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Medical Pathophysiology > Week 6 - Lecture 2a - Alterations in vision > Flashcards

Week 6 - Lecture 2a - Alterations in vision Flashcards

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USMLE® Step 1 flashcards
1
Q

What percentage of the body’s sensory receptors are in the eye

A

70%

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2
Q

3 step of vision

A
  1. light/energy strikes the retina
  2. converts energy into action potentials
  3. relayed to brain for processing
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3
Q

Where is vision processed in the cerebral cortex

A

primary visual cortex and visual association areas

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4
Q

pathway of light entering eye

A
  1. lights enters the cornea
  2. light enters the aqueous humor of the anterior chamber
  3. light passes through the pupil of the iris
  4. light enters the lens which sits on top of the vitreous humor
  5. light is refracted to the retina which is composed of an entire layer of photoreceptors
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5
Q

Describe how the pupils control light

A

Think of pupils as a window, the iris is a sphincter, the iris contracts or relaxes (constricts or dilates the pupil) to allow more or less light in.

  1. close vision and bright light- sphincter pupillae (circular muscles) contract; pupils constrict
  2. distant vision and dim light - dilator papillae (radial muscles) contract; pupils dilate - sympathetic fibres - pupil dilation also occurs in response to change in emotional state
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6
Q

Rods

A

more numerous (20 rods for 1 cone)

  • Mostly in peripheral retina
  • vision in dim light
    • very sensitive to light
    • best suited for night vision and peripheral vision
    • contain single pigment : rhodopsin (opsin + retinae)
      • contract with light breaks down rhodopsin
    • perceived input in gray tones only
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7
Q

cones

A

less numerous
mostly in central retina
visual acuity (clarity), bright light and colour
- need bright light for activation, breaking down photopigments
-react more quickly
- have 1 of 3 photopigments for coloured view

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8
Q

vision photopigments

A

erythrolabe : red cone absorbs light at 625-nm wavelength

chlorolabe : green cone absorbs light at 530nm wavelength

cyanolabe : blue cone absorbs light at 455nm wavelength

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9
Q

colour blindness

A

lack of one or more cone pigments

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10
Q

Refraction

A

bending of light rays

  • due to change in speed when light passes
  • occurs when light meets surface of different medium at an oblique angle

light refracted 3 times along pathway

  • entering cornea
  • entering lens
  • leaving lens

majority of refractory power in cornea

change in lens curvature allows for fine focusing

light passing through convex lens (as in eye) is bent so that rays converge at focal point
- image formed at focal point is upside down and reversed right to left

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11
Q

Control of eye movements

A

6 muscles - originate from bony orbit ; insert on eyeball

  1. enable eye to follow moving objects
  2. maintain shape of eyeball
  3. hold in orbit

4 rectus muscles - originate from common tendinous ring : name indicates movement
superior, inferior, lateral, medial rectus muscles

2 oblique muscles move eye in vertical plane and rotate eyeball
- superior and inferior oblique muscles

innervation
CN III, IV, VI

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12
Q

5 movement types

A

saccades : looking from object A to object B
Pursuit : smoothly following a moving object
Convergence/divergence : both eyes turning inward/outward simultaneously
Vestibular : eyes sensing and adjusting to head movement via connections with nerves in the inner ear
Fixation maintenance: minute eye movements that position and accommodate both eyes

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13
Q

Eyebrows function

A

protection/shade eye from sunlight

prevent perspiration from reaching eye

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14
Q

eye lids function

A

protect eye anteriorly
blink reflexively every 3-7 seconds
spread secretions to moisten eye

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15
Q

eye lashes function

A

nerve endings of follicles initiate reflex blinking

lubricating glands associated with eyelids

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16
Q

conjuntiva function

A

transparent mucous membrane

produces a lubricating mucous secretion

17
Q

Lacrimal glands

A

primary producers of tear

lacrimal secretion (tears) - 1ml/day

dilute saline solution containing mucous, antibodies and lysosome

Protection:
enzymes in tears neutralise harmful substances, protect against bacterial infection
provide nutrients and moisture
remove debris and waste from the eye
-closure of eyelid (blinking) forces tear downward, toward nose
-tears enter paired lacrimal canaliculi via lacrimal
- - taste of eye drops

18
Q

Aqueous humor role

A

maintain eye pressure
provide nutrient to the cornea

production: ciliary body
reabsorption (main) : trabecular network (meshwork)
Reabsorption (minor) : uveal- scleral outflow pathway

