Eileen - eye, visual pathways, age related macular degeneration Flashcards
What is the definition of sightlessness?
Having less than 1/10 of normal vision in the more efficient eye when refractive problems are fully corrected
What develops in retinopathy?
- Microaneurysms
- Neovascularisation
- Heamorrhage
- Retinal opacities
How do micro-aneurysms appear?
Outpouring of the retinal vasculature that appear as minute, unchanging red dots associated with blood vessels
How do micro-aneurysms occlude vision?
- Leak plasma, resulting in localised oedema that gives the retina a hazy appearance
- They also bleed - contributing to oedema
- Affected vision if they encroach on the macula and cause degeneration before they are absorbed
How does neovascularisation occur?
Formation of new blood vessels from the choriocapillaries, entering between the pigment and sensory layer, or from the retinal veins, extending between the sensory retina and vitreous cavity and sometimes into the vitreous body - growth factors, signalling systems and VEGF involved
How can neovascularisation causes problems with sight?
The vessel are fragile, lead protein and are likely to bleed- blurred vision if covers the macular and can cause degeneration
What are opacities?
Loss of retinal transparency due to haemorrhages, exudates, cotton wool spots, oedema and tissue proliferation - exudate result from inflammatory processes
How do exudate damage vision?
Destroy the underlying retinal pigment and choroid layer
What are cotton wool patches?
Retinal opacities with hazy irregular outlines
These occur in the nerve fibre layer and contain cell organelles
Associated with
- retinal trauma
- severe anaemia
- papillodema
- diabetic retinopathy
What are the investigations that can be done to diagnose age related macular degeneration?
Funds fluorescein angiogram
- IV fluorescein injection
- fluorescein binds to albumin which remains in normal capillaries
- use blue and yellow filter to see details of retinal circulation
Optical coherence tomography
- low power laser interferometry
- generates detailed cross-section image of retina
Name and describe the two types of AMD.
Exudative - new blood vessel formation under retina/above choroid
- rapid
- metamorphopsia
Atrophic
- atrophy of the outer retina
- slow
- blurring
How does AMD cause blindness?
Blood vessels and scar tissue grow under the retina
Leaking vessels cause retinal oedema
This blocks transport of oxygen and nutrients from the choroid to the avascular macular
Eventual scarring causes destruction of photoreceptors
What are the risk factors for AMD?
- Smoking
- Age (over 70)
- Diet - high doses of vitamin A, C and zinc may be protective
- Family history
- Genetics
What’s the most common genetic cause of AMD?
Polymorphism sin complement factor H gene- regulates inflammation and prevents complement mediated attach on own cells - inflammation is significant in AMD
What are other genes that are thought to cause AMD?
- Complement genes - CFB, CF1, C2 and C3
- Lipids - genes for LDL and HDL
- ECM - collagen and matrix metalloproteinase
What is thought to start the process of AMD?
Photoreceptors continue to produce photosensitive pigment throughout life
The ends of photoreceptor cells decay and are removed by retinal pigment epithelium constantly
- in some cases the end products accumulate causing drusen
- increases AMD risk
What is the best treatment for AMD?
Monoclonal antibodies that inhibit VEGF - to prevent neovascularisation
What are the licensed and unlicensed treatments for AMD?
- Ranibizumab - licensed (more expensive)
- Bevacizumab - unlicensed (less expensive)
Both are anti-VEGF drugs given locally via intravitreal injections
How is ranibizumab administered?
Intravitreal injection by opthalmologists in a sterile room
How often is ranibizumab administered?
Once a month for three months
How many people on ranibizumab improved?
40% improved by 15 or more letters on a visual acuity test
What are the differences between ranibizumab and bevacizumab?
- Injections given as required, not every month
- Yearly cost of R=£9656 while B=£1509
- Same improvements
What are the economic problems or blindness?
- Unable to work - loss of employment and income
- Requires increased care - more likely to be put in a nursing home
- Finance - bank statements and bills
What are the problems people with independence that blind people have?
