age related macular degeneration Flashcards
what is the function of the eye
complex optical system that sits within the orbit
collects light from environment and forms an image on the retina which is passed on via optic nerve and visual pathways to the visual cortex in the occipital lobe.
Describe the structure and function of eye structures
- look at pictures*
- Conjuctiva/sclera: whiter outer coat of eye
- Cornea: clear front window of the eye which transmits and helps to focus light
- Iris: coloured part of the eye, helps control how much of light enters the eye
- Pupil: Reacts to amount of light available
- Lens: Focuses light rays onto retina, normally transparent (clouding of lens = cataract)
- Vitreous chamber: clear gel filling central cavity of eye
- Retina: layer of nerve cells lining the back of the eye
- Choroid: layer at back of eye between retina and sclera. contains blood vessels, associated with macular degeneration
- Macula: located near centre of retina. responsible for detailed central vision. deteriorates with age
- optic nerve: 2nd CN, > 1 million ganglion cells of retina.
what are the photoreceptor cells of the retina
Rods and Cones
- specialised neuroepithelial cells
- lie adjacent to retinal pigment epithelium and choroid
Rods:
Functions in dimmer light (scotopic)
120 million
Cones:
Brighter light
high acuity tasks e.g. reading (photopic)
6 million
Describe the histology of the macula
layered structure
10 basic layers
*starting from top to bottom* Nerve fibre layer Ganglion cells Bipolar cells Photoreceptors (rods and cones) Retinal pigment epithelium Choroid
Describe the structure and function of the macula
no blood vessels- dependent on choroid for O2 and metabolic resources
subdivided into:
- fovea (only has cones photoreceptors- responsible for high resolution vision)
- parafoveal
- perifoveal
Role; phototransduction
- converts light into neural impulses via a cascade of chemical and electrical events
- AP generated in retinal ganglion cells
- Onto visual cortex via visual pathway
what type of vision does normal and abnormal macula allow
Normal Macula: Detailed vision (reading, facial recognition, visual acuity)
Disturbance of macula (e.g. blood+fluid):
Disrupts RPE + photoreceptor function
Causes distortion (metamorphopsia)
Reduced central vision
what is age related macular degeneration (ARMD)
- degenerative condition affecting macula
- blurred central vision
- peripheral vision not affected
- leading cause of blindness in >50s
what are the 2 main types of ARMD
- Wet
- exudative
- gradual/ rapid loss of central vision
- distortion - Dry
- non exudative
- gradual loss of central vision
- distortion
what is the pathophysiology and treatment of wet ARMD
Pathophysiology:
- choroidal neovascularisation (stimulated by VEGF)
- new vessels originating from choroid
- grow under RPE
- causes haemorrhage + leaking -> ultimately fibrosis
- disrupts photoreceptor and retinal function
- loss of vision - profound if untreated
Treatment:
- Anti VEGF antibodies
- intravitreal injection (into vitreous cavity)
- inhibits angiogenesis
- reduces leakage + haemorrhage
- preserve vision/ delay visual loss
what is the pathophysiology and treatment of dry ARMD
Pathophysiology:
- accumulation of cellular debris (drusen) between retina and choroid
- causes atrophy of RPE and photoreceptors
Treatment:
- No treatment, only management
- lifestyle advice
- smoking cessation
- magnifiers
- visual aids
- partial sight registration
how does amd cause blindness?
Blood vessels and scar tissue grow under retina
Leaking vessels cause retinal oedema
Block transport of O2 and nutreitns from choroid
Eventual scarring causes destruction of photoreceptors
what are the risk factors of ARMD
age genetics family history smoking diet CV disease obesity
what investigations are used for ARMD
Visual acuity
Metamorphopsia
DIlated fundus exam
OCT
how is visual acuity tested
- measured on snellen chart
- measures central vision
- tested at 6m
- recorded as a fraction:
numerator- distance in metres from pateint to chart
denominator- distance at which normal eye could see the same line
how is visual field tested
- measures peripheral vision
Normal (monocular) human visual field: - horizontally extends 60 degrees nasally and 100 degrees temporally
- vertically extends 60 degrees above and 75 degrees below
