TONOMETRY &ITS RECENT ADVANCES

TONOMETRY & ITS RECENT ADVANCES

TonometryTonometry is the procedure performed to determine the intraocular pressure (IOP)

HISTORY1826: William Bowman used digital tonometry as a routine examination test.

1863: Albrecht von Grafe designed the first instrument to attempt to measure intraocular pressure.Further instruments followed, notably by Donders in 1865 and Preistly-Smith in 1880.All were of the indentation type and rested on the sclera1885:Maklakov designed an applanation tonometer. Used for a number of years in Russia and Eastern Europe.1905: Hjalmar Schiotz produced his indentation tonometer. This made tonometry a simple and routine clinical test.

Albrecht von Grafe tonometer

Donders tonometer

Ideal tonometerShould give accurate and reasonable IOP measurement Convenient to use Simple to calibrate Stable from day to day Easier to standardise Free of maintenance problems

All clinical tonometers measure the IOP by relating a deformation of the globe to the force responsible for the deformation.The two basic types of tonometers differ according to the shape of the deformation: indentation and applanation (flattening).

INDENTATION TONOMETERThe shape of the deformation with this type of tonometer is a truncated cone.These instruments displace a relatively large intraocular volume. As a result, conversion tables based on empirical data from in vitro and in vivo studies must be used to estimate the IOP.Prototype- Schiotz tonometer

APPLANATION TONOMETERSThe shape of the deformation with these tonometers is a simple flattening, and because the shape is constant, its relationship to the IOP can, in most cases, be derived from mathematical calculations. The applanation tonometers are further differentiated on the basis of the variable that is measured.

VARIABLE FORCEThis type of tonometer measures the force that is required to applanate (flatten) a standard area of the corneal surface. Prototype- Goldmann applanation tonometer, which was introduced in 1954.

VARIABLE AREAOther applanation tonometers measure the area of the cornea that is flattened by a known force (weight).Prototype- Maklakoff tonometerGoldmanntype tonometers have relatively minimal displacement of intraocular volume, whereas that with Maklakoff-type tonometers is sufficiently large to require the use of conversion tables.

Applanation Tonometer with variable areaTONOMETERDESCRIPTION/USEMaklakoff-KalfaPrototypeApplanometerCeramic endplatesTonomatDisposable endplatesHalberg tonometerTransparent endplate for direct reading: multiple weightsBarraquer tonometerPlastic tonometer for use in operating roomOcular tension indicator

Uses Goldmann biprism and standard weight, for screening (measures above or below 21 mm Hg)Glaucotest

Screening tonometer with multiple endplates for selecting different cutoff pressures

NON-CONTACT TONOMETERA third type of tonometer uses a puff of air to deform the cornea and measures the time or force of the air puff that is required to create a standard amount of corneal deformation.The prototype was introduced by Grolman in 1972.

Schiotz Indentation TonometryIt consists of a footplate that rests on the cornea and a weighted plunger that moves freely (except for the effect of friction) within a shaft in the footplate with the degree to which it indents the cornea indicated by the movement of a needle on a scale. A 5.5-g weight is permanently fixed to the plunger, which can be increased to 7.5,10, or 15 g by adding additional weights.

Parts of schiotz tonometer

Scale needleWeight 5.5g plungerholderFoot plate lever3mm diameterROC 15mmTonometer weight = 11gAdditional weights 7.5,10,15g

Schiotz tonometry - characteristicsThe extent to which cornea is indented by plunger is measured as the distance from the foot plate curve to the plunger base and a lever system moves a needle on calibrated scale. 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 IOPEach scale unit represents 0.05 mm protrusion of the plunger.

PRINCIPLEThe weight of tonometer on the eye increases the actual IOP (Po) to a higher level (Pt). The change in pressure from Po to Pt is an expression of the resistance of the eye (scleral rigidity) to the displacement of fluid.

IOP with Tonometer in position Pt =

Actual IOP Po + Scleral Rigidity E (P (t) = P(o) + E)

Determination of Po from a scale reading Pt requires conversion which is done according to Friedenwald conversion tables.

