Vision

Question 1

If I stuck a needle into the eye, what is the order of structures it would hit?

Answer 1

1. conjunctiva
2. cornea (epithelium)
3. aqueous humor
4. lens
5. vitreous humor
6. blood vessels
7. retina:
   a. ganglion cell axons
   b. ganglion cell bodies
   c. bipolar cells (inner nuclear area)
   d. rod/cones - cell bodies in own layer, outer segment (PR part) in PR layer
   e. retinal pigmented epithelium (RPE) ** this is as far as the light can go
   f. choroid
8. sclera

Question 2

Retinal Cell types and functions

Answer 2

• pigmented epithelium: separates choroid from neural retina; also supports rods/cones, eat the outer segment q10d
• photoreceptors: rods, L-cones (red), M-cones (green), S-cones (blue)
• interneurons: horizontal cells (outer plexiform) and amacrines (inner plexiform); help converge the millions of PR signals to hundreds of bipolar cells
• bipolar cells: input is from horizontal cells; output is to tens of ganglions cells, functions to converge/consolidate signals; there are separate bipolar cells for collecting rods and others for collecting cones
• ganglion cells: axons form the optic nerve

Question 3

Chemistry of phototransduction

Answer 3

1. light hits PR, converts 11-cis-retinal to all-trans-retinal
2. all-trans retinal removes arrestin from rhodopsin and thereby allows it to be activated [phosphorylated by rhodopsin kinase]
3. active rhodopsin activates GTP
4. GTP activates cGMP PDE
5. active PDE lowers cGMP enough to hyperpolarize the membrane which closes Na/Ca channels
6. with enough Ca within the cell the Na-Ca exchanger starts working, cGMP levels go back up, and the membrane is re-polarized; channels are re-opened

Question 4

Describe the types of ganglion cells

Answer 4

Alpha:

• predominate in periphery
• most input from rods
• large axons, many dendrites
• project to Magnocellular layer of LGN for object location

Beta:

• mostly in central retina
• color stimuli
• small receptive fields, small dendritic arbors
• project to parvocellular region of LGN for color/texture of object

Question 5

drusen

Answer 5

• autofluorescence of the PR
• this happens when the RPE cells become less effective at eating the outer segment and leave behind protein, which autofluoresces
• more common as we age
• key factor in dry macular degeneration
• “look like little pebbles in/around the optic disc”

Question 6

Why is there melanin in the RPE cells?

Answer 6

to help absorb some light that comes to the PRs

Question 7

3 stages of HTNive Retinopathy

Answer 7

1. Mild
   - arterial narrowing, wall thickening with opacification
   - copper wire
   - AV nicking
2. Moderate
3. Severe

Question 8

What is the cell composition of the fovea?

Answer 8

• contains only the outer segments of PR cells
• ratio is 1 PR : 1 BC : 1 GC
• *NO horizontal or amacrine cells**
• rest of signaling pathway projects radially away from fovea

Question 9

Describe visual pathway from retina to cortex

Answer 9

1. retinal ganglion cell axons project to form optic nerve
2. nasal fibers decussate to take Right VF to L cortex and vice-versa
3. project to LGN
   • M path: magnocellular to LGN layers 1/2, layer 4C -a in cortex (location info)
   • P path: parvocellular cells to LGN layers 3-6, layer 4C-b in cortex (form)

Question 10

Diabetic retinopathy

Answer 10

• high glucose causes damage to endothelial cells and pericytes in the retinal arteries
• leads to hypoxia; damages PRs and GCs
• leads to activation of GFs; angiogenesis
• patchy loss of vision in the VF

Question 11

Age-related macular degeneration

Answer 11

• damage to choroidal vasculature leading to exudate through the RPE and damage to macula
• central vision is lost
• biggest cause is age; also smoking, genes contribute
• Dry: drusen, little vision loss, often 1 eye
• Wet: blood vessels grow into the macula; sometimes drusen, vision loss, progresses to both eyes
• Dry can progress to wet

Question 12

Amarosis fugax

Answer 12

transient loss of vision for 5-10min

Question 13

Sudden unilateral loss of vision in older adults/young people is most likely what?

Answer 13

• older adults: arterial or venous occlusion

- younger: optic neuritis (*r/o demyelinating disease)

Question 14

What are the most common causes of blurred vision?

Answer 14

• need for glasses
• cataracts
• macular degeneration
• ambylopia (lazy eye)
• trauma

Question 15

What does a positive RAPD indicate?

Answer 15

• usually associated with diseases of the optic nerve, chiasm, or optic tract anterior to the exit of the pupillary fibers
• often associated with arterial or venous occlusive disease, aka widespread retinal disease

Question 16

What change in vision is produced by a lesion in each of these locations:

• optic nerve
• optic chiasm
• post-optic chiasm

Answer 16

• nerve = uni-/ipsilateral scotoma
• chiasm = bitemporal hemianopsia
• post-chiasm = homonymous hemianopsia (left or right VF)
  • congruous = defect is same shape and extent in both eyes
  • incongruous = not the same shape and extent, related to more anterior lesion

Question 17

Optic neuritis

Answer 17

• inflammation of the optic nerve, usually because of swelling/destruction of myelin sheath
• MCC is multiple sclerosis and this may be presenting sign of MS
• usually occurs in young-middle aged adults, presents as sudden central vision loss in 1 eye that progresses over ~2wk, improves over ~3wk (>90% prognosis for return of vision)
• also: pain on eye movement, APD, color desaturation (red); the disc may appear normal (retrobulbar neuritis) or swollen (papillitis)
• tx: IV corticosteroids

Question 18

MCC of chiasm lesion

Answer 18

pituitary adenoma

+ also: meningioma, craniopharyngioma, aneurysm

Question 19

Papilledema

Answer 19

• bilateral swelling of optic discs 2/2 increased ICP; disc appears large and hyperemic
• ddx: pseudotumor cerebri; brain tumor
• normal VA but inc. physiologic blind spot, no pain on eye movement, not APD; other sx related to inc. ICP (N/V/HA); +/- CN6 palsy
• may also see hemorrhages, exudates and cotton-wool spots (CWS=universal sign of ischemia), BUT WON’T see spontaneous venous pulsations

Question 20

Ischemic optic neuropathy

Answer 20

• ischemia of optic disc 2/2 vessel occlusion (central retinal artery or vein)
• almost always unilateral at first
• presents like optic neuritis but in older patients

Question 21

Central retinal artery occlusion - appearance of retina

Answer 21

PALE 2/2 swelling 2/2/ ischemia

• arteries narrowed
• macula has cherry red spot - ischemic infarct
• tx rarely effective

Question 22

Central retinal vein occlusion

Answer 22

RED 2/2 hemorrhagic infarct

• disc swollen/hyperemic
• venous distention
• flame-shaped and other hemorrhages
• no treatment; pts with hyperviscous blood (DM, glaucoma, etc.)

Question 23

Optic atrophy

Answer 23

• many causes; basically anything that would hurt the optic tract
• pale disc
• usually poor vision and VF defect
• if accompanied by large optic cup, think glaucoma

Question 24

Chronic open angle glaucoma

Answer 24

• increased IOP, damage to optic nerve
• this type is chronic, due to age, and is caused by a clog that develops in the drainage path of the aqueous humor
  EARLY –> VF loss (asx, hence screening)
  LATE –> central vision loss

Question 25

Closed/Narrow-angle glaucoma

Answer 25

• increased IOP, damage to optic nerve
• the iris/pupil opening is too small to allow aqueous humor to exit through it, so fluid builds up in/around the lens; this puts pressure on the iris/cornea and pushes shut the drainage path