(The material in this article was produced in collaboration with
Daniel Finkelstein, M.D., Associate Professor of Ophthalmology, The
Wilmer Eye Institute, Johns Hopkins University, Baltimore, Maryland.)

This article includes information on only the most commonly encountered eye conditions, and it reduces a great deal of medical and technical detail to language understandable to the lay inquirer. Many medical facts have, in the interest of clarity and brevity, been simplified; therefore, this text is not in any way intended as a basis for self-diagnosis or lay diagnosis of anyone's eye condition, a job which belongs to the eye doctor. If you are wondering whether anything is wrong with your eyes, we urge you to see an eye doctor at once. If you cannot afford to pay, there are many agencies and funds which can help you.

Some of the conditions described below are hereditary (passed on from one generation to another through genes and chromosomes), but there are many patterns of heredity, even for a single disorder. Specific questions about the inheritance of a particular characteristic in an individual or family should be directed to a genetic specialist. A family physician can usually provide the proper referral.

Following are descriptions of the specific eye conditions about which we receive the most frequent questions. A longer list is treated in our free publication Blindness and Disorders of the Eye. Generally we have not attempted to rank causes by percentages or numbers of cases since such statistics are constantly changing, and the analysis of them is a complex matter.

Albinism

Albinism is a hereditary condition in which there is a lack of normal pigment in part or all of the body. The hair is white, the skin very fair, and the iris of the eye white or pinkish. The person with albinism usually has poor vision, an imperfectly developed retina, over sensitivity to light, and nystagmus (abnormal muscle movement causing constant twitching or jerking of the eyes).

Specially treated lenses can improve vision and can also lessen discomfort by reducing the amount of light entering the eye. There is no cure for albinism. Amblyopia

The general term amblyopia applies to poor vision which is not due to any observable disease and which cannot be corrected by glasses. It may be congenital (present at birth) or may develop later.

Sometimes the cause is unknown; however, blindness can often be prevented if proper steps are taken. A common example is lazy eye or amblyopia ex anopsia, affecting perhaps one to two percent of children. This condition occurs in a young child if one eye becomes so dominant that the other is suppressed and deteriorates through lack of use. This is likely to happen if the two eyes focus differently because of strabismus (see below) or because one eye is much more nearsighted than the other. One eye may fall into disuse as the person relies on images from the other. However, the eye may appear perfectly normal to others. If this problem is discovered and treated early enough (generally before the age of six or seven), often the unused, weak eye will regain its strength. Otherwise permanent loss of vision may result. Many service organizations provide free preschool screening tests in an effort to find and refer these children before the vision of one eye is completely lost. Treatment generally consists of correcting the basic inequality of the eyes through surgery or corrective lenses and/or putting a patch over the strong eye temporarily to force the weak one to work again. Eye examinations at birth and again by three years of age are recommended to increase chances of early detection and effective treatment.

Cataracts

The lens of the human eye is comparable to the lens of a camera or magnifying glass, and for obvious reasons it needs to be transparent and free of flaws. The development of what are called cataracts, or opacities and clouding of the eye's lens, blocks the passage of light through the eye.

Various forms of cataract make up one of the leading causes of blindness in this country today. Although some cataracts are congenital (present at birth), likelihood of developing them increases with age. Chemical changes in the lens and diabetes have also been associated with development of cataracts, and chemical burns and heredity may cause them. All of the causes, however, are not known. Like the majority of eye conditions, they are not contagious.

Cataracts are not painful. In fact the only symptom as far as the patient is concerned is from very slight to virtually total interference with vision. Cataracts may be manifested in dimmed, blurred, or double vision or a need for frequent changes of glasses. Typically a person with cataracts experiences difficulty in satisfactorily adjusting light for activities like reading while at the same time having to avoid glare. Driving at night may for this reason become very difficult. Not all cataracts require surgery, some being small enough that they do not seriously affect vision. For those large enough to cause visual problems, medication does not help; and the only effective treatment is surgery to remove the affected lens, implantation of an intraocular lens, and the subsequent wearing of conventional eyeglasses or strong contact lenses.

