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MD4001 > The eye and vision (special senses) > Flashcards

The eye and vision (special senses) Flashcards

1
Q

Identify the main anatomical components of vision.

A 
• Eyeball - optical front end and retina/optic disc at the back
ANTERIOR SEGMENT (cornea, iris, ciliary body, and lens)
(made of anterior chamber, between the posterior surface of the cornea (i.e. the corneal endothelium) and the iris, and the posterior chamber (between the iris and the front face of the vitreous))
POSTERIOR SEGMENT (vitreous humor, retina, choroid, and optic nerve)
  • Connections - optic nerve, chiasm, optic tract, LGN, radiation
  • Brain – occipital, temporal, parietal and frontal lobes
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2
Q

What is the function of the cornea ?

A

Major light focusing element of the eye

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3
Q

When might corneal oedema occur ?

A

After surgery for cataract.

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4
Q

Define refraction of the eye. Which parts of the eye are involved in this ?

A

Refracting the light

• Cornea
– largest element (40D)
– Interfaces with air (low RI) - big difference (lot of refraction occurs here)

• Lens
– lesser element (20D)
– Interfaces with aqueous (similar RI) – small difference (less refraction occurs here)
– But can vary in power (accommodation)

• Whole Eye Ball
– About 60D

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5
Q

State the near triad.

A

Triad of things occurring when looking at something up close:

  1. Miosis (because reduces aberration with lens, get pinhole effect, so sharper vision)
  2. Convergence (eyes turning in)
  3. Accommodation
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6
Q

Identify the main refractive errors. Which is the most common ?

A

1) Presbyopia (most common form of refractive error)
2) Myopia
3) Hypermetropia

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7
Q

What is presbyopia ?

A

Failure to accommodate (e.g. when you are older) due to weaker ciliary muscles and stiffer lens, meaning eye gets fixed in the distance

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8
Q

What is myopia ? How does it arise ?

A

Distant objects appear blurred (short-sightedness). Eye is bigger, so when parallel light comes into the eye it focuses short of the back of the eye. Need minifying (minus/concave) lens for the light to hit back of the eye more clearly.

Can be due to:

▫ Open angle glaucoma
▫ Retinal detachment (eye gets bigger but retina remains same size, so retina becomes stretched, and detaches)

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9
Q

What is hypermetropia ? How does it arise ?

A

Nearby objects appear blurred (long sightedness). Associated with squint and lazy eye (amblyopia). Eye is smaller so when light comes in from far, it focuses to point behind retina, so blurred vision. Need magnifying (plus) lens to make it converge more to hit back of the eye.

Can be due to:

▫ Angle closure glaucoma (structures are anatomically crowded in the anterior segment. System can get blocked due to crowding, and increase intraocular P)
▫ Ischaemic optic neuropathy= “Disc at risk”

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10
Q

What is the main type of uncorrected refractive error ?

A

Presboyopia (failure to accommodate when you are older)

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11
Q

What is the main cause of blindness in the world ? of visual impairment ?

A

Cataracts (followed by glaucoma). Other causes include Age-related Macular Degeneration, Refractory Error, Trachoma, Corneal Blindness

Refractive error (followed by cataracts). Other causes include glaucoma, age-related macular degeneration

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12
Q

What is the distance for Arclight ? How does this change the scoring system for visual acuity ?

A

• The Arclight chart is 50% smaller chart with 50% smaller letters and so you use the chart at 50% distance = 3m

Top line is still called the 60 line and so you document the vision as 6/60 and 6/36

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13
Q

What is the standard distance for normal Snellen charts ?

A

6M

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14
Q

Identify steps in testing distance visual acuity.

A
  1. Measure out 3m
  2. Cover left eye with palm of hand
  3. Ask patient to read from the top of the chart
  4. Chart must be held perpendicular to patient in good lighting, smoothed out and flat
  5. Record the ‘number’ of the smallest line that can be seen
  6. If cannot read even the top letter then go to 1.5m and repeat
  7. If cannot read even the top letter at 1.5m then go to 0.5m and repeat
  8. If cannot see at 0.5m then try counting fingers (CF) at 1m, hand movements (HM) then perception of light and classify with projection or with no projection of perception of light then finally no perception of light
  • Repeat for fellow eye
  • Repeat with both eyes together
  • Repeat with pinhole and with glasses
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15
Q

Identify the main categories of visual acuity scores.

