Anatomy and Refractive Conditions Flashcards
What is the canal of Schlemm?
It is a circular canal near the junction of the cornea and iris. Is it a passageway for aqueous humour from the anterior chamber into the venous bloodstream
What is Low Vision?
A condition caused by an eye disease where BCVAs are less than 20/70 or worse
What is legally blind?
BCVAs of 20/200 (the big E) or the visual field is 20% or less
What are the 3 tunics of the eye?
Sclera, Choroid and Retina
Describe the Sclera
The outermost protective, firm and fibrous layer of the eye. It is made of collagen
Describe the Choroid
The middle, nourishing layer of the eye. This layer has blood vessels to nourish the eye
Describe the Retina
The light sensitive, perceptive layer of the eye. 10 layers that convert light into electrical impulses. It is held in place by pressure from the vitreous humour
Describe the Ciliary Body
The ciliary body is made of the ciliary muscle and the ciliary body.
What is the ciliary muscle?
A ring-shaped muscle that is continuous with the choroid and helps in the process of accommodation and helps shape the crystalline lens
What is the ciliary process?
A part of the ciliary body that secretes aqueous fluid
Describe the Aqueous Humour
A watery fluid (n=1.336) that is found between the lens and the iris (in the anterior and posterior chambers) and nourishes the cornea. This is constantly being regenerated
Describe the Cornea
An avascular “window” at the front of the globe. This is the main refractive medium in the eye
What is the average refractive power of the cornea? How thick is the entire cornea?
+43.00 D (+42.00 to +45.00 D) which is equivalent to 2/3 of the eye’s refractive power. It is only 0.5 mm thick
What are the 5 (6) layers of the cornea?
1) Epithelium
2) Bowman’s Membrane
3) Stroma
4) Descemet’s Membrane
> 5) Dua Layer
6) Endothelium
Describe the Crystalline Lens
A semi-rigid, bi-convex transparent tissue that refracts the light to focus on the retina. The structure is similar to an onion as it is made of layers. It is held in place by the suspensory ligaments. This has a refractive power of ~12-16 D
What is the index of refraction of the crystalline lens?
n = 1.42
Describe the Iris
Iris = rainbow (in Greek). Circular, pigmented muscle that controls the pupil size with sphincter and dilator muscles
Describe the pupil
The central opening in the iris
What is the average size of the pupil (in normal lighting)?
3-6 mm
What is the trabecular meshwork?
A “screen” over the canal of Schlemm
What is the index of refraction of the cornea?
n = 1.376
What are the two types of photoreceptors in the retina?How many of each are in the eye?
Rods (130 million) and Cones (7 million)
What are Rods responsible for?
Low light vision (scotopic), peripheral vision and detecting movement
What is unique to the function of Rods?
They are interconnected therefore they have aplification
What are Cones responsible for?
Photopic vision (normal lighting), colour and central vision. Most of your cones are found in the macula
What is unique to the function of Cones?
Each cone is colour specific and they do NOT have amplification
Describe the Macula
5 mm diameter zone of the retina that is responsible for central acute vision. At the middle there is the fovea centralis
Describe the Fovea Centralis
The very central part of the macula that is responsible for extreme acute fine details ~ 3000 cones are found in the fovea
Describe the Ora Serrata
Junction fo the retina and ciliary body. This is a landmark (no function). This is a common place for retinal detachments
What is the Uveal Tract?
The 3 pigmented parts of the eye: the choroid, Ciliary body and iris
What is Uveitis? What are common symptoms?
Inflammation of the uveal tract. Light sensitivity and troubles with accommodation
Describe the Vitreous Humour
A jelly-like watery fluid int eh posterior cavity that uses negative pressure to hold up the retina. This is NOT replenished
Describe the Optic Disc
This is the head of the Optic Nerve found 3-4 mm nasal to the fovea. This is a natural “blind spot” as there are no rods of cones found here therefore there is no perception of light. This is where nerve fibres leave the eye and blood vessels enter the eye
Describe the Optic Nerve
The nerve that carries information from the eye to the brain
Describe the Optic Chiasm
The location in the brain where the right and left optic nerves cross
Describe Optic Radiations
When the optic nerves reach the brain they fan out into radiations. The brain brain combines the visual impulses from these radiations with memory cells to produce a mental image onto the occipital lobe.
What is a special function of the macula?
It absorbs Extra UV light - it acts as a natural sunblock
Describe the Conjunctiva
A thin mucous layer on top of the sclera and lining the eye lids. Important for lubrications and protection
Describe the Zonules of Zinn
A.K.A The suspensory Ligaments
Suspensory fibres that connect the crystalline lens to the ciliary body
Describe the Limbus
The dark border between the sclera and the iris. This contains blood vessels and supplies nutrients to the outer edge of the cornea. It contains stem cells
Describe the Outer Adnexa
All the visible parts of the eye.
