The eye and vision (special senses) Flashcards
What makes up vision?
Eyeball - optical front end & retina/optic disc at the back
Connections - optic nerve, chiasm, optic tract, LGN, radiation
Brain - occipital, temporal, parietal & frontal lobes
How does the eye focus and sense?
Anterior and posterior chamber (containing aqueous humour) is responsible for focusing
Posterior - Vitreous body (contains vitreous) is responsibkle for sensing (retina)
What are the different parts of the Anterior segment of the eye?
Anterior Segment;
- rectus muscle
- posterior chamber
- ciliary body
- suspensory ligament
- lens
- anterior chamber
- cornea
- aqueous humour
- iris
What is the function of the Cornea ?
Cornea;
- Major light focusing lement of the eye - 40D of ‘fixed’ power
- The cornea has multiple layers, aligned specifically to maintain trasnparency - called Stroma
Cornea constantly replenished epithelium which gets damaged by light
Stroma gives cornea rigidity and transparency so its not opaque
Endothelium doesn’t replenish and is reduced as you age
How does refraction of the eye occur?
Cornea;
- largest element (40D)
- Interfaces with air (low RI) - big difference
Lens;
- lesser element (20D)
- Interfaces wit aqueous (similar RI) - small difference
- But can vary in power (accommodation)
Whole eye ball;
- About 60D
D = refractory index
What is accommodation ?
Accommodation is the ability of the eye to change the focal length of the lens by changing the curvature of the eye lens. Accommodation allows the eye to automatically adjust focus from seeing things at a distance and “tune” it to seeing nearer objects. (lens gets fatter)
How is the Iris innervated?
Dual innervation
Sympathetic (dilation) and parasympathetic (constrict) innervation
Compression on nerves can cause change in constriction
What is the near triad?
1). Miosis (constriction)
2). Convergence (eyes come together)
3). Accommodation (lens gets fatter)
What is Presbyopia?
A refractive error. When older people start needing glasses for reading - age related
Ability to focus on something is lost as lens get thicker and lest plastic so cannot change as easily and muscles don’t work as well so can see at a distance if don’t have a cataract but cannot see up close
What is Myopia and Hypermetropia ?
Myopia - short sighted (can generally see without glasses but cannot see at distance) - eye is “too powerful for them”
Myopia at risk of;
- Open angle glaucoma
- Retinal detachment
Can tell by looking at edge of glasses - gap
Hypermetropia - (eye isn’t powerful enough so need glasses to bring rays of light forward in front of retina\
Hypermetropia at risk of;
- Angle closure glaucoma
- Ischaemic optic neuropathy
(“disk at risk”)
Can tell by looking at edge of glasses - no gap
What is uncorrected refractive error (URE)
Uncorrected refractive error (URE) - (Not having glasses)
- Main cause of visual impairment world wide
- Presbyopia (failure to accommodate when you are older) is the main type of URE
- Myopia (short-sightedness) growing epidemic in industrialised regions especially Asia and associated with retinal detachment
- Hypermetropia (long sightedness) is associated with squint and lazy eye (amblyopia) and acute closed angle glaucoma
What are the main causes of visual impairment and blindness world wide?
Visual impairment;
- RE (refractive error)
- Cataract
Blindness;
- Cataract
(cataract is treatable tho!)
What is the Visual Acuity Assessment?
Visual Acuity Assessment;
- Distance 3m chart & matching card
- Cloth - compact, easily, washed
- Sloan letters - easily ‘drawn’ in air
- Broken Cs
- Matching card
How do we record visual activity ?
6m is the standard distance for ‘big’ Snellen charts
The archlight is 50% smaller chart with 50% smaller letters and so you use the chart at 50% distance = 3
But the top line is still called the 60 line and so you document the vision as 6/60 and 6/36 etc
20/20 vision - top no is how far someone with normal vision is away from the letter and can see (20 feet). Bottom is patient number.
What are the steps in testing distance visual acuity ?
