Visual Impairment

Question 1

What is visual impairment and visual disability?

Answer 1

Visual impairment ranges from a total absence of sight to partial sight loss that cannot be further improved by surgical procedures or wearing spectacles.

Visual disability refers to reduced functionality as a result of sight loss.

Question 2

What is the pupil?

Answer 2

The pupil is, literally, a hole through which light enters the eye.

Question 3

The pupil is surrounded by a solid structure:

Answer 3

The iris, which has the appearance of a coloured disc.

The iris is made of smooth muscle tissue and has pigment cells that bestow the characteristic colour. Contraction and relaxation of the iris muscles alters the size of the pupil, thereby regulating the amount of light that enters the eye.

Question 4

What are sclera and cornea?

Answer 4

Sclera: the ‘white of the eye’, continues with the
Cornea: transparent (for pupil and iris) so that light can pass

Question 5

The cornea lies in front of the iris, separated from it by a transparent fluid called…

Answer 5

aqueous humour

Question 6

The gelatinous fluid with a thick consistency within the eyeball that helps maintain eyeball shape is called…

Answer 6

vitreous humour

Question 7

The transparent structure behind the iris is:

Answer 7

the lense

Question 8

The light falls though the several transparent layers on the..

Answer 8

retina, the innermost layer of the wall of the eye and the layer that contains visual receptor cells

Question 9

Any light that penetrates through the retina is captured very effectively by the next layer of the eye:

Answer 9

the choroid

Question 10

Light can travel through many different substances or…

Answer 10

media (air, water, glass)

Question 11

The process where light passes through a substance is known as…

Answer 11

transmission

Question 12

Not all media transmit light; some can stop light, a process known as

Answer 12

absorption (like curtains on windows absorbing the light)

Question 13

What is light?

Answer 13

A form of electromagnetic radiation.

This is a type of energy that can be described both as a wave, and as a flow of ‘packets’ of energy.

Question 14

What is a vacuum?

Answer 14

A volume in which no atoms or molecules are present and therefore has zero pressure

Question 15

Electromagnetic radiation is often explained as a wave. What does this mean?

Answer 15

A wave is a constantly repeating variation that transfers energy from one position to another.

The distance between one peak and the next is known as the wavelength.

The number of times per second that this variation occurs at a stationary point is known as the frequency of the wave.

The distance that the wave travels in one second is the speed of the wave.

Question 16

The frequency and wavelength of a wave are related by its speed, expressed by the equation:

Answer 16

speed = wavelength × frequency

m s−1) = (m) x (s−1 or Hz

Question 17

What is accommodation?

Answer 17

The process of changing the lens shape to focus on objects at different distances.

Question 18

How are the photoreceptors in the retina called?

Answer 18

Rods: more sensitive to light, only black and white (monochromatic vision), more of them in the retina

Cones: work best under bright daylight conditions, three types, concentrated in the central foveal region

Question 19

What is the macula lutea?

Answer 19

A yellow area in the retina with a radius of about 2 mm, with the small fovea inside that is consisting exclusively of cones packed tightly together.

Question 20

Visual information carried from the eyes by the optic nerve reaches a region of the brain called the…

Answer 20

primary visual cortex, found in the occipital lobe of the brain, which is at the very back of the head

Question 21

Name the components of the eye that correspond to the following parts of the pinhole camera and explain their purpose: (a) pinhole; (b) front wall of the camera box; (c) back wall of the camera box.

Answer 21

(a) The pupil is the opening at the front of the eye and determines the amount of light entering the eye.
(b) The sclera prevents other extraneous light from transmission through the eyeball;
(c) The retina is the light-sensitive tissue onto which an image is projected at the back of the eye.

Question 22

You wave goodbye to relatives on board a train as it leaves the station platform. What is happening to the thickness of the lens in your eye as you watch the train travel into the distance? What structures are involved in producing this effect?

Answer 22

Your lens must accommodate to keep the train in focus. As the train moves further into the distance, the lens of the eye must become progressively thinner to maintain focus. This effect is produced by the ciliary muscles relaxing and allowing the suspensory ligaments to be pulled taut, flattening the curvature of the lens.

Question 23

What black-coloured pigment absorbs any extraneous light within the eye that is not captured by the photoreceptor cells? Why is this important?

