Ophthalmology and Anatomy of the Eye
Ophthalmology is the branch of medicine, which deals with the diseases of the eye and their treatment. The word ophthalmology comes from the Greek root ophthalmos meaning eye and logos meaning word; ophthalmology literally means "the science of eyes."
Eyes are the organs of sight or vision in our body. They are contained in the bony or-bit of the skull. Each eyeball is oval in shape, about an inch in diameter, and is protected by appendages such as upper and lower eyelids, eyelashes, conjunctiva, eyebrows, meibomian glands, and lacrimal apparatus. The eye is constructed to receive the stimuli of rays of light on the retina and, by means of optic nerves, to transmit these to visual centers of brain for interpretation.
Anatomy of the Eye
The structure of the human eye owes itself completely to the task of focusing light onto the retina. All of the individual components through which light travels within the eye before reaching the retina are transparent, minimizing dimming of the light. The cornea and lens help to converge light rays to focus onto the retina. This light causes chemical changes in the photosensitive cells of the retina, the products of which trigger nerve impulses which travel to the brain.
Light enters the eye from an external medium such as air or water, passes through the cornea, and into the first of two humours, the aqueous humour. Most of the light refraction occurs at the cornea, which has a fixed curvature. The first humour is a clear mass, which connects the cornea with the lens of the eye, helps maintain the convex shape of the cornea (necessary to the convergence of light at the lens) and provides the corneal endothelium with nutrients. The iris, between the lens and the first humour, is a colored ring of muscle fibers. Light must first pass though the centre of the iris, the pupil. The size of the pupil is actively adjusted by the circular and radial muscles to maintain a relatively constant level of light entering the eye. Too much light being let in could damage the retina, too little light would be blinding. The lens, behind the iris, is a convex, springy disk which focuses light, through the second humour, onto the retina.
To clearly see an object far away, the circularly arranged ciliary muscles will pull on the lens, flattening it. Without muscles pulling on it, the lens will spring back into a thicker, more convex, form. Humans gradually lose this flexibility with age, resulting in the inability to focus on nearby objects, which is known as presbyopia. There are other refraction errors arising from the shape of the cornea and lens, and from the length of the eyeball. These include myopia, hyperopia, and astigmatism.
On the other side of the lens is the second humour, the vitreous humour, which is bounded on all sides: by the lens, ciliary body, suspensory ligaments and by the retina. It lets light through without refraction, helps maintain the shape of the eye and suspends the delicate lens.
Three layers, or tunics, form the wall of the eyeball. The outermost is the sclera which gives the eye most of its white colour. It consists of dense connective tissue filled with the protein collagen to both protect the inner components of the eye and maintain its shape. On the inner side of the sclera is the choroid, which contains blood vessels that supply the retinal cells with necessary oxygen and remove the waste products of respiration. Within the eye, only the sclera and ciliary muscles contain blood vessels. The choroid gives the inner eye a dark colour, which prevents disruptive reflections within the eye. The inner most layer of the eye is the retina, containing the photosensitive rod and cone cells, and neurons.
To maximize vision and light absorption, the retina is a relatively smooth (but curved) layer. It does have two points at which it is different; the fovea and blind spot. The fovea is a dip in the retina directly opposite the lens, which is densely packed with cone cells. It is largely responsible for colour vision in humans, and enables high acuity, such as is necessary in reading. The blind spot is a point on the retina where the optic nerve pierces the retina to connect to the nerve cells on its inside. No photosensitive cells exist at this point, it is thus "blind."
Cytology of Vision & Visual Acuity
Cytology of Vision
The retina contains two forms of photosensitive cells - rods and cones. Though structurally and metabolically similar, their function is quite different, though they are equally important to vision. Rod cells are highly sensitive to light allowing them to respond in dim light and dark conditions. These are the cells, which allow humans and other animals to see by moonlight, or with very little available light (as in a dark room). However, they do not distinguish between colors, and have low visual acuity (a measure of detail). This is why the darker conditions become, the less colour objects seem to have. Cone cells, conversely, need high light intensities to respond and have high visual acuity. Different cone cells respond to different colors (wavelengths) of light, which allows an organism to see colour.
The differences are useful; apart from enabling sight in both dim and light conditions, humans have given them further application. The fovea, directly behind the lens, consists of mostly densely-packed cone cells. This gives humans a highly detailed central vision, allowing reading, bird watching, or any other task, which primarily requires looking at things. Its requirement for high intensity light does cause problems for astronomers, as they cannot see dim stars, or other objects, using central vision because the light from these is not enough to stimulate cone cells. Because cone cells are all that exist directly in the fovea, astronomers have to look at stars through the "corner of their eyes" (averted vision) where rods also exist, and where the light is sufficient to stimulate cells, allowing the individual to observe distant stars.
Rods and cones are both photosensitive, but respond differently to different frequencies of light. They both contain different pigmented photoreceptor proteins. Rod cells contain the protein rhodopsin and cone cells contain different proteins for each colour-range. The process through which these proteins go is quite similar - upon being subjected to electromagnetic radiation of a particular wavelength and intensity (i.e. a colour visible light) the protein breaks down into two constituent products. Rhodopsin, of rods, breaks down into opsin and retinal; iodopsin of cones breaks down into photopsin and retinal. The opsin in both opens ion channels on the cell membrane, which leads to the generation of an action potential (an impulse, which will eventually get to the visual cortex in the brain).
This is the reason why cones and rods enable organisms to see in dark and light conditions.
Colour is distinguishable when breaking down the iodopsin of cone cells because there are three forms of this protein. One form is broken down by the particular EM wavelength that is red light, another green light, and lastly blue light. In simple terms, this allows human beings to see red, green and blue light. If all three forms of cones are stimulated equally, then white is seen. If none are stimulated, black is seen.