- Health risks - falls and fractures
- Domestic - cooking, eating, dressing and telephone
- Shopping
- Navigation - safety
What are the problems with communication people with blindness have?
- Hearing impairment
- Non-verbal communication
- Social interactions
- TV, film and media
What are the psychological problems people with blindness have?
- Depression
- Anxiety
- Increased isolation
What is visual acuity?
Recorded as: the distance chart is read/distance at which it should be read
What measures must be made when checking visual acuity?
Check if the patients need distance glasses - get them to wear them
6 meters from the chart
One eye at a time, if vision is imperfect - use a pinhole
What extra tests should be done is the recorded vision is less than 6/60?
- Count finger
- Hand motions
- Light perception
What information do the rods in the eyes pass to the CNS?
They tell the CNS about the absence or presence of photons without regard to wavelength
Where does the transduction of light energy into a receptor potential occur in the photoreceptors?
The outer segments of both rods and cones
Describe the plasma membrane in the outer segment of the cones.
Folds back and forth in a pleated fashion
Describe the plasma membrane in the outer segment of the rods.
The pleats formed pinch off from the plasma membrane to form discs - around 1000 stacked discs piled in each outer segment
How are the outer segments of the rods renewed?
One to three new discs are added to the base each hour, while old discs are court off the tip and phagocytosed by pigment epithelial cells
What is contained in the inner segment of the photoreceptors?
- Mitochondria
- Golgi complex
- Nucleus
Describe the proximal end of the photoreceptors.
Expands into a bulblike synaptic terminal filled with synaptic vesicles
What is the first step in signal transduction?
Absorption of light by a photopigment
What is a photopigment?
A coloured protein that undergoes structural changes when it absorbs light in the outer segment
What is the photopigment found in rods?
Rhodopsin
What are the photopigments in the outer section of the cones?
Three different types - one for each colour - colour vision comes from selective activation of these pigments
What do all photopigments in the eye have in common?
Their two constituent parts- Opsin (glycoprotein)- retinal (pigment)
Out of retinal and Opsin, which part of photopigments varies depending on cone colour or rod?
Opsin (4 different types)- small variations in amino acid sequence of the different opsins permit the rods and cones to absorb different wavelengths of light
Describe retinal.
Vitamin A derivative formed from caroteneThis is the light absorbing party of all visual photopigments
What is the shape of retinal in darkness?
Retinal has a bent (cis-retinal) shape which fits into the opsin portion of the photoreceptor
What happens to cis-retinal when it absorbs a photon of light?
It undergoes isomerisation - where it straightens out to become trans-retinal
What happens after isomerisation of trans-retinal?
Several unstable chemical intermediates form and disappear - leading to the production of a receptor potential
What happens to the retinal after it has been straightened.
It detaches from the opsin in a method known as bleaching (because the final products look colourless)
What is the function of retinal isomerase?
It converts trans-retinal back into cis-retinal, so it can bind to opsin (reforming a functional photopigment) in a process called regeneration
How does the pigmented area contribute to regeneration?
It stores a large quantity of vitamin A for the retinal in the rods
Describe the release of neurotransmitters by the photoreceptors in darkness.
1) cGMP gated sodium channels open 2) sodium influx 3) membrane potential increases (partial depolarisation) 4) causes constant glutamate release at the synaptic terminals and inhibition of the bipolar cells
Describe neurotransmitter release during light transduction.
1) isomerisation of retinal activates an enzyme that breaks down cGMP2) cGMP gated sodium channels close 3) sodium inflow slows down4) hyperpolarisation induces a receptor potential 5) this turns off neurotransmitter release, exciting the bipolar cell
What happens to glutamate release in dim light?
Small and brief receptor potentials are activated that only partially turn off glutamate release
How many rods synapses with a single bipolar cell?
6-600 depending on the area - this increases the light sensitivity of rod vision, but slightly blurs the perceived image
How many bipolar cells do cones synapse to?
Normally just one- this is less sensitive to light, but yields a sharper image
What happens to the cone bipolar cells when light enters the eye?