19
Q

3 steps for aqueous humor

A

aqueous humor forms by filtration from the capillaries in the ciliary processes

aqueous humor flows from the posterior chamber through the pupil into the anterior chamber

aqueous humor is reabsorbed into the venous sinus through the trabecular meshwork and scheme canal (some reabsorbed into larger blood vessels through anterior ciliary body)

20
Q

REVIEW visual lesions on page 14

A

3

21
Q

What are eyes best adapted for

A

distant vision

far point of vision - distance beyond which no change in lens shape needed for focusing - 6m for emmetropic (normal) eye- cornea and lens focus light precisely on retina

ciliary muscles relax - sympathetic activation

22
Q

Close focus

A

light from close objects (<6m) diverges as approaches eye

requires eye to make active adjustments using 3 simultaneous processes

  1. accommodation
  2. constriction
  3. convergence
23
Q

what is accommodation

A

changing lens shape to increase refraction

presbyopia - loss of accommodation over age 50

24
Q

what is constriction

A

accommodation pupillary reflex constricts pupils to prevent most divergent light rays from entering eye

25
Q

what is convergence

A

medial rotation of eyeballs toward object being viewed

26
Q

what is myopia

A

nearsightedness

eye focuses an image in front of retina

correction: biconcave lens to promote divergence of light

27
Q

what is hyperopia

A

farsightedness

focusing image behind the retina

correction: biconvex lens to promote the convergence of light

28
Q

What is astigmatism

A

irregular curvature of the lens
prevents the focusing of an image
blurring vision
cause: corneal scar, scattering in the crystalline lens

correction : glasses, contact lenses, surgery

29
Q

What is presbyopia

A

farsightedness associated with ageing
in ability of ciliary muscle and lens to accommodate for near vision
Correction: bifocals

30
Q

What is strabismus

A

“crossed eyes”
misalignment of visual axes results in inability to focus on a single object (there are many types)
lack of coordination between extra ocular muscle function
prevents the eyes in lining up in the same direction

31
Q

what is amblyopia

A

“lazy eye”

loss of visual detail from uncoordinated eye movement and focus

  • visual axes are misaligned in children
  • may result form strabismus to avoid diplopia

The brain suppresses on of the images

  • visual impairment of the ignored eye
  • from dimness of vision to permanent loss of one eye
  • the impairment may not be fully correctable
32
Q

what is Diplopia

A

double vision

perception of 2 images of a single object

images fall on non-corresponding areas of the retina

may be the result of strabismus

lack of coordination of extraocular muscle

  • mechanical problems
  • disorders of neuromuscular junction
  • CN 3,4,6 palsy

(lens problems can cause diplopia as well : cataract)
(temporary diplopia : alcohol intoxication)

33
Q

What is nystagmus

A

involuntary oscillation of the eye

caused by abnormal functioning of the brain, labyrinth of the inner ear or vestibular pathways responsible for controlling eye movement

congenital : oculomotor abnormalities during neural development
post-natal - acquired : any visual input disruptions
Head injury
space occupying lesion that interferes with vestibular pathways/cerebellar function
stroke
meunière disease
MS
drug, alcohol toxicity

34
Q

Conjunctiva function

A

transparent mucous membrane - produces a lubricating mucous secretion

conjuntivitis "pink eye" 
inflammation of the mucous membrane lining of the eye due to 
viral 
bacterial
or allergy 
causes
35
Q

what is cataract

A

clouding of lens
-altering vision focus by scattering incoming light
protein component of lens (crystallin) clumps or aggregate
increasing opaque area, increasing impairment

36
Q

cataract causes

A

nuclear

  • most common
  • form in the centre, or nucleus of the lens
  • caused by ageing

cortical

  • forms in the lens cortex
  • extends from outer lens towards the centre

posterior sub scapular

  • begins at the back of the lens
  • may result from diabetes and severe hyperopia

other risk factors

  • heavy smoking
  • frequent exposure to intense sunlight
  • some congenital
  • large doses of vitamin C increases cataract formation
37
Q

manifestations and evaluation of alterations in vision

A

decreasing acuity an claret occurs over time

visual acuity : far and near vision
far vision : snellen or E chart

near vision

  • handheld card
  • jaegar chart
38
Q

treatment of alterations in vision n

A 
specific to identified impairment 
muscle imbalances 
-glasses
-patching 
-surgery

errors in refraction

  • glasses, contact lenses
  • surgery

myopia, hyperopia, astigmatism
-laser assisted in situ keratomileusis (LASIK)

cataract
-lens and cataract removal, lens implantation

Medical Pathophysiology (30 decks)

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