Visual acuity (VA) is one of many components of the visual perception sense and is defined as the eye's ability to resolve fine details. VA is a quantitative measure to see an in-focus image at a certain, standarized distance. VA is the most common measurement of visual function that is performed in a clinical setting.
Visual acuity can be measured with several different metrics.
Cycles per degree (CPD) measures how much an eye can differentiate one object from another in terms of degree angles. It is essentially no different from angular resolution. To measure CPD, first draw a series of black and white lines of equal width on a grid (similar to a bar code). Next, place the observer at a distance such that the sides of the grid appear one degree apart. If the grid is 1 meter away, then the grid should be about 8.7 millimeters wide. Finally, increase the number of lines and decrease the width of each line until the grid appears as a solid grey block. In one degree, a human would not be able to distinguish more than about 12 lines without the lines blurring together. So a human can resolve distances of about 0.73 millimeters at a distance of one meter. A horse can resolve about 14 CPD (0.62 mm at 1 m) and a rat can resolve about 1 CPD (8.7 mm at 1 m).
A diopter is the unit of measure of focus.
Many humans have one eye that has superior visual acuity over the other. If a person cannot achieve a visual acuity of 20/200 (6/60) or above in the better eye, even with the best possible glasses, then that person is considered legally blind in the United States. A person with a visual field narrower than 20 degrees in diameter also meets the definition of legally blind.
Visual acuity is registered documenting if the test was for distant or near vision, the eye evaluated and if corrective lenses (i.e. spectacles or contact lenses) were used:
Distance from the chart
D (distant) for the evaluation done at 20 feet (or 6 meters).
N (near) for the evaluation done at 14 inches (or 35 cm).
OD (lat. oculus dexter) for the right eye.
OS (lat. oculus sinister) for the left eye.
OU (lat. oculi uterque) for both eyes.
Related Parts & Eye Movements
In many species, the eyes are inset in the portion of the skull known as the orbits or eye sockets. This placement of the eyes helps to protect them from injury.
In humans, the eyebrows redirect flowing substances (usually rainwater) away from the eye. Water in the eye can alter the refractive properties of the eye and blur vision. It can also wash away the tear fluid - along with it the protective lipid layer - and can alter corneal physiology, due to osmotic differences between tear fluid and freshwater. This is made apparent when swimming in freshwater pools, as the osmotic gradient draws 'pool water' into the corneal tissue, causing edema, and subsequently leaving the swimmer with "cloudy" or "misty" vision for a short period thereafter. It can be reversed by irrigating the eye with hypertonic saline.
Eyelids wipe the eye and prevent dehydration. They spread tear fluid on the eyes, which contains substances which help fight bacterial infection as part of the immune system. Some aquatic animals have a second eyelid in each eye, which refracts the light and helps them see clearly both above water and below it. Most creatures will automatically react to a threat to its eyes (such as an object moving straight at the eye or a bright light) by covering the eyes, and/or by turning the eyes away from the threat. Blinking the eyes is, of course, also a reflex.
Eyelashes prevent fine particles from entering the eye. Fine particles can be bacteria, but also simple dust, which can cause irritation of the eye, and lead to tears and subsequent blurred vision.
Animals with compound eyes have a wide field of vision, allowing them to look in many directions. To see more, they have to move their entire head or even body.
The visual system in the brain is too slow to process that information if the images are slipping across the retina at more than a few degrees per second. Thus, for humans to be able to see while moving, the brain must compensate for the motion of the head by turning the eyes. Another complication for vision in frontal-eyed animals is the development of a small area of the retina with a very high visual acuity. This area is called the fovea, and covers about 2 degrees of visual angle in people. To get a clear view of the world, the brain must turn the eyes so that the image of the object of regard falls on the fovea. Eye movements are thus very important for visual perception, and any failure to make them correctly can lead to serious visual disabilities. To see a quick demonstration of this fact, try the following experiment: hold your hand up, about one foot (30 cm) in front of your nose. Keep your head still, and shake your hand from side to side, slowly at first, and then faster and faster. At first you will be able to see your fingers quite clearly. But as the frequency of shaking passes about one hertz, the fingers will become a blur. Now, keep your hand still, and shake your head (up and down or left and right). No matter how fast you shake your head, the image of your fingers remains clear. This demonstrates that the brain can move the eyes opposite to head motion much better than it can follow, or pursue, a hand movement. When your pursuit system fails to keep up with the moving hand, images slip on the retina and you see a blurred hand. Having two eyes is an added complication, because the brain must point both of them accurately enough that the object of regard falls on corresponding points of the two retinas; otherwise, double vision would occur. The movements of different body parts are controlled by striated muscles acting around joints. The movements of the eye are no exception, but they have special advantages not shared by skeletal muscles and joints, and so are considerably different.
Each eye has six muscles that control its movements: the lateral rectus, the medial rectus, the inferior rectus, the superior rectus, the inferior oblique, and the superior oblique. When the muscles exert different tensions, a torque is exerted on the globe that causes it to turn. This is an almost pure rotation, with only about one millimeter of translation (Carpenter, 1988). Thus, the eye can be considered as undergoing rotations about a single point in the center of the eye.
Rapid eye movement
Rapid eye movement typically refers to the stage during sleep during which the most vivid dreams occur. During this stage, the eyes move rapidly. It is not in itself a unique form of eye movement.
Saccades are rapid refocussing actions of the eyes. Many animals are able to quickly look at a point in space (prompted by memory, peripheral vision or an audio cue) without actively looking at anything in between. The eyes simply jerk into a new position. Saccades move the eye at up to 900°/s in adult humans, and take roughly 250 milliseconds to be initiated by the neural network.
Even when looking intently at a single spot, the eyes drift around. This ensures that individual photosensitive cells are continually stimulated in different degrees. Without changing input, these cells would otherwise stop generating output. Microsaccades move the eye no more than a total of 0.2° in adult humans.