Answer 23

The pigment melanin absorbs this light in the choroid and RPE. This is important because otherwise light rays would reflect within the eye creating extra images (or extra parts of an image) on the retina – like the double exposure effect on camera film where a second photograph overlays the first.

Question 24

What is the difference between colour deficiency and colour blind?

Answer 24

Colour deficiency: difficult to distinguish certain colours, most commonly red and green

Colour blind: can see no colour at all

Question 25

How dooes the human eye detect colours?

Answer 25

• Two processes: absorption and reflection
• When visible light hits an object, some of the wavelengths in the light are absorbed by the molecules in the object because they interact with the electrons in these molecules
• The wavelengths that are not absorbed are reflected and thence fall on the eye and are detected
• If all the light hitting an object is absorbed then that object appears black to a viewer. The combination of the reflected frequencies gives the perceived colour of the object. If no wavelengths are absorbed but all are reflected, the object appears white.

Question 26

If you mix together red, green and blue light, then where all three colours overlap white is seen. These three colours are known as the…

Answer 26

additive primary colours

Question 27

What is trichromacy?

Answer 27

By varying the intensity of the overlapping spots (of the additive primary colours), any colour in the visible spectrum can be produced; this phenomenon is known as trichromacy.

Question 28

What happens when light reaches the molecules on the cone cells?

Answer 28

There are three types of cone cells in which the photopigment, retinal, is attached to three different opsin molecules.

When the retinal molecule absorbs light, it changes shape causing a cascade of events resulting in electrical signals which can be processed and transmitted to the brain via the optic nerve.

Question 29

What are photopigments and how do they work?

Answer 29

Photopigments are coloured pigments contained within rods and cones and they absorb particular wavelengths of light.

Rod cells: cis-retinal molecule attached to a protein called opsin

Cone cells: also consist of cis-retinal but each has attached a slightly different opsin molecule

Question 30

What happens when light is absorbed by the cis-retinal molecule?

Answer 30

When light is absorbed by the cis-retinal molecule, the energy imparted by the light moves (excites) the electrons in the double bonds, causing the molecule to change shape at one carbon atom - it is then trans-retinal.

Then change in the shape of the attached protein, opsin, and further cascade of events, generating an electrical signal which is processed by the retinal neuron cells, and thence to the retinal ganglion cells which send the signals to the brain via the optic nerve.

The trans-retinal, which becomes a free molecule during these reactions, finally converts back into the cis- form and reattaches to the opsin, and the whole process can begin again.

Question 31

Why is Vitamin A so important?

Answer 31

Because the cis-retinal in the retina is made from vitamin A

Question 32

When an object appears white, which wavelengths of visible light are absorbed and which are reflected?

Answer 32

When an object appears to a viewer to be white then all the visible light is reflected and none is absorbed.

Question 33

What is visual acuity?

Answer 33

The minimum distance that two lines (or points) must be separated, when viewed at a test distance, to appear as two distinct objects rather than one.

Visual acuity is measured by referring to a Snellen letter chart which consists of rows of letters of decreasing size.

Question 34

The visual acuity is referred to as ‘6/6’. What do these numbers mean?

Answer 34

First number: the furthest distance at which the individual being tested can read a line of letters

Second number: the furthest distance at which a person with ‘normal’ vision can read the same size letters

Question 35

What are refractive errors?

Answer 35

These are defects caused by imperfections in the workings of the front region of the eye (the cornea and lens).

If you have a refractive error, the refraction process, which re-directs light rays to form the image, does not perform optimally.

Question 36

What is myopia?

Answer 36

Short-sightedness (unable to focus clearly on distant objects)

Question 37

Myopia results from one of two causes. Which are they?

Name one environmental factor.

Answer 37

1. The front-to-back distance of the eyeball is too long
   OR
2. The anatomy at the front of the eye refracts light rays to a greater extent than necessary

Nearwork activity in childhood increase an individual’s likelihood of developing myopia.

Question 38

What is the optical power, its unit and how is it measured?

Answer 38

Optical power is the quantity – measured in the unit dioptres (D) – that an optometrist uses to describe the lens performance.

The optical power (in dioptres, D) is the inverse of the focal length (in metres).

optical power (D) = 1 / focal length (m)

Question 39

Spectacles place a further lens in front of the cornea and the natural lens inside the eye. How can the combined optical power of two lenses that are close together be calculated?

