Vision tests Health Dictionary

See SQUINT.... double vision

There are several tests for pregnancy (see PREGNANCY AND LABOUR) in its early stages, and these can be done on blood or urine; some of the urine tests may be carried out at home. Most tests are based on the detection of HUMAN CHORIONIC GONADOTROPHIN (HCG) in the woman’s urine. They are nearly 100 per cent accurate and may show positive as early as 30 days after the ?rst day of the last normal period.

The haemagglutination inhibition test This, and the subsequent tests to be mentioned, are known as immunological tests. They are based upon the e?ect of the urine from a pregnant woman upon the interaction of red blood cells, which have been sensitised to human gonadotrophin, and anti-gonadotrophin serum. They have the great practical advantage of being performed in a test-tube or even on a slide. Because of their ease and speed of performance, a result can be obtained in two hours.

Enzyme-linked immunosorbent assay (ELISA) This is the basis of many of the pregnancy-testing kits obtainable from pharmacies. It is a highly sensitive antibody test and can detect very low concentrations of human chorionic gonadotrophin. Positive results show up as early as ten days after fertilisation – namely, four days before the ?rst missed period.

Ultrasound The fetal sac can be detected by ULTRASOUND from ?ve weeks, and a fetal echo at around six or seven weeks (see also PRENATAL SCREENING OR DIAGNOSIS).... pregnancy tests

Tests to assess how the LUNGS are functioning. They range from simple spirometry (measuring breathing capacity) to sophisticated physiological assessments.

Static lung volumes and capacities can be measured: these include vital capacity – the maximum volume of air that can be exhaled slowly and completely after a maximum deep breath; forced vital capacity is a similar manoeuvre using maximal forceful exhalation and can be measured along with expiratory ?ow rates using simple spirometry; total lung capacity is the total volume of air in the chest after a deep breath in; functional residual capacity is the volume of air in the lungs at the end of a normal expiration, with all respiratory muscles relaxed.

Dynamic lung volumes and ?ow rates re?ect the state of the airways. The forced expiratory volume (FEV) is the amount of air forcefully exhaled during the ?rst second after a full breath – it normally accounts for over 75 per cent of the vital capacity. Maximal voluntary ventilation is calculated by asking the patient to breathe as deeply and quickly as possible for 12 seconds; this test can be used to check the internal consistency of other tests and the extent of co-operation by the patient, important when assessing possible neuromuscular weakness affecting respiration. There are several other more sophisticated tests which may not be necessary when assessing most patients. Measurement of arterial blood gases is also an important part of any assessment of lung function.... pulmonary function tests

The list of disorders resulting in poor or dim vision is huge. Disturbance of vision can result from an uncorrected refractive error, disease or injury of the cornea, iris, lens, vitreous, retina, choroid or sclera of the EYE. It may also result from disease or injury to the structures comprising the visual pathway from the retina to the occipital cortex (see VISION – Pathway of light from the eye to the brain) and from lesions of the structures around the eye – for example, swollen lids, drooping eyelids. (See EYE, DISORDERS OF.)... vision, disorders of

Refer: ALCOHOLISM, CATARACT, CONJUNCTIVITIS, DIABETES, ECLAMPSIA, GLAUCOMA, IRITIS, MIGRAINE, MULTIPLE SCLEROSIS, RETINITIS, SHOCK. ... blurred vision

Broadly speaking, vision is the ability to see.

Pathway of light from the eye to the brain Light enters the EYE by passing through the transparent cornea, then through the aqueous humour ?lling the anterior chamber. It then passes through the pupil, through the lens and the vitreous to reach the retina. In the retina, the rod and cone photoreceptors detect light and relay messages in the form of electrochemical impulses through the various layers of the retina to the nerve ?bres. The nerve ?bres carry messages via the optic nerve, optic chiasma, optic tract, lateral geniculate body and ?nally the optic radiations to the visual cortex.

Here in the visual cortex these messages are interpreted. It is therefore the visual cortex of the BRAIN that ‘sees’.

Visual acuity Two points will not be seen as two unless they are separated by a minimum distance. This distance is such that the objects are so far apart that the lines joining them to the eye enclose between them (subtend) an angle of at least one minute of a degree. This amount of separation allows the images of the two points to fall on two separate cones (if the light from two points falls on one cone, the two points would be seen as a single point). There are many tests of visual acuity. One of the more common is the Snellen test type. This is made up of many letters of di?erent size. By conventions the chart is placed 6 metres away from the patient. Someone able to see the lowest line at this distance has a visual acuity of 6/4. If they are only able to see the top letter they have 6/60 vision. ‘Normal’ vision is 6/6.

