Structure & Function of the Eye - Part 3 Flashcards
Which lens converges light rays?
Convex lens (converging lens)
Which lens sprays out light rays?
Concave lens (diverging lens)
What do you call eyes which have perfect refractory power?
Emmetropia (light rays converge exactly on the back of the retina)
What do you call short sightedness?
Myopia
What do you call far sightedness?
Hyperopia
Compare what happens to the light rays in hypermetropia vs myopia
Hypermetropia - light rays converge behind retinal surface
Myopia - light rays converge in front of retinal surface
State which type of lenses can correct hypermetropic eyes and how this occurs
Convex lenses - provide additional converging power
Give two causes of hypermetropia
Axial hypermetropia (short globe) Refractive hyperopia (flat corneal surface) Can be inherited, but not clear
State two causes of myopia
Axial myopia (long globe) Refractive myopia (high corneal curvature)
Which type of lenses can correct myopic eyes, and how?
Convex lenses - spray light rays out a little further until they reach the back of the eyes
What is astigmatism?
When each eye has different light refraction powers at different orientations, therefore the parallel rays come to focus in two different lines
How does a patient with astigmatic eyes see distant objects?
As blurred ellipses
How can you correct regular astigmatism?
Cylinder lenses
Give three symptoms of myopia
Blurred distant vision
Headache
Squinting
Give symptoms of hyperopia
Visual acuity - can vary from inability to read fine print to near vision is clear but suddenly intermittently blurry Asthenopic symptoms: -Eye pain -Headache -Burning sensation in eyes -Eye strain -Blepharoconjunctivitis
When are the symptoms of hyperopia more noticeable?
When patient is tired,
Lighting is weak
Printing is inadequate
What is blepharoconjunctivitis?
Blepharitis - inflammation of eyelids + conjunctivitis
What is the technical term for eye strain due to weak eyes?
Asthenopia
What can result from uncorrected hyperopia?
Ambylopia - when the eyes cannot be corrected (lazy eye)
What are the causes of astigmatism?
Hereditary
What are the symptoms of astigmatism?
Asthenopic symptoms
Blurred vision
Distortion of vision
Head tilting and tuning
How can you correct irregular astigmatism?
Rigid contact lenses
Surgery
Describe the near response triad of adaptation for near vision
Pupillary miosis via sphincter pupilae to increase depth of field
Convergence to align eyes to a near object
Accommodation via circular ciliary muscle to increase refractive power
What do you call age related vision loss of accommodation?
Presbyopia
Describe the vision in presbyopia and what kind of lenses are needed to correct it
Distant vision is intact Convex lenses to increase refractive power of eye Bifocal Trifocal Progressive power glasses
Describe the types of spectacle lenses
Monofocal lenses
Multifocal lenses
What type of monofocal lenses are there?
Spherical lenses
Cylindrical lenses
Give some advantages of using contact lenses
Higher quality of optical image
Less influence on the size of retinal image than spectacle lenses
Cosmetic
Useful for athletic, occupational, activities
Give indications for contact lenses
Cosmetics Athletics Occupation Irregular corneal astigmatism High anisometropia Corneal disease
What is anisometropia?
When two eyes have different refractive powers
Give disadvantages for contact lenses
Requires careful daily cleaning to avoid infections
State some complications of contact lenses
Infectious keratitis
Giant papillary conjunctivitis
Corneal vascularisation
Severe chronic conjunctivitis
State four types of optical correction
Spectacles
Contacts
Intraocular lenses
Surgical correction
Give indications of intraocular lenses
Replacement of cataract crystalline lenses
Best correction for aphakia (lack of lens due to surgical removal - spectacles would be too distorting)
State some types of surgical corrections for eye conditions
Thermokeratoplasty Keratorefractive surgery Photorefractive keratectomy (PRK) Astigmatic keratotomy LASIK (laser) Intracorneal ring (ICR)
Intraocular surgery (lens extraction) Phakic IOL (implantable lens)
What happens in clear lens extraction surgery
Lens extracted e.g. in cataract
Implantation of artificial lens
What is the consequence of cataract extraction?
Loss of accommodation (patient will need reading glasses)
Explain the mechanism of accommodation
Ciliary muscle contracts
Relaxation of zonules (note these are NOT active contractile bands, they’re passive elastic bands between the lens and the ciliary body)
Lens becomes convex
Increased refractive power of the lens
Which nerve mediates accommodation?
Third cranial nerve
