HNN Topic 20 - Eyes and Vision Flashcards
What is the function of the bipolar cells of the retina?
Transmit excitatory and inhibitory signals vertically through the retina
Describe the inner layer of the eyeball
- Retina - light detecting part of the eye
- 2 layers - neural and pigmented
Describe the features of exudative AMD
- New blood vessels growing under retina from choroid
- Rapid
- 10-15% of cases, 90% of cases of severe vision loss in AMD
- Blood vessels break, bleed and leak fluid, damaging the macula and causing it to lift away from the choroid
- Metamorphosia - distorted vision, straight lines appear wavy
Why is the visual field of both eyes limited in the inferior medial quadrant?
Lateral side of the nose blocks vision in this area
List the steps in diagnosis of AMD
- Visual acuity test
- Fundus fluorescein angiogram
- Optical coherence tomography
What is optical coherence tomography?
- Low powered laser interferometry
- Generates detailed cross-sectional image of retina
Describe how neovascularisation causes visual disturbances in AMD
- Choroidal neovascularisation begins in choroid
- CNV enters gaps in Bruch’s membrane, vessels begin growing in subretinal space
- Subretinal haemorrhage, retinal distortion
- Intraretinal haemorrhage, intraretinal fluid
- Thicking and elevation of macula
List the layers of the eyeball
- Fibrous
- Vascular
- Inner
How do genetics contribute to AMD?
- Polymorphisms in complement factor H gene strongly linked to AMD - complement factor H regulates inflammation, prevents complement attack of own cells
- Othe genes
- Complement - CFB, CF1, C2/3
- Lipids - genes for HDL and LDL
- ECM - collagen and matrix metalloproteinase
What is the result of a lesion to Meyer’s loop
Homonymous upper quadrantanopia - loss of vision in same upper quadrant of visual field in both eyes
What anatomical landmark does the calcarine sulcus indicate?
Primary visual cortex
Describe the structural features of the retina seen during an ophthalmoscopy
- Macula - slightly off-centre highly pigmented area
- Depression in central 500 microns of macula = fovea
- High concentration of cone cells - high acuity vision (reading, facial recognition)
- No blood vessels - dependent on choroid for O2 and metabolic support
- Optic disc - area where optic nerve leaves retina, no light detecting cells
Describe the location of the eyeball
- Bilateral spherical organ
- Lies in bony orbit - bony cavity within facial skeleton
Describe the structure and function of the iris
- Circular structure with aperture in centre (pupil) - diameter altered by smooth muscle fibres within iris, innervated by ANS
- Between lens and cornea
Describe the pathogenesis of AMD
- Photoreceptors continue to produce photosensitive pigment throughout life
- Ends of photoreceptor cells decay and are removed by retinal pigment epithelium
- End products accumulate, causing drusen
Describe how visual acuity tests are carried out
- Recorded as the distance chart is read/distance at which it should be read
- 6/6 is normal - reads at 6m what should be seen at 6m
- 6/12 - reads at 6m what should be seen at 12m
- 6/36 - reads at 6m what should by seen at 36m
- Wear distance glasses if needed, 6m from chart, one eye at a time, if vision not perfect use pinhole
- If vision <6/60 - count fingers, hand motions, light perception
Describe the appearance of the retina in exudative AMD
Neovascularisation, leaking fluid
Describe the path of light as it enters the eyeball
- Incident light passes through cornea to enter eye
- Moves through aqueous humour of anterior segment
- Passes through lens
- Passes through vitreous humour of posterior segment
- Hits retina
What is the result of an optic radiation lesion?
Homonymous hemianopia
Describe ion transport in rod cells in normal conditions (no light)
- Extracellular fluid surrounding rod cells is high in sodium ions and low in potassium ions, opposite in cells - maintained by sodium/potassium pump
- In resting state K moves out - negative charge inside
What is the result of a complete optic nerve lesion?
Blindness in that eye
What is the function of the horizontal cells in the retina?
Inhibitory neurons that provide lateral inhibition - increases spatial resolution (i.e. visual contrast)
How is visual field tested?
Goldmann Perimetry test - outline shows borders of peripheral vision
What are the ganglion cells of the retina?
Axons from optic nerve after exiting through the optic disc
What causes optic nerve lesions?
- Acute optic neuritis (inflammation of optic nerve) - Multiple Sclerosis
- Indirect traumatic optic neuropathy
- Optic atrophy - loss of nerve fibres e.g. due to ischaemia
Are dendrites afferent or efferent neuronal cell processes?