A

Normal to Mild Visual Impairment (Group 0 in WHO Classification): <6/18

Moderate Visual Impairment (G1): 6/18 to 6/60

Severe visual impairment (G2): 6/60 to 3/60

Blindness (G3): 3/60 to 1/60

Blindness (G4): 1/60 to HM, CF, PL

Blindness (G5): NPL

Unspecified or observed visual behaviour (G9): N/A

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16
Q

Define accommodation in the context of vision.

A

Process by which eye changes optical power to maintain a clear image or focus on an object as its distance varies.

  • When looking at something in distance, ciliary muscles relax, zonules tighten up, and pull on lens which makes it flatter (less convex shape)
  • When looking at something close, clilary muscle contracting, zonules which hold lens relax, lens more spherical, bends light more
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17
Q

What is the white of the eye made of ?

A

Sclera (continuation of cornea) made up of tough collagen.

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18
Q

Why is the sclera white given that cornea (both made of collagen) clear ?

A

In sclera, collagen fibrils are oriented in such a way that it becomes opaque

In cornea, fibrils are very parallel to each other, which means the light can travel through, and it’s a clear structure

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19
Q

What is the relation between sclera and conjunctiva ?

A

On top of sclera lies the conjunctiva, which is a clear layer (between the two lie BVs) which also lines eyelids

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20
Q

Identify the main components of the iris.

A

Two muscles, one constrictor and one dilator, which form a circular hole (pupil) which is black (hence pupil is formed by iris)

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21
Q

Define hypopyon.

A

Inflammatory cells build up in anterior chamber as yellow layer of cells called hypopyon.

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22
Q

Define hyphema.

A

Layer of red cells at the bottom of anterior chamber of the eye due to bleed

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23
Q

Describe the layers of the cornea.

A

1) Epithelium
2) Thick layer of collagen
3) Endothelium

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24
Q

What is the function of the endothelium of the cornea ?

A

Pumping fluid out of stroma of the collagen layer, to keep it dry, and keep collagen fibers parallel and regularly spaced, to keep clarity of cornea

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25
Q

Describe possible consequences of damage to the endothelial layer of the cornea.

A

If damage to endothelial layer (e.g. due to surgery for cataract) to cornea, fluid builds up in cornea, becomes swollen and thickened, and fibers are not spaced apart the right way, cornea becomes opaque (and corneal oedema)

26
Q

Describe dual innervaiton of the iris.

A

DILATION:
-SNS fibers synapse in superior cervical ganglion, and run with V1

CONSTRICTION
-PSNS fibers synapse in the ciliary ganglion, and run with CNIII

27
Q

Describe possible causes, and effects of sympathetic palsy to the eye.

A

CAUSES

  • Car accident, whiplash, stretched neck and stretched SNS chain on internal carotid artery, resulting in rupture of the chain so decreased/no SNS supply to head and neck, especially the eyes
  • Pancoast tumor

EFFECTS
-Horner’s syndrome

28
Q

What is the main cause of visual impairment worldwide ?

A

UREs

29
Q

What does it tell you if pinhole helps someone overcome refractive error ?

A

Suggests they need glasses

30
Q

What is the name of the charts usually used for visual acuity recordings ?

A

Snellen charts

31
Q

What is Trachoma ?

A

Infectious disorder of the lining of the eye caused by Chamydia. Causes inflammation and scarring of conjunctiva and eyelid. Latter turns in and rubs on cornea, leading to corneal scarring and infection, leading to corneal blindness

4th main cause of blindness in the world

Preventable AND treatable

32
Q

How do cataracts cause blindness ?

A

Lenticular blindess, lens becomes hazy

33
Q

Describe red reflex in different pathologies.

A

LEUCOKORIA (white pupil reflex) may indicate:

  • Retinoblastoma
  • Corneal opacity
  • Hyphema
  • Congenital cataract (back lines)

BLACK OR ABSENT REFLEX may indicate:

  • Corneal scar
  • Complete Cataracts
  • Haemorrhage at the back of the eye

Any asymmetry of the red reflex: may indicate a
refractive error

34
Q

Describe the pathway of the fluid in the eye.

A

Fluid in the eye is created by ciliary body. It flows in posterior chamber, between lens and iris, into the anterior chamber, and then exists through the trabecular meshwork (drains the eye)

35
Q

Explain why acute closed angle glaucoma may happen with age, and identify the main symptoms of this.

A

As get older lens naturally becomes fatter and bulkier because of aging changes, so begins to bulge forward, the iris rubs on the lens, and the fluid going from here gets stuck by this resistance and so
fluid P here builds up, the iris starts bowing forward and it blocks off the trabecular meshwork.

