The Eye B&B

Question 1

describe the parasympathetic control of miosis (pupillary constriction) - name the specific nuclei involved (2)

Answer 1

2 neuron pathway:
1. begins at Edinger-Westphal nucleus in midbrain, near CN III nucleus
2. nerve fibers enter orbit with CN III
3. synapse at ciliary ganglion, which signals to sphincter pupillae and activates muscarinic receptors (ACh)

Question 2

contrast the clinical significance of the following:
a. eye down and out + pupillary dilation
b. eye down and out + absence of pupillary dilation

Answer 2

[eye down/out = CN III lesion]

parasympathetic fibers control pupillary constriction and run on the outside of CN III

CN III lesion + dilation = parasympathetic nerves impacted, such as by compression by mass (Pcomm aneurysm)

CN III lesion alone = ischemia, such as by diabetes (neuropathy damages interior of the nerve, sparing PNS fibers on the outside)

“Rule of the Pupil”

Question 3

Adie’s Tonic Pupil

Answer 3

unilateral dilated pupil due to blocked parasympathetic innervation

mostly idiopathic, can be caused by disorders of ciliary ganglion (tumor, inflammation, trauma, surgery, infection)

Question 4

which adrenergic receptors control mydriasis (pupillary dilation)?

Answer 4

norepinephrine binding alpha1 receptors —> activate dilator pupillae

Question 5

what are the steps of the pathway controlling mydriasis (pupillary dilation)? (3)

Answer 5

1. post hypothalamus to spinal cord, ending at ciliospinal centre of Budge (C8-T2)
2. spinal cord to superior cervical ganglion - exits at T1 and crosses apical pleura of lung, then travels with cervical sympathetic chain near subclavian
3. superior cervical ganglion to dilator pupillae, courses with internal carotid artery and passes through cavernous sinus

Question 6

how can cocaine be used as a diagnostic test for Horner’s Syndrome?

Answer 6

cocaine blocks NE reuptake, but for this to work it still requires NE to be released

Horner’s Syndrome = impaired sympathetic innervation to face

test: topically apply cocaine to the eye - normal eye will dilate, Horner Syndrome eye will be unable to dilate

Question 7

anisocoria

Answer 7

difference in pupil sizes

Question 8

what nuclei are involved in the pupillary light reflex? (3)

Answer 8

1. light signals to pretectal nucleus in the midbrain
2. pretectal nucleus signals to bilateral Edinger Westphal nuclei
3. EWN travel with CN III and signal to ciliary ganglion on each side to activate sphincter pupillae

does not involve cortex

Question 9

Marcus Gunn Pupil

Answer 9

relative afferent pupillary defect (RAPD) - shining light in one eye produces less constriction than shining light in the other eye, caused by a lesion in the afferent component of light reflex (not sensing light appropriately)

diagnosed with the swinging flashlight test

Question 10

what is Marcus Gunn Pupil classically caused by?

Answer 10

optic neuritis: inflammatory, demyelinating disease commonly occurring in multiple sclerosis

Marcus Gunn pupil = relative afferent pupillary defect (RAPD) - shining light in one eye produces less constriction than shining light in the other eye

Question 11

Argyll Robertson Pupil

Answer 11

bilateral constricted pupils - no constriction to light, but WILL constrict for accommodation (“light-near dissociation”)

strongly associated with tertiary neurosyphilis - causes damage to pretectal nucleus, which is part of light reflex but not accommodation

Question 12

PERRLA

Answer 12

pupils equal, round, reactive to light and accommodation

Question 13

most refraction of the eye is performed by the ____

Answer 13

cornea (fixed)

[some refraction is performed by adjustable lens as well]

Question 14

what type of collagen is found in the lens of the eye

Answer 14

type IV collagen

avascular structure, uses anaerobic metabolism (glucose —> lactic acid)

Question 15

effect of ciliary muscle relaxation vs contraction

Answer 15

ciliary muscles relaxed = flat lens = far sight

ciliary muscles contract = round lens = near sight

Question 16

what is the cause of myopia? how is it corrected?

Answer 16

aka nearsightedness: focal point is in front of the retina because the eye is too long or the cornea is too curved

corrected with a negative lens (concave, spreads light out more as it hits the cornea)

Question 17

what is the cause of hyperopia? how is it corrected?

Answer 17

aka farsightedness: focal point is behind retina because the eye is too short or the cornea is too flat

corrected with a positive lens (convex, causes light beams to converge more as it hits cornea)

Question 18

which 2 systemic disorders are associated with ectopia lentis (lens dislocation)?

Answer 18

1. Marfan’s (most commonly): AD mutation in fibrillin —> upward/outward lens dislocation
2. homocystinuria: deficiency of cystathionine beta synthase —> high homocysteine levels, Marfanoid body habitus, intellectual disability, downward/inward lens dislocation

Question 19

how can diabetes lead to cataracts?

Answer 19

via polyol pathway: high glucose or galactose is converted to sorbitol or galactitol, respectively, by aldose reductase

sorbitol and galactitol are osmotically active, accumulation can cause lens damage leading to cataracts

Question 20

which 2 inherited metabolic disorders can present with cataracts and why?