The vestibulo-ocular reflex (VOR) is a reflex eye movement that stabilizes images on the retina during head movement by producing an eye movement in the direction opposite to head movement, thus preserving the image on the center of the visual field. For example, when the head moves to the right, the eyes move to the left, and vice versa. Since slight head movements are present all the time, the VOR is very important for stabilizing vision.
Smooth pursuit movement
The eyes can also follow a moving object around. This is less accurate than the vestibulo-ocular reflex as it requires the brain to process incoming visual information and supply feedback. Following an object moving at constant speed is relatively easy, though the eyes will often make saccadic jerks to keep up. The smooth pursuit movement can move the eye at up to 100°/s in adult humans.
The optokinetic reflex is a combination of a saccade and smooth pursuit movement. When, for example, looking out of the window in a moving train, the eyes can focus on a 'moving' tree for a short moment (through smooth pursuit), until the tree moves out of the field of vision. At this point, the optokinetic reflex kicks in, and moves the eye back to the point where it first saw the tree (through a saccade).
Accommodation is the process by which the eye increases optical power to maintain a clear image (focus).
The accommodation reflex is a reflex action of the eye, in response to focusing on a near object, then looking at distant object (and vice versa). It is dependent on cranial nerve II (afferent limb of reflex), higher centres and cranial nerve III.
A near object (for example, a computer screen) appears large in the field of vision, and the eye receives light from wide angles. When focusing on a near object, the pupil constricts in order to prevent diverging light rays from hitting the periphery of the retina and resulting in a blurred image. As the pupil constricts, the lens becomes more spherical to allow for the diverging light rays.
Physical Assessment & Pathological conditions
A thorough physical examination of the eye includes detailed examination of the eyeball, eyelids, eyelashes, ocular movements, and visual fields, and inspection of the fundus and retina of the eye. The firmness or the intraocular tension is measured by palpation of the eyeballs through closed lids. The lids are inspected for evidence of any deformity, masses, swelling, or discoloration and for disorders of lacrimal apparatus. After this cursory exam, a more detailed examination ensues. The physician can diagnose any abnormalities of lens, iris, cornea, power of accommodation, shape of pupils, opacities of lens, and lesions of cornea with a hand lens.
The sclera is examined for signs of systemic infections and diseases as in jaundice. Extraocular movements of the eyes are checked by holding an object in front of subject and asking the patient to follow its movement with his gaze. An eye examination also includes examining the fundus of the eye. An instrument called funduscope is used by the examiner to look through the pupil. Funduscope has a light source and a set of magnifying lenses. The examiner can clearly inspect the retina, optic disc, retinal artery, retinal vein with the help of ophthalmoscope and can detect any damage to these parts. For clear and proper examination, pupils are priorly dilated by ophthalmic drops to have clear retinal vision.
Pathological Conditions of the Eyes
· Macular Degeneration
Acataract is a progressive eye disorder in which the lens becomes cloudy or opaque due to intrinsic physical or chemical changes. The vision of the person suffering from cataract deteriorates, as the clouding of the lens does not allow the passage of the light to the back of the eyeball. Other than the rare cases of the babies being born with congenital cataracts. Cataracts can very well be a normal part of the aging process.
A person with cataract will have blurred or double vision and he will frequently feel the need to change his eyeglasses. He will also experience the scattering of light coming from one single source. Cataracts, even the incipient ones, can be readily detected during the routine eye examinations. The treatment available for the cataract is surgical and is done mostly on an outpatient basis. The urgency of the cataract surgery depends on the severity of the disease and the need of the patient.
There are three different ways pf removing a cataract: extracapsular surgery, phacoemulsification, and intracapsular surgery. Phacoemulsification is a variation of the extracapsular surgery in which only the core of the lens is removed with the help of the ultrasound.
A cataract is an opacification or cloudiness of the eye's crystalline lens due to aging, disease, or trauma that typically prevents light from forming a clear image on the retina. If visual loss is significant, surgical removal of the lens may be warranted, with lost optical power usually replaced with a plastic intraocular lens (IOL). Due to the high prevalence of cataracts, cataract extraction is the most common eye surgery. The two main types of cataract extraction are intracapsular cataract extraction (ICCE) and extracapsular cataract extraction (ECCE).
Intracapsular cataract extraction involves the removal of the lens and the surrounding lens capsule in one piece. The procedure has a relatively high rate of complications due to the large incision required and pressure placed on the vitreous body, thus is rarely performed in countries where operating microscopes and high-technology equipment are readily available . Cryoextraction is a form of ICCE that freezes the lens with a cryogenic substance such as liquid nitrogen . Although it is now used primarily for the removal of subluxated lenses, it was the favored form of cataract extraction from the late 1960s to the early 1980s.
Extracapsular cataract extraction involves the removal of the lens while the elastic lens capsule is left partially intact to allow implantation of an intraocular lens . There are two main types of extracapsular surgery: conventional ECCE and phacoemulsification. Conventional extracapsular cataract extraction involves manual expression of the lens through an incision made in the cornea or sclera. Although it requires a larger incision and the use of stitches, the conventional method is indicated for patients with very hard cataracts or weak corneal epithelial tissue. Phacoemulsification involves the use of an ultrasonic vibrating handpiece to shatter and break up a cataract, thus making it easier to remove. Because a smaller incision is required, few or no stitches are needed and the patient's recovery time is usually shorter.
A capsulotomy is a procedure to open a portion of the lens capsule. An anterior capsulotomy refers to the opening of the front portion of the lens capsule, whereas a posterior capsulotomy refers to the opening of the back portion of the lens capsule. In an extracapsular surgery, the surgeon performs an anterior capsulotomy, or capsulorhexis, to create an opening through which the lens nucleus can be removed and the intraocular lens implant inserted. An opacification or clouding of the posterior lens capsule frequently occurs in those who have had an extracapsular cataract extraction procedure, therefore, a laser posterior capsulotomy, or YAG laser capsulotomy, is used to clear the back implant surface.
