visual systems 2 Flashcards
what is refraction?
light bends as it goes from one medium to another
it is faster in less dense mediums (light bends towards the normal when going into a more dense medium)
refraction index (n) n= speed of light in a vacuum (c) / speed of light in a medium (Vm)
n=c/Vm
the denominator will always be smaller, it is unit-less
all substances have a unique refraction index
some light is also reflected at the boundary
angle of incidence = angle of reflection
angle of incidence > than angle of refraction
what are the two types of lenses?
convex:
converging
gathers light rays and brings them to a point
real focal point, focal length is on other side to incident light
concave:
diverging
takes light rays and spreads them
virtual focal point, focal length is on same side as the lens
what is emmetropia?
when in a healthy eye adequate correlation between axial length and refractive power
parallel light rays fall on the retina (no need for accommodation)
what is ametropia?
refractive error
mismatch between axial length (length of eye) and refractive power
parallel light rays dont fall on the retina (without accommodation)
near sightedness (myopia) (most common)
far sightedness (hyperopia)
astigmatism
presbyopia
what is myopia?
short sightedness
parallel rays converge at the focal point ANTERIOR to the retina
image doesnt fall on the surface of the retina
aetiology: not clear, genetic factor
causes:
excessive long globe (axial myopia)- more common
excessive refractive power (refractive myopia)
what are the symptoms of myopia?
blurred distance vison
squint in attempt to improve uncorrected visual acuity when gazing into the distance
headache
how is myopia treated?
- correction with a diverging (concave) lens (negative lenses)
- correction with contact lenses
- correction by removing the lens to reduce the refractive power of the eye
what is hyperopia?
parallel rays converging at a focal point posterior to the retina
aetiology: not clear, inherited
causes:
excessive short globe (axial hyperopia) - more common
insufficient refractive power (refractive hyperopia)
what are the symptoms of hyperopia?
visual acuity at near tends to blur relatively early
nature of blur varies from inability to read fine print to clear vision with a sudden intermittent blur
blurred vision is more noticeable if the person is tired, printing is weak, or low light
asthenopic symptoms: eye pain, headache in frontal region, burning sensation in the eyes, blepharoconjunctiveitis
amblyopia - uncorrected hyperopia > 5D
unlike myopia, these people cant really read
how is hyperopia treated?
- correction with converging (convex) lenses (positive lenses)
- correction with positive lens and cataract extraction(no lens)
- correction with contact lens
- correction with intraocular lens
what is astigmatism?
parallel rays come to focus in 2 focal lines rather than a single focal point
aetiology: heredity
cause: refractive medium is not spherical (cornea is irregular) ->
refracts differently along one meridian than along meridian perpendicular to it ->
two focal points (punctiform object is represented as 2 sharply defined lines)
between the two focal lines (perpendicular), there is a “circle of least confusion” (Kr), the location with the least loss of image definition
what are the symptoms of astigmatism?
asthenopic symptoms (headache, eye pain)
blurred vision
distortion of vision
head tilting and turning
what is the treatment of astigmatism?
regular astigmatism: cylinder lenses with or without spherical lenses (convex or concave), Sx
irregular astigmatism: rigid cylinder lenses, surgery
what is the near response triad?
adaptation for near vision
- pupillary miosis (sphincter pupillae) to increase depth of field
- convergence ( medial recti from both eyes) to align both eyes towards near object
- accommodation (circular ciliary muscles) to increase the refractive index power of lens for near vision (more curved). (zonules relax)
what is presbyopia?
naturally occurring loss of accommodation (focus for near objects). due to loss of elasticity
onset from 40 years
distant vision intact
corrected by reading glasses (convex lenses) to increase the refractive power of the eye
how is presbyopia treated?
convex lenses in near vision:
reading glasses
bifocal glasses
trifocal glasses
progressive power glasses
why might contact lenses be used instead of glasses?
higher quality of optical images and less influence on the size of retinal image than spectacle lenses
indication: cosmetic, athletic activities, occupational, irregular corneal astigmatism, high anisometropia, corneal disease
what are different types of optical correction?
contact lenses:
disadvantages - careful daily cleaning and disinfection, expense
complications - infectious keratitis, giant papillary conjunctivitis, corneal vascularisation, severe chronic conjunctivitis
intraocular lenses:
replacement of cataract crystalline lens
gives best optical correction for aphakia (no lense), avoid significant magnification and distortion caused by spectacle lenses
what is surgical correction?
ketorefractive surgery: RK, AK, PRK, LASIK, ICR, thermokeratoplasty
intraocular surgery: clear lens extraction (with or without IOL), phakik IOL
what is the general process of (laser) surgical correction?
- pre operative eye
- initial cutting of corneal flap
- cutting of corneal flap
- flipping of corneal flap
- photorefractive treatment (laser)
- corneal stroma reshaped post laser
- corneal flap back in position
- treatment completed
what is the staar intra-collamer lens?
ICL
inserted into the eye in front of real lens
correction of myopia and astigmatism
what is clear lens extraction and intraocular lens?
clear lens extraction + IOL
same as cataract extraction (except lens is softer)
implantation of artificial lens
lose accommodation (patient will need reading glasses)
