Means of IOP measurement

Means of IOP measurement
By Dr Kalyani & Dr Ankit

1. MEANS OF IOP MEASUREMENT PRESENTERS: DR KALYANI & DR ANKIT MODERATOR: DR KHER MA’AM CONDUCTOR: DR NAGPURE MA’AM

2. BASICS INTRAOCULAR PRESSURE: Pressure exerted by the intraocular fluids on the coat of the eye ball. NORMAL IOP: the pressure that doesn’t induce glaucomatous changes to optic nerve head. Normally it is estimated to be 10-21 mmHg. WHY 21? The distribution in general population resembles a Gaussian curve but skewed towards right . Mean IOP is considered to be 15.5 +/- 2.57mmHg. Two standard deviation above the mean is approximately 20.5 mmHg as approximately 95% of the area under the Gaussian curve lies between the mean +/- 2 SD . Thus the concept of normal IOP limits is viewed as only a rough approximation .

3. FACTORS AFFECTING LEVEL OF IOP: Rate of aqueous secretion, Resistance encountered in outflow channels, Level of Episcleral Venous Pressure (N- 8-10mmHG) The Aqueous humor Nutrition to lens, cornea and iris Medium for removal of metabolic toxic products Has an RI of 1.33 Inflates the globe and maintains IOP Facilitates cellular and humoral response of eye towards inflammation & infection.

4. AQUEOUS FORMATION: The Ciliary epithelium is a bilayer of polarised epithelial cells that line the surface of the ciliary body processes are the site of formation. The two cell layers are Nonpigmented epithelium and the Pigmented epithelium. The NPE forms a part of the blood-aqueous barrier through tight junctions, thereby preventing paracellular transport from the ciliary stroma into the posterior chamber. The PE is a leaky epithelium that allows solutes to move through the space between the PE cells. Aqueous humor is secreted by the NPE from a substrate of blood plasma . Flow rate is 2-3 microlitre/ min .

5. The aqueous enters the posterior chamber by: 01 ACTIVE : Energy dependant secretion of certain ions and substrates 02 PASSIVE: Diffusion & Ultrafiltration

6. DRAINAGE CONVENTIONAL OUTFLOW: 75-90% Trabecular Meshwork  Schlemm’s Canal  Collector Channels (Intrascleral Plexus- Indirect)  Episcleral and Conjunctival Veins  Cavernous Sinus UVEOSCLERAL OUTFLOW: 10-25% Ciliary Body  Supra-Choroidal Space  Venous Circulation of Ciliary Body  Choroid  Sclera  Orbit The outflow is approx. 0.3 ml/min and is independent of IOP changes

7. ELEVATED EVP: Obstruction of venous drainage: Thyroid eye disease Radiation Pseudotumor Cavernous sinus thrombosis Jugular vein obstruction Superior vena cava obstruction Pulmonary venous obstruction Arteriovenous fistulas Carotid-cavernous fistula Sturge-Weber syndrome Dural fistula Venous varix

8. CONDITIONS THAT INFLUENCE THE IOP : Diurnal variation Postural variation Exertional influences Lid and eye movement Intraocular conditions Systemic conditions Environmental conditions General anesthesia Food and drugs

9. DIURNAL VARIATION: The most common pattern is that the IOP is maximum in the morning and decreases as day progresses and becomes minimum in the evening because of endogeneous catecholamines. It once again starts to rise as the night progresses. The variation is about 3 to 6 mm Hg in normal individuals and about 10 mm Hg or more in a glaucomatous eye . It is due to cyclic fluctuation of blood levels of adrenocortical steroids . Maximum IOP is reached 3 to 4 hours after the peak of plasma cortisol. The night time elevated IOP is due to the supine position along with the fluctuating cortisol levels.

10. There are four types of diurnal variation curves. Falling type : maximal at 6–8 am followed by a continuous decline Rising type : maximal at 4–6 pm Double variation type : with 2 peaks 9–11 am and 6 pm Flat type of curve. POSTURE: The IOP rises (0.3–6 mm Hg) when a person is lying down . This may be because of increase in the episcleral venous pressure in the supine posture. EXERTION: Valsalva maneuvers increases IOP (by increasing episcleral venous pressure) while prolonged exercise decreases IOP (by metabolic acidosis and increased colloid osmotic pressure).

