3.1.6 Uses both non-contact and contact tonometer to measure IOP and analyses and interprets results

Question 1

What is the mode of action of topical anaesthetics?

Answer 1

Blocks the initiation and propagation of action potentials by preventing
the voltage dependent increase in Na+ via blocking Na+ channels on the nerve cell
membrane. Cationic portion of the Topical Anaesthetic binds causing a physical occlusion
and preventing an axon depolarisation.

Question 2

What is the advice given when instilling topical anaesthetics?

Answer 2

– Rapid onset maximum within 30 secs (lidocaine up to 60) – lasting 20 to 30 minutes. (proxy, oxy approx. 15 mins, tetra 20, lido 30)
No CL reinsertion till 30mins post TA administration
Corneal melt (diffuse or local desquamation) is a rare
but a possibility – VA severe reduced (sudden onset blurred vision) – recovery with artificial tears
Allergic response takes form of blepharoconjunctivitis usually. Note any previous adverse reaction to
anaesthetics. TA with ester linkage more likely to cause allergic response.

Question 3

What are the uses for topical anesthetics?

Answer 3

• Anaesthesia prior to contact tonometry
  Þ Goldmann’s or Perkins
• Anaesthesia prior to foreign body removal
  Þ In either cornea or sub-tarsus
• Anaesthesia prior to punctual plug insertion or removal
• Anaesthesia prior to eye impressions
  Þ Scleral Contact Lenses
  Þ Ocular Prosthetics
• Anaesthesia prior to diagnostic procedures
  Þ Gonioscopy
  Þ Schirmer test
  Þ Pachymetry
• Irrigation of the eye
  Þ Foreign body removal
  Þ Burns
  Þ Chemical Injuries

Question 4

What do topical anesthetics do to the eye?

Answer 4

• Will sting
• Can cause redness
• Softens the cornea, affecting IOP
• Reduces blink rate
• Delays cell regeneration
• Always warn patient of foreign body risk after instillation

Question 5

What are the contraindications for anaesthetics?

Answer 5

• Known hypersensitivity
• Premature babies (don’t have the enzymes to metabolise amides especially)
• Global penetrating injuries
• Pregnancy & Breastfeeding
• Where wound healing would be compromised

Question 6

Describe oxybuprocaine?

Answer 6

• 0.4%
• Anaesthetic Duration: 15 mins
• Ester
• Biotransformed in most cells by Esterases
  General Uses
  Often used for Tonometry in practice because
  Oxybuprocaine produces less punctate staining than tetracaine
  Can be stored at room temperature

Question 7

Describe lidocaine?

Answer 7

• 4% (or with 0.25% Sodium Fluorescein)
• Anaesthetic Duration: 30 minutes
• Amide
• Biotransformed in the liver, so must be transported there first
  Use where a patient has had an adverse drug reaction of any of the Ester anaesthetic
  group.

Question 8

Describe proxymetacaine?

Answer 8

Least anti-bacterial action
* 0.5%
* Anaesthetic duration: 15 mins
* Ester
* Biotransformed in most cells by Esterases
Proxymetacaine is often used because:
* It stings the least
* Produces less punctate staining than tetracaine
Disadvantages of Proxymetacaine
* More potent than tetracaine
* Has to be stored between 2 - 8 degrees.

Question 9

Describe tetracaine?

Answer 9

0.5% or 1%
* Anaesthetic Duration: 20 minutes
* Ester
* Biotransformed in most cells by esterases
Contraindications
Above 1% can damage the cornea
Patients:
* with known hypersensitivity
* On sulphonamides
Þ They get hydrolysed in the body to p-amino-benzonic acid
* Premature babies
Þ The enzyme system used to metabolise this ester-type anaesthetic is
too immature

Question 10

What is the principle behind GAT?

Answer 10

– Probe applanates the cornea using a probe 3.06mm - linear relationship between force applied and IOP in mmHg (Force (g) x 10)
– Using a 3.06mm probe the surface tension is equal to corneal rigidity and thus cancels out
– Prism used to line up mires with the aid of fluorescein to accurate measure IOP

Question 11

Describe calibration of Perkins tonometer?

Answer 11

1. Take off battery pack
2. Place instrument on its back, with black disc under head of the instrument
3. Place 2g or 5g weight on the probe
4. The probe should lift at 2 and 5.
5. If it does not, compensate by removing/adding the difference. As long as the difference is
   equal between the 2 and 5g weight, then the instrument can be used.

Question 12

Describe calibration of Goldmann tonometer?

Answer 12

1. Place metal rod into the side of the probe
2. The probe should rock at 2g and 6g (first line and second line)
3. Make sure biprism probe is horizontal
4. Slit-lamp should be cobalt blue and wratten 32

Question 13

What are the common errors with applanation tonometry?

Answer 13

• Calibration
• Incorrect alignment
• lids touching the probe
• Quantity/quality of tears,
• high astigmatism (distorted mires)
• Repeated measurements: reduce IOP – 2-4mmhHg.