Tonometry Lecture by Ellie Livings, OPT505

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Questions and Answers

What is the primary purpose of measuring intraocular pressure (IOP)?

To assess the overall health of the visual system

To evaluate the effectiveness of eye medication

To estimate the depth of the anterior chamber

To detect potential ocular diseases and conditions (correct)

Which of the following is a characteristic of contact tonometry?

It is non-invasive and does not require eye contact

It typically provides an accurate measurement of IOP (correct)

It measures IOP using air puff technology

It does not require prior patient preparation

What are the two main chambers of the eye that contain fluid?

Scleral and corneal chambers

Anterior and posterior chambers (correct)

Lens and vitreous body

Ciliary and conjunctival spaces

When is it most appropriate to refer a patient for medical attention concerning IOP?

If there is a consistent elevation of IOP beyond the normal limits Signup and view all the answers

Which of the following statements accurately describes the process related to aqueous fluid in the eye?

Aqueous fluid is generated and drained in a continuous cycle Signup and view all the answers

What is the minimum distance between the inner edges of the semi-circles when the applanated diameter is correct?

3.06mm Signup and view all the answers

What percentage of tonometers are typically incorrectly calibrated at any given time?

40-50% Signup and view all the answers

Which of the following conditions is NOT related to intraocular pressure (IOP) issues?

Cataracts Signup and view all the answers

What action should be taken if there is an error of 2.5mmHg or more during tonometer calibration?

Return the tonometer for service Signup and view all the answers

What is the main form of glaucoma associated with an IOP of 24 mmHg or more?

Primary open angle glaucoma (POAG) Signup and view all the answers

What should be done if the mires are unequal in size during tonometry?

Move the probe to the largest mire Signup and view all the answers

Which of the following is a disadvantage of Perkins Contact Tonometry?

Requires skilled operator Signup and view all the answers

In the Perkins procedure, what is the diameter that needs to be adjusted for the applanated area?

3.06 mm Signup and view all the answers

What is required for proper patient setup before using Perkins Contact Tonometry?

Corneal anaesthesia must be obtained Signup and view all the answers

What action should be taken if the mires appear too thick during the procedure?

Remove the probe and blot with tissue Signup and view all the answers

Which of the following describes the light management of Perkins Tonometry compared to Goldmann tonometry?

Inferior magnification and lighting Signup and view all the answers

What is one of the advantages of Perkins Contact Tonometry?

Directly comparable to Goldmann readings Signup and view all the answers

What setting should the dial be adjusted to for the Perkins setup before commencing the procedure?

Approximately 16 mmHg Signup and view all the answers

What is the primary site of aqueous production in the eye?

Ciliary epithelial cells of the pars plicata Signup and view all the answers

What percentage of aqueous humor exits through the trabecular meshwork and canal of Schlemm?

70-90% Signup and view all the answers

What can happen if intraocular pressure (IOP) becomes too high?

It can damage eye structures Signup and view all the answers

Which pathway accounts for the secondary exit of aqueous humor?

Uveoscleral outflow pathway Signup and view all the answers

Which statement is true regarding narrow angle patients?

Low-risk narrow angle patients may not need intervention. Signup and view all the answers

What condition can result from low intraocular pressure (IOP)?

Unsupported eye structures Signup and view all the answers

What is the primary function of the trabecular meshwork?

To drain aqueous humor Signup and view all the answers

Which of the following statements about aqueous circulation is false?

Most aqueous humor is produced in the anterior chamber. Signup and view all the answers

When should IOP be measured in patients?

In patients aged 40 and over or with a family history of glaucoma Signup and view all the answers

Which of the following conditions is a contraindication for pupil dilation?

Patient is pregnant in the first trimester Signup and view all the answers

What is considered a normal intraocular pressure (IOP) range?

8 to 21 mmHg Signup and view all the answers

Which technique is used to check the anterior chamber angle?

Van Herrick's technique Signup and view all the answers

In what cases might you need to consult with a hospital before dilation?

If the patient has a known drug allergy Signup and view all the answers

Why is it necessary to record the time and date of eye drop administration?

To monitor the effectiveness of the eye drops over time Signup and view all the answers

What is a significant factor in determining whether the IOP is normal or abnormal?

Guidelines from the NHS and knowledge of average IOP Signup and view all the answers

What does IOP stand for in the context of eye assessments?

Intraocular pressure Signup and view all the answers

Study Notes

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Lecture Recording Information

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Lecture Topic: Tonometry

Topic: Tonometry, a lecture by Ellie Livings, OPT505.

Intended Learning Outcomes

Understand intraocular pressure.
Recognize factors influencing intraocular pressure (IOP).
Understand the significance of measuring IOP.
Learn how to measure IOP.
Understand various tonometry techniques.
Compare and contrast contact and non-contact tonometry.
Determine when and how to refer patients.