11. HARD LID SQUEEZING: Increases IOP because of increased orbicularis tone . INTRAOCULAR CONDITIONS: Acute anterior uveitis causes a slight reduction in IOP because of decreased aqueous humour production. Rhegmatogenous retinal detachment also causes a reduction because of reduced aqueous humor production as well as shunting of aqueous humour from the posterior chamber through the vitreous and retinal hole into the subretinal space. SYSTEMIC FACTORS CAUSING DECREASED IOP: Pregnancy Hyperthyroidism Myotonic dystrophy SYSTEMIC CONDITIONS CAUSING INCREASED IOP: Systemic hypertension Systemic hyperthermia ACTH and growth hormone stimulation Hypothyroidism Diabetes

12. ENVIRONMENTAL CONDITIONS: Exposure to cold decreases IOP (because of lowered episcleral venous pressure) while reduced gravity increases IOP ANESTHETIC AGENTS: In general, general anesthetic agents reduce IOP. However, tricholoroethylene , ketamine, succinylcholine and suxamethonium increase IOP FOOD AND DRUGS: Factors increasing IOP: i . Caffeine ii. Tobacco smoking Factors decreasing IOP: i . Alcohol ii. Heroin and marijuana HEREDITY: IOP tends to be higher in individuals with enlarged cup-disk ratio and in relatives of open-angle glaucoma

13. TONOMETRY

14. In 1865 Donders  designed the first tonometer intended for use against the sclera and Priestley Smith in 1884, came up with something similar. In 1885, Applanation tonometer was designed by Maklakoff . In 1897, an Impression tonometer for the Sclera was developed by Schiotz, later updated to corneal plunger device in 1905. In 1954, Applanation tonometer was invented by Goldmann. In 1972, Grollman invented the Non Contact Tonometer

15. FACTORS INFLUENCING TONOMETRY: CENTRAL CORNEAL THICKNESS: increase in corneal rigidity  increased IOP. ASTIGMATISM CORNEAL CURVATURE OCCULAR RIGIDITY: It is the measurement of the resistance to deformation of the ocular coats. It is decreased in Acutely elevated IOP, Osteogenesis imperfecta, Miotic therapy, Vasodilator therapy, Myopic eyes. The increase in ocular rigidity is seen in ARMD, Hypermetropic eyes and long standing glaucoma.

16. DIGITAL TONOMETRY

17. It is the response of the intact eyeball to the pressure exerted on it by the pulp of finger Alternately just enough pressure is applied on the eyeball to indent it with the index finger while feeling the compliance with the other. It is easy to perform; no equipment required; no anesthesia is required; and helps to estimate the IOP in irregular corneas, where applanation tonometry isn’t possible. It is inaccurate and often subjective; leads to over or under estimation of the IOP.

18. IDEAL TONOMETER: Must be accurate in its measurement Should be convenient to use C alibration should be simple Easy Standardization Maintenance should be hassle-free

19. MANOMETRY: The only direct measurement of IOP. A needle is introduced into the AC or the vitreous. And then connected to a mercury or water manometer Uses: Continuous measurements of IOP; Used in animal eyes for research and experimental purposes Disadvantages: An impractical method for use on humans; Needs general anesthesia; Introduction of needle produces breakdown of blood aqueous barrier and release of Prostaglandins which alter the IOP

20. Indentation Tonometry

21. The shape of deformation is a truncated cone. It is based on the principle that the plunger would indent a soft eye more than the hard one SCHIOTZ TONOMETER is the most popular, it was devised in 1905 and had a lot of refinement ever since. When the tonometer is placed on the cornea, the following forces become relevant : W - Weight of the tonometer A - Area Vc - Volume of fluid displaced after indentation T- Tensile forces of the outer coats of the eyeball

22. The resting intraocular pressure (P0) which raises to a new artificial value (P1), the scale reading of tonometer actually measures the artificially raised Intraocular Pressure. The conversion of P1 to P0 is elicited from the conversion tables developed by Friedenwald . The calibration was carried by experiments in cadaveric eyes connected with manometer through cannula FRIEDENWALD FORMULA: Log Pt2 / Pt1 = K ( V2 – V1 ) Pt1 & V1 represent the tonometric pressure & volume of the indentation caused by the bar in the determination made with the first weight. Pt2 and V2 represent the tonometric pressure & volume of the indentation as obtained with the second weight. The observation were plotted on semilog scale ,which serve as Friedenwald nomogram where the indicated scale reading and the plunger weight are converted to an IOP measurement, More the plunger indents the cornea, higher the scale reading and lower the IOP.