Types of cataracts
Classified by etiology
· Age-related cataract
· Congenital cataract
· Secondary cataract
· Traumatic cataract
Classified by location
· Anterior cortical cataract
· Anterior polar cataract
· Anterior subcapsular cataract
· Nuclear cataract
· Posterior cortical cataract
· Posterior polar cataract
· Posterior subcapsular cataract
Uveitis specifically refers to inflammation of the middle layer of the eye, termed the "uvea" but in common usage may refer to any inflammatory process involving the interior of the eye.
Uveitis is estimated to be responsible for approximately 10% of the blindness in the United States. Uveitis requires a thorough examination by an ophthalmologist.
Uveitis is usually categorized anatomically into anterior, intermediate, posterior and panuveitic forms.
Anywhere from two-thirds to 90% of uveitis cases are anterior in location (anterior uveitis), frequently termed iritis - or inflammation of the iris and anterior chamber. This condition can occur as a single episode and subside with proper treatment or may take on a recurrent or chronic nature. Symptoms include red eye, injected conjunctiva, pain and decreased vision. Signs include dilated ciliary vessels, presence of cells and flare in the anterior chamber, and keratic precipitates ("KP") on the posterior surface of the cornea.
Intermediate uveitis consists of vitreitis - inflammatory cells in the vitreous cavity, sometimes with snowbanking, or deposition of inflammatory material on the pars plana.
Posterior uveitis is the inflammation of the retina and choroid.
Pan-uveitis is the inflammation of all the layers of the uvea.
Glaucoma is a group of diseases of the optic nerve involving loss of retinal ganglion cells in a characteristic pattern of optic neuropathy. Although raised intraocular pressure is a significant risk factor for developing glaucoma, there is no set threshold for intraocular pressure that causes glaucoma. One person may develop nerve damage at a relatively low pressure, while another person may have high eye pressures for years and yet never develop damage. Untreated glaucoma leads to permanent damage of the optic nerve and resultant visual field loss, which can progress to blindness.
The most common type, primary open angle glaucoma (POAG), frequently has no symptoms and has been nicknamed "the sneak thief of sight". One factor may be a relative obstruction on the outflow of aqueous humour from the eye. Aqueous humour is produced by the ciliary body of the eye, and then flows through the pupil and into the anterior chamber. The trabecular meshwork then drains the humour to Schlemm's canal, and ultimately to the venous system. All eyes have some intraocular pressure, which is caused by some resistance to the flow of aqueous through the trabeculum and Schlemm's canal. If the intraocular pressure (IOP) is too high, (>21.5 mm Hg), the pressure exerted on the walls of the eye results in compression of the ocular structures. However, other factors such as disturbances of blood flow in the optic nerve head may interact with IOP to affect the optic nerve. In one third of cases of POAG there is statistically normal IOP. This is called normal tension glaucoma (NTG). Because optic nerve examination and perimetry testing are not always done in addition to IOP measurement in those at risk, NTG is under diagnosed and the condition presents late.
Another type, acute angle-closure glaucoma, is characterized by an acute rise in the intraocular pressure. This occurs in susceptible eyes when the pupil dilates and blocks the flow of fluid through it, leading to the peripheral iris blocking the trabecular meshwork. Acute angle-closure glaucoma can cause pain and reduced visual acuity (blurred vision), and may lead to irreversible visual loss within a short time. This is an ocular emergency requiring immediate treatment. Many people with glaucoma experience halos around bright lights as well as the loss of sight characterized by the disease.
Primary congenital glaucoma or buphthalmos is a rare genetic disease affecting infants. Newborns present with enlarged globes and clouded corneas. It is thought that reduced trabecular permeability is the cause of increased intraocular pressure. Surgery is the treatment.
Secondary glaucoma occurs as a complication of various medical conditions such as eye surgery, advanced cataracts, eye injuries, some eye tumors, uveitis, diabetes or use of corticosteroid drugs.
Conjunctivitis or pink eye is the inflammation of the conjunctiva, the thin membranous lining on the inner surface of the eyelid and the outer part of the eyeball. During conjunctivitis, the eye becomes reddened and has a runny discharge. This discharge makes the eyelids stick together during sleep and may cause itching and hypersensitivity to light. The causes of conjunctivitis can range from viral infection to bacteria and gonorrhea and Chlamydia.
· Blepharoconjunctivitis is the combination of conjunctivitis with blepharitis.
· Keratoconjunctivitis is the combination of conjunctivitis and keratitis.
There are three common varieties of conjunctivitis, viral, allergic, and bacterial. Other causes of conjunctivitis include thermal and ultraviolet burns, chemicals, toxins, overuse of contact lenses, foreign bodies, vitamin deficiency, dry eye, dryness due to inadequate lid closure, exposure to chickens infected with Newcastle disease, epithelial dysplasia (pre-cancerous changes), and some conditions of unknown cause such as sarcoidosis.
Viral conjunctivitis is spread by aerosol or contact of a variety of contagious viruses, including many that cause the common cold, so that it is often associated with upper respiratory tract symptoms. Clusters of cases have been due to transfer on ophthalmic instruments which make contact with the eye (e.g., tonometers) and have not been adequately sterilized.
Allergic conjunctivitis occurs more frequently among those with allergic conditions, with the symptoms having a seasonal correlation. It can also be caused by allergies to substances such as cosmetics, perfume, protein deposits on contact lenses, or drugs. It usually affects both eyes, and is accompanied by swollen eyelids.
Bacterial conjunctivitis is most often caused by pyogenic bacteria such as Staphylococcus or Streptococcus from the patient's own skin or respiratory flora.