They can either be excited or inhibited depending on the cone they are attached to
What is the function of the lateral inhibitions by horizontal cells?
It sends inhibitory signal a to bipolar cells in the area lateral to the excited rods and conesIt does this because it enhances contrast of the visual scene between the areas that are strongly stimulate and adjacent areas that are more weakly stimulated
What do amacrine cells do?
These are excited by bipolar cells, synapse with ganglion cells and transmit information to them that signals a change in the level of illumination of the retinaThis (and excitement of the bipolar cells) depolarises the ganglion cells and initiate nerve impulses
Where do the axons of the optic nerve terminate?
In the lateral geniculate nucleus of the thalamus
What does the optic nerve axons synapse with in the lateral geniculate nucleus?
Neurons whose axons form the optic radiations
Where do the optic radiation a project to?
The primary visual areas in the occipital lobes and visual perception begins
What are some of the other fibres in the optic tract that synapse with the optic nerve in the lateral geniculate nucleus of the thalamus?
Some project to the superior colliculus - control extrinsic eye musclesOther projects to the pretectal nucleus - controls pupillary and accommodation reflexes
What is a visual field?
Everything that can be seen by one eye- the two visual fields overlap considerably
What are the two regions the visual field can be divided into?
Nasal and temporal halves (A.k.a. Central and peripheral halves)
Where do light rays from the nasal and temporal parts of the visual field fall in the retina?
Light from the nasal part of the visual field falls on the temporal part of the retinaVice versa
Which side of the brain does visual information from the right half of each visual field travel?
The left
Which side of the brain does visual information from the left half of the visual field travel to?
The right side
At the optic chiasm, what happens to axons from the temporal half of the retina?
They do not cross
At the optic chiasm, what happens to axons from the nasal half of the retina?
They cross the optic chiasm and travel to the opposite side of the brain for interpretation
What are the things processed by different systems to produce visual signals?
Shape of objectsColour of objectsMovement, location and spatial organisation
What are the three layers of the eye?
RetinaFibrous tunicVascular tunic
What are the layers of the fibrous tunic?
Superficial layerAnterior cornea Posterior sclera Junction of sclera and cornea
Describe the cornea.
Transparent coat that covers the iris Curved to help focus light on the retina The central parts of the cornea receives oxygen from the outside air - contacts must be permeable to allow oxygen through them
Describe the components of each layer of the cornea.
Outer surface consists of non-keratinised stratified squamous epitheliumMiddle coat - collagen fibres and fibroblasts Inner coat - simple squamous epithelium
Describe the sclera.
The white of the eyeA layer of dense connective tissue composed of collagen fibres and fibroblasts Covers the whole eye ball except the cornea - gives the eyeball shape and makes it rigid (protects the inner parts)An attachment site for extrinsic eye muscles
What is found in the junction of the sclera and cornea?
Sclera like venous sinus - aqueous humour drain into this
What is the function of the fibrous tunic?
Mechanical support and physical protectionAttachment site for extrinsic eye musclesAssists focusing
What are the components of the vascular tunic?
ChoroidCiliary bodyIris
What is the most posterior part of the vascular tunic?
Choroid
Where is the choroid in relation to the sclera?
Lines the internal surface of the sclera
Why does the choroid appear dark brown?
Contains melanocytes that produce high quantities of melanin
Why does the choroid contain such high quantities of melanin?
So it absorbs lots of light rays, preventing reflection and scattering of the light within the eyeball- causing the image to be cast on the retina by the cornea to remain sharp and clear
What happens to the choroid in the anterior portion of the vascular tunic?
It becomes the ciliary body
Where is the ciliary body?
It extends from the jagged anterior margin of the retina (or a serrata) to just posterior to the junction of the sclera and cornea
What are the two parts of the ciliary body?
Ciliary processes and ciliary muscle
Describe the ciliary processes
Protrusions or folds in the inner surface of the ciliary body Contains blood capillaries that secrete aqueous humour
What are zonular fibres?