Main Relevant GOC Core Competencies

Safe and appropriate ocular examinations using proper techniques and procedures.
Clinical decision-making within the scope of practice.
Diagnostic analysis of visual function to create a management plan for ocular and systemic diseases.
Timely identification of patients needing referrals for urgent or emergency medical attention.

Tonometry = Measuring IOP

The human eye is a fluid-filled sphere with two main chambers (anterior and posterior).
The aqueous fluid generated and drained in a continuous process.
Intraocular pressure (IOP) results from the rates of aqueous generation and drainage.
Ideal IOP remains within a safe limit, neither too high nor too low.
High IOP squashes structures, low IOP means structures lack support.

Aqueous Circulation

Aqueous is produced by non-pigmented ciliary epithelial cells.
70-90% of aqueous exits through trabecular meshwork and Schlemm's canal.
10-30% of aqueous exits through the uveoscleral outflow pathway.
Aqueous flows into the anterior chamber and exits the eye.

Anterior Chamber Angle

Diagrams depict open and closed angle structures of the eye.
Importance of understanding various types and the clinical implications of each type.

Anterior Chamber Angle Pathology

Majority of narrow angle patients are low risk.
Common practice is to offer peripheral iridotomies to all patients with narrow anterior angles.

Gonioscopy

This procedure uses an instrument to view the angle between the iris and the cornea.
Key structures visible during gonioscopy include the trabecular meshwork, scleral spur, and Schwalbe's line.

Dilation, ACA, and IOP

Before dilation, check intraocular pressure (IOP), anterior chamber angle (ACA) using the Van Herricks technique, and allergies/contraindications.
Record information regarding drops, time, and date.
Provide patient's contact details, relevant information regarding use of drops, and post-dilation IOP.
Note the patient's driving status.

Caution: When Not to Dilate

Review potential risks and alternatives for patients with varying conditions (drug allergies, pregnancy, specific eye conditions).

When Do We Measure IOP?

Patients aged 40 or over.
Patients with a family history of glaucoma.
Patients with suspicious optic discs.
Prior to dilation.
Assisting with identifying ocular diseases
Patients with narrow anterior chamber angles.
Patients with painful, red eyes.

What is a Normal IOP?

Normal range of Intraocular pressure (IOP) is between 8 and 21 mmHg.
Significant fluctuations in IOP are possible even in healthy individuals.
Significant overlap exists between normal and glaucomatous IOPs.
IOP damage threshold varies, with higher IOP not always indicative of problems.
Clinical guidelines and average IOP data from healthcare providers will help determine normal values for a given patient

Ideal IOP

Aim for an IOP under 21 mmHg.
Referral criteria may lower this value to 24 mmHg if only non-contact tonometry (NCT) is used.
IOP of less than 5 mmHg difference between eyes (R and L).
Stable (ish) over time.
Makes sense in the context of the patient's medical history.

Normal IOP Variations

IOP increases with age and cataract formation.
IOP varies throughout the day, influenced by factors such as the pulse and seasonal changes.
Factors such as lid closure, anxiety, accommodation, and exercise may impact IOP.
Medications like tobacco, marijuana, and alcohol impact IOP, while others may increase or decrease IOP.
Corneal surgery may alter IOP readings.

How to Record IOPs

IOP measurements are recorded in mmHg.
Typically, three or four IOP readings are taken and averaged.
Time of day is crucial, as IOP naturally varies.
The type of tonometry instrument used and what drops, if any, were used, must be recorded properly.

How We Measure Pressure: Tonometry

The basic concept of tonometry involves measuring the force necessary to flatten the eye's surface using various techniques.

Applanation

This technique measures the force needed to flatten a specific area of the cornea.
Contact tonometry techniques (Goldmann and Perkins) use direct contact.
Non-contact tonometry techniques (Pulsair/NCT) use air pulses.

Applanation Theory: Imbert-Fick Law

Imbert-Fick law describes the relationship between force, area, and pressure during applanation.
The cornea isn't a perfect sphere (it's wet, not dry, it has thickness).

Surface Tension

Tear film surface tension acts on the probe, creating a constant force.
Cornea resistance pushes in an opposing direction
Force needed to flatten the cornea balanced against these forces
Diameter of 3.06mm, for these to balance and thus be calculated

Factors Affecting Measurement: Corneal Biomechanics

Corneal thickness, rigidity/elasticity, curvature, and hydration influence IOP measurements.

Corneal Thickness

Vary corneal conditions affect IOP measurement accuracy.
Thin corneas underestimate IOP, thick corneas overestimate IOP.

Corneal Thickness Data

Tables present age-related variations in corneal thickness.

Factors Affecting Measurement: Operator and Patient

Applying pressure to the eye, improper technique, and patient factors (drugs, time of day) impact IOP readings

What Can Make the IOP Higher or Lower?

Various conditions, such as pressure on the globe, stress, holding one's breath, and aging, can raise IOP.
Conditions associated with lower IOPinclude corneal surgery, alcohol consumption, relaxation activities, trauma, glaucoma medications, cycloplegics, and mydriatics

NCT: Keeler Pulsair

Handheld device with rapid air pulse to applanate cornea.
Optical detection of applanation.
Uses a diode system that monitors corneal curvature.