Irritant, toxic, thermal and chemical conjunctivitis are associated with exposure to the specific agents, such as flame burns, irritant plant saps, irritant gases (e.g., chlorine or hydrochloric acid ('pool acid') fumes), natural toxins (e.g., ricin picked up by handling castor oil bean necklaces), or splash injury from an enormous variety of industrial chemicals, the most dangerous being strongly alkaline materials.
Xerophthalmia is a term that usually implies a destructive dryness of the conjunctival epithelium due to dietary vitamin A deficiency—a condition virtually forgotten in developed countries, but still causing much damage in developing countries. Other forms of dry eye are associated with aging, poor lid closure, scarring from previous injury, or autoimmune diseases such as rheumatoid arthritis, and these can all cause chronic conjunctivitis.
Redness, irritation and watering of the eyes are symptoms common to all forms of conjunctivitis. Itch is variable.
Conjunctivitis sometimes requires medical attention. The appropriate treatment depends on the cause of the problem. For the allergic type, cool compresses and artificial tears sometimes relieve discomfort in mild cases. In more severe cases, non-steroidal anti-inflammatory medications and antihistamines may be prescribed. Some patients with persistent allergic conjunctivitis may also require topical steroid drops.
Bacterial conjunctivitis is usually treated with antibiotic eye drops or ointments that cover a broad range of bacteria.
Macular degeneration is a medical condition where the light sensing cells in the macula malfunction and over time cease to work. According to the American Academy of Ophthalmology, it is the leading cause of central vision loss (blindness) in the United States today for those over the age of fifty. There are two basic types of the disease: Standard Macular Degeneration (MD) and Age Related Macular Degeneration (ARMD), with ARMD being the most common form of the condition. Macular degeneration that is not age related is most commonly caused by an inherited condition. These forms are sometimes called Juvenile macular degeneration (JMD). In macular degeneration the final form results in missing or blurred vision in the central, reading part of vision. The outer, peripheral part of the vision remains intact.
Age related macular degeneration
ARMD is further divided into a "dry," or non-exudative, form and a "wet," or exudative, form. Eighty five to ninety percent of cases are categorized as "dry" macular degeneration where fatty tissue, known as drusen, will slowly build up behind the retina. Ten to fifteen percent of cases involve the growth of abnormal blood vessels under the retina. These cases are called "wet" macular degeneration due to the leakage of blood and other fluid from behind the retina into the eye. Wet macular degeneration usually begins as the dry form. If allowed to continue without treatment it will completely destroy the macula. Medical, photodynamic, laser photocoagulation and laser treatment of wet macular degeneration are available.
· Pigmentary alterations
· Exudative changes: hemorrhages, hard exudates, subretinal/sub-RPE/intraretinal fluid
· Atrophy: incipient and geographic
· Visual acuity drastically decreasing (two levels or more) ex: 20/20 to 20/80.
· Holes visible in fundus photographs.
· Blurred vision: Those with non-exudative macular degeneration may by asymptomatic or notice a gradual loss of central vision, whereas those with exudative macular degeneration often notice a rapid onset of vision loss.
· Central scotomas
· Distorted vision (i.e. metamorphopsia) - A grid of straight lines appears wavy and parts of the grid appear blank.
· Trouble discerning colors; specifically dark ones from dark ones and light ones from light ones.
Fluorescein angiography is a special study of the blood vessels in the retina, and it helps the doctor in detecting an overgrowth of the blood vessels. Exudative laser surgery, if carried out in time, can cure the exudative macular degeneration. Newly-formed abnormal blood vessels are sealed off by laser photocoagulation.
Retinopathy or retinitis pigmentosa is a general term designating the diseases affecting the retina with loss of vision and often due to systemic disorders.
The two major kinds of retinopathy are hypertensive and diabetic. People with chronic hypertension undergo degenerative retinal changes due to the impairment of the blood supply to the retina. As a result, arteriosclerosis develops, and there are other apparent physical changes due to the vascular damages.
As for diabetic retinopathy, it can be proliferative as well as non-proliferative. Proliferative retinopathy is characterized by the neovascularization of the retina enhancing the chances of vitreous hemorrhage and retinal tears.
Physicians make use of fluorescein angiography to detect the retinopathy, and the treatment is done through the laser coagulation or surgery. Non-proliferative retinopathy is characterized by retinal edema, venous dilation, microaneurysms, hard exudates, and retinal hemorrhages. This is treated with laser coagulation.
Strabismus, also known as "heterotropia", "squint", "crossed eye", "wandering eye", or "wall eyed", is a disorder in which the eyes do not point in the same direction. It typically involves a lack of coordination between the extraocular muscles, which prevents bringing the gaze of each eye to the same point in space, preventing proper binocular vision, which may adversely affect depth perception. The cause of strabismus can be a disorder in one or both of the eyes; for example, nearsightedness or farsightedness, making it impossible for the brain to fuse two different images.
When strabismus is congenital or develops in infancy, it can cause amblyopia, in which the brain ignores input from the deviated eye although it is capable of normal sight. Since strabismus can cause amblyopia, which is sometimes referred to as lazy eye, it is sometimes itself inaccurately referred to as lazy eye.
Strabismus may be concomitant or incomitant. Concomitant strabismus means that the strabismus is equal regardless of which direction the gaze is targeted. This indicates that the individual extraocular muscles function individually, but that they may simply not be aimed at the same target. Concomitant strabismus in a child under the age of 6 rarely indicates serious neurologic disorder. Blindness in one eye usually causes concomitant strabismus, with the eye of a child turning inward, and that of an adult turning outward.
Incomitant strabismus occurs when the degree of misalignment varies with the direction of gaze. This indicates that one or more of the extraocular muscles may not be functioning normally. Types of strabismus include:
· Esotropia, or one eye turning inward;
· Exotropia, or one eye turning outward;
· Hypertropia, or one eye turning upward.