Friedenwald formula

Friedenwald generated formula for linear relationship between the log function of IOP and the ocular distension.Pt = log Po + C VThis formula has C a numerical constant, the coefficient of ocular rigidity which is an expression of distensibility of eye. Its average value is 0.025V is the change in volume

Friedenwald developed a set of conversion tables, referred to as the 1948 and 1955 tables for IOP.Subsequent studies indicated that the 1948 tables agree more closely with measurements by Goldmann applanation tonometry.

Friedenwald conversion table

Plunger LoadScale Reading5.5 g7.5 g10 g15 g3.024.435.850.681.83.522.433.046.976.24.020.630.443.471.04.518.928.040.266.25.017.325.837.261.85.515.923.834.457.66.014.621.931.853.66.513.420.129.449.97.012.218.527.246.57.511.217.025.143.28.010.215.623.140.28.59.414.321.338.19.08.513.119.634.69.57.812.018.032.010.07.110.916.529.6

TECHNIQUEPatient should be anasthetised with 4%lignocaine or 0.5% proparacaine With the patient in supine position, looking up at a fixation target examiner separates the lids and lowers the tonometer plate to rest on the anesthetized cornea so that plunger is free to move vertically .The examiner observes a fine movement of the indicator needle on the scale in response to the ocular pulsations. The average between the extremes of these is taken. The 5.5 gm weight is initially used.If scale reading is 4 or less, additional weight is added to plunger.Conversion table is used to derive IOP in mm Hg from scale reading and plunger weight.

Cause of errorBecause conversion tables were based on an average coefficient of ocular rigidity (C), eyes that deviate significantly from this C value give false IOP measurements. Another variable that affects accuracy is expulsion of intraocular blood during indentation tonometry . A relatively steep or thick cornea causes an increased displacement of fluid during indentation tonometry, which leads to a falsely high IOP reading

CALIBRATIONThe instrument should be calibrated before each use by placing it on a polished metal sphere and checking to be sure that the scale reading is zero. If the reading is not zero, the instrument must be repaired.

STERILIZATIONThe tonometer is disassembled between each use and the barrel is cleaned with 2 pipe cleaners, the first soaked in isopropyl alcohol 70 % or methylated spirit and the second dry. The foot plate is cleaned with alcohol swab. All surfaces must be dried before reassembling.The instrument can be sterilized with ultraviolet radiation, steam, ethylene oxide.As with other tonometer tips, the Schiotz can be damaged by some disinfecting solutions such as hydrogen peroxide and bleach.

Differential tonometryIt is done to get rid of ocular rigidity factor.A reading is taken with one weight on the Plunger and then a second reading' in taken with a different weight.Making a diagnosis of glaucoma in a pt. with myopia presents unusual difficulties. The low ocular rigidity in these eyes result in Schiotz readings within normal limits.

5.5g10gOcular rigidityIOP18 mm Hg15 mm Hglower>1818 mm Hg21 mm Hghigher3 mm of area gives artificial elevation of IOP.

The most commonly used Mackay-Marg-type tonometer today is the Tono-Pen, a handheld instrument with a strain gauge that creates an electrical signal as the footplate flattens the cornea(microstrain gauge technology). It averages 4 to 10 readings to give a final digital readout. It also provides the percentage of variability between the lowest and highest acceptable readings from 5% to 20%.

Tonopen

Tonopen

PneumotonometerHere a central sensing device measures the IOP, while the force required to bend the cornea is transferred to a surrounding structure. The sensor, is air pressure.It has a sensing device that consists of a gas chamber covered by a polymeric silicone diaphragm.A transducer converts the gas pressure in the chamber into an electrical signal that is recorded on a paper strip. As the sensing nozzle touches the cornea and when the area of contact equals that of the central chamber, an initial inflection is recorded, which represents the IOP and the force required to bend the cornea. With further enlargement of the corneal contact, the bending force is transferred to the face of the nozzle, which is interpreted as the actual IOP.

Pneumotonometer

NONCONTACT TONOMETERThe noncontact tonometer was introduced by Grolman and has the advantage over other tonometers of not touching the eye.After proper alignment of the patient, a puff of room air creates a constant force that momentarily deforms the central corneaDetected by an optoelectronic system of a transmitter, which directs a collimated beam of light at the corneal vertex, and a receiver and detector, which accepts only parallel, coaxial rays reflected from the cornea.