Afferent - recieve information
Describe the sequence of events which occurs when light hits a rod cell
- Light hits rod cell, absorbed by rhodopsin
- Rhodopsin breaks down into scotopsin and 11 cis-retinal = bleaching
- 11 Cis-retinal absorbs photon, changes to trans-retinal
- Trans-retinal activates the enzyme scotopsin
- Large amount of G protein transducin produced, activates enzyme phosphodiesterase
- Phosphodiesterase hydolyses cGMP which stops the flow of sodium ions inside rod cells - causes hyperpolarisation of cells
- Hyperpolarised rod cells transmit the neural signal to bipolar cells
- Bipolar cells, amacrine cells and ganglion cells process the neural signal and generate action potentials to transmit to the brain via the optic nerve
Describe the features of atrophic AMD
- Slow, blurring
- 80-90% of cases of AMD
- Small white/yellow deposits called drusen form on the retina, beneath the macula, causing it to deteriorate or degenerate over time
Describe the contents of the left and right optic tracts
- Right optic tract - temporal fibres from right eye and nasal fibres from left eye (left hemifield)
- Left optic tract - temporal fibres from left eye and nasal fibres from right eye (right hemifield)
Describe the mechanism of action of cone cells
- Similar neural activity to rod cells
- 3 different types ofo cone cell (each contains different photopigment), sensitive to red, green or blue
- Iodopsin is photpigment, composed of 11 cis-retinal + photopsin
- Final percieved colour is combination of all three types of cone cells depending on level of stimulation
- White = combination of all cone cells
- Black = absence of light
Which cranial nerves pass through the cavernous sinus?
Oculomotor, trochlear and abducens nerves
Describe the main projections of the extra-geniculate pathways
- Pretectal nucleus of midbrain - pupillary light reflex
- Superior colliculus of midbrain - coordinated reflex head and eye movements in response to visual stimulus
- Suprachiasmatic nucleus of hypothalamus - regulates day-night cycle, circadian rhythm
How are the visual fields perceived within the eye?
- Image is reversed and inverted in the eye
- Visual field of each eye divided into nasal (medial) and temporal (lateral)
- Fields cross and are recieved by opposite sides of the retina
Define homonymous hemianopia
Loss of one side of the visual field - same side in both eyes
Describe the structure of rod cells
- Light sensitive pigment = rhodopsin
- Combination of protein scotopsin and light sensitive small molecule retinal
- Retinal is a carotenoid molecule, derivative of Vitamin A (retinol)
- Retinal exists in cis- and trans- forms according to light condition
How long are the optic nerves?
Approx 50mm
What sign can be see during an ophthalmoscopy which indicates raised intracranial pressure?
Papilloedema
Explain the psychosocial effects of visual impairment
- Economic - financial, unemployment, increased care required
- Indepence - domestic, navigation
- Communication + social - non-verbal communication, TV/film/media
- Psychological - isolation, anxiety, depression
List the visual pathways
- Geniculate pathway
- To occipital lobe, for concious visual processing
- Extra-geniculate pathways
- To midbrain for reflex responses
- To hypothalamus for modulation of day-night cycles
Describe the appearance of the retina in atrophic AMD
Drusen deposists, areas of retinal pigment epithelium thinning/depigmentation
Describe the structure and function of the choroid
Connective tissue and blood vessels, nourishes outer layer of retina
What is the result of a visual cortex lesion?
Homonymous hemianopia
What causes RAPD?
Disease of optic nerve or retina e.g. optic neuritis, large retinal detachment
Describe the decussation of the optic nerves at the optic chiasm
- 50-60% decussation
- Fibres from nasal retina cross, fibres from temporal retina don’t cross
Define hippus
Normal pupillary response to light - eye constricts rapidly, dilates a little then constricts to a constant diameter
What is the result of a partila optic nerve lesion?
Ipsilateral scotoma
Describe convergence in the eye
- Binocular vision - two eyes percieve single image
- Two eyeballs turn slightly inwards to focus on close objects so both images fall on corresponding points on the retina at the same time
Describe accommodation of the lens for a near object
Contraction of ciliary muscles, less tension on suspensory ligaments, lens thicker and rounder
What is the afferent pathway of the pupillary light response?