Colour vision ‘White light’ is made up of component colours. These can be separated by a prism, thereby producing a spectrum. The three cardinal colours are red, green, and blue; all other colours can be produced by a varying mixture of these three. Colour vision is a complex subject. The trichromat theory of colour vision suggests that there are three types of cones, each type sensitive to one of the cardinal colours. Colour perception is based on di?erential stimulation of these cone types. The opponent colour theory suggests that each cone type can generate signals of the opposite kind. Output from some cones can collaborate with the output from others or can inhibit the action of other cones. Colour perception results from these various complex interactions.

Defective colour vision may be hereditary or acquired, and can occur in the presence of normal visual acuity. HEREDITARY DEFECTIVE COLOUR VISION is more common in men (7 per cent of males) than women (0·5 per cent of females). Men are affected, but women convey the abnormal gene (see GENES) to their children. It occurs because one or more of the photopigments of the retina are abnormal, or the cones are damaged. Red-green colour defect is the most common. ACQUIRED DEFECTIVE COLOUR VISION is the result of disease of the cones or their connections in the retina, optic nerve or brain – for example, macular disease, optic neuritis. Col-our vision can be impaired but not lost as a result of corneal opaci?cation or cataract formation (see under EYE, DISORDERS OF).

TESTS OF COLOUR VISION These use specially designed numbers made of coloured dots surrounded by dots of confusing colour (e.g. plates).... vision

Loss of the peripheral visual field to the extent that only objects straight ahead can be seen clearly. Tunnel vision is most commonly caused by chronic glaucoma. Retinitis pigmentosa is another possible cause.... tunnel vision

When the eye looks at a speci?c point or object, that point is seen clearly. Other objects within a large area away from this ?xation point can also be seen, but less clearly. The area that can be seen around the ?xation point, without moving the eye, is known as the ?eld of vision. The extent of the ?eld is limited inwards by the nose, above by the brow and below by the cheek. The visual ?eld thus has its greatest extent outwards from the side of the head. The ?eld of vision of each eye overlaps to a large extent so that objects in the centre and towards the inner part of each ?eld are viewed by both eyes together. Because the eyes are set slightly apart, each eye sees objects in this overlapping part of the ?eld slightly di?erently. It is because of this slight di?erence that objects can be perceived as three-dimensional.

Defects in the visual ?eld (scotomas) can be produced by a variety of disorders. Certain of these produce speci?c ?eld defects. For example, GLAUCOMA, some types of brain damage and some TOXINS can produce speci?c defects in the visual ?eld. This type of ?eld defect may be very useful in diagnosing a particular disorder. The blind spot is that part of the visual ?eld corresponding to the optic disc. There are no rods nor cones on the optic disc and therefore no light perception from this area. The blind spot can be found temporal (i.e. on the outer side) of the ?xation point. (See also EYE.)... vision, field of

Tests to screen for and diagnose bleeding disorders, usually resulting from deficiencies or abnormalities of blood coagulation factors or of platelets (see blood clotting). Tests are also used to monitor treatment with anticoagulant drugs.... blood-clotting tests

Tests performed to investigate kidney disorders.

Urinalysis is a simple test in which a urine sample is examined under a microscope for blood cells, pus cells, and casts (cells and mucous material that accumulate in the tubules of the kidneys and pass into the urine).

Urine may be tested for substances, such as proteins, that leak into the urine when the kidneys are damaged.

Kidney function can be assessed by measuring the concentration in the blood of substances, such as urea and creatinine, that the kidneys normally excrete.

Kidney function may also be assessed by kidney imaging techniques.... kidney function tests

Tests of blood chemistry that can detect changes in the way the liver is making new substances and breaking down and/or excreting old ones.

The tests can also show whether liver cells are healthy or being damaged.... liver function tests

Analysis of a sample of blood to give information on its cells and proteins and any of the chemicals, antigens, antibodies, and gases that it carries. Such tests can be used to check on the health of major organs, as well as on respiratory function, hormonal balance, the immune system, and metabolism. Blood tests may look at numbers, shape, size, and appearance of blood cells and assess the function of clotting factors. The most important tests are blood count and blood group tests if transfusion is needed. Biochemical tests measure chemicals in the blood (see acid–base balance; kidney function tests; liver function tests). Microbiological tests (see immunoassay) look for microorganisms that are in the blood, as

in septicaemia. Microbiology also looks for antibodies in the blood, which may confirm immunity to an infection. blood transfusion The infusion of large volumes of blood or blood products directly into the bloodstream to remedy severe blood loss or to correct chronic anaemia. In an exchange transfusion, nearly all of the recipient’s blood is replaced by donor blood. Before a transfusion, a sample of the recipient’s blood is taken to identify the blood groups, and it is matched with suitable donor blood. The donor blood is transfused into an arm vein through a plastic cannula. Usually, each unit (about 500 ml) of blood is given over 1–4 hours; in an emergency, 500 ml may be given in a couple of minutes. The blood pressure, temperature, and pulse are monitored during the procedure.

