Neuro - Visual System pt. 2 Flashcards
What is refraction?
when light passes through one medium to another, its velocity changes
How is the index of refraction calculated?
index of refraction (n) = speed of light in a vacuum / speed of light in a more dense medium = c/v
What are the basic types of lenses?
→ convex
→ concave
What is a convex lens?
→ almond shaped
→ converging lens
→ bring light rays to a point
What is a concave lens?
→ diverging lens
→ spreads light rays away
What are some applications of lenses?
cameras, etc.
What is emmetropia?
→ adequate correlation between axial length + refractive power
→ parallel light rays fall on the retina without accommodation
What is ametropia?
→ refractive error
→ mismatch between axial length + refractive power
→ parallel light rays don’t fall on the retina (no accommodation)
What are some examples of ametropia?
→ myopia (near-sightedness)
→ hyperopia (far-sightedness)
→ astigmatism
→ presbyopia
What is myopia?
→ short-sightedness
→ parallel rays converge at a focal point anterior to the retina
What are symptoms of myopia?
→ blurred distance vision
→ squint in an attempt to improve uncorrected visual acuity when gazing into distance
→ headache
What are causes of myopia?
→ not clear, possible genetic factors
→ excessive long globe (axial myopia)
→ excessive refractive power (refractive myopia)
What is myopia treatment?
→ correction with diverging lenses
→ correction with contact lens
→ correction by removing the lens to reduce refractive power of the eye
What is hyperopia?
→ long-sightedness
→ parallel rays converge at focal point posterior to the retina
What are causes of hyperopia?
→ not clear, possible genetic factors
→ excessive short globe (axial hyperopia)
→ insufficient refractive power (refractive hyperopia)
What are symptoms of hyperopia?
→ visual acuity at near tends to blur relatively early
→ nature of blur varies from inability to read fine print, to near vision is clear but suddenly and intermittently blurred
→ blurred vision is more noticeable if person is tired, printing is weak or light inadequate
→ eyepain, headache in frontal region, burning sensation in the eyes, blepharoconjunctivitis
What is amblyopia?
uncorrected hyperopia > 5D
How is hyperopia treated?
→ correction with converging lenses (positive lenses)
→ correction with positive lens + cataract extraction
→ correction with contact lenses
→ correction with intraocular lenses
What is astigmatism?
parallel rays come to focus in 2 focal lines rather than a single focal point
What are causes of astigmatism?
→ hereditary
→ 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 asthenopic symptoms?
→ fatigue → pain in or around the eyes → blurred vision → headache → occasional double vision
What is asthenopia?
→ eye strain
→ non-specific disorder
What are symptoms of astigmatism?
→ asthenopic symptoms
→ blurred vision
→ distortion of vision
→ head tilting + turning
How do you treat regular astigmatism?
cylinder lenses with or without spherical lenses
How do you treat irregular astigmatism?
→ rigid cylinder lenses
→ surgery
What is the Near Response Triad?
how near vision is mediated by 3 separate but simultaneous pathways:
→ pupillary miosis
→ convergence
→ accommodation
What is the Pupillary Miosis pathway?
→ sphincter pupillae muscle in iris contracts
→ causes the pupil to constrict
→ increases depth of field
What is the Convergence pathway?
→ medial rectus muscle contract
→ both eyes adduct medially
→ aligns both eyes towards a near object
What is the Accommodation pathway?
→ circular ciliary muscle
→ lens accommodates, increasing refractive power required to focus on a near object
What is presbyopia?
→ naturally occurring loss of accommodation (focus for near objects) due to loss of lens elasticity
→ onset from age 40 years
→ distant vison intact
How can presbyopia be corrected?
→ corrected by reading glasses (convex lenses) to increase refracting power of the eye
→ bifocal glasses
→ trifocal glasses
→ progressive power glasses
What are the advantages of having contact lenses?
higher quality of optical image + less influence in size of retinal image than spectacle lenses
What are the disadvantages of having contact lenses?
→ careful daily cleaning and disinfection → expense complication → infectious keratitis → giant papillary → conjunctivitis → corneal vascularisation → severe chronic conjunctivitis
What are the advantages of intraocular lenses?
→ replacement of cataract crystalline lens
→ give best optical correction for aphakia
→ avoid significant magnification and distortion caused by spectacle lenses
What are the different surgical options for correction?
Keratorefractive surgery :RK, AK, PRK, LASIK, ICR, thermokeratoplasty
Intraocular surgery : clear lens extraction (with or without IOL), phakic IOL
What is the method of laser surgical correction?
→ 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
What is the ICL?
→ Staar Intra-Collamer Lens
→ inserted into the eye for the correction of myopia + astigmatism
