3.1.6 Uses both a non- contact and contact tonometer to measure intraocular pressure and analyses and interprets the results.

Question 1

Does goldmann underestimate pressures?

Answer 1

• I think it does. If you push too hard, then it may overestimate so who knows
  
  • Elliot says that For a very thick cornea, GAT tends to overestimate IOP, and for very thin corneas it underestimates IOP.

Question 2

How accurate is NCT & how many readings needed?

Answer 2

• 3-4 readings is accurate
• NCT does fluctuate & can overestimate IOP at higher IOPs
• Contact tonometry also takes into account arterial pulse which can be averaged during the procedure whereas NCT doesn’t & can differ by up to 4mmHg as a result

Question 3

Why is Goldmann used? Why is it gold standard?

Answer 3

Due to its accuracy & numerous clinical trials

Question 4

Introduction to px:

Answer 4

“Hi, so today I will be measuring the pressure in your eye(s) using a special piece of equipment to give me a more precise reading. I will be using a yellow dye & some anaesthetic to numb the eye(s) during the procedure. Have you had any bad reactions to eyedrops in the past? Any allergies? Any injuries, infections or surgeries done to your eyes? Any health conditions you have you’re aware of or any medications you’re taking?”

Question 5

Start with staining

Answer 5

1. Explain that you will be putting a yellow dye onto their lower eyelid which allows you to look at the front surface of their in eye(s) in more detail to ensure it will be suitable for the measurement of the pressure
2. Use a wratten filter & 16X mag! MAKE SURE TO RECORD YOUR FINDINGS! CHECK EXPIRY DATE OF NAFL
3. RECORD: NaFl used, Expiry date 08/2022. Record staining found e.g. few superificial micropunctates

Question 6

INSTRUCTION FOR ANAESTHETIC

Answer 6

• “So I’ll be instilling some anaesthetic into each eye so your eye doesn’t feel anything during the procedure. Have you had any bad reactions to eye drops in the past? Any allergies or health issues? Any eye related issues or any prev trips to HES for your eyes/infections/injuries?
• It may sting a little initially but that will wear off very quickly. I advise not to rub your eyes, or to get any dust or grit into them for at least 30 minutes after the drops go in as your eyes. It is very unlikely for you to have an adverse reaction to the drops but if you do experience any pain, blurred vision, severe redness then please let us know immediately or A&E.”
• GRAB A TISSUE FOR THE PX BEFORE PUTTING DROP IN!!!
• Rehearse the anaesthetic leaflet!!
• YOU MUST KNOW BASICS OF ANAESTHETIC

Question 7

Imbert Fick Principle

Answer 7

P = W/A

P = Pressure, W = Weight or Force of probe, A = Area being applanated

The law assumes that the cornea is completely spherical, dry, infinitely thin & perfectly elastic…which it isn’t. A modified version of the law was then formed

Through clinical trials, it was found that an applanation diameter of 3.06mm allowed the surface tension of the probe to be equal to the tension caused by corneal rigidity and tear film compression. Because this cancelled out, it could be said that force = pressure

Also, it works out nicely as Force (g) x 10 = Pressure in mmHg

Question 8

Indications

Answer 8

 African/Caribbean
 Family history glaucoma
 Cupping of >0.5 or 0.2 difference between eyes
 CRVO – 100 day glaucoma
 Steroid use
 Diabetes
 HBP
 Suspicious fields
 Before/after dilation
 Red eye
 Haloes around lights
 >40s

Question 9

NCT and GAT disadvantages

Answer 9

NCT
Causes anxiety/artificially raised IOP
Doesn’t take into account CCT
Needs 3 readings minimal

GAT
1mmHg effort for every 4D of astigmatism
Doesn’t consider CCT
Subject to user error/subjective

