Introduction: Eye Disorders

Introduction: Eye Disorders: Excerpt from Professional Guide to Diseases (Eighth Edition)

Vision, the most complex sense, has recently been the focus of some of the greatest medical and surgical innovations. Disorders that affect the eye generally lead to vision loss or impairment; routine ophthalmic examinations and early treatment can help prevent it.

Review of anatomy

The visual system consists mainly of the bony orbit, which houses the eye; the contents of the orbit, including the eyeball, optic nerves, extraocular muscles, cranial nerves, blood vessels, orbital fat, and lacrimal system; and the eyelid, which covers the eye, moistens it, and protects it from injury.

The orbit (also called the socket) encloses the eye in a protective recess in the skull. Its seven bones — frontal, sphenoid, zygomatic, maxillary, palatine, ethmoid, and lacrimal — form a cone. The apex of this cone points toward the brain, and the cone’s base forms the orbital rim. The periorbita covers the bones of the orbit.

Extraocular muscles hold the eyes in place and control their movement as described below:

❑ superior rectus: elevates the eye upward; adducts and rotates the eye inward

❑ inferior rectus: depresses the eye downward; adducts and rotates the eye outward

❑ lateral rectus: abducts or turns the eye outward (laterally)

❑ medial rectus: adducts or turns the eye inward (medially)

❑ superior oblique: rotates the eye inward; abducts and depresses the eye

❑ inferior oblique: rotates the eye outward; abducts and elevates the eye.

The actions of these muscles are mutually antagonistic: As one contracts, its opposing muscle relaxes.

ELDER TIP Eye structure and activity change with age. The eyes set deeper in their sockets or have laid down more fat, and the eyelids lose their elasticity and become saggy and wrinkled.

Ocular layers

The eye has three structural layers: the sclera and cornea, the uveal tract, and the retina. (See Cross section of the eye, page 1154.)

The sclera is the dense, white, fibrous outer protective coat of the eye. It meets the cornea at the limbus (corneoscleral junction) anteriorly, and the dural sheath of the optic nerve posteriorly. The lamina cribrosa is a sievelike structure composed of a few strands of scleral tissue through which the optic nerve bundles pass. The sclera is covered by the episclera, a thin layer of fine elastic tissue.

ELDER TIP In older adults, lens changes occur typically with formation of a cataract. The vitreous body liquefies and pulls away from the retina, generating floating vitreous debris and peripheral vitreous detachments.

The cornea is the transparent, avascular, curved layer of the eye that’s continuous with the sclera. The cornea consists of five layers: the epithelium, which contains sensory nerves; Bowman’s membrane, the basement membrane for the epithelial cells; the stroma, or supporting tissue (90% of the corneal structure); Descemet’s membrane, containing many elastic fibers; and the endothelium, a single layer of cells that acts as a pump to maintain proper dehydration or detumescence of the cornea. Aqueous humor bathes the posterior surface of the cornea, maintaining intraocular pressure (IOP) by volume and rate of outflow. The anterior cornea is kept moist by the tear film. The cornea’s sole function is to refract light rays.

ELDER TIP Corneal sensitivity to touch decreases with age, and corneal curvature probably changes slightly also. It’s believed that atrophy of the dilator muscle fibers and increased rigidity of the blood vessels of the iris reduce pupil size, decreasing the amount of light that reaches the retina. Consequently, higher levels of illumination may be needed to improve uncorrected visual acuity in the older adult.

The middle layer of the eye, the uveal tract, is pigmented and vascular. It consists of the iris and the ciliary body in the anterior portion, and the choroid in the posterior portion. In the center of the iris is the pupil. The sphincter and dilator muscles control the amount of light that enters the eye through the pupil, and the pupil itself allows aqueous humor to flow from the posterior chamber into the anterior chamber.

The angle formed by the anterior iris and the posterior corneal structures contains many minute collecting channels of the trabecular meshwork. Aqueous humor drains through these channels into an encircling venous system called the canal of Schlemm.

The ciliary body, which extends from the root of the iris to the ora serrata, produces aqueous humor and controls lens accommodation through its action on the zonular fibers. The choroid, the largest part of the uveal tract, is made up of blood vessels bound externally by the suprachoroid and internally by Bruch’s membrane.

