Week 5: IOP Flashcards
What is IOP?
The dynamic fluid force within the eye that reflects a balance between aqueous production and aqueous outflow
What factors determined IOPs?
○ Rate of aqueous secretion
○ Resistance to aqueous outflow
○ Level of episcleral venous pressure
Difference in pressure between the arteries entering the tissue and the veins leaving it
Higher venous pressure reduces outflow
What is normal IOPs?
Mean IOP = 15.5+/- 3.0mmHg
But can range between 10 - 21 mmHg
Difference between eye <3 - 5 mmHg
What effects IOPs results?
Diurnal variation - 3 - 5 mmHg
Sitting to supine: Increases 2 -4 mmHg
Arterial Pulse oscillation: 2 - 4 mmHg
Heart rate, blood pressure, age, weight, ocular pathology
How do we detect and manage anomalous IOP?
Ocular hypertension
○ Defined by the Ocular Hypertension Treatment study (OHTS) as IOP 24 - 32 mmHg
○ Monitored and managed by Optoms
Ocular Hypotension
○ Low IOP <6mmHg
This is often associated with corneal edema, retinal edema, chorioretinal folds and choroidal detachment
What causes Ocular Hypertension?
Excessive Production
Impaired outflow
Why is measuring IOP important?
Elevated IOP may damage ocular structure
○ Especially nerve fibre layer at the optic disc, leading to loss of sight
However, individual susceptibility varies
Is IOP a Glaucoma Test?
Not exactly, normal IOP doesn’t mean the patient doesn’t have glaucoma, and high IOPs Is not a diagnosis of glaucoma
What is Glaucoma?
is “an optic neuropathy characterised by classic optic nerve heard changes and corresponding visual field changes for which IOP is the only known modifiable risk factor
What is included in Glaucoma Assessement?
- Functional tests
- objective measures sujc as IOPs, visual assessment of the health of the optic nerve
List the 6 types of Tonometry?
Applanation, Non-contact, rebound, electronic indentation, indentation and Implantable
What is the types of Applanation tonometry?
Biomicroscope mounted (Goldmann Applanation Tonometry (GAT) )
Hand held (Perkins Tonometry)
What is a type of non-contact tonometry?
Pneumatic
What is a type of rebound tonometry?
iCare
What is a type of Electronic indentation tonometry?
tonopen
What is a type of indentation tonometry?
Shiotz
What is contraindication of tonometry?
- Active infection/inflammation (or use disposable prism if available)
- Significant corneal abrasions/erosions
- Significant epithelial basement membrane dystrophy
- Lacerated or perforated globes
- Hyphaemia
Known sensitivity to anaesthetics
What errors can be made with mires?
- Incorrect amount of fluorescein
- Incorrect calibration
- Pressure on the globe
- Central corneal thickness
- Corneal oedema
- Astigmatism
- Wide pulse pressure
- Patient comfort
If the mires too thin = falsely high IOP
If the mires are too thick = falsely low IOP
What are the advantage of Goldmann Applanation Tonometry?
Considered the Gold Standard
Accurate
High Degree of Repeatability
What are the disadvantage of Goldmann Applanation tonometry?
Risk of cross infection (disposable prisms)
Mires more difficult to visualise as light intensity varies with battery power
Non portable
Patient must sit behind biomicroscope
Ocular rigidity affects applanation
Age, ocular disease, high myopia, eye surgery
High rigidity over estimates Low rigidity under estimates
What are the advantage of Rebound tonometry?
Well tolerated and safe
No anaesthetic required
Reasonably accurate
Extremely fast
Contamination risk low as probes are disposable
Objective test
Portable, uninvasive (children, disabled people)
Small cornea are required (can avoid scarring, distortin)
What are the disadvantage of Rebound Tonometry?
Does not balance pulsatile variation ( 6 readings required)
What test is designed to check the diurnal varition of IOP?
Water Drinking Test
It assesses the eyes ability to increase aqueous outflow in response to increased production.
A rise in IOP of 6-8mmHg or 30% from baseline is considered positive.
Physiology
Not fully known
Episcleral venous pressure doubles in 10 minutes following WDT
20% increase in choroidal expansion
Why is Goldmann the gold standard for tonometry?
Goldmann applanation tonometry is the gold standard.
Best for accuracy, reliability, repeatability.
Repeatability of successive readings is ± 2.2 to 2.5mmHg 1