Answer 39

By adding together the optical power of each individual lens (called D1 and D2), i.e.

DTOTAL = D1 + D2

Question 40

What is presbyopia and how can it be treated?

Answer 40

Presbyopia is the term used to describe the decreased ability of the eye lens to accommodate and change optical power. This results from loss of elasticity in the structure of the lens.

Bifocal lenses can be used then. These usually have a flat or concave shape but have an additional convex lens attached to the lower portion, in the region of the spectacle lens that one looks through while reading.

Question 41

What is astigmatism?

Answer 41

Astigmatism is when the cornea is not perfectly symmetrical, with curvature of the cornea in the horizontal direction different from that in the vertical direction. As a result, the cornea is shaped as if slightly compressed or stretched in one direction.

Thus vision will appear blurred, but this blurring tends to occur predominantly in one direction (vertically or horizontally).

Question 42

What is PRK and how does it work?

Answer 42

Photorefractive keratectomy (laser surgery)

The cutting away or excising part of the cornea to improve refractive errors, using photons

Question 43

Why is the cellular respiration of the cornea special?

Answer 43

There is no blood supply, it gets the oxygen directly from the air.

The oxygen dissolves in the tears on the surface of the eye and then diffuses into the cornea.

Question 44

What is a polymer?

Answer 44

A polymer is a long-chain molecule made up of many repeating units. Each individual repeat unit is formed from a small monomer molecule, usually containing a double bond, and many thousands of units can link together to make the chain.

Question 45

Why does the lens gradually becomes thicker and less flexible as mature lens cells accumulate over a lifetime?

What is the consequence of this?

Answer 45

The outermost cell layer of the lens is formed of epithelial cells. As there is no blood supply, the lens epithelial cells have a very low level of metabolic activity. Oxygen
and nutrients, such as glucose, are able to diffuse into these cells from the surrounding fluids, but only very slowly. Thus, very slowly, these epithelial cells make new lens cells and becomes thicker.

It becomes more difficult to adjust focus (accommodate) and to form a clear image. The individual is said to have presbyopia

Question 46

What is Cataract?

Answer 46

Cataract, globally the leading cause of blindness, is caused by deterioration of lens proteins. However, cataract surgery is a relatively inexpensive and successful treatment.

Question 47

What is Glaucoma?

Answer 47

Raised intraocular pressure leads to glaucoma, of which there are several types. By definition, they all lead to damage of the optic nerve. Damage to the optic nerve cannot be repaired. Damage can go unnoticed for some time because the brain ‘fills in’ for ‘holes’ in the visual field. Once detected, the progression of the condition can be arrested by appropriate medication. Treatment must be continuous and for life.

Question 48

What is AMD?

Answer 48

In age-related macular degeneration (AMD) the retinal blood vessels deteriorate, leading to death of the rods and cones because their supplies of oxygen and nutrients are inadequate and waste products are not removed. There is no cure for AMD but it progresses slowly, painlessly and rarely leads to complete loss of sight.

The rarer and more serious condition of wet AMD can be arrested by laser surgery to seal leaky blood vessels.

Question 49

How can diabetes lead to eye problems?

Answer 49

Diabetes, a chronic condition where raised blood pressure and blood glucose levels lead to leaking or over-growing blood vessels (diabetic retinopathy), can cause visual impairment with consequences for quality of vision that are similar to those of AMD. The detrimental effects can be reduced using laser treatment.

Question 50

Can eyesight be affected by infections?

Answer 50

Eyesight can be affected not just by structural defects, but also by infections.

The most common eye pathogen is the bacterium Chlamydia trachomatis. It infects millions of people worldwide. C. trachomatis causes conjunctivitis, but when left untreated the infection can progress to trachoma, with trichiasis and blindness ensuing.

Question 51

Distinguish between open-angle and closed-angle glaucoma and say which ethnic groups are particularly at risk from each of these eye diseases.

Answer 51

In both open-angle and closed-angle glaucoma, the drainage angle becomes blocked preventing fluid from leaving the aqueous cavity and leading to raised intraocular pressure, but the cause of the blockage differs in each case.

In open-angled glaucoma the trabeculae silt up, whilst in closed-angle glaucoma the drainage angle closes because the iris and cornea become too close to one another.

Africans are at particular risk from open-angle glaucoma and the Chinese are at particular risk from closed-angle glaucoma.