If mismatched blood is accidentally introduced into the circulation, antibodies in the recipient’s blood may cause donor cells to burst, leading to shock or kidney failure. Less severe reactions can produce fever, chills, or a rash. Reactions can also occur as a result of an allergy to transfused blood components. All

blood used for transfusion is carefully screened for a number of infectious agents, including HIV (the AIDS virus) and hepatitis B and hepatitis C.

In elderly or severely anaemic patients, transfusion can overload the circulation, leading to heart failure.

In patients with chronic anaemia who need regular transfusion over many years, excess iron may accumulate (haemosiderosis) and damage organs such as the heart, liver, and pancreas.

Treatment with desferrioxamine to remove excess iron may be needed.... blood tests

See pulmonary function tests.... lung function tests

Questionnaires designed to define various personality traits or types. Tests may be designed to detect psychiatric symptoms, underlying personality traits, how outgoing or reserved a person is, and predisposition to developing neurotic illness.... personality tests

See pulmonary function tests.... respiratory function tests

Procedures for determining the body’s reaction to various substances by injecting a small quantity of the substance under the skin or by applying it to the skin (usually on patches).

Patch tests are used in the diagnosis of contact allergic dermatitis.

They can also be used to test immunity to certain infectious diseases (such as in the tuberculin test).... skin tests

A group of blood tests used to evaluate the function of the thyroid gland and to detect or confirm any thyroid disorder. The thyroid hormones T3 and T4 are measured, as well as thyroid-stimulating hormone (TSH), the pituitary gland hormone that stimulates the thyroid gland.... thyroid-function tests

The ability to see different parts of the colour spectrum. Light perceived by the human eye consists of electromagnetic radiation (energy waves) with a spectrum of different wavelengths

between about 400 and 700 nanometres. Different wavelengths produce sensations of violet, indigo, blue, green, yellow, orange, and red when they fall on the retina and stimulate nerve signals, which are processed in the brain.

As light falls on the retina, it strikes light-sensitive cells called rods and cones. The rods can detect all visible light, but only the cones can distinguish colour. There are 3 types of cones: red-sensitive, blue-sensitive, and green-sensitive. Each responds more strongly to a particular part of the light spectrum. Because the cones are most concentrated in a central area of the retina called the fovea, colour vision is most accurate for objects viewed directly and is poor at the edges of vision. When light hits a cone, it causes the cone to emit an electrical signal, which passes to the brain via the optic nerve. Colour perception requires a minimum level of light, below which everything is seen as shades of grey. (See also colour vision deficiency; eye; perception; vision.)... colour vision

Any abnormality in colour vision that causes difficulty distinguishing between certain colours. Total absence of colour vision (monochromatism) is rare. The most common types of colour vision deficiency are reduced discrimination of red and green. Most cases of red and green colour vision deficiency are caused by defects in the light-sensitive cells in the retina. These defects are usually inherited, although occasionally defects are caused by retinal or optic nerve diseases or injury. The inherited defects tend to be sex-linked (see genetic disorders), which means that the majority of sufferers are male. A person with a severe green deficiency has difficulty distinguishing oranges, greens, browns, and pale reds. In severe red deficiency, all reds appear dull. A much rarer deficiency in which blue cannot be distinguished may be inherited or may be due to degeneration of the retina or optic nerve.... colour vision deficiency

Tests carried out to assess hearing. Hearing tests are performed as part of a routine assessment of child development and when hearing impairment is suspected. The tests are sometimes included in a general medical examination. Hearing tests may also be used to identify the cause of tinnitus or dizziness.

An audiometer (an electrical instrument) is used to test an individual’s ability to hear sounds at different frequencies and volumes. The lowest level at which a person can hear and repeat words (the speech reception threshold) is tested, as is the ability to hear words clearly (speech discrimination). The type of hearing loss (see deafness) is determined by holding a tuning fork to different parts of the ear.... hearing tests

Tests designed to provide an estimate of a person’s mental abilities. The most widely used are Wechsler tests of which there are 2 basic types: the Wechsler Adult Intelligence Scale (WAIS) and the Wechsler Intelligence Scale for Children (WISC). Each is divided into verbal (concerned with language skills) and performance sections, including measures of constructional ability and visual-spatial and perceptual ability (interpretation of shapes). Other tests include the Stanford–Binet test, which is used mainly as a measure of scholastic ability.

In most intelligence tests, scoring is based on mental age (MA) in relation to chronological age (CA). The intelligence quotient (IQ) is MA divided by CA, multiplied by 100. The tests are devised to ensure that 3 in 4 people have an IQ between 80 and 120. They are standardized so that the score indicates the same relative ability at different age levels.