Taking the Reading (Keeler Pulsair)

Detailed procedure of using the tonometer, from initial setup to taking a reading.

Importance of Calibration

Calibration must be regular for accurate measurements.
Daily and periodic checks are essential.

Contact Tonometry Types

Categorization of common, less common, and historical contact tonometry methods (Goldmann, Perkins, iCare, Tonopen, Schiotz, Tonomat and MackayMarg)

iCare Rebound Tonometer

Handheld, light, portable, and quick.
No anesthesia required.
Good correlation with Goldmann applanation tonometry (GAT).

Indentation Tonometry: Schiotz

Pressure-volume relationship where the depth of indentation measures IOP.
Cheap, robust, and portable, but influenced by supine position, ocular rigidity, and procedure type.

Electronic Applanation Device: Tonopen

Provides electronic measurements using a micro strain gauge transducer.
Short contact time, allowing a reading to be derived with multiple readings taken consecutively.
Flexible and adaptable for various body positions.

Tonomat Tonometer

A cheap, portable device, but subjective, with potential for inaccuracies and requiring anaesthetic.

GAT Instrument Setup

Steps for proper GAT instrument setup prior to taking measurements of intraocular pressure (IOP).

Preparing the Patient

Explaining the procedure.
Allergy checks.
Drop administration procedure for anaesthesia.
Using fluorescein to observe mires.

GAT Procedure

Steps in the procedure for taking measurements using the Goldmann applanation tonometer (GAT).

Errors

Common errors in measuring IOP (too little/too much fluorescein, corneal edema, astigmatism).
How to address different error types.

Troubleshooting

Addressing issues when mires are unequal or not centered in the Goldmann applanation tonometry procedure.

Perkins Contact Tonometry

Measures IOP using the same principle as GAT.
Using its own light and viewing system, and offering a coiled spring mounting and suitability with patient sitting or lying down.

Perkins Disadvantages

Corneal anaesthetic needed for procedure
Requires specific skillset from operator
Magnification and lighting of slit lamp may not be as ideal
Less stable and consistent readings than GAT, and may be more difficult to obtain consistent readings
Problems in using this tonometry device on people with scarred corneas, nystagmus,
Subjective endpoint from device and potential for operator error

Perkins Setup

Steps for the preparatory setting-up of this tonometry device.

Perkins Procedure

Detailed steps to gather accuracy in readings from the device

Calibration

Requirements for regular calibration checks for accuracy and maintenance of tonometers.
Percentage of tonometers that are improperly calibrated.
Verification procedures given and how to check if there is a problem or errors in calibration.

When IOP is a Problem (High/Low)

Possible eye conditions that are connected to high / low IOP such as intraocular inflammation, choroidal detachment, glaucoma, fluid leakage and retinal detachment).

IOP and Glaucoma

A higher IOP increases the risk of glaucoma.
IOP over 30 mmHg is considered dangerous.
Primary open-angle glaucoma (POAG) is the main type associated with optic nerve damage and visual field loss.
Normal Tension Glaucoma (NTG) - has a lower IOP, but can still lead to damage.
Ocular Hypertension - is a raised IOP, but no damage

When to Refer

NICE guidelines: Refer if IOP is 24mmHg or higher in either eye.
Do not use ONLY non-contact tonometry (NCT) readings to make this decision
Refer when pressure reaches 24 mmHg or more, via Goldmann Applanation Tonometry (GAT), and after patients have been discharged from HES, after being screened for Glaucoma.
Refer if any signs of optic nerve damage or repeated visual field defects are observed at any IOP level, as this may suggest pre-existing glaucoma.

Refining the Referral

If IOP is 24mmHg or more by non-contact tonometry (NCT) and there are no risks of acute conditions, repeat the test.
If no GAT device is available for the patient, refer them to a service or accredited optometrist who can complete a GAT measurement
Recommend continuation of regular checkups to patients exhibiting IOP readings below 24 mmHg

Referring High IOPs

Refer patients with IOPs of 35 mmHg or higher urgently due to potential closed-angle glaucoma or central retinal vein occlusion risks; refer urgent if IOP is 40 mmHg or above.

Example Cases

Presented examples of patient cases and suggested handling protocols.

Assessment Information

Details of OSCE station requirements:
Setting up Goldmann applanation tonometry (GAT).
Performing calibration checks.
Preparing drop selection.
Demonstrating safe technique using a model eye.
Correctly recording findings.

Round Learning

Importance of learning the concepts of tonometry and Imbert-Fick law.
Guidance on additional learning resources on Moodle for the subject matter.

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Description

Join us for a comprehensive lecture on tonometry presented by Ellie Livings in OPT505. This session covers important concepts including intraocular pressure, measurement techniques, and factors influencing IOP. Learn how to safely conduct ocular examinations and when to refer patients for further care.