Introduction to ophthalmology By Dr.Bakhtiar Q. Jaf
Objective of the lecture <ul><li>To give a simple introduction to clinical anatomy, physiology & embryology of the eye </li></ul><ul><ul><li>TO recognize clinical approach to the eye complaints </li></ul></ul>
Anatomy of the eye The eyeball , or globe , sits in a protective bony structure known as the orbit. Lined with muscle, connective and adipose tissues. the orbit is about 4 cm in height, width, and depth and is shaped roughly like a four-sided pyramid surrounded on three sides by the sinuses : The ethmoid (medially), the frontal (superiorly), and the maxillary (inferiorly). The optic nerve and the ophthalmic artery enter the orbit at its apex through the optic foramen.
Eyelids protect the anterior portion of the eye , composed of thin elastic skin that covers striated and smooth muscles & the tarsal plates. Tears are vitally important to the health of the anterior segment of the eye. They are formed by the main lacrimal gland and the accessory lacrimal glands. The conjunctiva , a mucous membrane, provides a barrier to the external environment and nourishes the eye.
The sclera , commonly known as the "white of the eye," is a dense fibrous structure that composes the posterior five sixths of the eye The cornea a transparent avascular domelike structure, forms the most anterior portion of the eyeball and is the main refracting surface of the eye. Behind the cornea lies the anterior chamber , filled with a continually replenished supply of clear aqueous humor, which nourishes the cornea.
The uvea consists of the iris, the ciliary body, and the choroid. The iris, or colored part of the eye, is a highly vascularized, pigmented collection of fibers surrounding the pupil. The pupil is a space that dilates and constricts in response to light.
Directly behind the pupil and iris lies the crystalline lens , a colorless and almost completely transparent biconvex structure held in position by zonular fibers. It is avascular and has no nerve or pain fibers. The lens is suspended behind the iris by the zonules and is connected to the ciliary body. The ciliary body controls accommodation through the zonular fibers and the ciliary muscles.
The posterior chamber is a small space between the vitreous and the iris. Aqueous fluid is manufactured in the posterior chamber by the ciliary body The choroid is layered between the retina and the sclera and is a highly vascularised tissue, supplying blood to the adjacent outer portion of the retina . The ocular fundus is the largest chamber of the eye and contains the vitreous humor, a clear gelatinous substance, mostly water, encapsulated by a hyaloid membrane, the vitreous humor .
visual pathway <ul><li>Good vision is not dependent solely on a healthy functioning eyeball but also on an intact visual pathway . This pathway is made up of the retina, optic nerve, optic chiasma, optic tracts, lateral geniculate bodies, optic radiations & the visual cortex of the occipital lobe. The pathway is an extension of the central nervous system. The optic nerve is the second cranial nerve. Its function is to transmit visual impulses from the retina to the higher centers in the brain. </li></ul>
Embryology of the eye The eye is formed from both ectoderm and mesenchyme . The neuroectoderm that is derived from the neural tube gives rise to (the retina, the fibers of the optic nerve, and the smooth muscle of the iris). The surface ectoderm on the side of the head forms( the corneal and conjunctival epithelium, the lens, and the lacrimal and tarsal glands). The mesenchyme forms( the corneal stroma, the sclera, the choroid, the iris, the ciliary musculature, part of the vitreous body, and the cells lining the anterior chamber).
The rudimentary eyeball develops as an ectodermal diverticulum from the lateral aspect of the forebrain. The diverticulum grows out laterally toward the side of the head, and the end becomes slightly dilated to form the optic vesicle , while the proximal portion becomes constricted to form the optic stalk . At the same time, a small area of surface ectoderm overlying the optic vesicle thickens to form the lens placode . The lens placode invaginates and sinks below the surface ectoderm to become the lens vesicle , the optic vesicle becomes invaginated to form the double-layered optic cup .
<ul><li>At end of 4 th week of pregnancy; Optic Vesicle contact with surface ectoderm & invaginate to form Optic cup & the lens vesicle </li></ul>
The optic nerve The ganglion cells of the retina develop axons that converge to a point where the optic stalk leaves the posterior surface of the optic cup. This site will later become the optic disc . The axons now pass among the cells that form the inner layer of the stalk. Gradually, the inner layer encroaches on the cavity of the stalk until the inner and outer layers fuse.
The Ocular Examination Visual Acuity After the patient's chief complaint and history have been established, visual acuity should be assessed. This is an essential part of the eye examination and a measure against which all therapeutic outcomes are based. Most health care providers are familiar with the standard Snellen chart. This chart is composed of a series of progressively smaller rows of letters and is used to test distance vision. The fraction 6/6 is considered the standard of normal vision. Most people can see the letters on the line designated as 6/6 from a distance of 6 meters.
