Porth's Essentials of Pathophysiology, 4e

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Disorders of Special Sensory Function: Vision, Hearing, and Vestibular Function

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Corneal Trauma The integrity of the epithelium and endothelium is necessary to maintain hydration of the cornea within a limited range. 8 The integrity of the endothelium is more important than the epithelium, and damage to the endo- thelial cells is far more serious, causing edema of the cornea and loss of transparency. With corneal edema, the cornea appears dull, uneven, and hazy; visual acuity decreases; and iridescent vision (i.e., rainbows around lights) occurs. Trauma that causes abrasions of the cornea can be extremely painful, but if minor, the abrasions usually heal in a few days. The epithelial layer can regenerate, and small defects heal without scarring. If the stroma is damaged, healing occurs more slowly, and the danger of infection is increased. Injuries to the Bowman mem- brane and the stromal layer heal with scar formation that impairs the transmission of light. Keratitis Keratitis , or inflammation of the cornea, can be caused by infections, misuse of contact lenses, hypersensitiv- ity reactions, ischemia, trauma, defects in tearing, and interruption in sensory innervation, as occurs with local anesthesia. Scar tissue formation due to keratitis is the leading cause of blindness and impaired vision through- out the world. Most of this vision loss is preventable if the condition is diagnosed early and appropriate treat- ment is begun. Keratitis can be infectious or noninfectious and non- ulcerative or ulcerative. 8,9 In nonulcerative keratitis, all the layers of the epithelium may be affected, but the epi- thelium remains intact. There are a number of causes of epithelial keratitis, including epidemic keratoconjuncti- vitis caused by adenoviruses 8 and 19 and ultraviolet (UV) light exposure. Most cases of UV keratitis occur in welders with inadequate eye protection, but may also occur with tanning booth and other UV lamp exposure, and from sun reflecting off snow. 10 Ulcerative keratitis is an inflammatory process in which parts of the corneal epithelium, stroma, or both are destroyed. Causes of ulcerative keratitis include infectious agents (e.g., S. aureus, S. pneumoniae, C. tra- chomatis ), exposure trauma, and use of extended-wear contact lenses (i.e., Pseudomonas and Acanthamoeba keratitis). Bacterial keratitis is aggressive and demands immediate care. Mooren ulcer is a chronic, indolent, but painful ulcerative keratitis that occurs in the absence of infection. It is usually seen in older persons and may affect both eyes. Although the cause is unknown, an autoimmune origin is suspected. Herpes Simplex Keratitis. Herpes simplex virus (HSV) keratitis, which usually is unilateral, is an important cause of ocular morbidity. 11,12 With the exception of neo- natal infections, which are usually due to HSV type 2 (genital herpes) acquired during the birth process, most cases of herpes keratitis are caused by HSV type 1 (labial [lip] herpes). The disease can occur as either a primary or recurrent infection. Primary epithelial infections are the

optical counterpart of labial herpes with similar immu- nologic and pathologic features as well as a similar time course (see Chapter 46). The first manifestations of recurrent herpes keratitis are irritation, photophobia, and tearing. Some reduc- tion in vision may occur when the lesion affects the central part of the cornea. Because corneal anesthesia occurs early in the disease, the symptoms may be mini- mal. A history of fever blisters or other herpetic infec- tion is often noted, but corneal lesions may be the only sign of recurrent herpes infection. Typically, the corneal lesions involve the epithelium and heal without scarring. Lesions that involve the stromal layer of the cornea pro- duce increasingly severe corneal opacities. Any person with herpes simplex keratitis and an acute red eye should be referred urgently to an ophthalmolo- gist. The treatment of HSV keratitis includes the use of topical antiviral agents to promote healing. 8 Oral antivi- ral agents (e.g., acyclovir) may be helpful for treatment of severe keratitis and as prophylaxis against recurrence, particularly in persons with compromised immune func- tion. Although corticosteroids may control the damag- ing inflammatory responses, they do so at the expense of facilitating viral replication. With a few exceptions, their use is contraindicated. 11 Varicella Zoster Ophthalmicus. Herpes zoster or shingles is a relatively common infection due to the her- pesvirus that causes varicella (chickenpox). 8,12 Herpes ophthalmicus, which represents 10% to 25% of all cases of herpes zoster, occurs when reactivation of the latent virus occurs in the ganglia of the ophthalmic division of the trigeminal nerve. 12 Immunocompromised persons, particularly those with human immunodeficiency virus (HIV) infection, are at higher risk for developing herpes zoster ophthalmicus than those with a normally func- tioning immune system. Herpes zoster ophthalmicus usually presents with malaise, fever, headache, and burning and itching of the periorbital area. These symptoms commonly precede the ocular eruption by a day or two. The rash, which is initially vesicular, becomes pustular and then crusting (see Chapter 46). Involvement of the tip of the nose and lid margins indicates a high likelihood of ocular involve- ment. Ocular signs include conjunctivitis, keratitis, and anterior uveitis, often with elevated intraocular pres- sure. Persons with corneal disease present with varying degrees of decreased vision, pain, and sensitivity to light. Treatment includes the use of oral and intravenous antiviral drugs. Initiation of treatment within the first 72 hours after the appearance of the rash reduces the inci- dence of ocular complications but not the postherpetic neuralgia 8 (see Chapter 35). Many cases of herpes zoster and herpes zoster ophthalmicus can now be prevented with the herpes zoster vaccine. Acanthamoeba Keratitis. Acanthamoeba is a free- living cyst-forming protozoan that thrives in polluted water containing bacteria and organic material. 8,13,14 Acanthamoeba keratitis is a rare but sight-threatening complication that typically occurs in people who wear