Fluid is building up, fluid cannot get out, so get acute high P in the eye (acute closed angle glaucoma)

SYMPTOMS: painful eye, loss of vision

36
Q

What is the function of the retina in the eye ? of the optic n ?

A

Light sensitive film

Optic nerve collects all the information from the retina and sends it out to the brain.

37
Q

Describe the organisation of the retina.

A

Center of retina = macula (full of cones)

Peripheral part of retina is full of rods

Fovea = Depression at the center of the macula, the area of most acute vision (PRs ultrapacked)

Photoreceptors are located in the pigmented epithelium of the retina (deepest layer)

PRs respond to light via electrical impulse which they sent down bipolar cells, to ganglion cells (axons of ganglion cells take the info out of the eyeball into the optic nerve, chiasm, tract, then brain)

Horizontal and Amacrine (interneurons) Cells:
Connect bipolar cells and allow for summation of information to allow detection of edges and contrast

Information from rods and cones is converged onto bipolar cells, then a ganglion cell (a lot less of those than PRs). Ganglion cell transfers the retinal information to the brain; optic nerve > chiasm > optic tract. First synapse is at LGN (eye and optic nerve stop and brain starts), part of thalamus (major relay station for sensory info)

Lateral cells and amacrine cells act to provide initial processing of the visual signal (but in fovea, the neural components are moved to one side, and the degree of convergence onto ganglion cells is minimal)

38
Q

Identify the main differences between cones and rods.

A

RODS:

  • 120 million in retina
  • High convergence to ganglion cells (lots of roads converge to only one or two ganglion cells)
  • One type (vision in greyscale) (cannot read color, only read one wavelength)
  • Very light sensitive (better at seeing poor illumination)
  • Widespread distribution in retina
  • Broad spectral sensitivity
  • Good at identifying mouvements

CONES:

  • 6 million in retina
  • Low convergence to ganglion cells (less cones going to ganglion cells than rods)
  • Three types (Blue, Green, Red) (responsible for color vision)
  • Only 1/30th the sensitivity of rods
  • Concentrated in macula
  • Narrow spectral sensitivity
  • Responsible for high acuity
39
Q

Draw the posterior chamber of the eye.

A

Refer to slide 47.

40
Q

Graph the number of photoreceptors depending on distance across the retina.

A

Refer to slide 49.

  • Lots of cones in fovea, in center
  • Where have greatest sensitbity
41
Q

What are opsins ? What is their significance in vision ?

A

They are the reason different PRs respond to different wavelength of light (each opsin gives sensitivity to a different range of wavelengths of light).

  • Protein retinal (derived from vitamin A) is found in all rods and cones
  • There are different opsin types, each specific to a different type of cone (3 colors with rod retinal binding an opsin called rhodopsin)
  • When light hits retinal it changes conformations, appears bleached
42
Q

What is the wavelength of visible light ?

A

400 to 700 nanometer

43
Q

When do PRs depolarise ? hyperpolarise ?

A

PRs are depolarised in the dark and hyperpolarise in the light

44
Q

At their most sensitive wavelength, how many photos of light can our eyes detect ?

A

At their most sensitive wavelength, our eyes can detect as little as 5 photons of light

45
Q

Describe the possible results of vitamin A deficiency on the eye.

A 
Vitamin A Deficiency: 
Night Blindness (because opsins are derivatives of vit A, so rods do not work properly) then TOTAL corneal blindness (cornea melts because depending on vitamin A) then Death
46
Q

Identify any current hopes for therapy for retinal disease.

A

Retina is metabolically more active when you are asleep - exploit as therapy for retinal disease, for instance through light-masks to prevent dark-adaptation in the treatment of early diabetic macular oedema

47
Q

Define adaptation.

A
  • Ability of retina to change its sensitivity in diff levels of illuminations.
  • Happens at the level of the retina by altering calcium channels (changes in Ca++ and cGMP levels within the cell alter the sensitivity of membrane channels)
48
Q

Describe cone fatigue.

A
  • When you stare at a specific colour for too long, the cells that detect that frequency of light will get fatigued.
  • The after image is a result your photoreceptors not being ‘in balance’.
  • As the photoreceptors become less tired, which takes between 10 and 30 seconds, the balance is recovered and the after image disappears.
  • At least pre-chiasmal phenomenon (because if cover one eye, does not occur in said eye, whereas if it was a post-chiasmal phenomenon it would occur bilaterally)
  • Retinal phenomenon
49
Q

Describe how bipolar cells work in the light and the dark.