Optic nerve
Describe the normal pupillary response to light
Constriction of that pupil (direct response) and of the contralateral pupil (consensual response)
List the major worldwide causes of blindness
- Cataract
- Glaucoma
- AMD
- Corneal scar
- Diabetic retinopathy
- Childhood blindness
- Trachoma
List the types of glial cells in the retina
- Muller cells - predominant
- Astrocytes - mostly in ganglion cell layer + nerve fibre layer
- Microglia - scattered throughout
Describe the components of the fibrous layer of the eyeball
- Sclera and cornea - continuous, provide shape and support
- Sclera - 85%, attachment of extraocular muscles, white of eye
- Cornea - transparent, located centrally at front of eye, refract light entering eye
What causes visible distortion of the optic disc?
Papilloedema or optic neuritis (inflammation of the optic nerve)
What is the functional contact point between two neurons called?
Synapse
Describe the structure and function of the ciliary body
- Ciliary muscle and ciliary process
- Ciliary muscle - smooth muscle attached to the lens by ciliary process, controls shape of the lens, forms aqueous humour
What is the efferent pathway of the pupillary light response?
Oculomotor nerve
Describe the pathway which leads to constriction of the pupils in response to light
- Parasympathetic pathway
- Pre-tectal nucleus projects bilaterally to Edinger-Westphal nuclei
- Preganglionic neurons travel on oculomotor nerve, synapse on ciliary ganglion
- Postganglionic neurons act on pupillary sphincter to constrict the pupils
- Both pupils contract as the optic tracts contain fibres from both eyes
Which parts of the retina are responsible for vision?
- Anteriorly - only pigmented layer, non-visual part of retina
- Posteriorly/laterally - both layers, optic part of retina
What promotes neovascularisation in AMD?
VEGF - vascular endothelial growth factors
Up-regulation promotes growth of new vessels
Describe the appearance of the lateral geniculate nucleus
6 layers of grey matter with alternating white matter - striped appearance
Describe the epidemiology of blindness
- 0.6% of population
- Mostly affects >50 year olds
- Higher prevelance in developing countries/poor populations
What suggests an efferent limb problem in the pupillary light response?
Efferent limb problem - no response in one eye but direct + consensual response in other eye
Describe the coverings of the optic nerve
- Dura mater, arachnoid mater and pia mater of the meninges form the outer, intermediate and inner sheaths of the optic nerve
- Subarachnoid space between the intermediate and inner sheaths
What are the requirements for legal blindness?
Severely sight impaired (blind) = 3/60
Sight impaired = 3/60-6/60
Compare Ranibizumab and Bevacizumab
- Ranibizumab - licensed, very costly (£750/dose), monthly injections (inconvenient, uncomfortable + risky)
- Bevacizumab - unlicensed, much cheaper (£49/dose), dose as required, just as effective
List the types of age-related macular degeneration
- Exudative (wet)
- Atrophic (dry)
List the steps in examining the pupils
- Inspect pupils - look for asymmetry or irregularity
- Check the direct and consensual reactions to light
- Swinging light test for RAPD
- Accommodation-convergence reflex
Describe and compare the two types of synapse
- Grey type 1
- Asymmetrical - presynaptic and postsynaptic membranes are different
- Spherical synaptic vesicles - excitatory e.g. glutamate
- Grey type 2
- Symmetrical - presynaptic and postsynaptic membranes are similar
- Elliptical synaptic vesicles - inhibitory e.g. GABA
What is the result of a bilateral macular cortex lesion?
Bilateral central scotoma - blind spot in centre of visual field of both eyes
Describe the treatment of AMD
- Anti-VEGF drugs - Ranizumab, Bevacizumab, Aflibercept
- Ranizumab used -
- Intravitreal injections
- Sterile procedure - needs clean room
- Repeated monthly for 3 doses, then as required
What is the function of the amacrine cells of the retina?
Many functional diverse subtypes that assisst in interpreting visual signals before they leave the retina
How is a fundus fluorescein angiogram performed?
- Inject fluorescein intravenously
- Fluorescein bound to albumin - remains within normal capillaries due to blood-retinal barrier
- Use blue flash and yellow filter to see details of retinal circulation
What causes lesions of the optic tract?
- Tumours
- Trauma
- Aneurysms of posterior cerebral artery
Describe the components of the anterior and posterior chamber
- Filled with aqueous humour
- Plasma-like fluid that nourishes and protects the eye
- Produced constantly, drains via trabecular meshwork
Describe refraction of light in the eye
- Light rays meet the convex surfaces of the cornea and lens and are angulated (via convergence) to a focal point on the retina
- Cornea is responsible for most of the refractive power, but changing the shape of the lens allows refractive power to be altered
How does AMD cause blindness?
