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Tonometry and Gonioscopy

Clinical glaucoma fellow at AEH

Chris285
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Tonometry and Gonioscopy

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  1. TonomETRy and Gonioscopy Presenter: Dr Chris Moderator: Dr Chetna 13/03/2025

  2. Tonometry Non-invasive measurement of intraocular pressure.

  3. Tonometers 1. Applanation: Goldmann, Perkins, Draeger, Mackay Marg, Tono pen, Pneumatic. 2. Non Contact: Air puff, ORA, CORVIS 3. Dynamic contour tonometry: Pascal 4. Indentation: Schiotz, Rebound 5.Trans palpebral 6. Continous IOP monitoring • Normal distribution of IOP & Factors influencing IOP • Diurnal variation • Difficulties in measurement

  4. ApplanationInstruments • Goldmann tonometer-international clinical standard for measuring IOP • Imbert-Fick Principle (Maklakov-Fick law): For an ideal sphere (perfectly spherical, dry, infinitely thin walled and perfectly flexible) P=F/A. Pressure (P) inside sphere force (F)required to flatten the surface area (A) of flattening GAT displaces about 0.5 ulof aqueous humor, which raises IOP by about 3%.

  5. Additional factors: • Force of capillary attraction between tonometer head and tear film • Force required to flatten the cornea( i.e , cornea offers resistance) When diameter of flattening of the cornea is 3.06 mm, 1)These two forces cancel out each other. 2)IOP (mmHg) = 10x the force applied to the cornea in grams. Inner borders touch at mid point of pulsations- The proper degree of applanation has been achieved.

  6. Potential sources of error: Red mark at flatter meridian 1mmHg inaccurate for every 3D

  7. Perkins’s tonometer • Portable and counterbalanced • Used in any position • The light comes from batteries • The force come from a spring, varied manually by the operator. Draegertonometer • The force for applanationis supplied by an electric motor • Different biprism • Portable • Can be used in any position.

  8. Mackay Marg Tonometer • Electronic AT with a 1.5 mm diameter plunger(quartz rod) • 6mm diameter applanation area. • Displaces around 8 ulof aqueous humor and raises IOP about 6–7 mmHg • The end point does not depend on optical regularity- useful in scarred, oedematous corneas. Height of notch = IOP A)Advancing plunger is opposed by IOP and corneal bending pressure C) Cornea is indented, IOP rises B) Notch indicates corneal bending pressure has been transferred to footplate.

  9. Tono- Pen • Portable version of Mackay Margtonometer • Area of applanation is small (1.5 mm diameter). • The IOP readings match well with GAT in the normal range. Overestimate IOP in eyes with low IOP Underestimate IOP in eyes with high IOP. • In a normal eye- little difference in IOP readings between central or peripheral cornea. • Accurate in eyes with- an amniotic membrance patch graft, BCL.

  10. Pneumatic tonometer Useful in: • Irregular /scarred/edematous corneal surface. • Can be used against the sclera near the limbus. • Used over therapeutic soft contact lenses

  11. Non-contact tonometers Air puff • Force of airjet increases linearly. • When an area of the cornea 3.6 mm in diameter is flattened, the light reflected is maximum. • The time required to produce the peak reflection depends on the force of the air jet .And hence depends on the IOP.

  12. IOP with NCT >> GAT and rebound tonometry. • Decrease in accuracy at higher IOP(>20mmHg). • Association between CCT and NCT, which is greater than that reported with GAT. • The IOP readings correlate fairly well GAT readings.

  13. Corneal Hysteresis • Corneal tissue is a complex visco-elasticstructure, characterized by Hysteresis: How a system responds to an applied force- biological shock absorbption. CH is a surrogate marker for hysteresis values, in the peripapillary sclera and the TM. Lower hysteresis values = faster rates of visual field loss* = progression of glaucoma *Susanna, C.N.; et.al; A Prospective Longitudinal Study to Investigate Corneal Hysteresis as a Risk Factor for Predicting Development of Glaucoma. Am. J. Ophthalmol. 2018, 187, 148–152

  14. Corneal Hysteresis Measurement Air pulse+a high-speed Scheimpflug camera Does not provide corneal hysteresis. Biomechanically corrected IOP(bIOP) Corneal deformation parameters considered Less influenced by the effects of CH and CCT . blOP is a useful substitute marker for IOPcc • Measures the corneal response to indentation by a rapid air pulse. • Address measurement of • corneal biomechanical properties • Corneal compensated IOPcc is measured considering CH.

