Longsightedness Health Dictionary

The process by which the refractive power of the lens of the EYE is increased by constriction of the ciliary muscle, producing an increased thickness and curvature of the lens. Rays of light from an object further than 6 metres away are parallel on reaching the eye. These rays are brought to a focus on the retina, mainly by the cornea. If the eye is now directed at an object

closer than 6 metres away, the rays of light from this near object will be diverging by the time they reach the eye. In order to focus these diverging beams of light, the refracting power of the lens must increase. In other words the lens must accommodate.

The lens loses its elasticity with age, and thus becomes less spherical when tension in the zonule relaxes. This results in an increased longsightedness (presbyopia) requiring reading glasses for correction. (See AGEING.)... accommodation

The use of animals in research to provide information about animal biology or, by inference, human physiology or behaviour. Animal research has contributed to the development of drugs, such as vaccines, and surgical techniques, such as transplant surgery. However, because of ethical concerns, alternative practices, such as cell cultures, are now used wherever possible.

animals, diseases from See zoonosis. anisometropia Unequal focusing power in the 2 eyes, usually due to a difference in size and/or shape of the eyes, that causes visual discomfort. For example, one eye may be normal and the other affected by myopia (shortsightedness), hypermetropia (longsightedness), or astigmatism (uneven curvature of the cornea). Glasses or contact lenses correct the problem in most cases. ankle joint The hinge joint between the foot and the leg. The talus (uppermost bone in the foot) fits between the 2 bony protuberances formed by the lower ends of the tibia (the shinbone) and the fibula (the outer bone of the lower leg). Strong ligaments on either side of the ankle joint give it support. The ankle allows for up-anddown movement of the foot.

An ankle sprain is one of the most

common injuries.

It is usually caused by twisting the foot over on to its outside edge, causing overstretching and bruising of the ligaments.

Violent twisting of the ankle can cause a combined fracture and dislocation known as Pott’s fracture.... animal experimentation

Indistinct or fuzzy visual images. Blurred vision, which should not be confused with double vision (diplopia), can occur in 1 eye or both, for episodes of varying lengths of time, and can develop gradually or suddenly. The usual cause of longstanding blurred vision is a refractive error such as astigmatism (unequal curvature of the front of the eye), hypermetropia (longsightedness), or myopia (shortsightedness), all of which can be corrected by glasses or contact lenses. After the age of 40, presbyopia (reduced ability to focus on near objects) becomes more common.

Vision may also be impaired or blurred as a result of damage, disease, or abnormalities of parts of the eye or its connections to the brain.

The most common causes of blurred vision as a result of disease are cataract and retinopathy.... blurred vision

Very thin, shell-like, transparent discs fitted on the cornea of the eye to correct defective vision. Generally, contact lenses are used to correct myopia (shortsightedness) and hypermetropia (longsightedness). There are several types.

Hard plastic lenses give good vision, are long-lasting and durable, inexpensive, and easy to maintain. However, they are sometimes difficult to tolerate and may fall out. Hard gas-permeable lenses are more comfortable because they allow oxygen to pass through to the eye, but are less durable. Soft lenses are the most comfortable because of their high water content. Disposable soft lenses are for single-use only; extended wear lenses are worn for up to a month.

Other types of lenses include rigid, scleral lenses that cover the whole of the front of the eye and are used to disguise disfigurement due to injury or disease; bifocal contact lenses; and interruptus; hormonal methods, including the use of oral contraceptives, implants, and injections (see contraceptives, injectable); intrauterine devices (see IUDs); postcoital methods (see contraception, emergency); or sterilization of the male (see vasectomy) or female (see sterilization, female).

contraception, barrier methods of

The use of a device and/or a chemical to stop sperm reaching an ovum, preventing fertilization and pregnancy. Barrier methods also help prevent the sexual transmission of diseases such as AIDS, genital herpes (see herpes, genital), and viral hepatitis (see hepatitis, viral).

toric contact lenses with an uneven sur- face curvature to correct astigmatism.

Hard plastic contact lenses may cause abrasion of the cornea if they are worn for too long. Soft lens wearers sometimes develop sensitivity of the eyes and lids. Other problems that may occur with any type of contact lens include infections and redness of the eye.... contact lenses

A unit of the power of refraction (“strength”) of a lens; the greater the power, the stronger the lens. Lenses that cause parallel light rays to converge have a positive dioptric number and are used to correct longsightedness

(see hypermetropia). Those that cause divergence have a negative number and are used to correct shortsightedness (see myopia).... dioptre

Optical devices that use lenses to correct focusing errors in the eyes to achieve clear vision. Lenses are made of glass or plastic, and the shape and thickness are chosen during a vision test. Convex lenses are needed for hypermetropia (longsightedness), and concave lenses for myopia (shortsightedness). Tinted lenses protect the eyes from sunlight. (See also bifocal; contact lenses)... glasses

Commonly known as longsightedness, hypermetropia is an error of refraction that initially causes difficulty in seeing near objects and then affects distance vision. Hypermetropia tends to run in families.

Hypermetropia is caused by the eye being too short from front to back, which results in images not being clearly focused on the retina. The error is present from birth, but symptoms generally do not appear until later life because the focusing power of accommodation, which compensates for hypermetropia, declines with age.

Glasses or contact lenses with convex lenses reinforce focusing power.... hypermetropia

An abnormal deviation of 1 eye relative to the other. Many babies have a squint because the mechanism for aligning the eyes has not yet developed. A squint that starts later in childhood is usually due to breakdown of the alignment mechanism. Longsightedness is a common factor. In some cases, the brainsuppresses the image from the deviating eye, leading to amblyopia.

In adults, squint may be a symptom of stroke, diabetes mellitus, multiple sclerosis, hyperthyroidism, or a tumour. A squint in adults causes double vision.

Treatment in children up to 6–7 years may include covering the normal eye with a patch to force the child to use the weak eye. Deviation of the squinting eye may be controlled by glasses and/or surgery. Sudden onset of a squint in adults may have a serious underlying cause and must be investigated promptly.... squint