The retina is the neural coat of the eye. It receives visual images and transmits them to the brain for interpretation. It extends from the ora serrata to the optic nerve; the retinal pigment epithelium (RPE) adheres lightly to the choroid. Located next to the RPE are rods and cones. Although both rods and cones are light receptors, they respond to light differently. Rods, scattered throughout the retina, respond to low levels of light and detect moving objects; cones, located in the fovea centralis, function best in brighter light and perceive finer details.

Three types of cones contain different visual pigments and react to specific light wavelengths: one type reacts to red light, one to green, and one to blue-violet. The eye mixes these colors into various shades; the cones can detect 150 shades.

ELDER TIP Many elderly patients experience a loss of ability to discriminate blue-greens, and white objects appear yellowish; these patients may also have difficulty discriminating among pastels, violets, and yellow-greens.

The lens and accommodation

The lens of the eye is biconvex, avascular, and transparent; the lens capsule is a semipermeable membrane that can admit water and electrolytes. The lens changes shape (accommodation) for near and far vision. For near vision, the ciliary body contracts and relaxes the zonules, the lens becomes spherical, the pupil constricts, and the eyes converge; for far vision, the ciliary body relaxes, the zonules tighten, the lens becomes flatter, the eyes straighten, and the pupils dilate. The lens refines the refraction necessary to focus a clear image on the retina.

The vitreous body, which is 99% water and a small amount of insoluble protein, constitutes two-thirds of the eye’s volume. This transparent, gelatinous body gives the eye its shape and contributes to the refraction of light rays. The vitreous is firmly attached to the ora serrata of the ciliary body (anteriorly) and to the optic disk (posteriorly). The vitreous face contacts the lens; the vitreous gel rests against the retina.

Lacrimal apparatus and eyelids

The lacrimal apparatus consists of the lacrimal glands, upper and lower canaliculi, lacrimal sac, and nasolacrimal duct. The main gland, located in a shallow fossa beneath the superior temporal orbital rim, secretes reflex tears, which keep the cornea and conjunctiva moist. These tears flow through 12 to 14 excretory ducts and contain lysozyme, an enzyme that protects the conjunctiva from bacterial invasion. Multiple sebaceous glands in the eyelids produce an oily secretion that prevents tears from evaporating. With every blink, the eyelids direct the flow to the inner canthus, where the tears pool and then drain through a tiny opening called the punctum. The tears then pass through the canaliculi and lacrimal sac and down the nasolacrimal duct, which opens into the nasal cavity.

The eyelids (palpebrae) consist of tarsal plates that are composed of dense connective tissue. The orbital septum — the fascia behind the orbicularis oculi muscle — acts as a barrier between the lids and the orbit. The levator palpebrae muscle elevates the upper lid. The eyelids contain three types of glands:

❑ glands of Zeis — modified sebaceous glands connected to the follicles of the eyelashes

❑ meibomian glands — are sebaceous glands in the tarsal plates that secrete an oily substance as a tear film component (About 25 of these glands are found in the upper lid and about 20 in the lower lid.)

❑ Moll’s glands — ordinary sweat glands.

The conjunctiva is the thin mucous membrane that lines the eyelids (palpebral conjunctiva), folds over at the fornix, and covers the surface of the eyeball (bulbar conjunctiva). The conjunctiva produces mucin, another component of the tear film. The ophthalmic, lacrimal, and multiple anastomoses of facial arteries supply blood to the lids. The space between the open lids is the palpebral fissure; the juncture of the upper and lower lids is the canthus. The junction near the nose is called the nasal, medial, or inner canthus; the junction on the temporal side, the lateral or external canthus.

ELDER TIP Age-related vision changes are usually first noticed during the fifth decade of life and may include the inability to focus, narrowing of the visual field, reduced peripheral vision, and loss of iris elasticity producing decreased response to light and dark. In addition, as people age, production of any of the three tear film components may decrease, causing dry eyes.

Depth perception

In normal binocular vision, a perceived image is projected onto the two foveae. Impulses then travel along the optic pathways to the occipital cortex, which perceives a single image. However, the cortex receives two images — each from a slightly different angle — giving the images perspective and providing depth perception.

Vision testing

Several tests assess visual acuity and identify visual defects:

❑ Ishihara’s test determines color blindness by using a series of plates composed of a colored background, with a letter, number, or pattern of a contrasting color located in the center of each plate. The patient with deficient color perception can’t perceive the differences in color or, consequently, the designs formed by the color contrasts.

