Tonometry

Tonometry

Introduction •Tonometry : It is a clinical technique that provides a measurement of the eye , which includes the combined resistance to deformity of its coats and the intraocular pressure (IOP).

•Tonometer –It is the device used for measuring the Intra ocular pressure.

IOP VARIATIONS

1. Physiological variations : the IOP normally fluctuates 2-5mmHg throughout the day :

with respiration and heart beat with the venous pressure with the arterial pressure with the osmotic pressure of blood.

2. Local mechanical factors :• dilatation of the pupil• changes in the solid content of the eye• pressure from outside

3. Pharmacological factors: The ciliary muscle is inserted into the trabeculae , so the

contraction of the ciliary muscle makes the trabecular meshwork more porous -> increases the facility of outflow -> reduces IOP

• outflow facility

• reduction of aqueous production

• atropine

Ideal tonometer

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

DirectIndirect

Manometer

DIGITAL INDENTATION(Schiotz)

APPLANATIONv

NON CONTACT TONOMETER

Types of Tonometer

GAT, Perkins, MMT, Tonopen,

Keeler pulsair Grolman airblast

static dynamicBallistic T

Vibrational T

MANOMETRY

• Manometry is the only direct measure of IOP

• In this method, needle is introduced in Anterior Chamber or in vitreous humor

• It is then connected to mercury or water manometer

Disadvantages:

• Not practical method for human beings

• Needs general anesthesia

• Introduction of needle produces breakdown of blood aqueous barrier and release of prostaglandins which alter IOP.

Uses

• It is used for continuous measurements of IOP

• Used in experiment, research work on animal eyes

Principle:- -Based on Palpation By the Examiner.

Procedure:- -Patient looks Down and the examiner palpates the eye. -Index finger of both hands are used ,one finger push the eye ball above the

tarsal plate and the other finger senses firmness.

oSoft and indents easily – low IOPoFirm to touch – normal IOPoHard to touch – high IOP

Digital Tonometry

•Advantages :- -Easiest -No equipment -No anaesthesia -No staining

• Disadvantages:- -Reading is not proper . -It depends on examiner . -Minor IOP can’t judged properly.

Indentation tonometer

(Schiotz) Principle:-

- Amount of the indentation depends upon IOP.

The higher IOP the lesser the indentation.

-The lesser the IOP the deeper the indentation.

Schiotz tonometer

It consists of :• Handle for holding the instrument in vertical position on the cornea .

• Foot plate which rests on the cornea.

• Plunger which moves freely within a shaft in the foot plate.

• Bent lever: whose short arm rests on the upper end of the plunger and a long arm which acts as a pointer needle .The degree to which the plunger indents the cornea is indicated by the movement of this needle on a scale

• Weights: a 5.5g weight is permanently fixed to the plunger ,which can be increased to 7.5 and 10gm.

Plunger of tonometer Indenting the cornea.

Base plate of tonometer resting on cornea.

In this the cornea is indented

Principle :

• The 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.

• P(t) = P(o) + E• IOP with Tonometer in position Pt = Actual IOP Po + Scleral Rigidity 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 ΔV

• This formula has ‘C’ a numerical constant, the coefficient of ocular rigidity which is an expression of distensibility of eye. Its average value is 0.025.

•ΔV is the change in volume.

It is based on fundamental fact that plunger will indent a soft eye more than hard eye.

-Less reading on scale High IOP

-Scale reading have to be converted to IOP with the help of chart (Friedenwald conversion table)

Friedenwald conversion table Plunger LoadScale Reading 5.5 g 7.5 g 10 g 15 g3.0 24.4 35.8 50.6 81.83.5 22.4 33.0 46.9 76.24.0 20.6 30.4 43.4 71.04.5 18.9 28.0 40.2 66.25.0 17.3 25.8 37.2 61.85.5 15.9 23.8 34.4 57.66.0 14.6 21.9 31.8 53.66.5 13.4 20.1 29.4 49.97.0 12.2 18.5 27.2 46.57.5 11.2 17.0 25.1 43.28.0 10.2 15.6 23.1 40.28.5 9.4 14.3 21.3 38.19.0 8.5 13.1 19.6 34.69.5 7.8 12.0 18.0 32.010.0 7.1 10.9 16.5 29.6

Technique Patient in supine position. 4%lignocaine / proparacaine. patient is made to look at a fixation target.• The 5.5 gm weight is initially used.

• Scale reading is measured.

• If scale reading is 3 or less, additional weight is added to plunger.

• Conversion table is used to derive IOP in mm Hg from scale reading and plunger weight. • Sterilization is done by dipping it in absolute alcohol , ether etc.

Advantages:-

- Fast - Very good for camp screening - Portable - Cheap - Easy to clean & maintain. - No requirement of Slit lamp / electricity.