Fibres that extend from the ciliary process to the lens The fibres consists of thin, hollow fibrils that resemble elastic connective tissues fibres
What is the ciliary muscle, and what is its function?
Circular band of smooth muscleContraction and relaxation of this muscle changes the tightness of the zonular fibres, which alters the shape of the lens, adjusting it for near or far vision
Where is the iris located?
Suspended between the cornea and the lens attached to the ciliary processes on its outer margin
What does the iris contain?
Blood vesselsPigment cellsLoose connective tissueCircular and radical smooth muscle fibres Melanocytes
What does the amount of melanin in the iris dictate?
Eye colour- large amounts of melanin = brown or black eyes- moderate amounts of melanin = green eyes- low amounts of melanin = blue eyes
What is the function of the iris?
Regulates the amount of light entering the eyeball through the pupil
Why does the pupil appear black?
It shows the inside of the eye- heavily pigmented choroid and retina can be seen
Why is light reflected (e.g. In photos) from the eyeballs seen to be red?
Blood vessels on the surface of the retina
When bright light stimulates the eye, what happens to the pupil, and how?
Parasympathetic fibres of CN III stimulate the circular muscles (sphincter pupillae) of the iris to contract Pupil constriction occurs
When dim light simulates the eye, what happens to the pupil, and how?
Sympathetic nerves stimulates the radical muscle (dilator pupillae) of the iris to contractThe pupil becomes dilated
What enters and leaves the eye at the optic disc?
Optic nerveCentral retinal artery (branch of the ophthalmic artery)Central retinal vein
What is the name of the two layers of the retina?
Pigmented layerNeural layer
What is the pigmented layer of the retina, and where is it located?
A sheet of melanin-containing epithelial cells found between the choroid and the neural part of the retina
What is the function of the pigmented layer of the retina?
Melanin in this layer helps absorb stray rays of light
What is the neural layer of the retina?
Multilayered outgrowth of the brain
What is the function of the neural layer of the retina?
Processes visual data extensively before sending nerve impulses into the axons that form the optic nerve
What are the three layers that form the neural part of the retina?
Photoreceptive layerBipolar cell layerGanglion cell layer
What are the two zones that separated the three layers of the neural part of the retina?
Outer an inner synaptic layers
Which order does light pass through the neural layer of the retina?
Ganglion cell layerInner synaptic layerBipolar cell layerOuter synaptic layer Photoreceptive layer
Name the two other cell types present in the bipolar layer, and what do they do?
Amacrine cells Horizontal cellsThey regulate communications between the rods/cones and ganglion cells- helps adjust retinal sensitivity to dim/bright light- changes the contrast
List the order of the layers from the outer layer of the eye, to the optic nerve axons on the inside.
Sclera Choroid Pigmented layer of the retinaPhotoreceptive layer of the neural layer of the retinaBipolar cell layer of the neural layer of the retinaGanglion cell layer of the neural layer of the retinaOptic nerve axons
What are the two types of photoreceptors?
Cones and rods
What is the function of the rods?
Allow vision in dim light (no colour vision) - black, white and all shades of grey - very light sensitive
What is the function of the cones?
Stimulates by brighter lights - three types of cones = red, green and blue - various combinations of these cones result in colour vision - gives sharper and clearer images- doesn’t see as well in dim light
Describe the cone and rod distribution of the eye.
Cones are found in greater concentrations at the back of the eye, and rods are found round the sides
What is the area of the eye with the highest concentration of cones called?
The macular densa
Define optic disc
Is the point of exit for ganglion cell axons leaving the eye.
Contains no cones or rods -> physiological blind spot
Describe the fovea centralis
A small depression in the centre of the macular densa which contains only cones - area of highest visual acuity
What makes up the refractive media of the lens?
Proteins called crystalline within the lens- arranged like layers in an onion - perfectly clear and transparent
What is the outer covering of the lens?
Clear connective tissue
What is the function of the lens?
Helps the focus the image on the retina by changing its own shape- facilitates clear vision
What are the two cavities within the eyeball called?