Much progress has been made recently in the development of improved forms of cataract surgery, including use of freezing probes and ultrasonic (not to be confused with laser) devices which make possible removal of the lens through a small opening in a process called phacoemulsification. The decision about whether and when to operate is dependent on factors such as the patient's age and occupation. The likelihood of regaining useful sight is generally good, with some estimates placing it as high as 95%, although complications may prevent successful treatment in some cases. It is no longer necessary to wait for cataracts to ripen before operating, and convalescence from the surgery is faster and safer than ever before.

Color Blindness

This familiar condition is predominant in males (in about 8% as opposed to around 0.5% of females) and most commonly is a sex-linked inherited characteristic transmitted through the male chromosome, although it may also result from poisoning or retinal disease. Generally affecting both eyes, it most often takes the form of a loss of perception of one or two fundamental colors (red-green color blindness is the most common congenital variety), but occasionally it is complete, so that the individual literally sees only in black and white.

The exact mechanism of color blindness has not been fully explained, but some writers simply attribute it to an absence of one or more of the cone pigments or an abnormal presence of a mixture of two of the color-sensitive pigments in one cone. In any event, use of various charts to test for color blindness is commonly included as part of an eye examination, especially for drivers and workers in industry and transportation, where the ability to distinguish colored signals is important. One of the most familiar tests consists of a circle containing dots of different colors, in which the subject with normal color vision is supposed to discern one number, while a person with defective color vision is likely to see another.

Some have found the pattern of inheritance for this recessive characteristic confusing. To put the matter simply, a female will usually not be color-blind herself unless there is an abnormal gene for color blindness on both of the X chromosomes carried by all females. On the other hand a male, who carries only one X chromosome, will be color-blind if it contains an abnormal gene. Moreover, the female with only one abnormal gene inherited from her father can become a "carrier" and pass the trait on to a male offspring without being color-blind herself.

There is no treatment for color blindness, and it cannot be overcome through any type of eye exercises.

Congenital Eye Defects

The term congenital means "present at birth"; hence, this term includes some or all cases of many conditions listed under other headings in this publication. Some congenital eye conditions like retinoblastoma are truly hereditary (passed on through the genes and chromosomes), while others are the result of a disease or deficiency during pregnancy for instance, German measles (rubella).

Often the exact cause of a congenital defect is unknown. Following are examples of conditions which may be seen at birth as a developmental deficiency of unknown cause:

aniridia absence or near absence of the iris

microphthalmos an abnormally small eye, usually with poor vision

megalophthalmos an abnormally large eye present at birth

anophthalmos absence of the eyeball (an artificial eye can usually be worn, and the eyelids are usually present.)

coloboma a cleft or slot in the iris and/or retina, as a result of incomplete growth.

Corneal Disease and Corneal Transplants

The cornea, a transparent layer over the front of the eye, functions as a refracting and protective "window" membrane through which light rays pass on the way to the retina. While there are no blood vessels in the cornea, there are many pain fibers, so that most injuries do cause severe pain. In addition, blurred vision results from injury to or disorder within the cornea. Problems in this area, resulting as they can in permanently blurred vision or blindness, are extremely serious and should receive immediate attention from an eye specialist.

Corneal ulcers Scarring or perforation due to corneal ulceration is a major cause of blindness throughout the world. Usually such ulcers can be treated if attended to quickly. Ulceration may be caused by bacteria such as streptococcus, viruses (herpes simplex keratitis being one of the most common), fungi, vitamin A deficiency, or other disorders. New drugs have aided the treatment of many of these conditions.

Degenerative corneal conditions Keratoconus, a rare degenerative condition which is inherited, causes a general thinning and an abnormal protrusion of the central cornea, as well as some scarring. Blurred vision results, and in advanced cases there may even be perforation of the cornea. While contact lenses (especially the new soft types) can aid vision in the early stages, corneal transplants are sometimes performed before extreme thinning takes place, and in such cases reading vision can usually be obtained.

Other degenerative conditions include corneal dystrophy and arcus senilis, an extremely common condition in elderly people.

Other types of corneal disorders may be indicated by pain, irritation, or blurred vision; however, some corneal ailments can be detected only by a trained eye specialist.