<ul><li>The External Eye Examination After the visual acuity has been recorded, an external eye examination is performed DIAGNOSTIC EVALUATION DIRECT OPHTHALMOSCOPY Is a hand-held instrument with varying plus and minus lenses. The lenses can be rotated into place, enabling the examiner to bring the cornea, lens, and retina into focus </li></ul>
Indirect Ophthalmoscopy Is an instrument commonly used by the ophthalmologist. It produces a bright and intense light. The light source is fixed with a pair of binocular lenses, which are mounted on the examiner's head. The ophthalmoscope is used in conjunction with a hand-held 20-dioptcr lens. This instrument enables the examiner to see larger areas of the retina, although in an unmagnified state.
Slit-Lamp Examination Is a binocular microscope mounted on a table. This instrument enabls the user to examine the eye with magnification of 10 to 40 times the real image. Color Vision Testing Because alteration in color vision is sometimes indicative of optic nerve problems, color vision testing is often performed in a neuro-ophthalmologic workup. Ultrasonography Lesions in the globe or the orbit may not be directly visible and are evaluated by ultrasound. Color Fundus Photography Fundus photography is a technique used to detect and document retinal lesions. The patient's pupils are widely dilated during the procedure, and visual acuity is diminished for about 30 minutes due to retinal "bleaching" by the intense flashing lights.
Tonometry Tonometry is used to measure IOP by determining the amount of force necessary to indent or flatten (applanate) a small anterior area of the globe of the eye. Gonioscopy Gonioscopy is used to visualize the angle of the anterior chamber to identify abnormalities in appearance and measurements. Perimetry Testing Perimetry testing is used to evaluate the field of vision. A visual field is the area or extent of physical space visible to an eye in a given position. Its average extent is 65 degrees upward, 75 degrees downward, 60 degrees inward, and 95 degrees outward when the eye is in the primary gaze.
IMPAIRED VISION REFRACTIVE ERRORS <ul><li>In refractive errors, vision is impaired because a shortened or elongated eyeball prevents light rays from focusing sharply on the retina. Blurred vision due to refractive error can be corrected with eyeglasses and contact lenses. </li></ul>
Those patients for whom the visual image focuses precisely on the macula and who do not need eye glasses or contact lenses are said to have emmetropia .
People with myopia have deeper eyeballs; the distant visual image focuses in front of, or short of, the retina. These people are nearsighted and are termed myopic and experience blurred distance vision.
When people have a shorter depth to their eyes, the visual image focuses beyond the retina; the eyes are more shallow and are termed hyperopic. People who have hyperopia are farsighted.
GLAUCOMA Glaucoma is the term used to describe a group of ocular conditions characterized by optic nerve damage, high intraocular pressure and visual field deffect. CLASSIFICATION OF GLAUCOMA Glaucoma can be open angle or angle closure, depending on which mechanisms cause the impairment of the aqueous outflow. Glaucoma can also be primary or secondary, depending on whether associated factors contribute to the rise in IOP.
CATARCT A cataract is a lens opacity or cloudiness Clinical Manifestations Painless blurring of vision is characteristic of cataracts.
Commonly Used Ocular Medications Topical Anesthetics One to two drops of proparacaine hydrochloride and tetracaine hydrochlorid are instilled before diagnostic procedures such as tonometry and gonioscopy and in minor ocular procedures such as removal of sutures or conjunctival or corneal scrapings. Mydriatics and Cycloplegics Mydriasis, or pupil dilation, is the main objective of the administration of mydriatics and cycloplegics
Anti-lnfectives Anti-infective medications include antibiotics, antifungals, and antivirals. Most are available as drops, ointments, or subconjunctival or intravitreal injections. Antibiotics include penicillin, the cephalosporins, aminoglycosides, and fluoroquinolones. The main antifungal agent is amphotericin B. Antivirals include acyclovir and ganciclovir.
Corticosteroids and Nonsteroidal Anti-Inflammatory Drugs The topical preparations of corticosteroids are commonly used in inflammatory conditions of the eyelids, conjunctiva, cornea, anterior chamber, lens, and uvea. In posterior segment diseases that involve the posterior sclera, retina, and optic nerve, the topical agents are less effective; hence, the parenteral and oral routes are preferred.
References 1. Lecture notes in ophthalmology 2. Parson’s disease of the eye