· Hypotropia, or one eye turning downward.
· Medial strabismus manifests as the inability to abduct (move laterally) one's eye. This is usually caused by damage to the abducens nerve or abducens nucleus. The result is that the eye in its normal resting state deviates medially, as the movements of the medial rectus muscle are less opposed by the denervated lateral rectus muscle.
Pseudostrabismus is the false appearance of strabismus. It generally occurs in infants and toddlers whose bridge of their nose is wide and flat. This causes the appearance of strabismus.
As with other binocular vision disorders, the primary therapeutic goal for those with strabismus is comfortable, single, clear, normal binocular vision at all distances and directions of gaze. If minor and detected early, strabismus can often be corrected with enforced use of an eye patch on the dominant eye and/or vision therapy. Advanced strabismus is usually treated with a combination of eyeglasses or prisms, vision therapy, and surgery, depending on the underlying reason for the misalignment. Surgery attempts to align the eyes by shortening, lengthening, or changing the position of one or more of the extraocular eye muscles, and is frequently the only way to achieve cosmetic improvement. Glasses affect the position by changing the person's reaction to focusing. Prisms change the way light, and therefore images, strike the eye, simulating a change in the eye position.
A stye (also spelled sty) is an inflammation of the sebaceous glands at the base of the eyelashes. They are harmless but can be very painful. They are generally caused by a Staphylococcus bacteria infection. They are particularly common in infants.
The stye may form on either the inside or the outside of the eyelid.
Most styes will drain on their own though this may be accelerated with a hot or warm compress. Medical professionals advise that patients not attempt to squeeze or apply excessive pressure to styes.
A scotoma (Greek: darkness) is an area or island of loss or impairment of visual acuity surrounded by a field of normal or relatively well preserved vision.
Every human eye has a scotoma in its field of vision, usually termed its blind spot. The presence of this normal scotoma does not intrude into consciousness because it is very small, but it can be demonstrated to oneself by the simplest of clinical methods.
Symptom-producing or pathological scotomata may be due to a wide range of disease processes, affecting either the retina (in particular its most sensitive portion, the macula) or the optic nerve itself. A pathological scotoma may involve any part of the visual field and may be of any shape or size. A scotoma may include and enlarge the normal blind spot. Even a small scotoma that happens to affect central or macular vision will produce a severe visual handicap, whereas a large scotoma in the more peripheral part of a visual field may go unnoticed by the bearer.
Common causes of scotomata include demyelinating disease such as multiple sclerosis (retrobulbar neuritis), toxic substances such as methyl alcohol and quinine, nutritional deficiencies, and vascular blockages either in the retina or in the optic nerve. Less common, but important because sometimes reversible or curable by surgery, are scotomata due to tumors such as those arising from the pituitary gland, which may compress the optic nerve or interfere with its blood supply.
Hemianopsia is the loss of half the vision in both eyes. It is categorized in to
· Binasal hemianopsia
· Bitemporal hemianopsia
Binasal hemianopsia is the medical description of a type of partial blindness that is associated with certain lesions of the eye, and of the central nervous system, such as congenital hydrocephalus. The absence of vision in half of a visual field is described as hemianopsia. The visual field of each eye can be divided in two vertically, with the outer half being described as temporal, and the inner half being described as nasal.
In binasal hemianopsia vision is missing in the inner half of both the right and left visual fields.
Binasal hemianopsia can be broken down as follows:
bi-: involves both left and right visual fields
nasal: involves the nasal visual field
hemi-: involves half of each visual field
Bitemporal hemianopsia is the medical description of a type of partial blindness that is associated with lesions of the optic chiasm, the area where the optic nerves from the right and left eyes cross near the pituitary gland.
The absence of vision in half of a visual field is described as hemianopsia.
The visual field of each eye can be divided in two vertically, with the outer half being described as temporal, and the inner half being described as nasal.
In bitemporal hemianopsia vision is missing in the outer half of both the right and left visual fields. Pituitary adenomata are associated with this kind of visual deficit, as are certain injuries.
Bitemporal hemianopsia can be broken down as follows:
bi-: involves both left and right visual fields
temporal: involves the temporal visual field
hemi-: involves half of each visual field
A pinguecula is a common, nonmalignant growth of the mucous membrane that lines the eyeball and underside of the eyelids, conjunctiva. It appears as a small, yellowish lesion derived from the conjunctiva. It can appear on either side of the cornea, but usually appears on the nasal side. It may increase in size over many years. The cause is unknown, but chronic sunlight exposure and eye irritation is shown to contribute in its development. Welding is a significant occupational risk.
The chief symptom is a yellow or white nodule on the conjunctiva near the cornea. An eye examination is often sufficient to diagnose this disorder. This condition is benign and often no treatment is required except for lubrication with artificial tears and temporary use of mild steroid eye drops. Rarely, it may need to be excised.
Proceduresof the Eyes
Fluorescein angiography, or fluorescent angiography, is a technique for examining the circulation of the retina. It involves injection of sodium fluorescein into the systemic circulation, and then an angiogram is obtained by photographing the fluorescence emitted after illumination of the retina with blue light at a wavelength of 490 nanometers.
Pathologic changes are recognized by the detection of either hyperfluorescence or hypofluorescence.
Causes of hyperfluorescence:
· Leaking (i.e. Capillary leakage, aneurysm, neovascularization)
· Pooling (serous retinal detachment)
· Transmission defects
· Abnormal vessels
Causes of hypofluorescence:
· Optical barrier (i.e. Blood)
· Filling defect (capillary blockage)
In order to detect injuries, ulceration, or foreign bodies, a fluorescein dye is applied to the cornea and conjunctivae. Then, the surface of the area is examined with the cobalt blue light.
