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Canadian Ophthalmological Society Evidence-based Clinical Practice Guidelines for the Management of Glaucoma in the Adult Eye Intraocular Pressure and its Measurement General factors that affect measured IOP General • Diurnal variation — IOP generally higher in the morning vs afternoon; normal fluctuation 2–5 mm Hg • Posture — higher in supine vs sitting position. Highest in head down position • Exercise — aerobic exercise generally lowers IOP, while isometric exercise can increase IOP Canadian Ophthalmological Society evidence-based clinical practice guidelines for the management of glaucoma in the adult eye. Can J Ophthalmol 2009;44(Suppl 1):S1S93. Systemic factors that affect measured IOP Systemic • Valsalva (e.g. with breath holding or in some obese patients) — generally increases IOP, although can decrease • Foods/drugs: Lower IOP — alcohol, fat-free diet, heroin, marijuana, some systemic vasodilators (e.g., nitroglycerin, beta blockers) Raise IOP — excessive water drinking, caffeine, tobacco, corticosteroids • General anesthesia — generally decreases (important exceptions: ketamine, succinylcholine [which may raise IOP]) Canadian Ophthalmological Society evidence-based clinical practice guidelines for the management of glaucoma in the adult eye. Can J Ophthalmol 2009;44(Suppl 1):S1S93. Tonometry — Goldmann tonometry for reproducibility Recommendation As Goldmann applanation tonometry is the most reproducible, it is recommended for IOP measurement in patients with healthy corneas [Level 31]. 1. Phelps CD, et al. Albrecht Von Graefes Arch Klin Exp Ophthalmol 1976;16;198:39–43. Canadian Ophthalmological Society evidence-based clinical practice guidelines for the management of glaucoma in the adult eye. Can J Ophthalmol 2009;44(Suppl 1):S1S93. Appendix E: How to test calibration of a Goldmann Tonometer • • 1. 2. 3. Standard method for measuring IOP1 Periodic calibration check recommended: at least twice yearly Set the tonometer in position on the slit-lamp stand, with the perspex biprism head in place and the tension on the circular dial on the right side (from the examiner’s side of the slit lamp) set at 5 mm Hg. The head should lean slightly forwards (away from the examiner). Slowly twirl the circular dial counter-clockwise until the head rocks back towards you. The tension should read 0 to 2 mm Hg below zero (Figure 1). Slowly twirl the dial clockwise until the head rocks forwards again. The tension should read 0 to 2 mm Hg (Figure 2). Figure 1 1. Garway-Heath DF. In: World Glaucoma Association: Intraocular Pressure. Consensus series 4. The Hague: Kugler Publications Copyright © 2008 SEAGIG, Sydney. Reproduced with Figure 2 Canadian Ophthalmological Society evidence-based clinical practice guidelines for the management of glaucoma in the permission from Asia Pacific Glaucoma Guidelines, 2nd ed. adult eye. Can J Ophthalmol 2009;44(Suppl 1):S1S93. Hong Kong: Scientific Communications, 208:1-117. Appendix E: How to test calibration of a Goldmann Tonometer 4. 5. 6. Remove the calibration rod from its box. Firmly screw into position the holding bracket that slides along the rod so that the closest mark in front of the centre one (on the other side of the centre from you) is aligned as exactly as you can (Figure 3). Slip the rod and its holder into the receptacle on the right side of the tonometer. The head will rock backwards towards you. Slowly twirl the circular dial clockwise until the head rocks forwards. Note the tension reading on the dial: it should be 20 to 23 mm Hg. Figure 3 Copyright © 2008 SEAGIG, Sydney. Reproduced with permission from Asia Pacific Glaucoma Guidelines, 2nd ed. Hong Kong: Scientific Communications, 208:1-117. Canadian Ophthalmological Society evidence-based clinical practice guidelines for the management of glaucoma in the adult eye. Can J Ophthalmol 2009;44(Suppl 1):S1S93. Appendix E: How to test calibration of a Goldmann Tonometer 7. 8. 9. Slowly twirl the circular dial counter-clockwise until the head rocks backwards. The tension on the dial should read 17 to 20 mm Hg. Remove the rod and holding bracket from the tonometer and reposition the bracket so that it is aligned exactly with the most forward mark on the rod—furthest away from you (Figure 4). Replace the rod in its bracket in the tonometer receptacle. The tonometer head should rock backwards, towards you. Figure 4 Copyright © 2008 SEAGIG, Sydney. Reproduced with permission from Asia Pacific Glaucoma Guidelines, 2nd ed. Hong Kong: Scientific Communications, 208:1-117. Canadian Ophthalmological Society evidence-based clinical practice guidelines for the management of glaucoma in the adult eye. Can J Ophthalmol 2009;44(Suppl 1):S1S93. Appendix E: How to test calibration of a Goldmann Tonometer 10. Slowly twirl the dial clockwise until the head rocks forwards. The tension should read 60 to 64 mm Hg. 11. Slowly twirl the dial counter-clockwise until the head rocks backwards—the tension should read 56 to 60 mm Hg. • The three threshold tension levels being used to test the tonometer’s calibration are 0, 20, and 60 mm Hg. • At each of these thresholds, you can gently twirl the dial backwards and forwards, reading the tension as the head responds. • These points should bracket the threshold level evenly—the higher the level being tested, the greater the interval is likely to be. Copyright © 2008 SEAGIG, Sydney. Reproduced with permission from Asia Pacific Glaucoma Guidelines, 2nd ed. Hong Kong: Scientific Communications, 208:1-117. Canadian Ophthalmological