At the moment that the central cornea is flattened, the greatest number of reflected light rays are received, which is recorded as the peak intensity of light detected. The time from an internal reference point to the moment of maximum light detection is converted to IOP.The time interval for an average noncontact tonometer measurement is 1 to 3 milliseconds (1/500th of the cardiac cycle) and is random with respect to the phase of the cardiac cycle so that the ocular pulse becomes a significant variable. For this reason, it is recommended that a minimum of three readings within 3 mm Hg be taken and averaged as the IOP.

New NCT, Pulsair is a portable hand held tonometer

Miscellaneous tonometers Rebound TonometerIn a new handheld tonometer, the Icare tonometer, IOP is determined by measuring the force produced by a small plastic probe as it rebounds from the cornea. The device uses an induction coil to magnetise the probe and fire it against the cornea. As the probe bounces against the cornea and back into the device it creates an induction current from which the intraocular pressure is calculated. As a screening tool in children. The ability to evaluate IOP without the use of topical anesthesia potentially provides the opportunity to monitor IOP at home.The rebound tonometer has been shown to have similar accuracy to the Tono-Pen, and it is comparable with Goldmann tonometry for IOPs over a reasonable range in adults.

Rebound tonometer

The Ocuton tonometer

The Ocuton tonometerHand-held tonometerWorks on the applanation principle Probe is so light that it is barely felt Needs no anesthetic in most patients. Used for home tonometry Useful to get some idea of the relative diurnal variation in IOP if the patient or relative can learn to use it.

Trans palpebral tonometry

Used in situations where other, more accurate, devices are not practical, such as in young children, demented patients and severely developmentally-challenged patients.It adds variables such as the thickness of the eyelids, orbicularis muscle tone and potential Intra palpebral scarring.

Transpalpebral tonometry does not involve contact with the cornea and does not require sterilization of the device or topical anesthetic during routine use. Only moderate correlation with those provided by applanation tonometryHome tonometer: Proview Phosphene Tonometer(Bausch & Lomb) is based on phosphene perception after eyelid indentation

Diaton tonometer (BiCOM, Inc)

Measuring intraocular pressure through the Eyelid The Diaton tonometer calculates pressure by measuring the response of a free falling rod The principle is based on Newton's second law, as it rebounds against the tarsal plate of the eyelid. The patient is positioned so that the tip of the device and lid are overlying sclera.

Ocular Response Analyser(ORA)Provides IOP measurement free from influence of corneal biochemical propertiesIt measures corneal hysteresis & corneal resistance factor & thus overcomes the demerits of GAT to some extend

Ocular Response AnalyzerIt directs the air jet against the cornea and measures not one but two pressures at which applanation occurs

1) when the air jet flattens the cornea as the cornea is bent inward and 2) as the air jet lessens in force and the cornea recovers.

Ocular response analyserThe first is the resting intraocular pressure. The difference between the first and the second applanation pressure is called corneal hysteresis Corneal hysteresis is a measure of the viscous dampening and, hence, the biomechanical properties of the cornea. The biomechanical properties of the cornea are related to corneal thickness and include elastic and viscous dampening attributes.

IOP correlate well with Goldmann tonometry but, on average, measure a few millimeters higher.Further , while IOP varies over the 24-hour day, hysteresis seems to be stable.Congdon et al found that a low hysteresis reading with the ORA correlates with progression of glaucoma, whereas thin central corneal thickness correlates with glaucoma damage.It has practical value in the management of glaucoma.

Pascals Dynamic Contour TonometryDynamic contour tonometry (DCT) is a novel method which uses principle of contour matching instead of applanation.Principle : By surrounding and matching the contour of a sphere (or a portion thereof ), the pressure on the outside equals the pressure on the inside. This is designed to reduce the influence of biomechanical properties of the cornea on measurement. These include corneal thickness, rigidity, curvature, and elastic properties.It is less influenced by corneal thickness but more influenced by corneal curvature than the Goldmann tonometer

Dynamic contour tonometer

The contour matched tip has a concave surface of radius 10.5mm, which approximates to the shape of a normal cornea when the pressure on both sides is equal. The probe is placed adjacent to the central cornea.The integrated piezoresistive pressure sensor automatically begins to acquire data, measuring IOP 100 times per second. A complete measurement cycle requires about 8 seconds of contact time. The device also measures the variation in pressure that occurs with the cardiac cycle. (Ocular pulse Amplitude)

The concept developed from a previous contact lens tonometer called the Smart Lens. It superior in accuracy to Goldmann tonometry and pneumotonometry .IOP is not affected by corneal thickness.IOP is not altered by corneal refractive surgery that thins the cornea.

