Toxic responses of the eye

Question 1

What are rods and cones? Where are they located?

Answer 1

They are photoreceptors located in the retina.

Question 2

Which parts of the eye are commonly exposed to xenobiotics?

Answer 2

Cornea, conjunctiva and eyelids

Question 3

Where is the first site of action for the eye? Why is it capable of doing so?

Answer 3

The tear film, a three layered structure above the cornea. The three layers are mucin, aqueous and lipid. They have both hydrophilic and hydrophobic properties to help flush out irritants. They also contain antimicrobial enzymes, e.g. lysozyme to protect from microbial infection.

Question 4

Which part of the brain is the visual processing centre?

Answer 4

Occipital lobe, located at the rear part of the brain (back of your head).

Question 5

How can light and UV induce phototoxicity?

Answer 5

Light and UV radiations are oxidizing agents. They can induce production of reactive oxygen species (ROS), which can accumulate over time and cause oxidative damage.

Question 6

Which UV radiation is the most damaging and why?

Answer 6

UV-C is the most damaging UV radiation because it has a shorter wavelength of 100 - 290nm. A shorter wavelength = higher energy = more damage.

Question 7

An easy one. What are the three types of UV radiation and their wavelength?

Answer 7

UV-A = 320 - 400nm
UV-B = 290 - 320nm
UV-C = 100 - 290nm

Question 8

How does our eye fight against phototoxicity?

Answer 8

Our eye reduces the amount of light that reaches the retina. This is performed by the cornea and lens. Cornea absorbs 45% of light with wavelength <290nm while lens absorbs most of the UV-A and only allows >400nm to pass through.

Question 9

Explain drug mediated photo-induced toxicity in cornea, lens or retina.

Answer 9

It is the development of intermediates, free radicals and ROS through the combined effects of chemicals and light absorbed. Can be potentiating, additive or synergistic.

An example is the generation of fluorophores and pigments that lead to yellow brown discoloration.

Question 10

What are the 3 essential function of the cornea?

Answer 10

1. Provide a clear refractive surface for the visual image to be focused at the retina
2. Provide tensile strength to maintain the globe shape of the eye.
3. Protect eye from external toxic chemicals.

Question 11

The cornea is sensitive to wavelength <290nm. True or False?

Answer 11

True. Anything above 320nm can be passed through the cornea. Anything lesser will be absorbed by the cornea.

Question 12

Exposure to UV-C on the cornea for a long period of time leads to____________.

Answer 12

Photokeratitis. When the cornea is damaged, the eyes become irritated and painful. Stopping UV exposure can reverse this within 48 hours.

Question 13

________ pH can cause severe ocular damage and permanent loss of vision.

Answer 13

Extreme pH, anything <2.5 and >11.5 are damaging.

Question 14

Acidic pH causes more damage to the eye.

Answer 14

False. Alkali causes more damage as it penetrates easily through the eye, causing injuries to both external and internal structures.

eg. Sodium hydroxide, Potassium hydroxide, Calcium hydroxide.

Question 15

Some acid burn is as damaging as alkali burn.

Answer 15

True. For example is hydrochloric acid burn, which is as dangerous as alkali burn.

Question 16

How do you treat an eye exposed to acid or alkali substances?

Answer 16

Irrigate the eye with large amounts of water or saline.

Question 17

Function of lens?

Answer 17

1. Focusing visual image

2. to maintain electrolyte and ionic balance within the eye.

Question 18

What is cataract?

Answer 18

When the lens has a decrease in optic transparency. This decreases the amount of light that reaches the retina. Eventually leading to visual disturbance.

Question 19

What are the different mechanisms hypothesized in the formation of cataracts?

Answer 19

1. Disruption of lens energy metabolism
2. electrolyte imbalance
3. oxidative stress due to generation of free radicals and ROS.

Question 20

Risk factor for cataracts are______

Answer 20

Aging, diabetes, low antioxidant levels and exposure to ocular toxins (corticosteroid, naphthalene, phenothiazine)

Question 21

Why is the retina so vulnerable to toxicant induced damage?

Answer 21

1. It is high fenestrated choriocapillaris and high blood flow rate. Toxicants are deliver quickly.
2. The retina contain many neurotransmitters and neuromodulatory systems. A good site for neurotoxicant to attach to.
3. Additive or synergistic effects of chemicals with light or UV.
4. High rate of oxidative mitochondrial metabolism

Question 22

What are the two chemotherapy drugs that can induce ocular toxicity?

Answer 22

Cisplatin and carboplatin.

They are both alkylating agents. The retina is vulnerable due to high metabolic activity and choroidal circulation.

Question 23

A common side effect of cancer chemotherapy is Ocular toxicity. True or False?

Answer 23

True

Question 24

What is the deal about chloroquine and hydroxychloroquine?

Answer 24

They are anti-malarial and anti-inflammatory drugs that can cause irreversible loss of retinal function.

Question 25

Name the 3 disease of the optic nerves.

Answer 25

Optic neuritis Optic neuropathy Optic nerve atrophy Each can happen on its own or even happen together.

Question 26

What is acrylamide commonly used for and what are the toxic effects?

Answer 26

Commonly used in laboratory assays, such as western blotting which requires polyacrylamide gels. Exposure lead to distal axonopathy (degeneration of terminal parts of sensory or motor neurons).

Question 27

This drug is used as anti-mycobacterial drug for treatment of tuberculosis. High dose can lead to dyschromatopsias (color blindness), decreased contrast sensitivity and visual field loss.

Answer 27

Ethambutol. A toxicant that affects the optic nerve. causes Ethambutol induced neuropathy.

Question 28

Jamie works at a factory that produces cellophane and viscose rayon. After 1 year, her vision starts to become blurry. What is happening to Jamie? What other eye problems will she have?

Answer 28

Jamie is exposed to carbon disulfide, a component found in cellophane and viscose rayon. Her vision is blurry because there are lesions in the retinal vasculature (arteries supplying inner retina). She might develop optic atrophy, disorders of color perception and depressed visual sensitivity as well.

Question 29

One day, holy realizes that her vision becomes darker as if she looks at the world through a tunnel. Doctor say she has tunnel vision. What is the possible cause?

Answer 29

She has methyl mercury poisoning. It is a toxicant that affects the central visual system. She is experiencing a progressive constriction of the visual field, something we call peripheral scotoma. She might also have poor night vision. **scotoma = alteration in field of vision.

Question 30

Methyl mercury can cause _______ and ________ as well.

Answer 30

Blindness and double vision.