Structure & Function of The Eye PART 2 Flashcards
What is the idea of refraction?
LIGHT is passing through one MEDIUM into another
Index of refraction?
Measured by:
n = speed of light in a vacuum (c)
———————————————
speed of light in a substance (v)
As c is a set limit AND the denominator can NOT be greater than it, value will ALWAYS be > = 1
What happens when light enters a new medium?
Light bends TOWARDS the normal when slowing down (entering a new medium)
Some light:
o reflects off the boundary
OR
o refracts through the boundary
What are the 2 basic types of lenses?
o Converging (convex)
- focuses light rays onto a SINGLE point
- used for long-sighted glasses & cameras
o Diverging (concave) - diverge light rays
Emmetropia?
Adequate correlation between axial length & refractive power
This is NORMAL - parallel light rays will always fall on the retina with NO accommodation required
Ametropia?
MISMATCH between axial length & refractive power
i.e. Parallel light rays do NOT fall on retina with NO accommodation
Accommodation?
The ability of the eye to change its focus from distant to near objects (and vice versa).
o this process is achieved by the lens changing its shape.
Accommodation is the adjustment of the optics of the eye to keep an object in focus on the retina as its distance from the eye varies
Forms of Ametropia?
- Myopia (near-sightedness)
- Hyperopia (far-sightedness)
- Astigmatism
- Presbyopia
Explain Myopia
Parallel rays converge as a focal point ANTERIOR to the retina
Aetiology is unclear BUT may be genetic
THIS ME!!!!!!!!!
Causes of Myopia?
o Axial myopia (long eyeball/globe)
- MORE common
o Refractive myopia (excessive refractive power)
Symptoms of Myopia?
Blurred DISTANCE vision
Squint in an attempt to improve uncorrected visual acuity
o this is as the CENTRAL LIGHT RAY does NOT refract so can be seen
Headache!
Treatments for Myopia?
o Concave lens (ONENOTE!!)
o Contact lens
o Laser treatment to remove part of the lens
Explain Hyperopia
Parallel rays converge at a focal point POSTERIOR to the retina
Aeitology unclear BUT may be inherited
Causes of Hyperopia?
Axial hyperopia (short eyeball/globe) - more common
Refractive hyperopia (insufficient refractive power)
Symptoms of Hyperopia?
o Visual acuity at near tends to blur relatively early
o Asthenopic symptoms
- e.g. eye pain, headache in frontal region, burning sensation in the eyes, blepharoconjunctivitis
Lazy eyes?
Lazy eye - if one eye is HYPEROPIC in childhood and goes unnoticed, the brain starts to ignore this eye and concentrates on the good eye so leads onto lazy eye if not identified
Amblyopia?
Uncorrected hyperopia > 5 dioptres
lens = 20 dioptres
Treatments for Hyperopia?
o CONVEX lens
o REMOVE lens with laser treatment and have a greater power lens
o CONTACT lens
o Intra-ocular lens
Astigmatism?
Parallel rays come to focus in TWO focal lines rather than a single focal point
Aetiology is hereditary
Cause of astigmatism?
Refractive media is NOT spherical SO rays refract differently along one meridian perpendicular to it HENCE producing more than one focal point
ONENOTE!!
Symptoms of astigmatism?
o Asthenopic symptoms
- e.g. headsche, eyepain etc.
o Blurred & distorted vision
o Head tilting & turning
Treatment for astigmatism?
Regular astigmatism
o cylindrical lens +/- spherical lens
Irregular astigmatism
o rigid contact lens +/- surgery
What is the Near Response Triad and what does it consist of?
Mediates the response for near-vision
Triad consists of:
1. Sphincter pupillae - pupillary miosis to increase depth of field
- Circular ciliary muscle - accommodation to increase refractive power of the lens
- Medial rectus - align BOTH eyes towards a near object
Presbyopia?
Naturally occurring loss of accommodation with age
Onset from 40years of age and DISTANT VISION remains intact
How can Presbyopia be treated/corrected?
Corrected by reading glasses (convex lens) to increase refractive power of the eyes
Can also treat with bifocal, trifocal and progressive power glasses
4 types of Optical correction?
Spectacle lenses
Contact lenses
Intraocular lenses
Surgical correction
Spectacle lenses?
Monofocal lenses - spherical OR cylindrical lenses
Multifocal lenses
Contact lenses?
Higher quality optical image (less influence on size of the retinal image than spectacle lenses)
Can be worn for sports/aesthetic reasons BUT require careful daily cleaning and are £££
Complications include:
o infectious keratitis
o giant papillary conjunctivitis
o severe chronic conjunctivitis
Intra-ocular lenses?
REPLACEMENT of cataract crystalline lens
Give the best optical correction for aphakia (do NOT have lens)
AND
avoid the magnification/distortion caused by spectacles
Surgical correction?
- Keratorefractive surgery
- Intra-ocular surgery (clear lens extraction with or without intra-ocular lens)
o small flap of eye made = laser changes refractive power of cornea = epithelial flap re-sewn onto eye
o extra intra-ocular lens can be placed on top OR old lens can be removed
LOSE accommodation so patients will need reading glasses
Mechanism of accommodation?
- Circular ciliary muscle CONTRACT
- RELAXES the ZONULES that are usually taught
- In absence of zonular tension, the LENS RETURNS to the natural CONVEX SHAPE
- This INCREASES natural power of lens
Mediated by CN III (oculomotor)
Zonules - are passive elastic band with NO active contractile activity