Corneal Transplant (Keratoplasty) When the cornea becomes scarred, hazy, or opaque or when there is danger of perforation of a corneal ulcer, an ophthalmic surgeon may remove the affected cornea and replace it with a healthy one taken from a donor. In many states it is easy for one to donate eyes for this purpose at death. Only the cornea is used in such cases. There is no immediate prospect of successful whole eye transplants. Surgeons would prefer to use the donated cornea immediately, but it may be used within sixty to seventy hours after death if handled properly. New techniques for this procedure involving surgery under a microscope and use of finer suture material have constituted a major advance in treatment of corneal disorders. Ordinarily using a knife called the trephine, which functions rather like a cookie cutter, the surgeon performs what is called a "penetrating keratoplasty." The defective cornea is cut with the knife and lifted out. A matching piece of the donor cornea is cut with the same knife , used to replace the excised piece of the defective cornea , and held in place by very fine sutures. The convalescence from this surgery is relatively brief and uncomplicated these days. Best vision returns when the sutures are removed about one year after surgery. Chances of rejection of the new cornea are rated by most authorities at from one to five percent.

Diabetic Retinopathy

This eye condition, a very common complication of diabetes, rivals glaucoma and cataracts as a leading cause of blindness in this country. Its incidence is rising as a result of the increased lifespan of diabetics, which has been made possible by improved medical technology and particularly by the synthesis of insulin in the early years of the twentieth century. The likelihood of retinopathy increases with the length of time a given person has had diabetes, so that many who have been diabetics for twenty or more years do become blind.

Long-term diabetes often brings about changes in the tiny blood vessels in the retina (the lining of the back of the eye). There are two forms of diabetic retinopathy. The milder form, background or nonproliferative, is the more common. It involves development of microaneurisms in retinal capillary blood vessels, which usually do not cause serious vision loss.

A relatively small number of diabetics develop the more severe, or proliferative, form of retinopathy, which can cause blindness. With its onset new, abnormal blood vessels may be formed; blood vessels may become engorged with too much blood and burst, or the retina may break loose from the back of the eye. In addition to the direct interference with vision caused by these events, blood from the burst vessels enters the vitreous (jelly-like) part of the eye and makes it cloudy instead of transparent.

Modern medical treatment administered as early as possible in the course of the disorder can often slow the course of diabetic retinopathy. Laser treatments sometimes can seal or weld broken blood vessels or seal a detached portion of the retina back into place. Photocoagulation treatment consists of scattering hundreds of small, quick flashes of intense laser light across the retina in order to seal or dry up new blood vessels. It must, however, be undertaken before there is bleeding into the vitreous or detachment of the retina. Although in many cases these kinds of treatment are wholly or partially unsuccessful, the techniques are continually being refined.

Medical science does not yet fully understand just what characteristics of diabetes bring about retinopathy. Although good adherence to a diet, regular administration of insulin, and other prescribed regimens will improve the patient's general health and help to lower the likelihood of complications in general, the most careful and conscientious patient may still become blind. On the other hand, some people have severe diabetes for many years and never develop eye problems.

German Measles (Rubella)

Many readers may be surprised to find that German measles is associated with blindness since it is commonly thought of as a very minor disease. Neither child nor adult is likely to be harmed permanently by a case of German measles, but an unborn baby may be very much affected. If an expectant mother has the disease during the first three months of pregnancy, she herself will probably hardly feel sick at all, but the disease is likely to infect her unborn child through the placenta, with serious results.

Many of these babies are born mentally retarded, and they also are likely to have physical problems. Heart disease, hearing disorders, and respiratory difficulties are particularly common. Many rubella children have cataracts or other eye problems like glaucoma, abnormally small eyes, or defects in the iris or retina.

A number of preventive efforts have been made to minimize this threat to the unborn. Children especially girls should be exposed to the disease or vaccinated if possible so that they may have mild symptoms and develop resistance to German measles for the future. Women who might be in the early stages of pregnancy should avoid contact with cases of German measles.

Glaucoma

Glaucoma is another of the leading causes of blindness in the United States, perhaps accounting for one in every seven or eight cases. In this condition, the transparent fluid inside the forward part of the eye does not drain normally, and excess pressure is built up within the eye. If the pressure is not controlled, the delicate structure of the eye is increasingly damaged, resulting in blurred vision, a narrowed field of sight, and eventually total blindness. Symptoms may include intermittent blurred vision, nausea, inability to adjust eyes to darkened rooms, seeing colored halos around lights, and reduced side vision.

The causes of glaucoma are not fully understood. Some cases are hereditary, while others arise as complications of other eye disorders. Glaucoma, not a contagious condition, is most common after the age of thirty-five.

