Ординатура / Офтальмология / Английские материалы / Ophthalmology A Short Textbook_Lang_2000

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that malignant melanomas in the choroid occur more frequently in patients with congenital melanosis.

4.5.5Conjunctival Lymphoma

Prominent areas of salmon-colored conjunctival thickening frequently occurring in the inferior fornix (Fig. 4.25) are often the first sign of lymphatic disease. Identifying the specific forms and degree of malignancy requires biopsy and histologic examination. Lesions may range from benign lymphoid hyperplasia to malignant lymphomas that are moderately to highly malignant. Because lymphomas respond to radiation, a combination of radiation therapy and chemotherapy is usually prescribed according to the specific histologic findings.

4.5.6Kaposi’s sarcoma

This is a prominent, light to dark red tumor in the conjunctival fornix or proceeding from the palpebral conjunctiva. It consists of malignant spindle cells and nests of atypical endothelial cells. Today Kaposi’s sarcomas are seen most frequently as opportunistic disease in patients with AIDS (Acquired Immune Deficiency Syndrome). The ophthalmologist can make a tentative diagnosis of AIDS on the basis of typical clinical signs on the conjunctiva and order further diagnostic studies (Fig. 4.26). Recently there has been evidence that herpes virus (HHV-8) is involved in the development of Kaposi’s sarcoma.

Conjunctival lymphoma.

Fig. 4.25 Typical salmoncolored conjunctival tumor in the inferior fornix.

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Kaposi’s sarcoma.

Fig. 4.26 Prominent dark red tumor in the conjunctival fornix in a patient with AIDS.

4.6Conjunctival Deposits

These can occur in both the conjunctiva and cornea. Some, like some tumors, lead to pigmented changes in the conjunctiva. However, their typical appearance usually readily distinguishes them from tumors (Fig. 4.24). The following conjunctival and corneal deposits and discolorations may occur:

Adrenochrome deposit (Fig. 4.24h). Prolonged use of epinephrine eyedrops (as in glaucoma therapy) produces brownish pigmented changes in the inferior conjunctival fornix and on the cornea as a result of epinephrine oxidation products (adrenochrome). This can simulate a melanocytic conjunctival tumor. Therefore, the physician should always ascertain whether the patient has a history of prolonged use of epinephrine eyedrops. No therapy is indicated.

Iron deposits (Fig. 4.24i). In women, iron deposits from eye make-up and mascara are frequently seen to accumulate in the conjunctival sac. No therapy is indicated.

Argyrosis. Prolonged used of silver-containing eyedrops can produce brownish black silver deposits in the conjunctiva.

Ochronosis (alkaptonuria: an inherited autosomal recessive deficiency of the enzyme homogentisate 1,2-dioxygenase). Approximately 70% of all patients with ochronosis exhibit brownish pigmented deposits in the skin of the eyelids, conjunctiva, sclera, and limbus of the cornea (Fig. 4.24j). The deposits increase with time. The disorder cannot be treated in the eye.

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Metallic foreign bodies in the conjunctiva. A metallic foreign body that is not removed immediately will heal into the conjunctiva, where it will simulate a pigmented change in the conjunctiva (Fig. 4.24g). Obtaining a meticulous history (the examiner should always enquire about ocular trauma) will quickly reveal the cause of the anomaly. The foreign body can be removed under topical anesthesia.

Jaundice. This will lead to yellowing of the conjunctiva and sclera.

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5 Cornea

Gerhard K. Lang

5.1Basic Knowledge

Fundamental importance of the cornea for the eye: The cornea is the eye’s optical window that makes it possible for humans to see. The ophthalmologist is only able to discern structures in the interior of the eye because the cornea is transparent. At 43 diopters, the cornea is the most important refractive medium in the eye.

Shape and location: The cornea’s curvature is greater than the sclera’s curvature. It fits into the sclera like a watch-glass with a shallow sulcus (the limbus of the cornea) marking the junction of the two structures.