This helps in the determination of the electrical potential of the retina in response to the light stimuli. It also identifies visual abnormalities due to retinal disease. It is used to measure the electrical responses of various cell types in the retina, including the light-sensitive cells (rods and cones) and the ganglion cells. Electrodes are placed on the cornea and the skin near the eye. During a recording, the patient is watching a standardized stimulus and the resulting signal is interpreted in terms of its amplitude (voltage) and time course. Stimuli include flashes (flash ERG) and reversing checkerboard patterns (pattern ERG).
Electroretinogram (ERG) is used for the diagnosis of various retinal diseases:
· Retinitis pigmentosa and related hereditary degenerations
· Retinitis pigmentosa sine pigmento
· Retinitis punctata albescens
· Leber's congenital amaurosis
Fundus refers to the rear interior of the eye, which contains the retina along with its blood vessels. Funduscopic examination is performed with an ophthalmoscope, which has a light source and some changeable lenses to Focus on the fundus. An ophthalmoscope is able to detect vascular disorders of the retina, swelling or cupping of the disk, and retinal detachment,
In order to identify intraocular or orbital foreign body, an orbital imaging is performed with X-rays or MRI of the skull emphasizing the orbits.
Perimetry is used to assess the peripheral vision by testing the patient's ability to notice the moving objects or flashing lights at the extreme periphery of the visual field.
This procedure gives the accurate determination of near and distant vision. The instrument allows the use of greater number of lenses in order to determine the correct lens for each eye and is more effective than the vision charts. This also helps in the detection and measurement of astigmatism:
Retinal arteriography is the process of recording the images of the retinal arteries with the fluorescein injected into the arm vein.
Slit lamp has a microscope and a built-in illumination, which is projected through a narrow slit. With the help of this slit lamp, a physician can view the cross sections of the anterior structures of the eye. Flares and cells can be detected through the slit lamp examination.
Test for color blindness
To test the color blindness, printed figures of different sized dots in different colors are used, which a normal person can perceive against a backdrop of differently colored dots. A person with color blindness perceives only the scattering of the dots.
Tonometry refers to the procedure eye care professionals perform use to determine the intraocular pressure (IOP), the pressure found inside the eye. It is an important test in the evaluation of ocular conditions such as glaucoma as well conditions such as phthisis bulbi, and iritis.
Methods of Tonometry
Applanation tonometry (Goldmann tonometry) is considered the gold standard in the determination of intraocular pressure. Applanation tonometry uses a special probe to flatten part of the cornea to measure eye pressure with a slit lamp. Because the probe makes contact with the cornea, a topical eye anesthetic, such as oxybuprocaine, tetracaine, Alcaine, proxymetacaine or proparacaine is introduced onto the surface of the eye in the form of one or a few eye drops. Yellow Fluorescein is also used to aid the examiner in determining the IOP.
Noncontact tonometry (or air-puff) employs a puff of air to flatten the cornea. This type of tonometry is the least accurate way to measure IOP. It is often used as a simple way to screen for high IOP and is the easiest way to test children.
The Tono-Pen is an electronic, digital pen-like instrument that determines IOP by making contact with the cornea, after use of topical eye anesthetic eye drops. This is especially useful for very young children and special population patients.
Indentation (Schiötz) tonometry historically was used to determine IOP. This type of tonometry makes use of a plunger to indent the cornea. The IOP is determined with small metal weights.
The standard way to test the vision is to place the Snellen chart 20 feet away from the subject, and visual acuity is recorded on the basis of the smallest line of type, which a person can accurately read more than half the letters of. Normal vision is designated as 20/20. A score of 20/80 indicates that the person can see an object clearly from a distance of 20 feet what a normal person can see from a distance of 80 feet. To test the near vision, cards with different-sized letters are used. For i1literates and children, these cards have pictures and symbols on them.
Visual field testing
A black felt sheet or screen mounted on a wall is used to assess the areas of impaired or absent vision. Doctors make use of the Amsler grid, which consists of a network of lines-usually white on black-around a central point at which the subject is instructed to gaze while the examiner moves a small object through the various parts of the visual field.
Amsler grid testing
The Amsler grid, used since 1945, is a grid of horizontal and vertical lines used to monitor a person's central visual field. It is a diagnostic tool that aids in the detection of visual disturbances caused by changes in the retina, particularly the macula (e.g. macular degeneration), as well as the optic nerve and the visual pathway to the brain.
Amsler grids can be obtained from an ophthalmologist or optometrist and may be used to test one's vision at home.
The original Amsler grid was black and white. A color version with a blue and yellow grid is more sensitive and can be used to test for a wide variety of visual pathway abnormalities, including those associated with the retina, the optic nerve, and the pituitary gland.
Pharmacology & Abbreviations
Antiglaucoma drugs are drugs that lower the intraocular pressure by reducing production of aqueous humor by the ciliary body. Most drugs of this class are beta-adrenergics. Drugs: timolol maleate, brimonidine brinzolamide, levobunolol, methazolamide, and pilocarpine.
Anti-inflammatory and antiallergics
In case of severe ocular inflammations, topical preparations containing antibiotics and antifungal agents are used with or without a combination with corticosteroid, to suppress the inflammation of the eye. Drugs: sodium Cromoglycate, flurbiprofen sodium, diclofenac ophthalmic solution.
Cycloplegics and mydriatics
Cycloplegics are agents that paralyze ciliary muscles and result in pupil dilation. Mydriatics cause the dilation of pupils. Most of the mydriatics cause cycloplegia also. Drugs: atropine, homatropine, cyclopentolate, scopolamine, and tropicamide.
Miotics are agents that cause constriction of the pupil of the eye mostly used in treatment of glaucoma. These agents are used to counteract mydriatics after having been used for testing refraction. Drugs: pilocarpine.