The acute type of glaucoma appears as a sudden attack, characterized by great pain and discomfort as the eye pressure rises quickly from blockage of drainage canals. Such an episode can damage the eyes severely in a short time. More common, however, is the chronic type (affecting perhaps 2.5 million Americans), in which there is no pain and in which the damage to vision is so slow that it may not be noticed by the patient for a long time. The intraocular pressure rises because the drainage canals are blocked and the fluid cannot drain properly.

Many cases are controlled by medication which relaxes and unblocks the drainage channels for the eye fluid. Sometimes surgery is necessary. In many cases, although not all, modern medical treatment can control the disease completely. As with other eye conditions, early detection and treatment to stop the progress of the disorder are important. Many sources recommend an eye examination every two or three years, especially for those over thirty-five. Developments such as optic disk topography mapping, the laser scanning ophthalmoscope, and color perimetry may assist in early detection and treatment of this disorder.

Injury (Trauma) and Burns

In an era when cures for disease are constantly being developed, injury remains a prominent cause of disability, and eye injuries are no exception. Preventable accidents injure the eyes of approximately 500,000 Americans each year.

Adequate protective eye guards (not just any goggles or eyeglasses) should always be worn during welding, in factories and laboratories, and in any other situation where the danger of burns or other injury exists. Children must be effectively taught not to point arrows, BB guns, or other sharp or dangerous objects toward the face. Farmers using anhydrous ammonia or other chemicals must observe suitable safety precautions since chemical burns are an important cause of injury on farms.

Everyone should know the basic principles of emergency first aid in case an injury does occur. In case of a chemical burn, the eyes should immediately be rinsed thoroughly with water for at least fifteen minutes, and emergency medical care should be secured at once. For other injuries, it is generally best to leave the eye alone or if necessary to cover it with a loose bandage while securing emergency medical care. Hyphema, a hemorrhaging into the anterior chamber, is a sign of serious injury. In such a case blood will be visible through the cornea, and a doctor's help should be sought at once.

In addition to direct damage due to the injury itself, the danger of infection is always present and may be even more serious. Moreover, injury to a single eye may result in pathologic symptoms in the other, a condition called sympathetic ophthalmia, which in the past often meant that total blindness ultimately resulted from injury to a single eye. Although the exact cause of sympathetic ophthalmia is not fully understood, it can usually be prevented today by prompt medical care.

We do not have the power to foresee and prevent all injuries. With proper precautions, however, a great many accidents can be prevented, and the effects of those which do occur can be minimized.

Macular Degeneration

As the inner surface or lining at the back of the eye, the retina serves a function similar to that of the film in a camera. The macula (the proverbial "apple" of the eye) is the part of the retina which forms the center of the "picture" and the sharpest image.

Degeneration or breakdown of the macula may come from many causes. Some cases are hereditary, and others are caused by diseases such as arteriosclerosis. The course of the deterioration may be slow or rapid; however, the patient generally keeps good peripheral vision that is, he can still see well around the edges of his visual field, although his central vision is blurred.

The most common form of this condition occurs in the elderly. In severe form this is called "age-related macular degeneration" (AMD), which occurs mostly in people over fifty years old. While a person with macular degeneration may retain substantial peripheral vision, the ability to read, sew, or drive may be seriously impaired. Magnifiers may help, and a small percentage of cases of a rare form of AMD may be candidates for laser treatment to seal off blood vessels which have grown beneath the retina or to repair the macula's weak spots by removing worn-out tissue and allowing new tissue growth.

Myopia (Nearsightedness)

Myopia results when the eyeball is longer than normal or when there is some change in the eyeball which causes the light rays to be bent abnormally. In either case when a distant object is observed, the parallel light rays passing through the lens into the eye tend to be focused in front of the retina rather than on it, and the result is a fuzzy image. Rays coming from nearby objects are more easily brought into focus, and thus the person is described as "nearsighted."

Most nearsighted people need only wear properly prescribed corrective lenses in order to see normally. These people are said to have simple myopia, which hardly ever causes blindness. A much less common condition, generally hereditary, is degenerative (or progressive) myopia. Persons with this condition may not see well even with glasses. Complications such as retinal detachment, cataracts, or secondary glaucoma may appear with degenerative myopia.

