21: Local Anesthetics

Question 1

how do topical anesthetics work?

Answer 1

they are surface-acting drugs that produce a reversible inhibition of the sensory nerve endings within the corneal and conjunctival epithelium, producing transient local anesthesia of the corneal and conjunctival surfaces

Question 2

what can topically applied anesthetics cause transiently?

Answer 2

-irregularity of the corneal epithelium and corneal disruption can interfere with subsequent procedures requiring visualization inside the eye, such as funds photography

Question 3

what should be measured before topical anesthesia

Answer 3

tear break up time

Question 4

does epinephrine and vasoconstrictors have any effect on topical anesthesia

Answer 4

no significant effect on the duration of topical anesthesia (shouldn’t be combined)
-only epi. is used with local infiltrative injections like in dentistry

Question 5

describe how local anesthetics are reversible

Answer 5

reversible conduction blockade of nerve impulses, therefore, the effects of local anesthetics are completely reversible, without any evidence of structural damage to nerve fibers

Question 6

the 2 ways in which local anesthetics produce anesthesia?

Answer 6

by inhibiting excitation of nerve endings or by blocking conduction in peripheral nerves

Question 7

how does local anesthesia interact with sodium channels ?

Answer 7

• reversible binding and inactivation of sodium channels
• sodium influx through these channels is necessary for the depolarization of nerve cell membranes
• when a nerve loses depolarization and capacity to propagate an impulse, the individual loses sensation in the area supplied by the nerve

Question 8

what is the main way local anesthetics prevent the generation and conduction of nerve impulses?

Answer 8

-reducing sodium permeability

Question 9

what are the 3 distinct structural components of local anesthetics

Answer 9

• aromatic lipophilic portion
• an intermediate alkyl chain or linkage (determines anesthetic class)
• hydrophilic amine group

Question 10

what structural component determines the class of anesthetic agent?

Answer 10

the intermediate linkage

Question 11

what does the intermediate linkage determine?

Answer 11

determines metabolism and other pharmacological properties

Question 12

what is PABAs role?

Answer 12

PABA esters metabolized in the plasma by esterases

Question 13

amides are metabolized by the

Answer 13

liver

Question 14

all topically-applied anesthetic agents are what class

Answer 14

esters (except for lidocaine)

Question 15

most injectable anesthetic agents are what class

Answer 15

amides

Question 16

in the middle of the structural components is a link or chain which is either an ___ or an ____

Answer 16

an ester or an amide

Question 17

3 options for linked to the aromatic group by an ester

Answer 17

• ester of PABA
• ester of meta-aminobenzoic acid
• ester of benzoic acid

Question 18

list 2 ophthalmic preparations that are esters of PABA

Answer 18

• tetracaine

- benoxinate (also in combo with fluorescein)

Question 19

list 2 ophthalmic preparations that are esters of meta-aminobenzoic acid

Answer 19

• proparacaine (wide use, comfort, less corneal compromise)

- proparacaine and fluorescein (Flucaine)

Question 20

list an example of an amide of benzoic acid

Answer 20

-lidocaine (Atken): an ophthalmic gel product

Question 21

allergic reactions to local anesthetics occur almost exclusively with which group?

Answer 21

to those with an ester linkage (PABA derivatives)

Question 22

although anesthetics in current ophthalmic use have relatively low systemic and ocular toxicity, what are some possible adverse effects?

Answer 22

• allergic reactions (both local and systemic)
• CNS stimulation
• cardiovascaular stimulation

Question 23

describe the mechanism of action of local anesthetics

Answer 23

stabilize neuronal membrane so the neuron is less permeable to sodium ions, preventing the initiation and transmission of nerve impulses

Question 24

compare/contrast proparacaine and tetracaine

Answer 24

• approximately equipotent
• both have rapid onset of anesthesia (within 10-20seconds)
• duration of action (10-20 mins)
• proparacaine may have slightly faster onset/longer duration

Question 25

prolonged use of local anesthetics could result in

Answer 25

• cause corneal epithelial erosions
• keratitis
• corneal opacification
• corneal perforation

Question 26

what is one unique ADE/ Caution for proparacaine and tetracaine

Answer 26

rare, severe immediate-type allergic corneal reaction has been reported
-characterized by acute diffuse epithelial keratitis with filament formation and sloughing of large areas of necrotic epithelium, diffuse stream edema, and iritis

Question 27

which appears to produce greater corneal compromise? proparacaine or tetracaine ?

Answer 27

tetracaine

Question 28

what is different about localized infiltrative injection and when is it indicated?

Answer 28

they are preferred over topical when a deeper more prolonged anesthesia is required for minor surgical procedures involving the eye and adnexa

Question 29

why may some injectable preparations include epinephrine?

Answer 29

to produce a longer acting block, to decrease systemic side effects of the anesthetic, and to provide for local hemostasis