A

• There are two types – ‘ON’ or ‘OFF’

In the dark, a photoreceptor (rod/cone) cell will release glutamate, which inhibits (hyperpolarizes) the ON bipolar cells and excites (depolarizes) the OFF bipolar cells.

In light, however, light strikes the photoreceptor which causes the photoreceptor to be inhibited (hyperpolarized). Hyperpolarizing the photoreceptor cell causes less glutamate to be released, which causes the ON bipolar cell to lose its inhibition and become active (depolarized), while the OFF bipolar cell loses its excitation (becomes hyperpolarized) and becomes silent.

50
Q

Describe the ganglion cell firing rate for the following:

  • No stimulation
  • Stimulation OFF
  • Stimulate OFF and ON
  • Stimulate ON more
  • Stimulate whole area
A

a. No stimulation – baseline
b. Stimulate ‘OFF’ - ↓
c. Stimulate ‘OFF’ and ‘ON’ - ↓↑
d. Stimulate ‘ON’ more - ↓↑↑↑
e. Stimulate whole area – small ↓↑↑

Refer to slide 73

51
Q

Describe possible findings of diabetes in the eye.

A

Diabetes can affect eye by new blood vessels growing, which are very leaky and fragile, so can bleed, and get hemorrhage. These blood vessels can leak fluid and lipid into retina (hard exudate), and cause maculopathy.
If fluid is found in the retina, retina becomes edematous and expands
and all connections can stretch, and then its function reduces so vision goes blurry.

52
Q

Describe possible findings of macular scar toxoplasma in the eye.

A

Central scar in the macula (can also be due to age-related macular degeneration) (center of macula is most metabolically active
part of retina, part that wears out first, so often can get scarring in that area)

53
Q

What is likely to be the leading cause of visual impairment in middle income countries in the one/two decades ?

A

Diabetic retinopathy (due to urbanisation, leading to adoption of Western lifestyle and diet)

54
Q

What is the optic nerve made of ? How does it exit the eye ?

A
  • Collection of all the ganglion cells (nerve fivers from individual PRs run towards optic nerve)
  • Exits the back of the eye through a hole in the sclera
  • The optic nerve head can be seen at the back of the eye (aka optic disc)
55
Q

Define optic disc, optic cup, and neuroretinal rim.

A

Optic disc: where the sensory fibers and vessels conveyed by the optic nerve (CN II) enter the eyeball (insensitive to light, so called the blind spot)

Optic cup: empty space

Neuroretinal rim: ‘tissue between the border of the cup and the disc’

56
Q

What do alterations in the cup to disc ratio mean ?

A

1) If increases (cupped optic nerve), more likely to have glaucoma (because size of cup proxy marker for the
areas of neuroretinal rim, which is directly proportional to the number of ganglion cells running out of the back of the eye). If ganglion cells are dying (as happens in glaucoma), will get less neuroretinal rim, so cup
will expand and get bigger. Normal ratio is 0.4, once cup to disc ratio past 0.6 ot 7, concern of possible
glaucoma

57
Q

What is the aim of direct ophthalmoscopy ?

A

Looking at the back of the eye through pupil using direct ophthalmoscope (can see optic nerve, main central retinal arteries, and veins)

58
Q

Identify possible causes of swollen optic nerve, describing the typical appearance of such a case.

A

Increased intracranial P –> Papilloedema

Edge of optic nerve has become indistinct and swollen

59
Q

Identify the cause of a pale optic nerve.

A

Lost axons running into optic nerve, so optic atrophy. May occur in patients with MS, who had optic neuritis inflammation within optic nerve demyelination episode related to MS, resulting in damaged optic nerve and loss of axons and ganglion cells, and thus optic atrophy.

60
Q

Identify techniques for testing pupils.

A

ASK PATIENT TO LOOK IN THE DISTANCE (otherwise get reflex miosis as part of triad)

  • Measure pupil diameter in light and dark conditions (the less reactive pupil is the abnormal one)
  • Test the direct response (shine light in one eye and examine the response in the same eye)
  • Test the indirect response (shine the light in one eye and examine the response in the other eye)
  • Check for a relative afferent pupillary defect (RAPD) (shine the light on one eye for 2-3 seconds, then rapidly move to the fellow eye. Normal response is either no change in size, or a brief constriction and returning to the same state = Hippus) (A pupil with an RAPD will paradoxically dilate when the light moves towards it)
  • Check for accommodation- alternating fixation on a distant then a near target
61
Q

How can you detect a cupped optic nerve ?

A

Through direct ophthalmoscopy

62
Q

What is the main cause of irreversible blindness ?

A

Glaucoma

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