Society evidence-based clinical practice guidelines for the management of glaucoma in the adult eye. Can J Ophthalmol 2009;44(Suppl 1):S1S93. Tonometry — role for finger tonometry for special circumstances Recommendation Consideration can be given to finger tonometry to estimate IOP as very low, normal, or very high in certain situations (e.g., eyes with flat anterior chambers [lens-cornea touch], eyes with keratoprosthesis) [Consensus]. Canadian Ophthalmological Society evidence-based clinical practice guidelines for the management of glaucoma in the adult eye. Can J Ophthalmol 2009;44(Suppl 1):S1S93. Tonometer tips — disinfection Recommendation Applanation tonometer tips should be disinfected between each patient [Consensus]. Canadian Ophthalmological Society evidence-based clinical practice guidelines for the management of glaucoma in the adult eye. Can J Ophthalmol 2009;44(Suppl 1):S1S93. Factors Influencing Accuracy of IOP Measurement Clinical features associated with measurement errors using applanation tonometers Clinical feature • Physiologically thin central cornea* • • • • • • • • • Insufficient or no fluorescein in tear film Corneal edema (stromal and epithelial) Physiologically thick central cornea* Excessive fluorescein in tear film Pressure on globe from examiner’s fingers and/or from eye lid spasm Obese patient Hair lying across cornea distorting mires Restrictive myopathy/muscle entrapment Breath holding or Valsalva manoeuvre *Assuming a structurally normal cornea; i.e., no change in rigidity/biomechanics such as with excessive hydration, scarring, or refractive surgery. Resulting error Underestimation of IOP (true IOP higher than what is measured) Overestimation of IOP (true IOP lower than what is measured) True IOP could be higher or lower than what is measured Canadian Ophthalmological Society evidence-based clinical practice guidelines for the management of glaucoma in the adult eye. Can J Ophthalmol 2009;44(Suppl 1):S1S93. Clinical features associated with measurement errors using applanation tonometers (cont’d) Clinical feature Resulting error • Lens-cornea apposition • Tonometer not well calibrated • Corneal abnormalities (scars, refractive surgery, corneal graft, edema, Keratoconus) • Marked corneal astigmatism • Small palpebral aperture • Nystagmus • Tremor Technical difficulties (interpret results with caution) Canadian Ophthalmological Society evidence-based clinical practice guidelines for the management of glaucoma in the adult eye. Can J Ophthalmol 2009;44(Suppl 1):S1S93. Appendix F: Tonometry mires Figure 1—Excess corneal applanation (IOP lower than tonometer reading) Copyright © 2008 SEAGIG, Sydney. Reproduced with permission from Asia Pacific Glaucoma Guidelines, 2nd ed. Hong Kong: Scientific Communications, 208:1-117. Canadian Ophthalmological Society evidence-based clinical practice guidelines for the management of glaucoma in the adult eye. Can J Ophthalmol 2009;44(Suppl 1):S1S93. Appendix F: Tonometry mires Figures 2 and 3—Insufficient corneal applanation (IOP higher than tonometer reading) Copyright © 2008 SEAGIG, Sydney. Reproduced with permission from Asia Pacific Glaucoma Guidelines, 2nd ed. Hong Kong: Scientific Communications, 208:1-117. Canadian Ophthalmological Society evidence-based clinical practice guidelines for the management of glaucoma in the adult eye. Can J Ophthalmol 2009;44(Suppl 1):S1S93. Appendix F: Tonometry mires Figure 4—Correct endpoint corneal applanation (IOP equals tonometer reading) Copyright © 2008 SEAGIG, Sydney. Reproduced with permission from Asia Pacific Glaucoma Guidelines, 2nd ed. Hong Kong: Scientific Communications, 208:1-117. Canadian Ophthalmological Society evidence-based clinical practice guidelines for the management of glaucoma in the adult eye. Can J Ophthalmol 2009;44(Suppl 1):S1S93. Tonometry — applanation with difficult positioning Recommendation Hand-held devices such as Perkins/Kowa, TonoPen, and hand-held noncontact tonometers are useful in children, and in those unable to come easily to the slit lamp (e.g., obese, bedridden, with postural difficulties) [Consensus]. Canadian Ophthalmological Society evidence-based clinical practice guidelines for the management of glaucoma in the adult eye. Can J Ophthalmol 2009;44(Suppl 1):S1S93. Central Corneal Thickness and its Measurement Documentation of IOP and influence of CCT Recommendation Correction nomograms that adjust GAT IOP based solely on CCT are neither valid nor useful in individual patients. IOP should always be communicated as measured IOP rather than “corrected” IOP, due to the lack of a universal correction nomogram. When recording and communicating IOP values, measured IOP should be recorded with CCT in a manner that is easily identifiable in the chart [Consensus]. Canadian Ophthalmological Society evidence-based clinical practice guidelines for the management of glaucoma in the adult eye. Can J Ophthalmol 2009;44(Suppl 1):S1S93. Central corneal thickness measurement Recommendation Measurement of CCT, preferably with ultrasonic means, should be performed on all patients with glaucoma and ocular hypertension. The variance from the mean in a given population may under- or overestimate the true value of IOP in a given individual, and may influence the risk of an individual with ocular hypertension converting to glaucoma [Level 11]. 1. Brandt JD, et al. Am J Ophthalmol 2004;138:717–22. Canadian Ophthalmological Society evidence-based clinical practice guidelines for the management of glaucoma in the adult eye. Can J Ophthalmol 2009;44(Suppl 1):S1S93.
