8.5 - Visual disorders Flashcards
(16 cards)
1
Q
Eye structure
A
Outer layer
Sclera
- Dense white layer that surrounds the eye and protects it
- Transparent part of the sclera is called the conjunctiva
Cornea
- Clear jelly-like structure at the front of the eye
- Protects front of the eye and focuses light that enters the eye
Middle area
Choroid layer
- Membrane layer containing blood vessels that nourish the eye
- Prevents light from scattering by absorbing light
Pigmented iris
- Suspended from the choroid layer
- A muscular sturcture that controls the amount of light that enters the eye by changing the size of the pupil (dilation or contraction)
Pupil
- The hole in the centre of the iris
Cilliary muscles
- Located behind the iris
- Cilliary muscles move the lens, which is responsible for the fine focus of light onto the back of the eye
Aqueous humour
- Clear water liquid that that transmits light and maintains pressure of the eye
- Found in front of the lens
Vitreous humour
- Jelly-like substance that is transparent, allows the light to pass through it and maintains the shape of the eye
- Found behind the lens, in the centre of the eye
Inner eye
Retina
- Detects light with photosensitive cells
Fovea
- Area at the start of the optic nerve
- Fovea centralis is the area of sharpest vision
Optic nerve
- Nerve at the back of the eye that carries nerve impulses from the retina to the visual cortex in the brain
Blind spot
- Point where the optic nerve leaves the retina
- So called because there are no photoreceptors at that point, so an image cannot be seen from there
2
Q
Photoreceptor cells
A
- Rods and cones
- Visual pigments present in each of these photoreceptor cells change light energy into electrochemical impulses to be sent to the brain for processing
- All pigments are composed of retinal (derivative of vitamin A) and opsin (protein). The type of opsin depends on the type of pigment
- Rods contain rhodopsin and are responsible for vision in low-light environments. Rods are distributed all over the retina
- Detection of light by pigments stimulates the formation of an electrochemical impulse and the splitting of the retinal and opsin. The pigment is only ready to receive more light energy when the two parts of the molecule have recombined
- Each cone contains one of three iodopsin pigments (red, blue, green). Cones are responsible for colour vision, and the multitude of colours detected is due to the combination of each of the cones
- Cones are found in clumps, with fewer on the edges of the retina
3
Q
Rods
A
- Contains many optic discs
- Rhodopsin proteins embedded throughout
- Much more abundant than cones, average retina has around 120 million rods
- Mostly found on the periphery of the eye
- Have a slow recovery time after the action potential has been fired
4
Q
Cone structure
A
- Has stacked optic discs, like rods
- Photopsin proteins embedded throughout
- Less abundant than cones, average retina has around 6 million cones
- Concentrated near the fovea
- Produce colour vision as each rod contains either red, blue, or green iodopsin pigment
- Red cones make up ~60% of all cones in the eye, green makes up ~30%, and blue makes up ~10%
- Have a fast recovery time: they can rapidly adapt to changes in illumination
5
Q
Detection of light energy/Refraction
A
- Human vision can only detect light in the visible light range, but other animals can see more, eg. snakes can detect infrared light
- Refraction: as light travels through a medium, it travells in a straight line. However, as it moved from one kind of medium to another that has a different optical density, the light path bends (refracts)
- Refraction only occurs at the boundary between the two mediums
- As light travels through the eye, each part of the eye has a different density, causing light to refract. The four refractive media in the eye are the cornea, aqueous humour, lens, and vitreous humour
- Light bends the most at the cornea-air boundary
- The result of these refractions is to focus light on the retina
6
Q
Accommodation
A
- The ability of the eye to change the shape of the lens and focus on objects whether they are near or far
- Achieved by the contraction of the ciliary muscles that control the movement of the lens
- When the ciliary muscles contract, the suspensory ligaments that hold the lens are released, and the lens becomes more rounded. This is fully accommodated and maximum refraction of light, causing near objects to be in focus
- When the ciliary muscles relax, the suspensory ligmaents are tense and the lens is flattened. Vision would be focused on far objects and the refractory power would be at a minimum
7
Q
Disorders that cause visual impairment
Hyperopia
A
- Far-sightedness
- Can view distant objects, but has difficulty seeing objects close to the eye
- Most common in old age due to the weakening of the cilliary muscles that control the tension of the suspensory ligaments, making the lens less flexible
Causes
- The eyeball is too rounded, causing light to be focused behind the retina
- The lens is too flat and unable to achieve the required convexity to properly focus light
- Refractive power of the cornea is too great for the shape of the eye
8
Q
Disorders that cause visual impairment
Myopia
A
- AKA shortsightedness
- Can see near objects, but not distant objects
- Light from distant objects is bent or refracted more than is necessary, causing the images of distant objects to be focused in front of the retina
- Most common during childhood or adolescence
Causes
- Eyeball may be too elongated or the cornea bulges out too much
- If the cornea bulges more than its customary curvature, light will be refracted more than usual, causing images of distant objects to be focused in front of the retina instead of on it. On the retinal surface, the image is hence not focused.