Nystagmus

Continuous jerky, involuntary movements of the eye muscles are called nystagmus. These oscillations may be in any or all directions, but are usually more pronounced in some directions than others and are usually fairly consistent in a particular individual. Sometimes dizziness is associated with nystagmus. The person with nystagmus has reduced visual acuity from being unable to maintain steady fixation on objects.

The exact cause of nystagmus is not fully understood, but it seems to be associated with poor vision in a kind of "vicious cycle" that is, nystagmus makes it more difficult to see, while at the same time poor vision increases the likelihood of nystagmus. Usually this disorder cannot be cured. The individual may find he or she is more comfortable and sees better if the head is tilted or moved slightly to compensate for the involuntary movements, and often this will be done unconsciously. Ophthalmia Neonatorum

The well-chosen name of this disease means "inflammation in the eyes of the newborn." Once a dreaded and very common cause of blindness, it is now preventable by means of modern hygiene and medical care.

Ophthalmia neonatorum appears soon after birth, but it is not hereditary. It is caused by the entrance of bacteria from the mother's birth canal into the baby's eyes. Inflammation appears in the eyelids and cornea and may spread further if not treated. Often the bacteria are those of gonorrhea, but other bacteria (such as staphylococcus) may be the cause as well.

A successful treatment to prevent the development of this disease was discovered in 1880, and this treatment or its equivalent is now required by law. The eyelids of every newborn baby are cleansed, and drops of a silver nitrate solution (sometimes penicillin and other antibiotics) are put into the eyes. Effort is also directed, of course, to promoting good health on the part of the expectant mother so that she will not transmit harmful germs to her baby.

Optic Nerve Atrophy and Optic Nerve Hypoplasia

These two conditions of the optic nerve have become a special concern of those who work with young children. In optic nerve atrophy the problem is damage to or degeneration of the optic nerve. Generally it results from a condition occurring after birth, but it can also be hereditary. Frequently, it results from pressure against the optic nerve and resultant loss of blood supply to it, for example from hydrocephaly or tumors. Without blood the nerve cells die from lack of nourishment.

In optic nerve hydroplasia there is a congenital deficiency of optic nerve fibers, which may exist independently or in association with other disabilities or defects. There is some evidence to suggest that this disorder is related to alcohol or drug use by mothers during their pregnancies, but in many cases the cause is unknown.

Retinal Breaks and Detachment

Sometimes as part of the aging process, sometimes through inheritance of a predisposition toward them, and sometimes from other causes, people may develop breaks in the retina which often lead ultimately to detachment of the various layers of the retina from each other. As with other eye conditions, early detection and treatment of these problems can help prevent blindness.

Sometimes there are not dramatic symptoms associated with breaks in the retina; however, such symptoms as "floaters," haziness or smokiness, or light flashes in the eye may indicate retinal problems. An immediate examination by an eye doctor is recommended.

Treatment for retinal breaks involves use of cryotherapy or lasers. In the former, a freezing probe is applied to the surface of the eye over the point of the retinal break. Only the area around the break is frozen, prompting the growth of scar tissue and reattachment of the separated retinal layers at the edges of the break. Lasers are used to burn tissue selectively so as to reattach the separated retinal layers in a process similar to welding.

Retinitis Pigmentosa

This condition is characterized by degeneration of the retina and the choroid, usually involving an abnormal development of excess pigment. It is hereditary, with a variety of patterns of inheritance and development.

The most common pattern of development is as follows: At approximately age ten or twelve, the youngster begins to experience some difficulty in seeing at night and in poorly lighted areas. His visual field also begins to narrow, although he may not realize this at first. The visual loss is progressive, so that the individual usually becomes legally blind by young adulthood and slowly loses more and more vision thereafter. Many adults with retinitis pigmentosa have a very tiny field of vision in which they see well under a good light but which is so small as to be of little use. Total blindness often results. There may or may not be additional problems, such as cataracts. There is no known treatment.