- Blood vessels and scar tissue grow under retina
- Leaking vessels cause retinal oedema
- Blocks transport of O2 and nutrients from choroid
- Eventual scarring causes destruction of photoreceptors
Describe the pigmented layer of the retina
- Outer layer
- Attached to choroid layer, supports neural layer
- Around whole inner surface of eye
Where do the visual pathways originate?
- Originates in retina due to stimulation of rod and cone cells, which activates bipolar/ganglion cells
- Magnocellular (M) cells - sensitive to movement and low contrast
- Parvocellular (P) cells - sensitive to fine details and colours
Describe the histological arrangement of the macula
- Ganglion cells (send neural signal to brain via optic nerve)
- Bipolar cells
- Photoreceptors (respond to light)
- Retinal pigment epithelium
- Choroid
List the types of neuron in the retina
- Photoreceptor cells - rods and cones
- Bipolar cells
- Ganglion cells
- Horizontal cells
- Amacrine cells
What is the swinging flashlight test used for?
Can identify asymmetry of afferent input in pupillary light reflex - Relative Afferent Pupil Defect (RAPD)
How is the shape of the lens changed?
Suspensory ligaments attach the lens to the ciliary muscles and through contraction and relaxation allows changes in the shape of the lens to increase or decrease refractive power
What is the function of rod cells?
- Predominant photoreceptor
- Don’t detect colour
- Poor resolution
- More sensitive than cones
Describe the neural layer of the retina
- Inner layer
- Contains photoreceptors - light detecting
- Posteriorly and laterally in the eye
What is the function of cone cells?
- Colour vision
- Sparse in comparison to rods, except in fovea of macula
- High resolution - high visual acuity
- Dark adaption much faster than rods
What is the simplest pathway which causes transmission of signals by the optic nerve?
3-neuron chain - photoreceptor cell to bipolar cell to ganglion cell
What passes through the blind spot and makes it blind?
Retinal ganglion cell axons travelling to the optic nerve
What is the result of an optic chiasm lesion?
Bitemporal hemianopia - loss of temporal visual field in both eyes
List the causes of AMD
- Environmental factors - smoking causes 30% of cases
- Age - rare in people <70%
- Diet? - possible that high doses of Vitamin A, C and zinc may be protective
- Family history - relative of those with AMD at greater risk
Where are the anterior and posterior chambers of the eyeball?
- Anterior is between cornea and iris
- Posterior is between iris and ciliary body
Where is the lens in the eyeball?
Anterior, between vitreous humour and pupil
Describe the vision loss caused by AMD
Central vision loss and metamorphosia
List the components of the vascular layer of the eyeball
- Choroid
- Cillary body
- Iris
What shows a RAPD during the swinging flashlight test?
When moving light from one eye to the other, pupil of eye which light is shining on dilates
Describe the geniculate visual pathway after leaving the retina
- Nerve fibres of ganglion cells from both eyes carry impulse along two optic nerves
- Optic nerves meet at optic chiasm
- Optic nerve form optic tracts after crossing at chiasm
- Synapses with neurons in the thalamus called lateral geniculate pathways
- Project through optic radiation, sweeps around lateral ventricle to the primary visual cortex on the medial side of the occipital lobe
What is the function of the lens?
- Refraction of light onto the retina
- Shape altered by ciliary body - alters refractive power
- Responsible for accommodation + convergence
What causes optic chiasm lesions?
- Tumours (pituitary adenoma, meningioma)
- Aneurysm
What mediates the changes in the shape of the lens?
Parasympathetic nerve fibres travelling on the oculomotor nerve
Describe the accommodation convergence reflex
- Pupil constriction and converge of the eyes when looking from a distant object to a close one
- Pupil constriction (constrictor pupillae)
- Lens accommodation (ciliary muscles)
- Convergence of eyes (contraction of both medial rectus muscles)
- Afferent pathway = optic nerve
- Efferent pathway = occulomotor nerve
Describe accommodation of the lens for a distant object
Ciliary muscles relax, more tension on suspensory ligaments, lens thinner and flatter
Describe the arterial supply of the eyeball
- Mostly via the ophthalmic artery (branch of internal carotid, arises immediately distal to cavernous sinus)
- Gives rise to many branches, central artery of retina most important - supplies internal surface of retina
