Glaucoma: Definition and IOP
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Questions and Answers

What is the main characteristic of glaucoma according to Hitchings' definition?

  • Deformations of the optic nerve head (correct)
  • Increased ocular hypertension
  • Elevated IOP
  • Abnormal corneal thickness

    • What is the typical IOP overlap between patients with glaucoma and normals?

  • Large overlap (correct)
  • No clear pattern
  • Small overlap
  • No overlap

    • What is the approximate prevalence of normo-tensive POAG in people over 40 years old?

  • 0.6% (correct)
  • 1.4%
  • 2.5%
  • 3.5%

    • What is the principle underlying applanation tonometry?

    <p>The Imbert-Fick law</p> Signup and view all the answers

    What is the average thickness of the human cornea?

    <p>540μm</p> Signup and view all the answers

    What percentage of referrals are typically deflected due to applanation tonometry?

    <p>77%</p> Signup and view all the answers

    What is the maximum acceptable difference in IOP measurement from the assessor's result?

    <p>±3 mmHg</p> Signup and view all the answers

    What is the effect on IOP measurement when the cornea is steeper than average?

    <p>Overestimation</p> Signup and view all the answers

    What is the normal diurnal variation in IOP?

    <p>Approx 5 mmHg</p> Signup and view all the answers

    What is the risk of glaucomatous field loss with IOP > 21 mmHg?

    <p>5x higher</p> Signup and view all the answers

    What is the effect of lid squeezing or finger pressure on IOP measurement?

    <p>Overestimation</p> Signup and view all the answers

    What is the effect of corneal astigmatism on IOP measurement?

    <p>Underestimation in WTR</p> Signup and view all the answers

    What is the normal inter-eye IOP asymmetry?

    <p>7 mmHg</p> Signup and view all the answers

    How often should the Goldmann applanation tonometer be calibrated?

    <p>Monthly</p> Signup and view all the answers

    What is the effect of breath-holding on IOP measurement?

    <p>Increase in IOP</p> Signup and view all the answers

    What is the recommendation for patients with IOP > 21 mmHg?

    <p>Check discs, fields, and angle</p> Signup and view all the answers

    What is the ideal applanation diameter for accurate results in contact tonometry?

    <p>3.06mm</p> Signup and view all the answers

    What is the purpose of the fluorescein in contact tonometry?

    <p>To visualize the edge of corneal contact</p> Signup and view all the answers

    What is the conversion factor used to convert the weight reading to intraocular pressure?

    <p>1g = 10mmHg</p> Signup and view all the answers

    What is the significance of the limbus shining with light during contact tonometry?

    <p>It indicates the probe is in contact with the apex of the cornea</p> Signup and view all the answers

    What is the purpose of adjusting the tension dial during contact tonometry?

    <p>To align the inner edges of the upper and lower semicircles</p> Signup and view all the answers

    What is the consequence of moving the slit-lamp too far toward the patient during contact tonometry?

    <p>The pressure arm will push against the eye with a low inoffensive force</p> Signup and view all the answers

    What is the significance of the meniscus rings in contact tonometry?

    <p>They indicate the alignment of the semicircles</p> Signup and view all the answers

    What is the competency requirement for hygiene in contact tonometry?

    <p>Demonstration of appropriate handwashing and clean chin-rest and headband</p> Signup and view all the answers

    What is the purpose of the biprism in contact tonometry?

    <p>To create two semicircles on the cornea</p> Signup and view all the answers

    What is the significance of the mires being approximately 10% of the arc width in contact tonometry?

    <p>It indicates the accuracy of the measurement</p> Signup and view all the answers

    Which of the following is a feature of glaucoma that distinguishes it from other diseases?

    <p>Deformations of the optic nerve head</p> Signup and view all the answers

    What is the primary source of error in applanation tonometry?

    <p>Corneal thickness and moisture</p> Signup and view all the answers

    What is the significance of the 21-22 mmHg IOP cut-off in glaucoma diagnosis?

    <p>It is the upper limit of normal IOP</p> Signup and view all the answers

    What is the main purpose of Perkins handheld tonometer?

    <p>To deflect unnecessary referrals</p> Signup and view all the answers

    What is the relationship between IOP and the force applied in applanation tonometry?

    <p>IOP is equal to the force per unit area of applanation</p> Signup and view all the answers

    What is the distribution of IOP in the general population?

    <p>Skewed with a large IOP overlap between normals and glaucomatous eyes</p> Signup and view all the answers

    What is the primary reason for selecting a circular zone of 3.06mm in diameter for applanation?

    <p>To minimize the relative error in IOP measurement</p> Signup and view all the answers

    What is the role of tear capillary action in applanation tonometry?

    <p>It balances the force of probe with IOP and ocular rigidity</p> Signup and view all the answers

    Why is it necessary to blink immediately prior to measurement in contact tonometry?

    <p>To moisten the cornea</p> Signup and view all the answers

    What is the purpose of anaesthetic and fluorescein in contact tonometry?

    <p>To anaesthetize the eye and visualize the cornea</p> Signup and view all the answers

    What is the consequence of not adjusting the tension dial during contact tonometry?

    <p>The inner edge of upper and lower semicircles will not align</p> Signup and view all the answers

    What is the significance of centering the semicircles in the field of view during contact tonometry?

    <p>It ensures accurate IOP measurement</p> Signup and view all the answers

    What is the purpose of multiplying the dial reading by 10 in contact tonometry?

    <p>To convert the weight reading to IOP</p> Signup and view all the answers

    What is the competency requirement for communication in contact tonometry?

    <p>Provision of adequate verbal information to the patient</p> Signup and view all the answers

    What is the significance of the meniscus rings being approximately 10% of the arc width in contact tonometry?

