Ophthamology #2 Flashcards
Difference between the cornea lens and the “lens” itself
Cornea: Fixed-power lens, can’t alter focussing properties
Lens: Variable power lens
The Lens,growth and progression
Has a thin ECM on the outside and the rest is Cells!
At the back of the anterior chamber.
Outside: Epithelium (single layer)
Middle: Long cells 12mm, look like fibres (they’re not) so “fibre Cells”
Central Part of lens is called the nucleus, although it doesn’t contain DNA it is made of 1000’s cells. Central cells laid down in utero, and surrounding layers are laid down throughout life, as the lens continues to grow.
Later in life you can get very thick lenses.
What is a Cataract.
A cataract is a clouding of the eye’s natural lens. Block the light coming through.
Cataracts are the most common cause of vision loss in people over age 40 and is the principal cause of blindness in the world.
Due mainly to age as lens proteins denature over time (central lens cells have been there since utero!)
Describe a Cortical and Nuclear Cataract
Nuclear Cataracts: Worse in the central area as it’s right in the visual axis.
Ciliary Body
Contains Ciliary Muscle that goes all around the periphery.
This is responsible for ‘accomodation‘, ability of lens to alter its focusing power.
- When ciliary muscles contract → diameter shrinks → loose zonules → thicker lens → able to focus on short distances
- When relaxed→ diameter increases → zonules under tension → focus is on objects further away where rays are parallel
As the lens is suspended within the muscle, as it’s ‘pole-to-pole’
The ciliary membrane is responsible for the production of _______
Aqueous Humour.
- Liquid which fills the anterior chamber of the eye, and the area between the lens and the iris.
- This production flow of liquid from the ciliary epi. is constant, flow through the pupil, into the anterior chamber and away to the trabeular meshwork.
- Trabecular meshwork acts as a filter where the fluid can flow out of the eye proper.
Clinically: If this flow gets blocked, fluid is unable to leave and accumulates in the anterior chamber, with more being still constantly produced. Pressure will slowly increase!
What’s the clinical purpose in knowing about aqueous humour flow?
Clinically: If this flow gets blocked, fluid is unable to leave and accumulates in the anterior chamber, with more being still constantly produced. Pressure will slowly increase!
The eye has two principle focusing structures, these are….
- Cornea (2/3): approximately 40 dioptres
- Lens (1/3) approximately 20 dioptres
**if a lens cataract is removed the focusing power needs to be replaced. This used to be done via glasses, now more by intraocular lens (eye surgery)
Briefly describe a crystalline lens
Surrounded by a very thin capsule and suspended by even thinner zonules. (1-2microns)
Social statistics of cataracts!
25 million blind from them Globally, extremely common!
Requires 36 mill. procedures
In Sweden: If you can’t drive, you get surgery
How to diagnose a cataract
- Entirely clinical:
opthalmascope
Slit Lamp - Reduced visual acuity
- Significant lens opacity
- No other ocular pathologies: may not be the main cause of their vision problems
Aetiology of Cataracts
- Age (majority) ~60%
- Congenital uncommon, but they’ll go blind if not treated
- Metabolic; diabetes get it earlier
- Toxic: corticosteriods (for asthma etc, you will get it earlier)
- Traumatic: irridation
Brunescent: hard brown cataract. If left alone will go black
Techniques of cataract surgery.
- Couching: needle in eye and poking lens into back of the eye
- Intracapsular: mainly in developing world removed
- Extracapsular high frequency US
- Phacoemulsification: v popular
- Femto-laser assisted.
Cataract/intraocular lens surgery steps?
- Small corneal incision made
- Circular hole is made in the lens capsule
- Inject fluid to seperate lens from capsule
- an US probe is used to probe and break up the cataract
- Broken into 4 pieces before removal
- Protective jelly
- IntraOcular Lens put in.
- AB injected under conjuctiva to prevent infection
The Retina is?
The retina is the light-sensitive tissue lining the back of our eye. Light rays are focused onto the retina through our cornea, pupil and lens. The retina converts the light rays into impulses that travel through the optic nerve to our brain, where they are interpreted as the images we see.
Fovea: central of our visual acuity
The retina can be split into 3 sections, these are…
Anterior: Ganglion cells** send image to brain **3rd
Middle: Bipolar linker cells** (like extension cables) **2nd
Posterior: ‘rods and cones’ photoreceptors 1st
Seems like a backwards system!!
Thats because of the pigment epithelium of the retina that supplies food/removes waste from the the photoreceptor cells (as these are highest energy consuming cells in body) Therefore in order to function they need to be at the back!
The fovea is?
The Bipolar cells and the ganglion cells blocking the light are moved out of the way at this point!
Specialised part of the retina where light can directly hit photoreceptor!
Macula lutea
Darker orange area surrounding the fovea, which contains special pigment that absorb that intense light and protect from light damage (from free radicals) to the fovea!
Stops light being able to bounce around.
Whats different about the Optic Nerve (CN II)
- No photoreceptors, specialised to gather all the axons of the ganglion cells and allow them to exit the eye, through the optic nerve, through the optic foramen, to the brain.
- Therefore we don’t perceive light at the optic nerve = Blind spot
But we don’t notice the blind spot because we have 2 eyes, our brain ‘fills in’ the image!
- **Also where the entry point for major blood vessels; Central Retinal artery/vein
Describe the photoreceptor density across the retina.
Cones: colour vision and visual acuity. These peak at the central fovea.
Rods: Either side of the fovea the rods quickly fill in for movment detection at the periphery.
Therefore the visual acuity falls away at the periphery, and only a small proportion of our vision is in focus at any one time!
Colour Absorbancy
Different types of cones for different colour frequencies.
Blue, green or red, all excited by different wavelengths.
Colour Blindness.
Mutations in genes that recognise red or green colouring
Gonioscopy
Lens that goes on the eye: Way of identifying what angle Glaucoma.
Looking at the angles.
What’s optic nerve cupping
Where you can lose most of your 1,000,000 optic nerve axons.
Half lost before any visual loss occurs.
Visual loss starts in the periphery and affects central vision last.
An ‘arcuate loss’ leading to tunnel vision.
Cup:disc ratio 0.5
Disc becomes ,0.9 big cup to disc ratio
The normal fundus
Darker you skin colour the darker your eye
Tigroid fundus, makes disc look lighter, for light protection