The DCT shows the magnitude of the difference between maximum and minimum IOP as the ocular pulse amplitude.OPA may be indicative of the status of ocular blood flow and be differentially affected in different types of glaucoma.Ocular pulse amplitude is increased over normals in most forms of glaucoma and may be related to the level of IOP.

IOP Monitoring DevicesThere is need for an IOP telemetry device without artificially altering the pressure. Several prototypesbased on a contact lens, an implantable device, or a scleral band device have been developed. Such a lens will help us monitor and manage individuals who are susceptible to wide IOP fluctuations, who have poor adherence to medical therapy, who perhaps are poor responders to medical therapy, and who have wide IOP fluctuations in the postoperative period

Continuous IOP monitoring devices: Contact lenses like Sensimed Triggerfish & the Smart contact lenses measure IOP by detecting changes in the corneoscleral curvature induced by IOP changes

Comparison of TonometersThe most precise method for evaluating the accuracy of a tonometer is to compare it with manometric measurements of the cannulated anterior chamber. Its use in largescale human studies has been limited. The alternative is to compare the tonometer in question against the instrument that previous studies have shown to be the most accurate. In eyes with regular corneas, the Goldmann applanation tonometer is generally accepted as the standard against which other tonometers must be compared. Even with this instrument, however, inherent variability must be taken into account. When two readings were taken on the same eye with Goldmann tonometers in a short time frame, at least 30% of the paired readings differed by 2 and 3 mm Hg or more. In another study, intraobserver variation was 1.5 1.96 mm Hg and interobserver variation was 1.79 2.41 mm Hg, which could be reduced by 9% and 11%, respectively, by using the median value of three consecutive measurements

Clinically, the most widely used methods for measuring IOP are by Goldmann applanation tonometry and with use of the Tono-Pen; the noncontact tonometer, Perkins tonometer, pneumotonometry, and the Schitz tonometer. In general, the Schitz tonometer reads lower than the Goldmann. The Perkins applanation tonometer compared favorably against the Goldmann tonometer.

The Tono-Pen resembles manometric readings in human autopsy eyes.Most studies agree that the Tono-Pen underestimates Goldmann IOP in the higher range and overestimates in the lower range

In multiple comparative studies, readings taken with the pneumotonometer correlated closely with those obtained by using Goldmann tonometers, although the pneumotonometer readings tended to be higher.Post-LASIK IOP measurements obtained by pneumotonometry were more reliable than those taken by Goldmann applanation.

Tonometry for Special Clinical Circumstances The pneumatic tonometer has been shown to be useful in eyes with diseased or irregular corneas.In eyes after penetrating keratoplasty, the Tono-Pen significantly overestimated Goldmann readings

Tonometry on Irregular Corneas

Tonometry over Soft Contact LensesIt has been claimed that pneumotonometry and the Tono-Pen can measure with reasonable accuracy the IOP through bandage contact lenses.

Tonometry with Gas-Filled EyesIntraocular gas significantly affects scleral rigidity. In a study with irregular corneas after vitrectomy and air-gas-fluid exchange, readings with the Tono-Pen and pneumotonometer were highly correlated.A manometric study with human autopsy eyes indicated that both instruments significantly underestimated the IOP at pressures greater than 30 mm Hg

Tonometry with Flat Anterior ChamberIn human autopsy eyes with flat anterior chambers, IOP readings from the Goldmann applanation tonometer, pneumotonometer, and Tono-Pen did not correlate well with manometrically determined pressures

Tonometry in Eyes with KeratoprosthesesIn patients at high risk for corneal transplant rejection, implantation of a keratoprosthesis is now a viable option for vision rehabilitation. However, given that most keratoprostheses have a rigid, clear surface, it is impossible to measure IOP by using applanation or indentation instruments.In such eyes, tactile assessment appears to be the most widely used method to estimate IOP