Retinopathy of Prematurity

Retinopathy of prematurity (ROP), which sometimes advances to a condition known as retrolental fibroplasia (RLF), was thought for a long time to be caused by exposing a newborn (frequently premature) baby to a high concentration of oxygen in an incubator, but there is now less agreement and certainty about the cause. In any case, an abnormal proliferation of blood vessels in the eye occurs, and there may be subsequent development of scar tissue , with bleeding and detachment of the retina. Total blindness may result. Glaucoma, uveitis, cataract, and degenerative lesions of the eye may occur months to years after onset of the RLF stage. Strabismus and myopia are commonly associated with cases in which the blood vessels partially heal. In the large majority of ROP cases (about 80%) abnormal blood vessels heal completely in the first year of life. In other cases scars from incompletely healed ROP result in either mild or severe RLF. In the most severe cases (about 5%) retinal detachment results from formation of scar tissue and the resultant pulling loose of the retina from the normal position in the back of the eye. Recently cryotherapy (freezing part of the retina while it is immature) has drawn some interest as a possible treatment for ROP and is currently under study. The history of ROP/RLF has affected the education of blind children in the United States. Because of the surge of RLF cases in the 1950's, large numbers of blind children reached school age at a time when social sentiment began increasingly to favor the mainstream placement of children with disabilities in the regular classroom. The parents of many RLF children worked successfully to arrange for education in the regular public schools, a trend which continues today.

Strabismus

Commonly called "crossed eyes" when the eyes turn in and "wall eyes" when they turn out, this condition is frequently due to muscle inequality. It can also result from trauma at birth, hereditary factors, or other disorders. It must be treated usually with corrective glasses, medication, surgery, patching of one eye, orthoptic exercises, or some combination of these.

Trachoma

Trachoma is a contagious disease, caused by a virus, which affects the eyelids and the cornea. Though still common in many less developed countries, it has become rare in the United States. It can be prevented through modern sanitation and medical care. The exact pattern of development varies with the individual, but the following symptoms are typical: The eye becomes painful, with burning sensations and oversensitivity to light. Vision is disturbed (or in some advanced cases destroyed) as the cornea becomes more and more opaque. An excess of tears is produced, and often an additional discharge. The eyelids develop muscle spasms; and the eyelashes may be turned inward, further irritating the cornea. In its early stages a case of trachoma can usually be cured by administration of suitable drugs and improved cleanliness and general health. Surgery is helpful in some cases. However, if the case is far advanced or if complications have set in, successful treatment may be impossible. The best way to control this disorder is of course through prevention.

Tumors

Not all tumors and growths are cancer. A "benign" or "nonmalignant" tumor is not much different from the surrounding tissue. It stops growing after it reaches a certain size and does not spread to other areas of the body. Some benign tumors in or near the eye cause no trouble at all. Others, however, do interfere with sight or cause pain. Such a tumor may be removed surgically, and in many cases the eye recovers to become completely normal. A cancerous tumor is much more threatening than a benign tumor. It is greatly different from surrounding tissue, grows rapidly without stopping, and often spreads through the lymph system to sprout growths in other parts of the body. There are many different kinds of cancers which may appear in or around the eye. The most common is melanoma, a malignant spot of color. (Note: Some benign tumors are also called "melanomas.") Retinoblastoma is a cancer of the retina. Usually hereditary, it appears in very young children and is believed to be present at birth in an incipient stage. If any history of this disease is known in a family, it is vital that each baby be examined frequently by an ophthalmologist. The eye specialist can detect the cancer before it is visible to the layman and before it causes any discomfort to the child. If the condition is untreated, the cancer will spread to the brain and elsewhere, causing death. Successful treatment of a tumor is always much more likely if it is begun early. Radiation, drugs, and/or surgery may be used, but frequently complete removal of the eye and any other affected tissue may be necessary.

Usher's Syndrome

An inherited disorder present at birth or early in life, Usher's Syndrome involves hearing loss and a progressive loss of vision caused by retinitis pigmentosa (discussed above). The hearing loss associated with this disorder, which may be from mild to profound, usually does not progress. The retinitis pigmentosa, however, follows the usual pattern of progressive loss first of night vision and gradually of peripheral vision. The incidence of Usher's Syndrome is estimated by the RP Foundation at about 1 in 15,000 to 30,000 births. There is no treatment for this disorder, but there has been some benefit from cochlear implants for those with severe hearing loss.

Uveitis

Inflammation of the uvea, the middle layer of the eye between the sclera and the retina, is called uveitis. Symptoms include light sensitivity, blurring of vision, pain, and redness of the eye. This condition can affect other parts of the eyeþcornea, retina, sclera, for example and may be serious enough to lead to loss of vision. It may come on slowly with little pain but with blurring of vision, or it may appear suddenly, accompanied by pain and redness of the eye.