Define Accommodation
The process by which the eye changes optical power to maintain a clear image as the object distance varies
What is happening when your eye is looking at an object in the distance?
There is tension on your suspensory ligaments and your ciliary body is moving AWAY and it is int he relaxed position. This causes the lens to be flatter therefore less power
What is happening your eye is looking at something at near?
The ciliary body contracts and and moves TOWARDS the the lens which loosens the suspensory ligaments. This causes the lens to bulge which creates more plus power
What causes eye fatigue?
The ciliary body is contracted and lactic acid builds up and causes discomfort
Define Amplitude of Accommodation
Range in power for the eye to see near and keep the object in focus
Define Near Point
Closest distance where you can hold an object and keep it in focus
Define Far Point
The furthest distance where you can keep an object in focus
Define Presbyopia
Gradual loss of the power of accommodation due to the inelasticity of the crystalline lens
Define Fusion
The ability to unite the images from each eye into one image
Define Diplopia
When the brain cannot fuse the images coming from each eye it leads to double vision
What are the two types of Glaucoma?
Open Angle - Most common, no symptoms, most dangerous
Closed Angle: Sudden onset, extreme within a couple hours. Blurred vision, photophobia, red eye, vomiting, halos. This occurs when the iris bends and sticks to the crystalline lens and IOPs increase in the posterior chamber
Define Cataracts
Over time the crystalline lens becomes cloudy ad harder which causes loss of vision when if left untreated it will cause complete blindness. it is normally caused by a chemical change in the composition of the lens
What are common symptoms of cataracts?
Blurry vision, impaired night vision, impaired depth perception, distorted colour vision
What are the three main causes of cataracts? Which is the most common in Canada?
Congenital - Rare in Canada
***Senile - Aging and long term exposure to UV
Trauma - Full cataracts can develop in 2-3 days
What are the 6 major Extrinsic Muscles of the Eye and what are their motions?
- Superior Rectus: Up and In
- Inferior Rectus: Down and In
- Lateral Rectus: Out
- Medial Rectus: In
- Superior Oblique: Down and Out
- Inferior Oblique: In and Out
Define Othophoria/Isophoria
“Normal” muscle balances resulting in normal binocular vision. When observing a distant object, the lines of sight are parallel and when looking at an object at near the lines of sight converge
Define Strabismus
Deviation of the eyes from the normal position usually caused by a muscle imbalance. NO BINOCULAR VISION because the images do not fall on corresponding parts of the retinas causing lack of fusion; the one eye is seeing a different image from the other
What are 3 difference names for Strabismus? What are the treatment options?
Heterotropia, cross-eyed, squint. Surgery and prism.
What are the two types of optical deviation?
Phoria: temporary deviation (tired, medication, alcohol induced. you can straighten your eyes with concentration
Tropia: permanent deviation
What are the 4 directions of movement of strabismus?
Hyper - up
Hypo - down
Exo - out
Eso - in
How does a prism bend light?
Light always refracts to the base, and the IMAGE always moves towards the APEX.
When correcting strabismus, how should you align the prism?
The BASE of the prims should always be OPPOSITE the deviation so you move the image to be in front of the eye
Define Amblyopia
A.K.A lazy eye
Poor vision in one eye with no physiological cause therefore the brain favours one eye over the other.
What are the leading causes of amblyopia?
- Strabismus
- Anisometropia
- Typically there is something wrong neurologically of within the optic tract
What are some behavioural signs indicating Amblyopia
- Covering one eye to see/read
- Squinting
- Turning head to face dominant eye to watch TV
- No pain/symptoms
What are 3 treatments for Amblyopia?
- Patching - cover the good eye to force the bad eye to work and train the brain to receive information from the poor eye
- Medicated drops - drops in the good eye to “freeze” accommodation to force the poor eye to work more
- Fogging - put excess plus in front of the good eye
What is important in the treatment of amblyopia?
Catching it early (before age 4)
Define Steropsis
The ability of the brain to interpret depth, width and height of an object. This is what allows for depth perception
Define Anisometropia
- Approximate same/similar prescriptions in both eyes (but not the exact same)
- Same Type (both + or both -)
Define Antimetropia
The two eyes have different types of power (one + and one -).
When answering m/c the BEST answer is the one with the biggest difference in power
Define Aniseikonia
Magnification imbalance between eyes, the brain interprets difference sizes or shapes of image which results in no fusion
What can causes Aniseikonia?
Trauma or amblyopia
Do the prescriptions have to be vastly different in order to cause aniseikonia?
NO - the prescriptions can be very similar, as sometimes this can be attributed to differences in the perceiving laters of the retina, or higher in the brain
How do you correct aniseikonia?
Iseikonic lenses - lenses customized for base curve and centre thickness
