Lab - Special Senses Practical: Vision Flashcards
What is Visual Acuity ?
Visual Acuity is the ability to resolve (see) a gap between two objects
How is visual acuity recorded?
Distance visual is recorded as distance tested at over the line seen: distance of patient/line seen
E.g A person with 20/50 vision can clearly see something 20 feet away that a person with normal vision can see clearly from a distance of 50 feet
Visual acuity is recorded as “6/X” in which the 6 indicated the 6 meter the patient is away from the chart and the X is what line of the chart that the smallest text that was seen
Normal healthy vision is 6/6 meaning that the subject is able to read 6 meters the line that is labelled number 6 on the chart.
If only the top line was seen this would be recorded as 6/60 as the top line is labelled as ‘60’ meaning someone with normal vision could see this line from 60 meters away. If they were standing 60 meters away this is all that a normal sight person would see and their vision would be 60/60. Overall ratio is 1.0 or 100%
Near vision- Near vision is recorded as ‘Nx. “x” is the number of text that can be read at 40cm. This is considered normal reading distance.
What are the different tests/charts used to test for visual acuity ?
Usually test vision with a standard full size Snellen chart from 6 meters away.
Arclight cloth chart letters can be used which are 50% smaller than the Snellen chart so you can stand 50% the distance which is 3m away
Near visual acuity - There are many near vision charts (we used Arclight near chart). This is considered normal reading distance and has been calibrated for visual acuity to be tested 1). unaided, 2). with prescribed glasses if worn or 3). with a pinhole
In babies (up to age of 24 months) and individuals of any age with learning difficulties we use black and white stripe test (gratings) - Lea paddles
In children between the ages of 2 and 4 years can be assessed by ‘matching pictures’ - Lea symbols
Children with higher level language and understanding can be tested like any literate adult using Snelllen chart.
Illiterate E tests are commonly used in rural locations in low resource countries
What must be remembered when taking results from Arclight cloth chart?
The top line is still labelled ‘60’ line so you document the vision as 6/60 and 6/36 despite testing at 3 meters
Why do we get patients to look at text through a pinhole?
A pinhole overcomes the optical errors of an eye and can improve unaided vision if a refractive error is present
What is Myopia ?
Nearsightedness (myopia) is a common vision condition in which near objects appear clear, but objects farther away look blurry. It occurs when the shape of the eye — or the shape of certain parts of the eye — causes light rays to bend (refract) inaccurately.
How do you test Visual Acuity ?
Test unaided, with glasses or contact lenses if worn and with a pinhole
If you tested at a distance of 6 meters then record the visual acuity as “6/x” in which 6 indicates the distance at which the test is performed and “x” denotes the number of the line of the smallest test that was seen
Near vision is simply recorded as “Nx’ where ‘x’ is the number of the paragraph of text that can be read at 40cm. This is considered a normal reading distance and is the distance the reading card is calibrated for.
What is the criteria to be registered as severely SI (Sight Impaired/blind)?
Must fall into one of the following categories while wearing any glasses for contact lenses that one may need;
- Visual acuity of less than 3/60 with a full visual field
- Visual acuity between 3/60 and 6/60 with a severe reduction of field of vision such as tunnel vision
- Visual acuity of 6/60 or above but with very reduced field of vision, especially if a lot of sight is missing in the lower part of the field
What is the criteria to be registered as SI (Partially Sighted)?
Must fall into one of the following categories while wearing any glasses for contact lenses that one may need;
- Visual acuity of 3/60 to 6/60 with a full field of vision
- Visual acuity of up to 6/24 with a moderate reduction of field of vision or with a central part of vision that is cloudy or blurry
- Visual acuity of up to 6/18 if a large part of your field vision for example a whole half of your vision is missing or a lot of your peripheral vision is missing
What are the vision requirements for driving ?
