the irregular corneal surface and corneal deposits. Corneal endothelial decompensation leads to corneal edema and blurring of vision. Persistent edema leads to bullous keratopathy, which is formation of blisters that cause pain and discomfort on rupturing. This condition is usually associated with primary open-angle glaucoma.

A bandage contact lens is used to flatten the bullae, protect the exposed corneal nerve endings, and relieve discomfort. Sympto-matic treatments, such as hypertonic drops or ointment (5% sodium chloride), may reduce epithelial edema; lowering the IOP also reduces stromal edema. Penetrating keratoplasty has a high success rate in advanced cases (see “Corneal Surgeries”). For dif-fuse bullous keratopathy, amniotic membrane transplantation may become the procedure of choice for patients with limited visual potential (Rapuano, 2000).

KERATOCONUS

Keratoconus is a condition characterized by a conical protuber-ance of the cornea with progressive thinning on protrusion and irregular astigmatism. The hereditary condition has a higher in-cidence among women. Onset occurs at puberty; the condition may progress for more than 20 years and is bilateral. Corneal scarring occurs in severe cases. Blurred vision is a prominent symptom. Rigid, gas-permeable contact lenses correct irregular astigmatism and improve vision. Advances in contact lens de-sign have reduced the need for surgery. Penetrating kerato-plasty is indicated when contact lens correction is no longer effective.

CORNEAL SURGERIES

Among the surgical procedures used to treat diseased corneal tis-sue are phototherapeutic keratectomy (PTK) and keratoplasty.

Phototherapeutic Keratectomy

PTK is a laser procedure that is used to treat diseased corneal tis-sue by removing or reducing corneal opacities and smoothing the anterior corneal surface to improve functional vision. PTK is a safer, more effective (when indicated) alternative than penetrat-ing or lamellar keratoplasty. PTK is contraindicated in patients with active herpetic keratitis because the ultraviolet rays may re-activate latent virus. Common side effects are induced hyperopia and stromal haze. Complications are delayed re-epithelialization (particularly in patients with diabetes) and bacterial keratitis. Postoperative management consists of oral analgesics for eye pain. Re-epithelialization is promoted with a pressure patch or thera-peutic soft contact lens. Antibiotic and corticosteroid ointment and NSAIDs are prescribed postoperatively. Follow-up examina-tions are required for up to 2 years.

Keratoplasty

Keratoplasty (ie, corneal transplantation or corneal grafting) involves replacing abnormal host tissue with a healthy donor corneal tissue. Common indications are keratoconus, corneal dystrophy, corneal scarring from herpes simplex keratitis, and chemical burns.

Several factors affect the success of the graft: ocular structures (eg, lids, conjunctiva), tear film function, adequacy of blinking, and viability of the donor endothelium. Tissue that is the possi-ble source of disease transmission from donor to recipient orcornea with functionally compromised endothelium is typically not used for grafting (Chart 58-7), nor is corneal tissue used from donors who have undergone laser-assisted in situ keratomileusis (LASIK) because the cornea is no longer intact. Conditions such as glaucoma, retinal disease, and strabismus (ie, deviation in oc-ular alignment) can negatively influence the outcome. Promising experimental therapies include stem cell transplants (Rongé, 2001) and autologous limbal epithelial cell transplants (Tsai et al., 2000).

The surgeon determines the graft size before the procedure, and the appropriate size is marked on the surface of the cornea. The surgeon prepares the donor cornea and the recipient bed, re-moves the diseased cornea, places the donor cornea on the recip-ient bed, and sutures it in place. Sutures remain in place for 12 to 18 months. Potential complications include early graft failure due to poor quality of donor tissue, surgical trauma, acute in-fection, and persistently increased IOP and late graft failure due to rejection.

Postoperatively, the patient receives mydriatic medications (2 weeks) and topical corticosteroids (12 months; daily doses formonths and tapered doses thereafter). Patients typically de-scribe a sensation of postoperative eye discomfort rather than acute pain.

Nursing Management

The nurse reinforces the surgeon’s recommendations and in-structions regarding visual rehabilitation and visual improvement by explaining why a technically successful graft may initially pro-duce disappointing results because the procedure has produced a new optical surface and only after several months do patients start seeing the natural and true colors of their environment. Correc-tion of a resultant refractive error with eyeglasses or contact lenses determines the final visual outcome. The nurse assesses the pa-tient’s support system and his or her ability to comply with long-term follow-up, which includes frequent clinic visits for several months for tapering of topical corticosteroid therapy, selective suture removal, and ongoing evaluation of the graft site and visual acuity. The nurse also initiates appropriate referral to community services when indicated.

Because graft failure is an ophthalmic emergency that can occur at any time, the primary goal of nursing care is to teach the patient to identify signs and symptoms of graft failure. The early symptoms are blurred vision, discomfort, tearing, or redness of the eye. Decreased vision results after graft destruction. Patients must contact the ophthalmologist as soon as symptoms occur. Treatment of graft rejection is prompt administration of hourly topical corticosteroids and periocular corticosteroid injections. Systemic immunosuppressive agents may be necessary for severe, resistant cases.

REFRACTIVE SURGERIES

Refractive surgeries are cosmetic, elective procedures performed to reshape corneal tissue and correct refractive errors so that eye-glasses or contact lenses are no longer needed. Current procedures include radial keratotomy, photorefactive keratectomy (PFK), and LASIK.

Refractive surgery alters the major optical function of the eye and thereby carries certain surgical risks. The patient must fully understand benefits, potential risks and complications, common side effects, and limitations of the procedure. Refractive surgery does not alter the normal aging process of the eye. If the reason for the procedure is occupational vision requirements, the results must satisfy both the patient and the employer. Precise visual out-come cannot be guaranteed with certainty. Typically, patients must be at least 18 years of age.