23. COMPONENTS OF SCHIOTZ Handle to hold the instrument in vertical ly on to the cornea Footplate that rests on the cornea l surface A Plunger that moves freely within a shaft in footplate A B ent lever whose short arm rests on upper end of plunger . A Long arm which acts as pointer needle Weights of 5.5 gm that is permanently fixed to the plunger and can be increased to 7.5 and 10 gm

24. 1. Patient should be anesthetis ed with 4% lignocaine or 0.5% proparacaine. 2. W ith the patient in supine position, looking up at a fixation target while the examiner would separate the lids and lower the tonometer plate to rest on the cornea so that plunger becomes free to move. 3. 5.5gm in the initial weight that is used. If the scale reading is 4 or less , additional weight is added. 4. IOP measurement is repeated until 3 consecutive readings agree within 0.5 scale units. Conversion table is used to derive IOP in mmHg from scale reading and plunger weight.2.

25. Scope of Error I n the instrument Might be d ue to the difference in shape , weight, size and curvature of footplate D ue to contraction of extra ocular muscles tend to increase IOP D ue to accommdation With the patient looking at the tonometer resulting in accommodation. Lowering of IOP because of contraction of ciliary muscle

26. Moses effect : On low scale reading the cornea might mould into the space between the Plunger and hole, giving high IOP reading. Due to ocular rigidity D ue to variation in corneal curvature : T hick cornea will cause greater displacement of fluid causing high IOP readings

27. A D V AN T A G E S Simple Techniq u e Elegant design Portable No need for slit lamp or power supply Reasonably priced Widely used tonometer DISADVANTAGES Falsely high/low IOP in Ocular rigidity Cannot be used in traumatic cases and corneal pathologies

28. APPLANATION TONOMETRY

29. The shape of deformation is flattening. It was introduced by Goldmann in 1954, It is based on IMBERT FICK LAW The prism applanates the cornea in an area of 3.06 mm diameter. Variable force, Fixed area of applanation For a very thin walled sphere, perfectly elastic and dry, the internal pressure is equivalent to the force per unit area of applanation. It states that the pressure inside an ideal sphere (P) is equal to force (W) reqired to flatten(A) P=W/A

30. Modifie d I mbert - Fick’ s Law W + S = P A1 + B W = Tonometer force S = Surface tension of pre-corneal tear film P = Intra-ocular pressure A1 = Inner corneal area of applanation B = Corneal rigidity GAT is influenced by corneal thickness, corneal curvature, structure of the cornea and the axial length

31. COMPONENTS of Goldmann Tonometer : Connects to the slit lamp Biprism (measuring prism) Feeler arm Control weight insert Housing Revolving knob & measuring drum

32. Technique Topical anesthesia is given, set correct eye height via the chin rest. The tear film is stained with fluorescein. The patient should blink the eyes once or twice to spread the fluorescein-stained tear film over the cornea. Then keep eyes open wide The cornea and biprisms are illuminated with cobalt blue light. Meticulously the apex of cornea is just touched by the biprism . At this point two fluorescent semicircles are viewed through prism.

33. Applanation force against cornea is adjusted until inner edges of two semicircles just touches.

34. Scope of error Falsely low IOP: too little flourecein , thin cornea, corneal edema, WTR astigmatism, prolonged contact, repeated tonometry and observer bias Falsely high IOP Tpp much fluorescein, thick cornea, steep cornea, ATR astigmatism, putting pressure on the lids while separating. Tonometer out of calibration

35. Advantages Highly accurate Do not requires supine position P ortable Disadvantages Not portable C ostly R eading error if the cornea is scarred

36. Perkins Tonometer It uses the same biprism as the Goldmann applanation. The light source is powered by battery. The readings are consistent and compared quite well with the Goldmann applanation. ADVANTAGES OVER GAT: Portable & counter-balanced, so it can be used in any position. Useful in examining Infants and children For bedside examination for non-ambulatory patients, who cannot be examined at the slit lamp.

37. Pneumatic tonometer Cornea is applanated by touching apex by silastic diaphragm covering sensing nozzle. It is connected to central chamber containing pressurized air. There is pneumatic to electronic transducer. It converts the air pressure to recording on paper strip and IOP is red.

38. Tono pen This is small, handheld Mackay Marg type computerised pocket tonometer The instrument is 18 cm in length and weighs 60 g. It converts IOP into electric waves Wave form is internally analyzed by a microprocessor Average of 3 to 6 readings of IOP

39. As the area of applanation of the Tonopen is smaller than GAT (2.36mm2 Vs 7.35mm2) therefore, theoretically the difference between applanating pressure & IOP is reduced due to reduced corneal resistance of a smaller contact area It is particularly useful in community eye camps, on ward rounds ,children, irregular surfaces, measuring through an amniotic membrane patch graft, to read from the sclera A disposable latex cover which is discarded after each use provides infection control

