The Human Eye and Its Defects

Partial or Total Blindness

Accidents or trauma to the eyesAdvancing ageCataractsDiabetesGlaucomaHereditary conditionMacular degeneration

Blindness has many causes. The most common include:

StrokeTumoursOptic neuritis, or inflammation of the optic nerve

The Human Eye And It's Defects38 Introduction Human Eye

The Human Eye And It's Defects4

Crossed EyesThe Human Eye And It's Defects5Diagnosing Visual Disturbances

If any of the visual disturbances begins suddenly and unexpectedly, see a doctor as soon as possible. In some cases, the visual disturbance may be the result of a minor problem, but many serious conditions, such as aneurysm, glaucoma, and brain tumors first cause vision problems.Your doctor will likely perform several diagnostic tests to determine the cause of your visual disturbance. These tests might include a physical exam, eye exam, and blood tests. Imaging tests, such as magnetic resonance imaging (MRI) or a computed tomography (CT) scan may also be used to confirm a problem or further investigate a suspected condition.

The Human Eye And It's Defects44Treating Visual Disturbances

The first step in treating a visual disturbance is figuring out the underlying problem that is causing it. Once you and your doctor have discovered the problem, you can develop a plan for treatment. In some cases, the disturbance will go away naturallyblurry vision caused by a headache will usually resolve when the headache recedes. However, your doctor may wish to prescribe medicine to prevent future headaches or medicine you can take when a headache begins causing visual complications.

There are several common treatments for visual disturbances. Medication can treat underlying conditions so they no longer cause symptoms. Dietary changes can prevent visual disturbances in people with uncontrolled diabetes. Glasses, contact lenses, or magnifying devices may be able to correct vision disturbances that cannot be corrected with another treatment. If necessary, surgery can help relieve or repair damaged nerves and muscles.

The Human Eye And It's Defects45Index of Crossed EyesIntroduction What Causes Crossed Eyes?Who Is at Risk for Crossed Eyes?Symptoms and Signs of Crossed EyesHow Are Crossed Eyes Diagnosed?How Are Crossed Eyes Treated?What Is the Long-Term Outlook for Crossed Eyes?

The Human Eye And It's Defects6What Causes Crossed Eyes?Crossed eyes occur when the numerous muscles around the eyes do not work in conjunction because some are weaker than others. When the brain receives a different visual message from each eye, it ignores the one coming from the weaker eye. Over time, you may lose vision in your weaker eye if the condition is not corrected.

Crossed eyes can also occur later in life and are usually triggered by physical disorders such as a brain tumor or an eye injury. If you have a lazy eye or are farsighted, it is also possible to develop crossed eyes as an adult because your eyes must strain in order to focus on objects. People with brain or nervous system disorders such as cerebral palsy are more likely to have strabismus.

The Human Eye And It's Defects8Who Is at Risk for Crossed Eyes?

You are at risk for strabismus if:your family members have the disorderyou have a brain disorder or brain tumoryour retina is damagedyou have suffered a stroke or brain injuryyou are diabeticyou have a lazy eye, are farsighted, or have vision loss

The Human Eye And It's Defects9Symptoms and Signs of Crossed EyesSymptoms may be constant or appear only when you (or your child) are tired or not feeling well. Your eyes might point inward or outward or focus in different directions.It is common for newborn babies to experience strabismus, but if it persists beyond 3 months of age, it is best to see a doctor. In addition to having crossed eyes, you might also:have impaired visionlose depth perceptionhave double vision

The Human Eye And It's Defects10How Are Crossed Eyes Diagnosed?For a strabismus diagnosis, you will visit an eye doctor who will perform a series of tests to check the health of your eyes, including a:corneal light reflex test to check for crossed eyesvisual acuity test to determine how well you can read from a distancecover/uncover test to measure deviation and eye movementretina exam to examine the back of your eyesEarly diagnosis is important for preventing vision loss. In young children, it is best to have an eye exam before age 3. If other physical symptoms appear along with crossed eyes, your doctor may examine your brain and nervous system for the presence of other conditions, such as cerebral palsy or Guillain-Barre syndrome.

The Human Eye And It's Defects11How Are Crossed Eyes Treated?

Treatment for strabismus depends on its severity and cause. If you have developed a lazy eye, you might need to wear a patch over your stronger eye to force the muscles of your weaker eye to work harder. Other common treatments include:eyeglasses, particularly in the case of farsightednesssurgery to strengthen certain eye muscles, particularly if glasses have not corrected the conditioneye exerciseseye drops to blur vision in the better eye According to Dr. James McDonnell from Loyola University, adults do not always need to have surgery to correct this condition. Botox, commonly used in cosmetic procedures, can also be applied to weaken the stronger eye muscle, giving the weaker muscle a chance to gain strength.In some cases, crossed eye symptoms may come and go, making it necessary to wear glasses and frequently do eye exercises to align your eyes. The condition sometimes occurs because of vision deterioration; in this case, you must have your vision loss corrected in order for strabismus surgery to be effective.

The Human Eye And It's Defects12What Is the Long-Term Outlook for Crossed Eyes?

In many cases, crossed eyes can be corrected with treatment, though you may still have vision problems after surgery and need to wear glasses. Seek treatment for the problem right away because it can sometimes lead to vision loss. The condition may also recur, so it is important to monitor it regularly.

The Human Eye And It's Defects13Night blindnessThe Human Eye And It's Defects14Index of Night blindnessIntroductionCauses of Night BlindnessSymptoms of Night BlindnessTreating Night BlindnessPreventing Night BlindnessThe Human Eye And It's Defects15Treating CatractsClouded portions of your eyes lens are called cataracts. Cataracts can be removed through surgery. Your surgeon will replace your cloudy lens with a clear, artificial lens. Your night blindness will improve significantly after surgery if cataracts are the underlying cause.

Treating Vitamin A DeficiencyIf your vitamin A levels are low, your doctor might recommend vitamin supplements. Take the supplements exactly as directed. Most people in the developed world do not suffer from vitamin A deficiency because they have access to proper nutrition.

The Human Eye And It's Defects20Treating Night Blindness Causes by a Genetic DefectGenetic conditions that cause night blindness, such as retinitis pigmentosa, are not treatable. The genetic defect that causes pigment to build up in the retina does not respond to corrective lenses or surgery. People suffering from this form of night blindness should avoid driving at night.

The Human Eye And It's Defects21Prevalence of night blindness and number of individuals affected among preschooll-age children and pregnant women in populations of countries at risk of vitamin A deficiency 1995-2005, globally and by WHO region

WHO regional estimates indicate that the highest proportion of preschool-age children affected by night blindness, 2.0%, is in Africa, a value that is four times of that estimated in South-East Asia (0.5%). This also means that Africa has the greatest number of preschool-age children affected with night blindness (2.55 million), and corresponds to almost half of the children affected globally. A comparable and high proportion of pregnant women affected by night blindness are in Africa (9.8%) and South-East Asia (9.9%), each of which is estimated to have over 3 million pregnant women affected, or one third of the pregnant women affected globally.

aPopulation subgroups: Preschool-age children (