The corneal structure must be normal and refractive error sta-ble. Patients are required to discontinue using contact lenses for a period before the procedure (ie, 2 to 3 weeks for soft lenses and 4 weeks for hard lenses). Patients with conditions that are likely to adversely affect corneal wound healing (eg, corticosteroid use, immunosuppression, elevated IOP) are not good candidates for the procedure. Any superficial eye disease must be diagnosed and fully treated before a refractive procedure.

Radial Keratotomy

Radial keratotomy (RK) is indicated for low myopia (less than 8D). The procedure involves making four to eight, deep, radial incisions in the paracentral and peripheral cornea with a metal or diamond blade. The corneal contour then becomes flatter. Glare, photosensitivity, fluctuations of vision during the day, and occa-sional diplopia are common side effects. As the popularity of laser refractive surgery grows, RK procedures decrease.

Laser Vision Correction Photorefractive Keratectomy

Laser vision correction photorefractive keratectomy (PRK) is a procedure used to treat myopia and hyperopia with or without astigmatism. The 193-mm argon fluoride excimer laser is ap-plied directly to the cornea according to carefully calculated measurements. For myopia, the relative curvature is decreased; for hyperopia, the relative curvature is increased. A bandage con-tact lens is placed over the cornea to promote epithelial healing and reduce pain similar to that of severe corneal abrasion. PRK requires a longer visual recovery period than RK, but PRK pro-vides more predictable and stable results. Except for the side effect of corneal haze and night vision problems, PRK has not been associated with the two major disadvantages of RK: hyper-opic drift and weakening of the structural integrity of the cornea.

Laser-Assisted In Situ Keratomileusis (LASIK)

An improvement over PRK, particularly for correcting high (severe) myopia, LASIK involves flattening the anterior curvature of the cornea by removing a stromal lamella or layer. The surgeon creates a corneal flap with a microkeratome, which is an auto-matic corneal shaper similar to a carpenter’s plane. The surgeon retracts a flap of corneal tissue less than one third of the thickness of a human hair to access the corneal stroma and then uses the ex-cimer laser on the stromal bed to reshape the cornea according to calculated measurements (Fig. 58-10). The corneal flap, a natu-rally adhering bandage, is rolled back and repositioned. LASIK also appears to be an effective, predictable, stable, and safe pro-cedure for correcting residual myopia after cataract surgery (Ayala et al., 2001).

LASIK causes less postoperative discomfort, has fewer side ef-fects, and is safer than PRK. The patient has no corneal haze and requires less postoperative care. With LASIK, however, the cornea has been invaded at a deeper level, and any complications are more significant than those that can occur with PRK.

PERIOPERATIVE COMPLICATIONS

Ablation-Related Complications.Ablation complications ofLASIK include an elevated area within the corneal treatment ab-lation zone (ie, central island). Signs and symptoms of this com-plication include ghosting, blurred vision, halo formation around lights, decreased visual activity, and contrast sensitivity in low light. Most of the island formations resolve over time; reablation is considered only after the island appears stable after repeated examinations for at least 3 months.

Diffuse Lamellar Keratitis. As LASIK increases in popularity andis performed more often, the vision-threatening complication known as diffuse lamellar keratitis (DLK) is reported more often. DLK is a peculiar, noninfectious, inflammatory reaction in the lamellar interface after LASIK. DLK is characterized by a white, granular, diffuse, culture-negative lamellar keratitis occurring in the first week after surgery. Studies suggest that, because no sin-gle agent appears to be solely the cause of DLK, the cause is multi-factorial (Holland et. al., 2000).

DLK is diagnosed by identifying cells in the lamellar interface by slit-lamp examination from postoperative day 1. Depending on the severity of the condition, treatment methods range from administering corticosteroid drops to intervening surgically.

Central Islands and Decentered Ablations.Decentered or ec-centric ablation involves a shift of the center of the ablation pat-tern from the pupil or visual axis to a more eccentric location. Symptoms include decreased visual acuity, halos, glare, and ghosting, especially in low-light settings.

LASIK Enhancements

LASIK enhancements are surgical options from improved tech-nology and software used to treat a wider range of myopia, hyper-opia, and astigmatism in eyes with a history of LASIK surgery. Astigmatic keratotomy continues to work well for patients with significant regular astigmatism. A newer procedure, Intacs im-plantation, is performed for patients left with significant myopia but who have thin corneas. Hyperopic excimer laser enhance-ments are indicated for patients who have undergone myopic LASIK and have consecutive hyperopia.

IMPLANTABLE DEVICES

Because the results of refractive surgery on high (severe) myopia, hyperopia, and astigmatism are less predictable, there has been in-creasing interest in the use of phakic IOLs. Anterior and poste-rior chamber IOLs are now in use, and design improvements continue to be made. Phakic IOL implantation does not com-promise the central optical zone and retains the normal aspheric contour of the cornea. Most importantly, it is reversible. Early re-search results on vision quality favor phakic IOL over LASIK. Po-tential complications include cataract, iritis or uveitis, endothelial cell loss, and increased IOP.

Intacs is an implantable intrastromal corneal ring used to cor-rect mild to moderate myopia. The intrastromal corneal ring seg-ments are placed in the corneal stroma outside of the central optical zone and reshape the anterior surface of the cornea.

Management

Patient satisfaction is the ultimate goal; therefore, patient educa-tion and counseling about potential risks, complications, and postoperative follow-up are critical. Minimal postoperative care includes topical corticosteroid drops. The length of postoperative follow-up depends on the refractive procedure, with PRK re-quiring a longer course, followed by RK and then LASIK.