  15. ORA • The two applanation events are delayed by the viscoelastic corneal damping. • Normal CH= 10.2 to 10.7

  16. Dynamic contour tonometry- PASCAL By surrounding and matching the contour of a sphere (or a portion), the pressure on the outside equals the pressure on the inside. concave contour tip Central sensor

  17. When “Contour matching” is reached, the pressure sensor directly measures the IOP. • No bias related to corneal factors. • Requires about 8-10 s of corneal contact. • Goldmannreadings are 3–5 mmHg higher. Diastolic IOP Quality score upto 3/5 Ocular pulse amplitude= Max-Min IOP

  18. Indentation tonometers • Schiotz • Rebound • Trans palpebral

  19. Schiotz The plunger, hammer, and needle weigh 5.5 g- can be increased if the scale reading is less than 3 units. Each scale unit represents a 0.05 mm protrusion of the plunger

  20. IOP reflects the dispensability of the ocular coat- ocular rigidity. • The tables assume a normal ocular rigidity-error . • High ocular rigidity: Hyperopia, longstanding glaucoma • Low ocular rigidity : Myopia, S/p retinal detachment surgery with compressible gas

  21. Rebound tonometer- iCare • Light-weight magnetic probe in an enclosed shaft that is propelled toward the cornea. Voltage is monitored by the sensor Shorter duration of impact in eyes with higher IOP. Probe decelerates and rebounds When compared with GAT, IOP reads slightly higher. Like GAT, RT Co-relates with CCT.

  22. Transpalpebral tonometry • Spring end presses on eyelid until patient sees phosphene • Then device is removed. • Pressure at which phosphene was seen is recorded -represents IOP • Found good correlation with GAT Proview transpalpebralphosphene tonometer

  23. Continous IOP monitoring Sensimed Triggerfish( 2016) • This device can measure small modifications at the corneo –scleral limbusdue to variations of IOP. • High correlation with GAT immediately after the insertion of CLS. • The correlation becomes poor after 24 h

  24. Implantable IOP sensors Telemetric devices- • EYEMATE-IO, was implanted within the ciliary sulcus (ARGOS-02 trial)* but cataract surgery is a prerequisite. • EYEMATE-SC IOP: Suprachoroidal space * Choritz, L.; Mansouri, K.; Van Den Bosch,; et al. Telemetric Measurement of Intraocular Pressure via an Implantable Pressure Sensor—12-Month Results from the ARGOS-02 Trial. Am. J. Ophthalmol. 2020, 209, 187–196.

  25. Distribution of intraocular pressure Gaussian curve with a skew toward the right Abnormal IOP must be defined empirically – that is, an abnormal pressure is one that causes optic nerve damage in a particular eye. 15.5 ± 2.57 mm Hg 20.5 mm( +2 SD)

  26. Factors influencing IOP Factors • Genetics • Seasonal • Environment

  27. Diurnal variation • Diurnal variation- Circulating Catecholamines affecting AH formation. Conflicting results: • Itis the level of IOP , not the magnitude of fluctuations, that is responsible for optic nerve damage.* • Large diurnal variation in IOP is a risk factor for progression of glaucoma^. *Jonas JB, et al: Diurnal intraocular pressure profiles and progression of chronic open-angle glaucoma, Eye 21:948, 2007. Epub Apr 7 2006. ^Asrani S, et al: Large diurnal fluctuations in intraocular pressure are an independent risk factor in patients with glaucoma, J Glaucoma 9:134, 2000.

  28. Tonometry in special situations

  29. Gonioscopy Gonio: “angle” or “corner”

  30. Gonioscopy-Overview • Principle • Lenses • Technique • Dynamic gonioscopy • Normal angle structures identification , corneal wedge • Grading systems • Normal variations and abnormalities

  31. Principle

  32. The total internal reflection at the cornea occurs when: i > critical angle. • The critical angle for the cornea–air interface is 46 degree. • Contact lenses have an index of retraction similar to the cornea.(1.4) • Light is refracted (goniolens) or reflected (gonioprism) beyond the contact lens–air interface.

