L5 LA and Stains (s)

Question 1

what are local anesthetics? how are they applied?

Answer 1

Anaesthetics are local agents that produce surface anaesthesia when applied.

• It can be applied in drop form OR administered by injection to a regional area.
• Local anaesthesia occurs without loss of consciousness.
  general anesthetics cause loss of vision

Question 2

what is the basic chemical structure of LAs?

Answer 2

LAs are weak bases consisting of a lipophilic group connected by an intermediate chain (an ester or an amide) to an ionizable group.

• The lipophilic section determines the:
• potency
• duration of action
• potential toxicity

Question 3

what is the basics of nerve conduction?

Answer 3

Sensory information passes along nerve fibres as “electrical impulses”

At rest, the nerve has a negative charge and is polarized. (electrical charge on the outside of the membrane is positive while the electrical
charge on the inside of the membrane is
negative).

• An action potential is generated by the influx
  of Na+ into the interior of the nerve

This is followed by the efflux of K+

• The nerve returns to it resting stage when Na+ and K+ are returned to their original sides: Na+ on the outside and K+ on the inside

Question 4

How do LAs produce numbing effect? (action potential)

Answer 4

LAs prevent the generation & conduction of nerve impulses by
blocking sodium channels.

Question 5

practice drawing of resting state and action potential

Answer 5

ok

Question 6

what are the 8 indications for topical LA use?

Answer 6

1. FB removal
2. Tonometry: Applanation & Schiotz (gold standard for iop measurement)
3. CL fitting e.g RGP
4. Schirmer test 2
   * Schirmer test 1: conducted without LA measures total reflex & basic tearing (to test quantity of tears)
   * Schirmer test 2: w/LA measures only basic secretion
5. Gonioscopy for ant chamber angle
6. Permit adequate examination in the presence of blepharospasm (uncontrollable eye spasm)
7. Nasolacrimal duct probing & irrigation
8. insertion of punctal plugs

Question 7

what are the common LAs for topical ophthalmic use? what are their duration of action and onset?

Answer 7

Tetracaine/Amethocaine (0.5-1.0%)
Duration of action: 20 min

Benoxinate/Oxybuprocaine (0.4%)
(available in combination with sodium fluorescein (Fluress),
Duration of action: 15 min

Proparacaine (0.5%)
Duration of action: 15 min
Alcaine

The onset for all 3 LA are from 20s – 1min

Question 8

what are 2 ocular side effects of LAs?

Answer 8

1. Momentary local stinging or burning
2. Localised or diffused epithelial desquamation (weaken)
   Most common is SPK (superficial punctate keratitis)

Question 9

what is the systemic side effects of LAs? when does it occur?

Answer 9

Toxicity affects CNS, cardiovascular & respiratory system.

Very rare
Occurs when:
* large dosage is used
* Rapid absorption
* Liver or kidney disease/damage

Question 10

Precautions for the use of LAs?

Answer 10

• never prescribe for home use
• Corneal abrasions will not heal properly in the presence of LAs. Epithelial regeneration is slowed
• Repeated drops can cause corneal staining: loosens epithelial cells
  *Use weakest concentration possible
• Make sure px does not rub eye while under local anaesthesia.

Question 11

what is the purpose of stains? what are the 5 types of stains?

Answer 11

Important information is provided by the presence & absence of
staining.

1. Fluorescein sodium
2. Rose bengal
3. Alcian blue
4. Trypan blue
5. Lissamine Green

Question 12

what is Fluorescein sodium?

Answer 12

Orange red dye
* Exhibits fluorescence when cobalt light is absorbed
The dye absorbs light in the blue range of the
visible spectrum, with absorption peaking at
490nm (blue) ,then emits light at 530nm (yellow)

susceptible to bacteria contamination, so need to be sterile

Question 13

what form is NaFl available in?

Answer 13

1. Solutions
2. Impregnated filter paper strips

Question 14

what are the 3 indications for NaFl use?

Answer 14

1) Assessment of epithelial integrity
* Important for detection of abrasions, ulcers,
dessication, surface irregularities etc.
* Normally high lipid content of corneal epithelial
cells prevents uptake of water soluble NaFl.
* However, NaFl passes into the corneal stroma if
there are :
- areas of epithelial loss OR
- if intraepithelial junctions are lost

2) Contact Lens fitting
* Used with rgp fitting
* Works by making the tear layer visible.

3) Applanation tonometry
* Use with anaesthetic

Question 15

how does rose bengal staining work? what is it’s use?

Answer 15

Rose bengal
* Pink/magenta in colour when viewed with white light

• Stains degenerated or dead cells, mucous strands (vital dye)
• Use: Diagnosis of dry eye
  Abrasions, ulcerations etc.