Steps in testing distance visual acuity;
1). Measure out 3m
2). Cover left eye with palm of hand
3). Ask patient to read from the top of the chart
4). Chart must be held perpendicular to patient in good lighting, smoothed out and flat
5). Record the ‘number’ of the smallest line that can be seen
6). If cannot read even the top letter then go to 1.5m and repeat
7). If cannot read even the top letter at 1.5m then go to 0.5m and repeat
8). If cannot see at 0.5m then try counting fingers (CFI at 1m, hand movements (HM) then perception of
light and classify with projection or with no projection of perception of light then finally no perception
of light
- Repeat for fellow eye
- Repeat with both eyes together
- Repeat with pinhole and with glasses
What are the WHO classification groups for eye sight? 0-5 (9)
WHO Classification Groups — 0 to 5 (9);
- Normal to Mild Visual Impairment - Group 0 - < 6/18
- Moderate Visual Impairment - Group 1 - 6/18 to 6/60
- Severe Visual Impairment - Group 2 - 6/60 to 3/60
- Blindness - Group 3 - 3/60 to 1/60
- Blindness - Group 4 - 1/60 to HM, CF, PL (hand movement, counting fingers, perception of light)
- Blindness - Group 5 - NPL (no light perception)
- Unspecified or “observed” Visual behaviour - Group 9 - N/A
Explain the structure of the retina?
Information from the rods and cones is converged to ganglion cells, lateral cells and amacrine act provide initial processing of the visual signal
In the fovea the neural components are moved to one side, and the degree of convergence onto ganglion cells is minimal
Outer nuclear layer;
- Pigmented epithelium
- Rods
- Cones
Plexiform layers;
- Lateral cells
- Bipolar cells
- Amacrine cells
Ganglion layer;
- Ganglion cells
Label the parts of this diagram of the eye ?
Image
Retina is whole thing
How are photoreceptors (rods and cones) distributed across the retina ?
Convergence différences of rods and cones onto ganglion cells is location dependent
- Rods start peripherally and increase the more central they come and at the fovea drop dramatically as the fovea increases
- Cones start almost non-existent peripherally then at the fovea dramatically spike and then fall again after
The blind spot has no rods or cones !
How does the retina detect light?
Our eyes can see visible light ranges from 400-700nm wavelength.
Each opsin gives sensitivity to a different range of wavelengths of light
At their most sensitive wavelength our yes can detect as little as 5 photons of light
Contrary to expectation, photoreceptors are depolarised in the dark and hyper polarise in light
What is Cone Fatigue ?
When you stare at a specific colour for too long, the cells that detect frequency of light will get fatigued
The after image will result in your photoreceptors not being ‘in balance’
As the photoreceptors become less tried which takes between 10-30 seconds the balance is recovered and the after image disappears
Hence if you stare at attached image then change to white screen will still see colours as cells are fatigued
This effect is restricted to one eye only!
This proves this is at least ‘pre-chiasmal’ phenomenon (before information crosses)
We know that it is definitely a retinal phenomenon
How do Retinal cells connect?
See image attached
What are the features of Bipolar Cells?
Bipolar cells;
There are 2 types - ‘ON’ or ‘OFF’
‘ON’ - neurotransmitter release in the light (when photoreceptor checks NOT releasing euro-transmitter) - switch the light ‘ON’
‘OFF’ - opposite of on
What are receptive fields?
Receptive fields - convergence of information from PR’s onto bipolar cells then a ganglion cell
Processing has already happened at the level of the retina by convergence of receptive fields (100 million photoreceptors converge info into only 1 million ganglia)
Ganglion transfers the retinal information to the brain; optic nerve > chiasm > optic tract
First synapse is at the lateral geniculate nucelus (LGN) - part of thalamus and is major relay station for sensory information
What are thee features of the optic nerve?
Optic nerve;
- Collection of all the ganglion cells
- Exits the back of the eye through a hole in the sclera
- The optic nerve head can be seen at the back of the eye (aka optic disc)
What are different pathologies you can see of the optic nerve?
Swollen Optic nerve
Cupped optic nerve
Pale optic nerve
What does Glaucoma look like in the eye?
Glaucoma is a common eye condition where the optic nerve, which connects the eye to the brain, becomes damaged. It’s usually caused by fluid building up in the front part of the eye, which increases pressure inside the eye. Glaucoma can lead to loss of vision if it’s not diagnosed and treated early.
What does optic nerve cupping look like?
The optic nerve sits in the back of your eye, and it’s surrounded by a dense network of other nerve fibers. When those smaller nerves die, the space they leave behind looks a bit like a cup. Doctors call this “optic nerve cupping.” Cupping can be a sign of glaucoma, and this condition always needs treatment