40. PASCAL DYNAMIC CONTOUR TONOMETER It is a slit lamp mounted device. The contact surface is contoured to compliment the average corneal profile. Contour matching- Independent of force applied or the contact area Hence, the device is insignificantly affected by corneal thickness, curvature, optical aberrations, or surface irregularity. Maintains the corneal touch over the course of the cardiac cycle (5-8 secs) The measured OPA is also displayed

41. PNEUMOTONOMETRY High displacement tonometer Can also be used to measure outflow facility Printouts of IOP over time- measuring IOP fluctutations possible. Measurements independent of CCT- Post LASIK

42. NON-CONTACT TYPE Air puff tonometer :- In this central part of cornea is flattened by a jet of air . This tonometer is very good for mass screening as there is no danger of cross infection and local anaesthetic is not required. Pulse air tonometer :- It is a non-contact tonometer that can be used with the patient in any position.

43. GROLMAN NON-CONTACT TONOMETER Introduced by Grolman in 1972 and works on the principle of a time interval. Measuring the time from initial generation of the puff of air to cornea gets flattened (in milliseconds) to the point where the timing device stops The NCT has 3 sub-systems: Alignment system: It aligns patient’s eye in 3 dimensions (axial/vertical/lateral) Optoelectronic applanation monitoring system: Comprises a transmitter, a receiver and detector & timer Transmitter directs a collimated beam of light at corneal apex Receiver & detector accept only parallel coaxial rays of light reflected from cornea Timer measures from an internal reference to the point of peak light intensity Pneumatic system: It generates a puff of room air directed against cornea

44. A puff of air jet creates a constant force that momentarily flattens the cornea The force of air jet generated by a solenoid-activated piston increases linearly over time, when the reflected light is at peak intensity, the cornea is presumed to be flattened. The time elapsed is directly related to the force of jet necessary to flatten the cornea & correspondingly to IOP The time from an internal reference point to the moment of flattening is measured & converted to IOP.

45. SPECIAL SITUATIONS In eyes with corneal edema , significant surface irregularity, scarring and recent corneal grafts: PNEUMOTONOMETRY TONOPEN, DCT (It gives more accurate measurement than the GAT) In children: PERKIN’S I-CARE (Rebound tonometry) TONOPEN

46. REBOUND TONOMETER It determines IOP by bouncing a small plastic tipped metal probe against the cornea. The device uses an induction coil to magnetize the probe and fire it against the cornea. The rebound tonometer is a handheld portable instrument with distinct advantages for the pediatric population because the light-weight probe makes only momentary contact with the cornea (i.e. 0.25 to 0.30 meters/seconds) its main advantage is that topical anesthesia is not required. It is particularly useful in children who do not tolerate Goldmann applanation tonometry. It has also been used in home settings, in situations when patients are unable to follow-up for frequent IOP assessments, and to monitor diurnal variations ( Icare HOME; Icare USA)

47. Post keratorefractive surgery: DCT Ocular Response Analyser

48. OCULAR RESPONSE ANALYSER It measures the corneal response to indentation by a rapid air pulse. A fully automated alignment system positions an air tube to a precise position relative to the apex of the cornea. Once aligned, a 25 millisecond air pulse applies pressure to the cornea. The air pulse causes the cornea to move inward, past applanation and into a slight concavity before returning to normal curvature. Corneal deformation is recorded via an electro-optical infrared (IR) detection system (similar to the classical air-puff tonometers ).

49. It directs the air jet against the cornea and measures not one but two pressures at which applanation occurs when the air jet flattens the cornea as the cornea is bent inward and as the air jet lessens in force and the cornea recovers. The first is the resting intraocular pressure. The difference between the first and the second applanation pressure is called corneal hysteresis. It is the "energy absorption capability" of the cornea This because of the speed at which the cornea is deformed during the dynamic bi-directional applanation process in ORA

50. In Supine position: PERKIN’s PNEUMOTONOMETER Less accurate but more practical and commonly used in TONOPEN Tonometry over Soft Contact Lenses PNEUMOTONOMETER TONOPEN can measure with reasonable accuracy the IOP through bandage contact lenses.

51. Tonometry in Gas-Filled Eyes Intraocular gas significantly affects scleral rigidity. In a study with irregular corneas after vitrectomy and air- gasfluid exchange, readings with the TONOPEN and PNEUMOTONOMETER were highly correlated. Tonometry in Flat Anterior Chamber In human autopsy eyes with flat anterior chambers, IOP readings from the GAT, PNEUMOTONOMETER and TONOPEN did not correlate well with manometrically determined pressures

52. REFERENCES American academy of ophthalmology. Parson’s diseases of the eye, 22 nd edition Comprehensive ophthalmology by A K Khurana Kanski book of clinical ophthalmology Google for images

53. THANK YOU!