Embryology: The corneal tissue consists of five layers. The cornea and the sclera are formed during the second month of embryonic development. The epithelium develops from ectoderm, and the deeper corneal layers develop from mesenchyme.

Morphology and healing (Fig. 5.1):

The surface of the cornea is formed by stratified nonkeratinized squamous epithelium that regenerates quickly when injured. Within a hour, epithelial defects are closed by cell migration and rapid cell division. However, this assumes that the limbus stem cells in the limbus of the cornea are undamaged. Regular corneal regeneration will no longer be possible when these cells are compromised. An intact epithelium protects against infection; a defect in the epithelium makes it easy for pathogens to enter the eye.

A thin basement membrane anchors the basal cells of the stratified squamous epithelium to Bowman’s layer. This layer is highly resistant but cannot regenerate. As a result, injuries to Bowman’s layer usually produce corneal scarring.

Beneath Bowman’s layer, many lamellae of collagen fibrils form the corneal stroma. The stroma is a highly bradytrophic tissue. As avascular tissue, it only regenerates slowly. However, its avascularity makes it an immunologically privileged site for grafting. Routine corneal transplants may be performed without prior tissue typing. An increased risk of rejec-

118 5 Cornea

Anatomy of the cornea.

Fig. 5.1 See discussion in text.

tion need only be feared where the recipient’s cornea is highly vascularized as may be the case following chemical injury or inflammation. Such cases require either a tissue-typed donor graft or immunosuppressive therapy with cyclosporin.

Descemet’s membrane and the corneal endothelium lie on the posterior surface of the corneal stroma adjacent to the anterior chamber. Descemet’s membrane is a relatively strong membrane. It will continue to define the shape of the anterior chamber even where the corneal stroma has completely melted (see Descemetocele). Because it is a genuine basement membrane, lost tissue is regenerated by functional endothelial cells. The corneal endothelium is responsible for the transparency of the cornea (see also Transparency below). A high density of epithelial cells is necessary to achieve this. The corneal endothelium does not regenerate; defects in the endothelium are closed by cell enlargement and cell migration.

Diameter: The normal average diameter of the adult cornea is 11.5 mm (10–13 mm). A congenitally small cornea (microcornea, diameter less than 10.0 mm) or a congenitally large cornea (megalocornea, diameter from 13 to 15 mm) is always an abnormal finding (see Corneal Size Anomalies).

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Nourishment: The five layers of the cornea have few cells and are unstructured and avascular. Like the lens, sclera, and vitreous body, the cornea is a bradytrophic tissue structure. Its metabolism is slow, which means that healing is slow. The cornea is nourished with nutritive metabolites (amino acids and glucose) from three sources:

1.Diffusion from the capillaries at its edge.

2.Diffusion from the aqueous humor.

3.Diffusion from the tear film.

Significance of the tear film for the cornea: The three-layer precorneal tear film ensures that the surface of the cornea remains smooth and helps to nourish the cornea (see above). Without a tear film, the surface of the epithelium would be rough, and the patient would see a blurred image. The enzyme lysozyme contained in the tear film also protects the eye against infection (see p. 50, for composition of the tear film).

Transparency: This is due to two factors.

1.The uniform arrangement of the lamellae of collagen fibrils in the corneal stroma and the smooth endothelial and epithelial surface produced by the intraocular pressure.

2.The water content of the corneal stroma remains constant at 70%. The combined action of the epithelium and endothelium maintains a constant water content; the epithelium seals the stroma off from the outside, while the endothelium acts as an ion pump to remove water from the stroma. This requires a sufficiently high density of endothelial cells. Endothelial cell density is age-dependent; normally it is approximately 2500 cells per mm2. At cell densities below 300 endothelial cells per mm2, the endothelium is no longer able to pump water out of the cornea, resulting in edema of the corneal stroma and endothelium.