Answer 20

classic galactosemia (liver failure + cataracts) and galactokinase deficiency (milder, cataracts)

via polyol pathway: high galactose is converted to galactitol via aldose reductase —> osmotically active, accumulation can cause lens damage leading to cataracts

Question 21

which TORCH infection classically causes infantile cataracts?

Answer 21

congenital rubella syndrome: cataracts + sensineural deafness + cardiac malformations + blueberry muffin rash (extramedullary hematopoiesis)

Question 22

what is contained in the macula

Answer 22

oval-shaped area near center of retina, contains fovea (high cone density for high-resolution color vision)

Question 23

retinitis pigmentosa - describe this disorder and how it appears on fundoscopy

Answer 23

inherited retinal disorder characterized by visual loss beginning in childhood due to loss of photoreceptors (rods/cones)

night/peripheral vision lost progressively, constricted visual field

fundoscopy shows bone-spicule pattern with lots of dark pigmented spots where photoreceptors are missing (around periphery)

Question 24

what is the classic cause of retinitis? in which patients does this occur? how does it appear on fundoscopy?

Answer 24

retinal edema/necrosis (can cause floaters) classically caused by cytomegalovirus (CMV) in patients with HIV/AIDS (low CD4 count, <50! Usually first sign of CMV infection in these patients)

also occurs in transplant patients on immunosuppression (also at risk for CMV!)

fundoscopy shows whitish appearance of retina (“pizza”)

Question 25

nonproliferative vs proliferative diabetic retinopathy on fundoscopy

Answer 25

blindness via pericyte degeneration (microaneurysms/hemorrhage)

nonproliferative (most common): “dot and blot hemorrhages” + “cotton wool spots”, can also be yellow hard exudates/macular edema (very dangerous risk of blindness)

proliferarive (retinal ischemia drives neovascularization): proliferation of many small vessels in the retina

Question 26

what are the treatment options for diabetic proliferative retinopathy? (3)

Answer 26

retinal ischemia drives neovascularization - but vessels are abnormal and can cause macula edema/ blindness

1. photocoagulation - use laser to stop vessel growth
2. vitrectomy - removing bleeding and debris
3. anti-VEGF (ranibizumab)

Question 27

how does retinal detachment appear on fundoscopy?

Answer 27

retina is seen crinkled together like tissue paper in one quadrant of the eye

Question 28

retinal detachment is often preceded by…

Answer 28

… posterior vitreous membrane detachment

as vitreous shrinks with age, it can pull on retina and cause holes/tears - this may cause floaters (black spots) or flashes of light

Question 29

what is the typical cause of a branch retinal vein occlusion (BRVO) vs a central retinal vein occlusion (CRVO)

Answer 29

BRVO = compression of branch vein by retinal arterioles at crossing points, associated with arteriosclerosis - fundoscopy will show hemorrhages in one area of retina

CRVO typically caused by primary thrombus disorder - fundoscopy will show hemorrhages throughout the retina

Question 30

what are the typical causes of retinal artery occlusion, and how does it appear on fundoscopy?

Answer 30

commonly caused by carotid artery atherosclerosis (internal carotid —> ophthalmic —> retinal)

can also be caused by giant cell arteritis (inflammation of blood vessels supplying head) or thrombus from cardiac source

fundoscopy shows cherry red spot - red circular area of macula surrounded by a halo (clearing caused by ischemia)

Question 31

what are 2 possible causes of a cherry red spot on fundoscopy?

Answer 31

1. retinal artery occlusion: halo caused by clearing of ischemia
2. Tay Sachs Disease (lysosomal storage disease): halo caused by accumulation of sphingolipids around the macula

Question 32

how does papilledema appear on fundoscopy?

Answer 32

blurred margins of optic disc

due to high intracranial pressure causing mass effect, usually bilateral

Question 33

what kind of visual defect occurs with macular degeneration?

Answer 33

loss of central vision (central scotomas) + distortion (metamorphosia)

Question 34

describe the pathogenesis of dry macular degeneration - how does it appear on fundoscopy?

Answer 34

accumulation of drusen (yellow extracellular material) between Bruch’s membrane (innermost choroid) and retinal pigment epithelium (next to choroid)

causes gradual loss in vision

fundoscopy shows soft yellow spots around macula

Question 35

describe the pathogenesis of wet macular degeneration - how does it appear on fundoscopy?

Answer 35

break in Bruch’s membrane (innermost choroid) - leads to blood vessels forming beneath retina, which hemorrhage can cause rapidly progressive vision loss

tx: laser therapy or anti-VEGF (ranibizumab)

fundoscopy shows hemorrhage in the macula portion of the retina

Question 36

overweight women of childbearing age + headaches + CN VI palsy =

Answer 36

pseudotumor cerebri - fundoscopy will show papilledema (blurred margins of optic disc)

recall CN VI (abducens) courses away from the other CN innervating the eye, and its location makes it susceptible to pressure forces