❑ The Snellen chart or other eye charts evaluate visual acuity. Such charts use progressively smaller letters or symbols to determine central vision on a numerical scale. A person with normal acuity should be able to read the letters or recognize the symbols on the 20/20 line of the eye chart at a distance of 20 feet.

Subjective testing

Several tests accomplish subjective testing of the eyes:

❑ B-mode ultrasonography delineates retinal tumors, detachments, and vitreous hemorrhages — even in the presence of opacities of the cornea and lens. A handheld B-scanner has simplified ultrasonic examination of the eye, making it possible to perform such studies in the ophthalmologist’s office.

❑ The convergence test locates the breaking point of fusion (before double vision occurs). For this test, the examiner holds a small object in front of the patient’s nose and slowly brings it closer to the patient. The point at which the eyes “break” is termed the near point of convergence and is measured in prism diopters.

❑ The cover-uncover test assesses eye muscle misalignment or tendency toward misalignment. In this test, the patient stares at a small, fixed object — first from a distance of 20'(6.1 m) and then from 1' (0.3 m). The examiner covers the patient’s eyes one at a time, noting any movement of the uncovered eye, the direction of any deviation, and the rate at which the eyes recover normal binocular vision when latent heterophoria is present.

❑ Duction test checks eye movement in all directions of gaze. While one eye is covered, the other eye follows a moving light. This test detects weakness of rotation due to muscle paralysis or structural dysfunction.

❑ Fluorescein angiography evaluates the blood vessels in the choroid and retina after I.V. injection of fluorescein dye; images of the dye-enhanced vasculature are recorded by rapid-sequence photographs of the fundus.

❑ Goldmann’s applanation, Tonopen tonometry, and Schiøtz tonometry all measure IOP. After instilling a local anesthetic in the patient’s eye, the examiner places the Schiøtz tonometer lightly on the corneal surface and measures the indentation of the cornea produced by a given weight. The Schiøtz tonometer has been largely replaced by an electronic Tonopen tonometer for bedside use. Applanation tonometry gauges the force required to flatten a small area of central cornea, and is the most accurate method of measuring IOP. For this test, a patient must be seated at a slit lamp and the cornea stained with fluorescein dye before the prism of the applanation tonometer touches the cornea and the examiner adjusts the controls until the two lines form an “S.”

❑ Gonioscopy allows for direct visualization of the anterior chamber angle.

❑ The Maddox rod test assesses muscle dysfunction; it’s especially useful in disclosing and measuring heterophoria (the tendency of the eyes to deviate). It can reveal horizontal, vertical and, especially, torsional deviations.

❑ Ophthalmodynamometry measures the relative central retinal artery pressures and indirectly assesses carotid artery flow on each side. This test has been largely supplanted by the color Doppler imaging test, which can assess blood flow velocities in ophthalmic vessels.

❑ Ophthalmoscopy — direct ophthalmoscopy or binocular indirect ophthalmoscopy allows examination of the interior of the eye after the pupil has been dilated with a mydriatic.

❑ Refraction tests may be performed with or without cycloplegics. In cycloplegic refraction, eyedrops weaken the accommodative power of the ciliary muscle. Lenses placed in front of the eye direct light rays onto the retina, thus focusing the image so that it can be transmitted along the visual pathway. A retinoscope may be used in the same way by directing a beam of light through the pupil onto the retina; the light’s shadow is neutralized by placing the appropriate lens in front of the eye.

❑ Slit-lamp biomicroscopic examination allows a well-illuminated examination of the eyelids and the anterior segment of the eyeball.

❑ Visual field tests assess the function of the retina, the optic nerve, and the optic pathways when both central and peripheral visual fields are examined.

Pictures

Book Source Details

• Book Title: Professional Guide to Diseases (Eighth Edition)
• Author(s): Springhouse
• Year of Publication: 2005
• Copyright Details: Professional Guide to Diseases (Eighth Edition), Copyright © 2005 Lippincott Williams & Wilkins.

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More About This Book: Title: Professional Guide to Diseases (Eighth Edition) Authors: Springhouse Publisher: Lippincott Williams & Wilkins Copyright: 2005 ISBN: 1-58255-370-X

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