Visual optics Flashcards
Whatn happens when light undergo refraction?
Give the formular for refractive index of an object.
it slows down and it changes the path towards original line.
Refreactive idnex = speed of light in vacuum/speed of light in the object
What 3 things happens to a ray of light when it enters a new medium
Reflected- angle of incidence is equal to angle of reflection
Refracted
Absorbed
what are the 2 basic types of lenses and what do they do
Convex- brings light rays together to a point. A magnifying glass uses this lens
Concave- takes light rays and spreads them outwards
what is Emmetropia
Adequate correlation between axial length and refracrtive power
the parralle light ryas fall on the retina (no accomodation)
what is Ametropia and what are the different types
Mismatch between axial length and refractive power; parallel rays don’t fall on the retina.
Types are:
- Near-sightedness (Myopia)
- Farsightedness (Hyperopia)
- Astigmatism
- Presbyopia
Desciribes the features of myopia. i.e. give etiology and causes
The parallel rays converge at a focal point ANTERIOR to retina.
Etiology: not clear, genetic factor
Causes:
- Excessive long globe (axial myopia)- COMMON
- refractive myopia (too much refracrtive power)
what are the symptoms of myopia?
How do u treat it?
- Blurred distance vision
- Squint in an attempt to improve uncorrected visual acuity when gazing into the distance
- Headache
TREATMENT:
Correction with DIVERGING LENS
Give the features of hypertropia
Parallel rays converge at a focal point posterior to the retina
Etiology : 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 is vary from inability to read fine print to near vision is clear but suddenly and intermittently blur
- blurred vision is more noticeable if person is tired , printing is weak or light inadequate
Asthenopic symptoms :
- eyepain,
- headache in frontal region,
- burning sensation in the eyes,
- blepharoconjunctivitis (eyelid and ocnjuctiva imflammed)
Amblyopia (lazy eye)– uncorrected hyperopia > 5 Dioptres
why is it important to find hypertropia in children
Can’t read because they can’t see properly. This can lead to learning diffculties.
Hence need treatment ASAP
what are the hypermetropia treatment options
Correction with converging (positive lenses)
Correction with positive lens + cataract extraction
Correction with contact lens
Correction with intraocular lens
Describe the features of Astigmatism including etiology and causes
Parallel rays come to focus in 2 focal lines rather than a single focal point
Etiology: Hereditary
Causes:
refractive media is not spherical
–>refract differently along one meridian than along meridian perpendicular to it
–>2 focal points (punctiform object is represent as 2 sharply defined lines)
what are the sympotms and treatment of Astigmatism?
Symptoms
- Asthenopic symptoms (headache , eyepain)
- blurred vision
- distortion of vision
- head tilting and turning
Treatment
- Regular astigmatism : cylinder lenses with or without spherical lenses (convex or concave), Sx
- Irregular astigmatism : rigid cylinder lenses or surgery
What is the Near resposne triad
- Pupillary Miosis (Sphincter Pupillae) to increase depth of field
- Convergence (medial recti from both eyes) to align both eyes towards a near object
- Accommodation (Circular Ciliary Muscle) to increase the refractive power of lens for near vision
What is Presbyopia and how can it be corrected
Naturally occurring loss of accommodation (focus for near objects) as they lose elasticity
Onset from age 40 years
Distant vision intact
Corrected by reading glasses (convex lenses) to increase refractive power of the eye
Describe the steps for surgical correction
LASIK
- 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 intra-ocular lens used to treat? what will evetually happen due to this treatment
Myopia and astigmatism
Eventually the ICL will be opaque and cataract will occur. Hence will need to remove both lens before operating.
Describe and explain the classification of lesions to the Oculomotor nerve
Medical lesions:
- typically affect the vasculature of the nerve and it spares the pupils
- Central portion of nerve typically affected
- microvascular diseases can cause this
Surgical lesions:
- usually a form of posterior communicating artery aneurysm
- this anuerysm compresses the outer portions of the nerve (the PNS fibres)
N.B; CN3 run in between posterior cerebral artery and superior cerebellar artery
How does pilocarpine work
it is an M3 agnoist (muscarinic) in iris sphincter muscle
causes miosis
it is independent of PNS
what is Adie’s pupil? what are the other presentations of the disease-causing Adies pupil?
It is described as having light-near dissociation.
There is damage to ciliary ganglion and lead to aberrant reinnervation. there is also upregulation of post synaptic receptors
The fibres targeted for the ciliary ganglion ends up in iris instead and therefore you get miosis with near accommodation rather than to light.
This is part of Adie’s syndrome which can cause absent knee jerk reflexes and impaired sweating and other autonomic nervous system damage if it affects the dorsla root ganglions