You must be able to read (with glasses for contact lenses if necessary) a car number plate made after September 2001 from 20 meters away
You also must meet the eyesight standard for driving having a visual acuity of at least 0.5(6/12) measures on the Snellen chart (with glasses or contact lenses, if necessary) using both eyes together or, if you have sight in one eye only, in that eye
What are the 2 main types of colour vision impairment ?
- Congenital red/green colour confusion
- Acquired colour vision dysfunction
What is Congenital red/green colour confusion ?
Congenital red/green colour confusion is the most common from of colour vision impairment affection 8% of the male population (its rare in females <1%)
Important genes for colour vision found on X chromosome and special forms of colour vision such as red/green colour confusion are inherited through genes on the X chromosome.
Ishihrara plates are used to asses this
What is Acquired colour vision dysfunction?
Acquired colour vision dysfunction is a much for rare form of colour vision. These can be due to drug toxicity of the retina or optic nerve, slowly developing but inherited forms of retinal disease and acutely acquired vascular damage to the brain where colour vision is consciously created.
A Farnsworth-Munsell test is used to asses this
How does colour exist?
Colour only exists in our minds!
There is no physical thing that is colour. There are only electromagnetic waves of different wavelengths which have no colour (they are invisible).
The brain creates a conscious perception of colour from a cascade of electrical activity started by EM waves hitting 3 different populations of cone photoreceptors. These are found in the central part of the retina called the macula.
How has colour vision evolved ?
Foraging for food, social behaviour and predation avoidance drove the evolution of colour vision. Mammal are generally dichromates (can see 2 colours) although most old world primates including humans are trichromats.
Wh is Red/Green colour confusion so common from an evolutionary stance?
Red/Green colour confusion is so common as it was an advantage when hunting and survival etc.
Those with Red/Green colour confusion are better at;
- Seeing shades of light brown/yellow (khaki colour)
- Distinguishing camouflaged objects amongst foliage
- Identifying patterns and textures in dim light conditions
How does the surroundings of an object influence the colour that the brain makes?
The type of colour the brain creates is influenced by surrounding colours. Despite the wavelength of light reflecting back from two different surface being identical the colour ‘seen’ is different. This is due to the surrounding colours
Remember hole cut out with grey in practice and when sheet was lifted looked white ?
What does lateral inhibition cause in the brain?
Lateral inhibition enhances contrast and colour
Groups of photoreceptors tends to inhibit the response of adjacent groups. A uniformly grey area can be perceived as a range of grey when the surrounding area varies. This is the same for colour
How are pupil size’s controlled?
When light is shone into an eye the optic nerve afferent pathway stimulates the parasympathetic efferent supply to BOTH iris sphincter muscles causing constriction (miosis) of the pupils.
Pupil dilation in the dark is largely passive process of inhibiting input to the iris constrictor muscle.
Arousal from brain input via the sympathetic system does however actively drive dilation (mydriasis) of the pupil
What is Miosis?
Constriction of pupils
What is Mydriasis?
Dilation of pupils
What muscles constricts the pupil?
Circular muscles - contract
What muscles dilates the pupil ?
Radial muscles contract
What are the symptoms of Third Cranial Nerve Palsy?
The affected eye turns slightly outward and downward when the unaffected eye looks straight ahead, causing double vision. The affected eye may turn inward very slowly and may move only to the middle when looking inward. It cannot move up and down.
What is Horner’s Syndrome?
Horner’s syndrome is diagnosed clinically by observing ptosis (drooping of upper and lower lids), miosis (constriction) of the ptotic eye and demonstration of dilation lag in the affected eye, and anhidrosis (can’t sweat) on the same side as the ptosis and/or mitosis
Dilation lag is a classic sign of Horner’s syndrome—when the slit lamp beam is turned off, the anisocoria will be most noticeable during the first four or five seconds of viewing. The abnormal pupil will slowly dilate or “lag behind” over 10 to 15 seconds, making the pupil asymmetry less evident.