    <p>It ensures accurate IOP measurement</p> Signup and view all the answers

    What is the consequence of moving the slit-lamp too far toward the patient during contact tonometry?

    <p>The mires will be too large</p> Signup and view all the answers

    What is the effect of corneal oedema on IOP measurement?

    <p>Underestimation</p> Signup and view all the answers

    What is the recommended action for patients with IOP > 21 mmHg?

    <p>Check discs, fields, and angle carefully to exclude glaucoma</p> Signup and view all the answers

    What is the significance of a diurnal variation of > 10 mmHg?

    <p>Pathological diurnal variation</p> Signup and view all the answers

    What is the contraindication to Goldmann applanation tonometry?

    <p>Irregular cornea</p> Signup and view all the answers

    What is the effect of tight clothing around the neck on IOP measurement?

    <p>Increased IOP</p> Signup and view all the answers

    How often should the Perkins applanation tonometer be calibrated?

    <p>Monthly</p> Signup and view all the answers

    What is the risk of glaucomatous field loss with IOP > 21 mmHg?

    <p>5x higher</p> Signup and view all the answers

    What is the normal inter-eye IOP asymmetry?

    <p>Up to 7 mmHg</p> Signup and view all the answers

    What is the effect of accommodation on IOP measurement?

    <p>Decreased IOP</p> Signup and view all the answers

    What is the significance of fluorescein in contact tonometry?

    <p>Enhances visibility of the mires</p> Signup and view all the answers

    Study Notes

    Glaucoma Definition and IOP

    • Glaucoma is a group of diseases characterized by deformations of the optic nerve head (glaucomatous cupping).
    • Modern definitions no longer include IOP as a diagnostic criterion.
    • There is a large overlap in IOP between people with glaucoma and those with normal vision.

    OHT and Normal Tension Glaucoma

    • OHT (Ocular Hypertension) is a condition where IOP is higher than normal, but there is no damage to the optic nerve.
    • False positives occur when someone with OHT is misdiagnosed with glaucoma.
    • False negatives occur when someone with normal IOP is misdiagnosed with OHT.
    • Using a cut-off of 21-22mmHg, the prevalence of OHT, hypertensive POAG, and normo-tensive POAG is 3.5%, 1.4%, and 0.6%, respectively, in people over 40.

    Applanation Tonometry

    • Applanation tonometry measures IOP by flattening the cornea with a force.
    • Perkins handheld and Goldmann style slit-lamp mounted tonometers are commonly used.
    • 77% of referrals are deflected due to inaccurate IOP measurements.

    Contact Tonometry

    • Contact tonometry measures IOP by applying a force to the cornea.
    • The Imbert-Fick law states that pressure is equal to the force per unit area of applanation for a spherical container.
    • The cornea is not a perfect sphere, but a 3.06mm diameter applanation zone is used to minimize errors.
    • Four forces act on the cornea during applanation: force of the probe, tear capillary action, IOP, and ocular rigidity.
    • A plastic biprism is used to create two semicircles, and the force is adjusted to align the inner margins of the semicircles.

    Measurement Technique

    • The patient should press their head firmly against the chin and forehead rest.
    • The patient should look straight ahead and fixate on a target.
    • The patient should breathe normally and blink immediately prior to measurement.
    • Anaesthetic and fluorescein are used to aid measurement.
    • The biprism is positioned to contact the apex of the cornea, and the lamp is moved to maximize illumination.
    • The semicircles are centered in the field of view, and the tension dial is adjusted to align the inner edges of the upper and lower semicircles.

    Competency Assessment

    • Practitioners must demonstrate competency in hygiene, communication, drug use, initial patient set-up, safety, and accuracy.
    • Hygiene includes handwashing, cleaning the chin-rest and headband, and using a new uncontaminated probe.
    • Communication includes providing adequate verbal information to the patient about the technique.
    • Accuracy includes measuring within ±3mmHg of the assessor's result.

    IOP Variation

    • Diurnal variation is 3-6 mmHg.
    • Pulse variation is approximately 3mmHg.
    • Respiration, medication, fluid intake, breath holding, and constricting clothing can affect IOP.
    • Modifiable factors include breath holding, tight clothing, and short-term steroid use.

    Sources of Error

    • Lid squeezing or finger pressure can overestimate IOP.
    • Excessive tears can create wide/broad fluorescein rings, leading to overestimation.
    • Insufficient tears can create narrow fluorescein rings, leading to underestimation.
    • Absence of fluorescein can make measurement difficult.
    • Corneal astigmatism can lead to underestimation in with-the-rule (WTR) and overestimation in against-the-rule (ATR) astigmatism.
    • Corneal curvature and thickness can affect IOP measurement.

    Contra-indications to GAT

    • Irregular cornea or compromised cornea.
    • Eye infection.
    • Corneal epithelial desquamation post anesthesia/trauma.
    • Infection (return when resolved).

    Risk of Glaucoma with IOP

    • The risk of glaucomatous field loss is 5x higher in those with IOP > 21mmHg.
    • IOP > 21mmHg is 12.8x more likely to develop POAG.
    • IOP > 30mmHg is 39x more likely to develop POAG.

    Inter Eye IOP Asymmetry

    • Normal inter-eye IOP asymmetry is 7mmHg.
    • IOL, NS cataract, PDS, PXF, trauma, ret detachment, and uveitis can cause IOP asymmetry.

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    Description

    This quiz covers the definition of glaucoma, its characteristics, and the relationship between intraocular pressure (IOP) and the disease. Learn about the overlap between normal and glaucoma IOP values and how it affects diagnosis. Test your knowledge of this important eye condition.

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