IOP Tonometry - Week 1 Flashcards
Define indentation
Measures IOP by direct pressure on the eyeball
- is an old technique, no longer used
Define Applanation
Measures IOP by determination of the force necessary to flatten a corneal surface of constant size
- can use Goldman/Perkins (contact tonometry)
What law does Applanation adhere to? Does Applanation always meet the criteria of this law?
Imbert-Fick law:
Pressure = W(g)/A(mm^2)
No Applanation does not always meet this criteria
Describe the law that Applanation adheres to
Imbert-fick law:
The pressure in a sphere filled with fluid (e.g eye) and surrounded by an infinitely thin, flexible membrane –> may be measured by the force that just flattens the membrane to a plane surface.
This pressure is equal to the Applanation force (W) divided by the Area
What are the 4 forces acting when a tonometer probe is in contact with the cornea?
- capillary attraction of tear film for the probe
- force of the probe
- fluid pressure behind the cornea
- corneal resistance
What is the collective name for the 4 forces that act when a tonometer probe is in contact with cornea?
And how do these forces relate to Fick Law?
They are called “Tissue Tension” forces
- they invalidate Fick Law
Which of the following is correct, during Applanation:
(A) internal volume of the eye is increased resulting in increased pressure
(B) internal volume of the eye is increased resulting in decreased pressure
(C) internal volume of the eye is decreased resulting in increased pressure
(D) internal volume of the eye is decreased resulting in decreased pressure
Answer C.
During Applanation, internal volume of eye DECREASES resulting in INCREASED pressure
… Not sure why as it sounds counter-intuitive. Find out why. (It was on the slides so it’s correct. Definitely)
What can surface tension attraction of probe result in?
DECREASED pressure
Define tonometry. What types can it be divided into?
- a clinical procedure used to measure the IOP of a patient
Can be divided into contact (goldmann/Perkins) or non-contact (air puff)
Can the IOP measurement from tonometer vary?
Yes. The measurement can vary both in the short and long term
What is the typical range for IOP?
10-21mmHg, with less than or equal to 3-4mmHg b/w eyes
What long term factors can influence IOP fluctuation?
- Age
- systemic B.P
- Race
- Body Weight
- Ethnic Origin
- Seasonal Variations (higher in winter)
Way to remember:
As seasons pass, Every race/ethnicity ages, gains weight and changes BP
or just SEBRAS – Systemic BP, Ethnic, Body Weight, Race, Age, Seasonal variation
What short term factors can influence IOP fluctuation?
- Diurnal variation (high morn, low evening)
- corneal thickness (thick overestimate, thin under)
- body posture (from lying down to sitting up)
- exercise (aerobic ex. decreases, weight lift increases)
- State of patient
- Cardiac + Resp. pulses
- Repeated measurements (decreases IOP)
- Medications
- Diet and Lifestyle habits
What effect do the following medications have on IOP:
A - BP drugs
B- steroids
BP Drugs: Decrease IOP
Steroids: Increase IOP
What effect do the following have on IOP: A - Excessive caffeine B- Excessive Water C- Excessive Cigarettes D- Excessive Alcohol E- Marijuana
Excessive Caffeine/Water/Cigarettes: Increase IOP
Marijuana/Excessive Alcohol: Decrease IOP
What is the normal corneal thickness?
545nm
What are the indications for tonometry/measuring IOP?
- First time/forty yr olds esp.
- (prior to) Dilation
- Glaucoma (risk factors/FOH/Diagnosed/sign+symp/manage)
- Ocular conditions
- Recent refractive change in 1 eye
- Systemic conditions (e.g. diabetes, HT, migraine)
- (previous) Surgery or ocular injury
- Frank Does Gardening On Relatively Sunny Sundays
What are the contraindications for tonometry/measuring IOP?
- Infections (presence of)
- (significant) Central corneal epithelial defects
- (significant) Epithelial basement membrane dystrophy
- Sensitivity to local anaesthetics/sensitive patients
- Trauma to eye/perforation of globe
INCEST
What are the procedural steps fro goldmann tonometry
- Alcohol swab the probe and let it air dry/wipe with kim wipes
- Explain technique to px
- Instill 1 drop Alcaine then fluoroscein
- Swing Goldmann into forward position
- Align the probe, based on if more/less than 3Dastig
- Set measurement drum to 14-15mmHg
- Beam 45-60deg from eyepiece – using max width + light intensity (cobalt blue) with 16xmag. Px’s face rest on forehead/chin rests
- Use joystick to adjust probe so that semicircles are centred in FOV and equally divided
- Make movements in direction of larger semicircle
- Turn measuring drum until inner borders of the 2 fluoroscein semicircles are touching
- Remove probe.
How do you align the probe for the Goldmann tonometer?
- if the px has less than 3D of astig: position the probe so that the white mark on the probe holder is continuous with the 0 degree line on the probe
- If px has >3D astig: align red mark on probe holder with the line on the probe corresponding to the ‘minus cylinder axis’ (43 deg to minus cyl axis). The error is 1mm for every 4D of corneal cylinder
True or False: goldmann and Perkins have the same accuracy
True.
Air puff is the one which has the different reading
Clinocal procedure for Perkins
Same as goldmann, but it’s handheld
What are mires?
A pattern used in an optical instrument to guide the observer
E.g the 2 fluorescein semicircles seen in an Applanation tonometer
What type of prism is used in a goldmann tonometer?
A bi-prism
What is the correct endpoint for the tonometer mires?
The inner borders of the 2 semicircles should be aligned
– shape will look like a horizontal upside down letter ‘S’