(Basically affected side has drooping of upper and lower eyelid, constriction of pupil, cant sweat on that side and when a light is turned off there will be unequal sized pupils for 4-5 secs and abnormal pull will dilate taking 10 - 15 secs)
How do you test visual fields?
Setup: Sit directly in front of your patient, at the same height, with your knees almost touching
Test binocular visual field:
a. Present your open palms in the 2 upper quadrants halfway between you and the
patient
b. Ask the patient “while looking at my nose, can you tell me how many hands I am
holding up”
c. Present your open palms in the 2 lower quadrants and repeat the question
d. Now waggle the fingers of one hand
e. As the patient “Point to the hand which is waving”
f. Repeat this 2 or 3 times to ensure a consistent response
Uniocular visual field assessment:
a. Hold up 1 or 2 fingers in front of the patient and confirm their vision is good enough
to see ‘counting fingers’ and that they understand the test
b. Emphasise they should keep looking at your open eye and not the fingers
c. With the patient’s left eye covered and your right eye closed, ask the patient to fix
on your open eye. Then, explain to the patient that you are going to hold up different
numbers of fingers. Ask them to tell you how many fingers they see.
d. Place your fist in the middle of one quadrant and ‘flash’ either 1 or 2 fingers
halfway between you and the patient
e. Test all 4 quadrants of one eye twice randomly presenting one or two fingers and not simply alternating
f. Repeat this for the fellow eye
Uniocular field testing: Testing right upper temporal quadrant
Remind the patient throughout the visual field examination to look at your open eye and not at the fingers.
The commonest error here is not keeping the examining hand halfway between examiner and patient.
A more in depth visual field assessment can be carried out to explore the meridia (horizontal and vertical) and also the blindspot using a red pin. However, the above points 1 to 3 are substantial enough for your stage in training.
What would be the cause of a complete loss of vision in one eye?
Optic nerve disease
What would be thee cause of a Bilateral Hemianopia ?
Disease of the optic chiasm
What would Inferior or Superior quadrantanopia be caused by?
Disease of the parietal or temporal lobes respectively (PITS)
What would cause a Hemianopia?
Optic tract or occipital lobe disease
How would you carry out a Retinal Reflection Test?
- Bright light, lens set to zero, arms length
- Observe both eyes at a time
- Comment on any asymmetry of brightness and colour or presence of opacity
What is wrong with this retina ?
- Dark Central Opacity
- Cataract
- Corneal scar
What is wrong with this retina ?
White reflex;
- Leucocoria
- Retinoblastoma
What is wrong with this retina ?
- Left central opacity
- Posterior polar cataract
What is wrong with this retina ?
- Left absent retinal reflection
- Complete Cataract
What is wrong with this retina ?
- Bilateral Leucocoria
- Retinoblastoma
How would you test the Optic Nerve?
- Low to medium bright light held in right hand
- Using right eye to their eye
- Start 15 degrees (blind spot) temporal on the horizontal with lens zero at arms length
- Slowly move in maintaining the rental reflection
- Until you can see the optic nerve
- Follow the vessel branch (arrows) to the disc
- If out of focus alter lens power
- Comment on margin, colour and cup
What is wrong with this Optic Nerve?
Swollen
What is wrong with this Optic Nerve?
New vessels
What is wrong with this Optic Nerve?
Cupped
What is wrong with this Optic Nerve?
Pale
How would you asses the Retina?
- Follow each major arcade vessel (1-4)
- Return to optic nerve each time until all 4 have been examined
- Finally ask the patient to look at light
- Examine the fovea (5) at the centre of macula (not too long as patient will see whiteness and stars)
- Comment on appearance of the vessels and retina noting any abnormal findings
What is wrong with this Retina?
Central Retinal Vein Occlusion
What is wrong with this Retina?
Maculopathy - Age related macular degeneration or Toxoplasma scar
What is wrong with this Retina?
Proliferative Diabetic Retinopathy