- Lens may not flatten enough when the ciliary muscles contract
9
Q
Disorders that cause visual impairment
Cataracts
A
- Lens of the eye is made up mostly of water and proteins, which are arranged to allow light to pass freely
- Sometimes proteins may clump togehter, clouding small areas of the lens. This obstructs the light from reaching the retina and causing vision problems
- A cataract is a progressive clouding of the lens and occurs over a prolonged period of time. Old age and diabetes can increase the risk or severity of cataract formation
10
Q
Disorders that cause visual impairment
Macular degeneration
A
- Disease that causes the degeneration of the cells beneath the retina, called the retinal pigment epithelium
- Degeneration of retinal pigment epithelial cells prevents light from being successfully focused on this area, leading to a loss of vision
- Affects central vision asosicated with the macula, but peripheral vision is unaffected
- Macular degeneration is responsible for 50% of all cases of blindness amd is the leading cause of legal blindness in Australia
- Risk of macular degeneration can be reduced through regular exercise, reducing UV exposure, maintaining a healthy, balanced diet, and not smoking.
11
Q
Technologies that assist with visual impairment
Glasses and contact lenses
A
Overview
- The most common way to correct eye conditions that impair focusing ability
- To correct hyperopia, a convex lens is placed in front of the eyes to help converge light rays further. This causes light to refract to a greater extent prior to entering the eye, therefore enabling light to focus correctly on the retinal surface
- To correct myopia, a concave lens is placed in front of the eye. This causes light rays to diverge just enough so that it will focus on the retina when it enters the eye
Benefits:
- Non-invasive, non-surgical, and mostly affordable
- Wearing glasses or contact lenses alleviates the eye strain that comes with squinting. This can help reduce vision degradation in the long term and prevent the worsening of the condition
- Can be used for a variety of focusing-related conditions, including hyperopia, myopia, presbyopia, and astigmatism
Disadvantages:
- Prescription lenses can be expensive
- Wearing glasses can be inconvenient for individuals who are often active, e.g. school-age children involved in sports can easily lose or damage their glasses
- Glasses or contact lenses are not effective in treating more serious vision impairment conditions, such as cataracts or macular degeneration
- This technology does not cure that eye damage that causes hyperopia/myopia, only treats the symptoms.
12
Q
Technologies that assist with visual impairment
Refractive laser surgery
A
Overview
- Refractive laser surgery can be used to change the shape of the cornea permanently, correcting the degree of light convergence or divergence by the eye.
- In this surgery, a flap of the cornea is cut and lifted, and a laser beamis used to reshape the rest of the cornea. The flap is then replaced.
- Modern laser technology uses a UV-argon laser to break intermolecular bonds in the corneal tissue, and thin layers are removed from the cornea, reshaping its curvature.
- The process is controlled by a computer for greater accuracy
Benefits:
- Refractive laser surgery treats the cause of hyperopia or myopia, unlike wearing glasses or contact lenses. The results are permanent and not contingent upon the wearing of focusing devices
- Recent technological advances, such as the use of computers in the surgery, and modern laser technology, has made the procedure safer and more efficient
Disadvantages:
- Any surgical operation carries with it the rist of complications. Corneal tissue may become permanently damaged, causing greater vision impairment than before
- Surgery is expensive and not accessible for individuals who have low income or restricted access to adequate medical services
13
Q
Technologies that assist with visual impairment
Phacoemulsification
A
Overview:
- A common method of cataract surgery
- A small incision is made where the cornea meets the sclera, and a probe that emits high frequency vibrations is inserted
- The vibrations break the lens into pieces, allowing it to be suctioned out and replaced with an artificial intraocular lens (IOL)
- The incision is usually so small that no stitching is required, leaving the rest of the eye intact
Benefits:
- Phacoemulsification permanently treats cataracts and prevents future cataracts, as the IOL cannot grow a cataract.
- The procedure has a high success rate, with millions of operations taking place each year, meaning it is a tried-and-tested method of treating cataracts
Disadvantages:
- Risk of surgical complications. The probe may be inserted incorrectly, causing other parts of the eye to be damaged by the high frequency vibrations.
- While cataracts cannot regrow after phacoemulsification, there is a risk of developing “secondary cataract”: a condition wherein the lens capsule becomes cloudy, leading to blurry vision
14
Q
Technologies that assist with visual impairment
Bionic eye
A
Overview
- A technology currently being developed by Bionic Vision Technologies to restore vision to people with retinitis pigmentosa, a condition in which the rods and cone cells degenerate gradually, and age-related macular degeneration
- The bionic eye is composed of a camera attached to a pair of glasses, which transmits high frequency radio signals to a microchip implanted in the eye
- Electrodes on the chip convert these signals into electrical impulses to stimulate the conversion of light rays into nerve impulses by cells in the retina. These impulses are sent to the brain via the optic nerve
Benefits:
- Can restore sight to people who have extensive eye impairment by bypassing most of the eye, relying only on the ability of the retina and optic nerve to still function normally
Disadvantages:
- Very expensive. The Argus II Retinal Prosthesis System, a bionic eye, costs $182,000 AUD, making it only available to few individuals who have the means to afford it. Hence, it is not accessible to the majority of the world’s severe or profoundly blind population
- Risk of surgical complications when implanting the chip into the eye
15
Q
Presbyopia
A
- Lens become inflexible, making accommodation difficult
- Common as individuals age, and can be alleviated with bifocal or mulitfocal lenses
16
Q
Astimagtism
A
- Condition where the lens or the cornea are irregularly shaped, causing blurry vision in both near and far vision
- Curvature of the lenses used in glasses or contact lenses can be varied to counteract the asymmetry in the affected eye/s