Question 10

Calibration of Perkins

Answer 10

 Remove battery pack
 Use flat surface - place instrument on back & have probe in situ
 Place black disc under head of instrument
 Check at 0, then 20, then 50 (use normal probe, then 2g, then 5g)
 Move wheel in small increments until probe lifts
 If calibration correct, can use instrument

Question 11

Errors of Perkins

Answer 11

 If astigmatism exceeds 3DC: needs to be compensated for as cornea is elliptical
 Causes misreading if this is not compensated for
 In this instance, the axis of the prism must be set to 43 degrees to the flattest meridian to ensure an area of 7.354mm2 is applanated
 This is achieved by aligning the minus cyl axis with the red mark on the cone holder (Goldman) / align with A (Perkins)

Question 12

Anaesthetics

Answer 12

 3 of the 4 main topical ocular anaesthetics are esters, meaning that if you have a reaction to one you will likely have a reaction to all (oxybuprocaine, tetracaine, proxymetacaine).
 In this case, lidocaine should be used as it has a completely different mechanism of action (benzoic acid amide).
 Oxybuprocaine 0.4% is a p-aminobenzoic acid ester and does not require refrigeration.
 Lidocaine 4% is a benzoic acid amide and is available in a solution with fluorescein. It does not require refrigeration. It is much stingier than esters on instillation! Lidocaine is hydrolysed and metabolised more easily, so it acts quicker, less chance of reaction (metabolise that caused the reaction)
 All of the above are expected to cause adequate surface anaesthesia within 2 minutes, which lasts around 30 minutes. They are not a common cause of ADRs, and any allergy should resolve in an hour. Repeated instillations could be toxic and cause diffuse SPK and discomfort. Should resolve in 24 hours

Question 13

Risks

Answer 13

 Px may have adverse reaction (RARE)
 Ocular
- Delayed healing & tear flow
- Lowering IOP
- Increase permeability & compromise integrity of cornea
- Desquamation/corneal melt risk if repeated drops instilled (VA suddenly reduces but recovers with artificial tears)
 Systemic
- Light headedness
- Tinnitus
- Occasional fainting
 Px may have allergic response (in form of blepharoconjunctivitis)

Question 14

Procedure

Answer 14

 Inform px of procedure; more accurate reading of pressure inside of the eye, which involves using anaesthetic and a small probe that touches front of the eye, won’t feel a thing
 Ask about allergies to anaesthetic
 Examine corneal integrity by slit lap with nafl
 Wash hands and instil anaesthetic
 Insert disposable probe without contaminating the tip, and align the cone; making any adjustments for astigmatism
 Instil nafl
 Position px comfortably with head supported
 Ensure px has fixation point
 Record reading, time, instrument
 Check for any abrasions by slit lamp

Question 15

After procedure advice

Answer 15

 Drops take 60 secs to work
 May last for up to 25 mins
 Do not rub eyes for at least 30 mins
 Avoid getting dust/grit in eyes
 Do not reinsert CLs until at least 30 mins post drops instillation
 If experience any unusual symptoms (pain/blurred vision), contact practice or seek medical advice
 Give leaflet

Question 16

Factors affecting accuracy

Answer 16

 Diurnal variation of up to 5mmHg; lowest in morning, increases throughout day
 The fluctuation of IOP with the heart rate, being equal to the difference between systolic and diastolic IOP is the ocular pulse amplitude (OPA)
 Stress / apprehension / eye squeezing (increase)
 Exercise (increase by up to 50%)
 Unstable tear film
 Accommodation can decrease IOP by 2-3mmHg

Question 17

Referral

Answer 17

Normal range = 10-21mmHg
 IOP <5mmhg = trauma/retinal detachment, hypotony

SIGN Referral Guidance
 IOP > 25mmHg – irrespective of CCT
 IOP between 21-25– when CCT is <555 (thin) & px 65 or under
 IOP < 26 & CCT > or equal to 555 (thick) – MONITOR

NICE guidelines
 IOP >24mmHg, asymmetry of 5mmHg