Protection and nerve supply: The cornea is a vital structure of the eye and as a result extremely sensitive. It receives its ample sensory supply from the ophthalmic division of the trigeminal nerve. The slightest tactile sensation causes an eye closing reflex. Any injury to the cornea (erosion, foreign body penetration, or ultraviolet keratoconjunctivitis) exposes sensory nerve endings and causes intense pain with reflexive tearing and involuntary eye closing.

The triad of involuntary eye closing (blepharospasm), reflexive tearing (epiphora), and pain always suggests a possible corneal injury (see Chapter 18).

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5.2Examination Methods

Non-ophthalmologists can evaluate the transparency of the cornea (opacities of the stroma and epithelium suggest scarring or infiltration of the epithelium), its surface luster (lack of luster suggests an epithelial defect), and possible superficial corneal injuries (see Fig. 19.1). A simple ruler may be used to measure the size of the cornea (see Anatomy), and sensitivity may be tested with a cotton swab (see Fig. 1.11, p. 11).

The ophthalmologist uses instruments to evaluate corneal morphology and function in greater detail.

5.2.1Slit Lamp Examination

The slit lamp is the primary instrument used in evaluating the cornea. The ophthalmologist chooses between eight and forty-power magnification for examining all levels of the cornea with a narrow beam of collimated light (Fig. 5.2).

5.2.2Dye Examination of the Cornea

Defects in the surface of the cornea can be visualized with fluorescein or rose bengal solution (in either case, administer one drop of 1% solution). Since these dyes are not usually absorbed by the epithelium, they may be used to visualize loss of epithelium over a wide area (such as corneal erosion) and extremely fine defects (as in superficial punctate keratitis). Illumination with a cobalt blue filter enhances the fluorescent effect.

Slit lamp examination of the cornea.

Fig. 5.2 The slit lamp (slit aperture) may be used to examine all levels of the cornea with a narrow beam of collimated light.

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These dye methods can reveal corneal epithelial defects (corneal erosion) even without the use of a slit lamp, which is helpful in examining infants.

5.2.3Corneal Topography

The keratoscope (Placido’s disk) permits gross evaluation of the uniformity of the surface of the cornea. This instrument consists of a round disk marked with concentric black and white rings around a central aperture. The examiner holds the disk in his or her hand and looks through the aperture. The mirror images of the rings on the patient’s cornea indicate the presence of astigmatism (in which case they appear distorted). However, this inexact evaluation method lacks the precision required for modern applications such as refractive surgery. Therefore, the surface of the cornea is now normally evaluated by computerized corneal topography (videokeratoscopy). In this examination, the contours of the cornea are measured by a computer in the same manner as the keratoscope. The refractive values of specific corneal regions are then represented in a color-coded dioptric map. Bright red, for example, represents a steep curvature with a high refractive power. This technique provides a contour map of the distribution of the refractive values over the entire cornea (Figs. 5.3a and b).

5.2.4Determining Corneal Sensitivity

Non-ophthalmologists may perform a simple preliminary examination of corneal sensitivity with a distended cotton swab (see Fig. 1.11, p. 11). This examination also helps the ophthalmologist confirm the diagnosis in the presence of a suspected viral infection of the cornea or trigeminal or facial neuropathy as these disorders are associated with reduced corneal sensitivity. Ophthalmologists may use an automatic Dräger esthesiometer for precise testing of corneal sensitivity and for follow-up examinations. This instrument can incrementally raise the sensitivity stimulus. This makes it possible to determine if and how rapidly corneal sensitivity increases following a corneal transplant.

5.2.5Measuring the Density of the Corneal Epithelium

A sufficiently high density of endothelial cells is very important for the transparency of the cornea (see Transparency). Gross estimation of the endothelial cell density is possible for a circumscribed area of the cornea using a slit lamp and indirect illumination. Both the viewing axis and illumination axis are offset from the visual axis. Precise quantification and morphologic evaluation of endothelial cells over large areas is only possible by means of specular microscopy, a technique designed especially for this purpose (Fig. 5.4). Exact

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Computerized corneal topography (videokeratoscopy).

Fig. 5.3 a Regular corneal astigmatism in a normal cornea.