Sem1 - 2 - TLA

Question 1

General structure of TLA

Answer 1

• Lipophilic aromatic ring
• Linkage site (amide or ester)
• Intermediate chain
• Hydrophilic amine

Question 2

TLA mechanism of action

Answer 2

• Weak base that’s ionised at physiological pH
• Lipophilic (uncharged) part penetrates membrane
• Charged part binds to Na+ channel.
• Blocks Na+ channels so nerves don’t depolarised to conduct their APs.

Question 3

Which nerve fibre class do TLAs target?
- Describe this class

Answer 3

Class C:
- Pain and temp
- Unmyelinated
- Slow 0.5-2m/s
Since they only target class C, have little effect on ocular function.

Question 4

What do onset and duration of TLA depend on?

Answer 4

Onset:
- Concentration + diffusion time
- Vasodilation = quicker
- Alkaline pH increases anaesthesia effects, inflammation decreases pH so less anaesthesia effects.
Duration:
- Vasoconstriction (prolongs LA as blood doesn’t get removed as quickly) but for ocular LAs, more dependent on tear removal than blood.
- Lipid solubility + protein binding.

Question 5

Different types of TLAs?
- Name the drugs.

Answer 5

Ester:
- Amethocaine/tetracaine
- Oxybuprocaine HCl 0.4% (main one)
- Proparacaine
Amide
- Lidocaine

Question 6

Difference between Ester and Amide TLAs

Answer 6

Esters can release PABA, a known allergen. Avoid esters in px taking anticholinesterase medication e.g. neostigmine for myesthenia gravis.
Amide is metabolised in liver so longer-lasting but have to avoid in liver disease.

Question 7

Onset, max effect, and recovery time of TLAs?

Answer 7

30s, 1min, 20min

Question 8

Side effects of TLA?

Answer 8

• Stings on instillation
• Allergic response possible
• Cornea epithelial desquamation/loss (by damaging TJs between epithelial cells)
• Slows corneal wound healing
• Reduces reflex tearing
• Decreases TBUT by disrupting corneal microvilli
• Max safe dose at 8 drops (typically usage of 1-2 drops)d

Question 9

What’s Fluress?

Answer 9

Combined TLA and NaFl

Question 10

Reasons for pre-treating w/ TLA?

Answer 10

• More permeation of other drugs.TLAs destroy TJs of epithelial cells which allow other drugs to more easily pass through cornea thus making the ocular eye drops more effective than w/o having used the TLA.
• Reduced stinging of other drugs. In pedriatics, can be used to reduce stinging from other eye drops like cyclopentolate for cycloplegia. This will increase compliance w/ children.

Question 11

What happens when misusing TLA?

Answer 11

Shouldn’t be self-adminstered by patients
- Superivison required
- Cases of severe keratitis and even eye loss reported
- Dnese corneal ring infiltrate w/ abuse of TLA.