Vision Flashcards
Define refraction
Light rays bend to form a sharp image on the retina
Define accommodation
Changing the focus from distant to near objects
What are refractive errors?
Mismatches between distance to retina and how much we bend the light rays, causing the image to be formed in the wrong place
What process describes how we see?
Phototransduction
What three things happen to comprise accommodation?
Lense changes shape (becomes thicker and more spherical)
Pupil constricts
Eyes converge
How does the lense become thicker?
Ciliary muscles contract which makes the ciliary body bulge
This allows the suspensory ligaments to become lax
Lense is no longer being stretched so becomes thicker and more spherical
What innervation causes ciliary body contraction and pupil constriction?
Parasympathetic innervation via the third cranial nerve (oculomotor nerve)
Which muscle constricts the pupil?
Sphincter pupillae
- concentric muscle around the border of the pupil
Which muscles contract to cause the eyes to converge?
Medial rectus muscles
Which extrinsic eye muscle are thicker and why?
Medial recti muscles because we spend a lot of time doing close work
Define myopia
Myopia: short-sightedness
Image is formed in front of the retina due to over-refraction or a too-short eyeball
Close objects look clear, distant objects look hazy
Define Astigmatism
Astigmatism: non-spherical curvature of the cornea or lense
Define presbyopia
Presbyopia: long-signtedness of old age
Define hyperopia
Hyperopia: long-sightedness
Image is formed behind the retina due to under-refraction of light or a long eyeball
Close objects look hazy, distant objects look clear
What are the symptoms of myopia?
Headaches
Complaining of not being able to see the whiteboard or distant objects etc.
Infants and preverbal children may develop a squint
Toddlers may lose interest in sports/people and be more interested in books/pictures
- may also lose interest in class if they can’t see the board
What are the symptoms of hyperopia?
Symptoms of eyestrain after reading or working on the computer
Convergent squint in children/toddlers
Define phototransduction
The conversion of light energy to an electrochemical response by photoreceptors (rods and cones)
Where in the rod/cone cells are the photoreceptors?
In the outer segment
What are the lamellae? Where are they located?
Cell membrane “disks” containing visual pigments
Located in the outer segment of the rod/cone cells
Name the visual pigments and where they are found
Rhodopsin - rod cells
Cone Opsin S, M and L - cone cells
What is Glaucoma? What is the main complication that it can cause?
Abnormally high intraocular pressure caused by a build up of fluid.
Blindness - nerve fibres die from the optic disk outwards
Name the two types of Glaucoma
Chronic: Primary Open Angle Glaucoma
Acute: Angle Closure Glaucoma (this is a medical emergency)
Describe the presentation of acute glaucoma (angle closure glaucoma)
Unilateral blurred vision accompanied by severe pain and a red eye.
Sudden onset (acute)
Patient may experience nausea and vomiting
Describe the underlying mechanisms involved in angle closure glaucoma
Some event causes the peripheral iris to block the angle so that aqueous humour cannot drain:
1) functional black in a small eye with a large lense
2) mid-dilated pupil - iris periphery crowds around the angle and obstructs outflow
3) the iris sticks to the pupillary border which causes the iris to balloon anteriorly and obstruct the angle
Describe the management of angle closure glaucoma
Decrease intraocular pressure
- IV infusion of carbonic anhudrase inhibitors
- constrictor eyedrops
- eyedrops of beta blockers and steroids
Analgesia
Laser iridotomy in both eyes
Describe the clinical presentation of primary open angle glaucoma
Raised intraocular pressure
Visual field defects
Optic disk changes on opthalmoscopy
What causes primary open angle glaucoma?
Blockage of the trabecular meshwork in the drainage angle, meaning that fluid cannot drain properly but is still being produced. The amount of fluid in the eye increases which causes intraocular pressure to also increase.
Describe the management of primary open angle glaucoma
Eyedrops to decrease intraocular pressure - prostaglandin anologues - beta blockers - carbonic anhydrase inhibitors Laser trabeculoplasty Trabeculectomy surgery (last resort)
Which parts of the eye make up the Uvea
ciliary body + iris + choroid
List possible causes of uveitis
Isolated illness
Non-infectious autoimmune disease
- e.g. HLA-B27
Infectious disease
- tends to be chronic diseases such as TB
Systemic disease
- tend to be inflammatory e.g. ankylosing spondylosis
Describe and explain the presentation of uveitis
Symptoms vary depending on location
Unilateral or bilateral depending on location
Inflamed blood vessels leak plasma and WBCs causing blurred vision
Can present with pain
List the possible causes of a corneal ulcer
Infection - bacterial, viral, fungal
Trauma - trauma, corneal degeneration, dysrophy
Describe the main features of non-inflammatory corneal disease
Mostly dystrophies
Bilateral, opacifying, genetically determined
May be due to accumulation of substances (e.g. lipids) within the corna