Various antibiotics are used in many eye infections caused by pathogens like bacteria and any of these antibiotics are used in combination with corticosteroids. These antibiotics are used to treat bacterial infection and fungal infections of the eye, conjunctivitis keratitis, blepharitis, meibomianitis, trachoma, dacryocystitis and differ according to the type of infection of the Drug: norfloxacin, ciprofloxacin, gentamicin, tobramycin, sulfacetamide, sodium, chloramphenicol, miconazole, silver sulfadiazine, acyclovir, ciprofloxacin, framycetin sulfate, and polymyxin.
Some Terms associated with ophthalmology
Accommodation: ability of the eye to focus on near and distant objection by changing the convexity of the lens
Achromatopsia: condition of color blindness of the eyes
Amblyopia: decreased vision
Astigmatism: visual condition in which light rays are unevenly focused on the retina due to abnormal curvature of the cornea
Blepharectomy: excision of lesion of eyelid
Blepharedema: swelling of eyelid
Blepharitis: inflammation of eyelid
Blepharoplegia: paralysis of muscles of eyelid
Blepharoptosis: drooping of eyelid
Blepharospasm: twitching or involuntary contraction of eyelid
Cataract: clouding of lens of eye which results in loss of transparency
Chemosis: accumulation of the lens of the eye which causes swelling around the cornea
Choroid: the vascular layer of eyeball between sclera and retina
Choroiditis: inflammation of choroid
Choroidopathy: disease of choroid
Conjunctiva: thin mucous membrane covering the front of the eyeball
Conjunctival: related to the conjunctiva
Conjunctivitis: inflammation of conjunctiva
Cornea: outer transparent portion of the eye, which is continuous with the sclera and through which the light passes
Dacryadenalgia: pain in tear glands
Dacryadenitis: inflammation of tear glands
Dacryorrhea: flow of tears
Diplopia: double vision
Epiphora: overflow of tears
Erythropia: red eye
Esotropia: condition in which both eyes turn inwards
Exophthalmos : protrusion of eyeballs
Exotropia: condition in which both eyes turn outwards
Fundus: a part of the interior of eyeball which is exposed to view through ophthalmoscope
Funduscope: instrument to view fundus
Funduscopic: related to funduscopy
Funduscopy: examination of the fundus of eye
Glaucoma: pathologic condition of eye in which intraocular pressure increases
Hordeolum: a small purulent inflammatory infection of sebaceous gland of eyelid; also called sty
Hypertropia: squint in which the eyes look upward.
Hypotropia: squint in which eyes look down words.
Intraocular: inside the eye
Iridoplasty: surgical repair of the iris
Iridoplegia: paralysis of the iris
Iridotomy: incision of the iris
Iris: pigmented portion of eye which regulates the amount of light coming through the pupil
Keratitis: inflammation of the cornea
Keratoplasty: surgical repair of the cornea
Keratotome: instrument to incise the cornea
Keratotomy: incision of the cornea
1aser iridotomy: cutting some fibers of the iris with laser
Macula: small spot on the retina which is the area of sharpest vision
Macule: a spot discoloration or thickening of the skin
Miosis: constriction of the pupil
Miotic: agents that constrict the pupil
Mydriatic: agent that dilates the pupil
Nystagmus: involuntary oscillation of the eyeballs
Ophthalmalgia: pain in the eyes
Ophthalmectomy: excision of an eyeball
Ophthalmodynia: pain in the eye
Ophthalmologist: specialist in diseases of the eye
Ophthalmomalacia: softening of the eye
Ophthalmoplegia: paralysis of the ocular muscles
Ophthalmoscope: instrument to view eyes
Optic: pertaining to the eye
Optic chiasm: point of crossing of optic nerves
Optometrist: person who tests visual acuity and prescribes corrective lenses
Phacoemulsification: process to disintegrate cataract
Photophobia: marked intolerance to light
Presbyopia: farsightedness associated with aging
Pterygium: outward growth of tissue of the eye
Pupil: the round opening in center of the iris through which light reaches the lens
Pupillary: relating to the pupil
Rectus: muscles of the eye
Retina: innermost layer of the eye, which has nerve endings of the optic nerve
Retinitis: inflammation of the retina
Retinoblastoma: malignant sarcoma or neoplasm of the retina
Retinopathy: any disease of the retina
Retinoscopy: visual examination of the retina
Retrobulbar: behind the eyeball
Sclera: white of the eye, outermost fibrous laver
Scleritis: inflammation of sclera
Scleromalacia: softening of sclera
Sclerostomy: surgical formation of opening in sclera
Sclerotomy: incision of sclera
Seton procedure: placing a tube in the anterior chamber to drain the excess fluid and decrease intraocular pressure.
Slit lamp: instrument consisting of a microscope and a bright thin beam of light to examine an eye
Strabismus: improper alignment of the eyes
Sty: infection of the sebaceous gland of the eyelid
Tonometry: test to measure intraocular pressure
Trabeculectomy: excision of the trabecular meshwork to treat glaucoma
Uveitis: inflammation of the uvea
Visual acuity: clearness of vision
Vitrectomy: excision of vitreous humor
Vitreous: relating to vitreous humor
Vitreous humor: thick, gelatinous substance within the eyeball
Xanthopsia: a condition of the eye in which everything appears yellowish
ARMD: age-related macular degeneration
DR: diabetic retinopathy
EOM: extraocular movement
IOL: intraocular lens
IOD: intraocular pressure
L&A: light and accommodation
OD: oculus dexter (right eye)
OS: oculus sinister (left eye)
OU: oculus unitas (both eyes)
PAN: periodic alternating nystagmus
PAM: potential acuity meter
PERRLA: pupils equal, round, and reactive to light and accommodation
REM: rapid eye movement
RD: retinal detachment
VA: visual acuity
VF: visual field
dacry/o: tear, lacrimal
presby/o: old age
-Iogist: specialist in study of
-scope: instrument to view
-spasm: involuntary contraction