Intelligence tests may be used to assess school or job aptitude. However, they have been criticized for their alleged bias regarding gender and race.... intelligence tests

Tests may be performed to investigate the cause of symptoms and establish a diagnosis, to monitor the course of a disease, or to assess response to treatment. A medical testing programme carried out on apparently healthy people to find disease at an early stage is known as screening.

The accuracy of a test is based on its sensitivity (ability to correctly identify diseased subjects), specificity (ability to correctly identify healthy subjects), and predictive value.

The predictive value is determined by a mathematical formula that involves the number of accurate test results and the total number of tests performed.

The best tests have both high specificity and high sensitivity, and therefore high predictive value.... tests, medical

Skin tests used to determine whether or not a person has been exposed to the bacterium that causes tuberculosis. Tuberculin tests are carried out for diagnosis of tuberculosis, and before BCG vaccination. A small amount of tuberculin (purified protein from the bacteria) is injected into the skin. A few days later, the skin reaction, if any, is noted. A reaction indicates previous exposure.... tuberculin tests

Hearing tests carried out to diagnose conductive deafness. In the Weber test, a vibrating tuning fork is held against the forehead. If there is conductive hearing loss, the sound seems louder in the affected ear. In the Rinne test, a vibrating tuning fork is held first near the ear, and then against the bone behind it. If it sounds louder when held against the bone, there is conductive hearing loss.... tuning fork tests

See urinalysis.... urine tests

the ability to focus both eyes on an object at the same time, so that a person sees one image of the object he is looking at. It is not inborn, but acquired during the first few months of life. Binocular vision enables judgment of distance and perception of depth. See also stereoscopic vision.... binocular vision

Inability to see. This may develop slowly or suddenly and may be temporary or permanent, depending on the cause. Vision loss may affect 1 or both eyes. It can cause complete blindness or may affect only peripheral, or only central, vision.

Progressive loss of visual clarity is common with advancing age and may be due to a number of disorders (see vision, disorders of).

Sudden loss of vision may be caused by disorders such as hyphaema, severe uveitis, vitreous haemorrhage, or retinal haemorrhage.

Optic neuritis can reduce vision in 1 eye.

Damage to the nerve connections between the eyes and brain, or to the visual area of the brain, can cause loss of peripheral vision and may be a result of embolism, ischaemia, tumour, inflammation, or injury.... vision, loss of

where clinical *negligence is claimed, tests used to determine the standard of care owed by professionals to those whom they serve, e.g. the standards of care provided to patients by doctors. The 1957 case of Bolam v Friern Hospital Management Committee established that if a doctor acts in accordance with a responsible body of medical opinion, he or she will not be negligent. In 1997 this standard of care test was amended by the case of Bolitho v City and Hackney Health Authority, which requires the doctor’s behaviour to satisfy the judgment not only of responsible medical opinion but also of a court’s own independent logical analysis.... bolam and bolitho tests

(DSTs) tests based on the principle that appropriate doses of *dexamethasone can suppress the output of cortisol from the adrenal glands in the normal state and that this ability is reduced or lost in *Cushing’s syndrome. In the overnight DST 1 mg of dexamethasone is administered at midnight and the serum cortisol level is measured at 9.00 am the next morning. Failure to suppress cortisol output may indicate Cushing’s syndrome but also occurs in patients with obesity and depressive illness. In the low-dose DST (0.5 mg dexamethasone every 6 hours for 48 hours), cortisol suppression occurs in patients with obesity and depression but not in those with Cushing’s syndrome. In the high-dose DST (2 mg dexamethasone every 6 hours for 48 hours), cortisol is suppressed in patients with Cushing’s disease (in which excess amounts of ACTH are secreted by the pituitary gland) but not in those with Cushing’s syndrome due to other causes. Although the low- and high-dose tests are unreliable, all three tests should be performed to aid the diagnosis of Cushing’s syndrome.... dexamethasone suppression tests

see visual field.... field of vision

tests for assessing the function of the kidneys. These include measurements of the specific gravity of urine, creatinine *clearance time, and blood urea levels; intravenous urography; and renal angiography.... renal function tests

(stereopsis) perception of the shape, depth, and distance of an object as a result of having *binocular vision. The brain receives two distinct images from the eyes, which it interprets as a single three-dimensional image.... stereoscopic vision

Standard Tests for Young Children and Retardates: tests to detect visual problems in children between the ages of six months and five years. They consist of a series of standardized balls, toys, or letters. The tests were developed by the paediatrician Mary Sheridan.... stycar tests

tests used to assess the ability of the adrenal cortex to produce cortisol. Serum cortisol is measured before and then 30 minutes (or 5 hours) after an intramuscular injection of 250 ?g (or 1 mg) tetracosactide (Synacthen), an analogue of *ACTH. The adrenal glands are considered to be inadequate if there is a low baseline concentration of cortisol or the rise is less than a certain predefined amount.... synacthen tests