The anterior and posterior cavity (or vitreous chamber)
What are the two chambers within the anterior cavity?
Anterior and posterior chambers
Where is the anterior chamber?
Between the cornea and the iris
Where is the posterior chamber?
Between the iris and the zonular fibres and lens
What is the contents of the anterior cavity?
Aqueous humour - transparent watery fluid which nourishes the lens and the cornea
How does the aqueous fluid enter and leave the anterior cavity?
Continually filters out from capillaries in the ciliary processes of the ciliary body - entering the posterior chamber It flows between the iris and the lens, through the pupil and into the anterior chamberIt then drains into the scleral venous sinus from the anterior chamber and into the blood
How often is aqueous humour replaced?
Completely replaced every 90 minutes
Where is the vitreous chamber?
Between the lens and the retina
Describe the vitreous fluid, and it’s function.
A transparent jelly-like substance that holds to retina flush against the choroid - gives the retina an even surface for the reception of clear images
What are the components of the vitreous body?
Water, collagen fibres, hyaluronic acid and phagocytes (to keep this part of the eye free from debris for uninterrupted vision)
What is the hyaloid canal?
A narrow channel (inconspicuous in adults) that runs through the vitreous body from the optic disk to the posterior aspect of the lens
What is the pressure in the eye called, and what is it normally?
The intraocular pressure is normally 16mmHg
Which component on the eye produces the intraocular pressure, and why is it needed?
The pressure is mainly produced by the aqueous humour and only partly by the vitreous body.It’s needed to maintain the shape of the eyeball and prevent collapse
Describe the anatomy of the eye
Describe the histology of the macula
- Central section = fovea
- Fovea contains NO rods, only CONE photoreceptors -> colour + detailed vision
- NO blood vessels - dependant on choroid for O2 & metabolic support
What investigations are proposed by a macular clinic when AMD is suspected?
- Visual acuity
- Metamorphopsia (when a group of straight lines look wavy)
- Dilated fundus exam
- Fundus fluoro angiogram (inject IV fluroscein -> binds to albumin -> should remain within normal capillaries because of blood-brain barrier
- Optical coherence tomography - detailed cross section of retina
What’s the diagnosis of this swinging flash light test?
Left occulomotor damage
What’s the diagnosis of this swinging flash light test?
Normal
What’s the diagnosis of this swinging flash light test?
Right optic nerve damage
What’s the diagnosis of this swinging flash light test?
Right optic & occulomotor damage
Describe the neural pathway for light reflex
Transmitted light rays focused onto retina -> retinal photoreceptors transduce light energy into changes in neuronal membrane potential -> signals transmitted to brain via optic nerve.
- Optic Chiasm
- Optic Nerve
- Optic Tract
- Lateral Geniculate Nucleus
- Optic Radiations
- Visual Cortex
- Midbrain
- Pituitary Stalk
What is the role of the visual pathway?
To relay visual field info from both eyes to the primary visual cortex
Give some examples of lesions of the visual pathway
What’s happening in this image and what can it be caused by?
Lesion of OPTIC NERVE
Partial/complete loss of vision in ipsilateral eye (same side)
Caused by:
- acute optic neuritis - Multiple Sclerosis
- indirect traumatic optic neuropathy
- optic atrophy
What’s happening in this image and what can it be caused by?
Lesion of OPTIC CHIASM
Bitemporal hemianopia: loss if vision in temporal visual field of both eyes
Caused by:
- tumours (pituitary adenoma, meningioma)
- aneurysms
What’s happening in this image and what can it be caused by?
Lesion of OPTIC TRACT, LGN, OPTIC RADIATION or VISUAL CORTEX
Homonymous hemianopia: loss if vision in same visual field of both eyes (can be left/right)
Caused by:
- tumours
- trauma
- aneurysms (of post cerebral artery)
Name the additional pathway that nerve fibres can project to apart from the LGN
- Superior colliculus -> eye movements
- Pretectal area of midbrain -> pupillary light reflex
What’s this?
Goldmann perimetry test - asseses the extent of the visual field using a light stimulus