  33. Contact lenses for gonioscopy Direct gonioscopy: The curve of the goniolens is such that the critical angle is not reached. Cornea=42D; Koeppe = 50 D The light rays are refracted at the contact lens–air interface.

  34. The Koeppe lens is the prototype diagnostic direct goniolens. • It magnifies x 1.5 + the x 16 magnification of the oculars=Total mag x24.

  35. Hoskins-Barkan surgical contact lens Swan-Jacobs surgical contact lens

  36. Indirect gonioscopy • Gonioprisms have an angled mirror. • Light rays from anterior chamber angle are reflected. • They emerge perpendicular to the lens–air interface. • May or may not require coupling agents

  37. The Goldmann lenses • Have a broad area of contact (approximately 12 mm) with cornea and under pressure may artificially close the angle. • A coupling material is required due to curvature-7.3mm • Modified Goldmann-type lens, with 8.4mm radius of curvature, requires no coupling agent. Goldmann 3 mirror Angles: 59 ° -Angles 67 ° -pars plana 73 ° -oraserrata Goldmann single mirror 62°

  38. Allen Thorpe gonioprism. • It has four prisms instead of mirrors and allows examination of the whole angle without rotation.

  39. Lenses for indentation gonioscopy Sussman four-mirror lens Posner four-mirror lens with fixed handle and plastic lens. Zeiss 4 mirror lens On Unger fork All the mirrors are tilted at 64 degrees Small area of contact with cornea- 9mm No coupling agent required Rotation of 11 degrees covers area between mirrors

  40. Mirrored gonio lenses provide a inverted image, but the image is not crossed.

  41. Surroundings • Gonioscopy should be performed in a relatively dark room-consensual pupillary reaction. • Get an overview of the angle structures. • Use a short (2–3 mm), not too wide (1 mm) and not too bright slit beam. • OtherwiseITCs may be missed.

  42. Illunmination techniques • Start with the inferior angle- pigmentation is more prominent. • Proceed clockwise. 1.Diffuse illumination 2. Focal illumination with broad beam( note the well defined iris process and contour of iris) 3.Focal illumination with narrow beam- Corneal wedge.

  43. Identification of the corneal wedge • A thin slit of light inclined 15–20° • Sharply focussed beam, projected on to the iridocorneal angle. • Two light reflections-from the external and internal surfaces of the cornea • These two reflections meet at the beginning of Schwalbe’s line. • This wedge can have a variable appearance, based on anatomy of sclera and cornea

  44. Normal angle structures Schalwbes’s line • It is a condensation of collagen tissue and highlights the end ofDescemet’s membrane • Normal vessels and normal tissue will not pass it. • PAS and neovascularizationmay pass Schwalbe’s ring. • It may be prominent with the appearance of a ledge, and is then called “embryotoxonposterius”.

  45. Schlemms Canal • Becomes visible only when filled with blood. • Blood can occasionally be found in Schlemm's canal in normal eyes, • When a contact lens with a large diameter is pressed too firmly against the eye, or • When the pressure in the episcleral venous system is high or when the IOP is low. Pathologic conditions- Hypotoniabulbi During resorption of hyphema Increased episcleral venous pressure Carotid-Cavernous fistula Sturge-Weber syndrome

  46. Trabecular meshwork • Grade the amount of pigmentation of the posterior trabecular meshwork at the 12 o’clock position. • Scheie’s system of grading : 0 none, +1 trace,+2 mild, +3 moderate, +4 severe; Aqueous humor outflow Also called the posterior TM Functional TM (White arrows) Non functional TM (Black arrows)

  47. Pigment along the anterior margin of the TM Non-pigmented angle -pale graycolor Heavy angle pigmentation, a wavy band of pigment anterior to SL- Sampaolesi’s line Deep pigments in the TM Heavily pigmented

  48. Scleral Spur • Ridge of collagen tissue of the sclera that lies anterior to the CBB. • White or light gray ,may have a yellowish cast in older individuals. • May be obscured by – iris processes(img) a high insertion of the iris iris bombe peripheral anterior synechiae or heavy pigmentation

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