Limitations - Reading is effected by corneo-scleral rigidity. - Can not be done in injury cases/early post-op.• Instrumental errors

•contraindication : corneal ulcer, conjunctivitis ,panophthalmitis ,corneal abrasion ,severe dry eye.•Variations in volume of globe•Microphthalmos• High Myopia• Buphthalmos• steeper or a thicker cornea • It can be recorded in supine position only

Applanation tonometer• It is more accurate than an indentation tonometer and is based on Imbert-

Ficks law.

• Instead of measuring the amount of indentation , applanation tonometer measures the amount of force needed to flatten or applanate the known area of the cornea.

• This concept is introduced by Goldmann in 1954.

• It consists of double prism mounted on a standard slit lamp

• it based on Imbert-Fick law it states that the pressure in a ideal sphere filled with fluid and

surrounded by an infinitely thin , flexible membrane , may be measured by the force that just flattens the membrane to a plane surface.

P = W/ A

The cornea is not an ideal sphere.

• Two extra forces acting on cornea -• Capillary attraction of tear meniscus (T), tends to pull tonometer towards cornea• Corneal rigidity (C) resists flattening

• Thus F = PA , becomes F + T = PA + C , or

P =( F + T - C) / A• These two forces cancel each other when flattened area has diameter of 3.06 mm

Parts of Applanation tonometer

Biprism (measuring prism)

Feeder arm

Housing Adjusting knob

Connected to the slit lamp

Control weight insert

Process:--slit lamp-4% xylocaine -Dark room- Fluorescein dye stains the tear film.-Cobalt blue light illumination

-Biprism applanation of the cornea with knobs reading at (1).The applanation force is adjusted until the inner edges of the two semicircles

just touch each other.-Readings on knobs scale is multiplied by 10 which gives the value of IOP in mmHg.

Calibration:-

Applanation tonometer is calibrated with calibration bar.• It should be done once in a weekly or monthly depending

upon usage.

Applanation tonometer

• The fluorescein rings should be approximately 0.25–0.3 mm in thickness – or about one-tenth the diameter of the flattened area.

• The mires should not be too thin or too thick because of excess or scarcity of fluorescein ,as the intraocular pressure will then be over or under estimated respectively .

Advantages:–

- Most accurate . -No indentation, so not much force is applied on

cornea. -Does not get affected by corneo-scleral rigidity .

-Readings are directly from knob . -Can be done on post op case /injury cases .Disadvantages :-

-Need slit lamp -Dark room .

- Need staining and cobalt light

PERKIN’S TONOMETER• It uses same prisms as Goldmann applanation tonometer.• It is counterbalanced so that tonometry is performed in any position.

• The prism is illuminated by battery powered bulbs.

• Applanated corneal surface is viewed by a magnifying lens.

• Being portable it is practical when measuring IOP in infants / children, bed ridden patients and for use in operating rooms.

.

Mackay-marg tonometer

• 1.5 mm diameter plunger • rigid spring ,rubber sleeve is 3mm in diameter.

• Movement of plunger is electronically monitored by a transducer and recorded on a moving paper strip.

• This instrument is useful for measuring IOP in eyes with scarred, irregular, or edematous corneas because the end point does not depend on the evaluation of a light reflex sensitive to optical irregularity, as does the Goldmann tonometer.

• It is accurate when used over therapeutic soft contact lenses.

Tonopen• This is handheld Mackay Marg type tonometer

• It is a computerized pocket tonometer

• It converts IOP into electric waves

• The wave form is internally analyzed by a microprocessor.

• Three to six estimations of the pressure are then averaged.

• The instrument is 18 cm in length and weighs 60 g.

Non contact tonometer• Noncontact tonometer (NCT) was introduced by Grolman. • Original NCT has 3 subsystems:• 1. Alignment system: It aligns patient’s eye in 3 dimensions.• 2. Optoelectronic applanation monitoring system:• 3. Pneumatic system: It generates a puff of room air directed against cornea

Air puff tonometer

Air Puff tonometer It is only for screening purpose

It applanates the cornea by means of a jet of air.

Once the instrument is properly aligned with the patient's eye, a fixed distance separates the cornea from the instrument.

An optical system measures the time that it takes for the air puff to flatten the cornea.

This can be correlated with the IOP. Mean IOP readings compare favorably with Goldmann tonometry

Advantages : The instrument is beneficial in mass glaucoma

screenings because it does not require topical anesthetic and, with proper use, there is no risk of injury to the cornea.

Dynamic Contour Tonometer

• The PASCAL (DCT) is a slit lamp–mounted device

• It measures IOP independent of corneal rigidity or thickness.• Dynamic contour tonometry (DCT) uses the principle of contour matching instead of

applanation. • The tip contains a hollow with the same shape as the cornea with a miniature

pressure sensor in its centre.• The probe is placed on the pre-corneal tear film on the central cornea and the integrated

piezoresistive pressure sensor automatically begins to acquire data, measuring IOP 100 times per second.

Diaton tonometer• Diaton tonometry is a unique approach to measuring intraocular pressure (IOP)

through the Eyelid. • Nowadays transpalpebral scleral tonometry has no other alternative in the

world.

• It is the most favourable method to carry out preclinical trials, moreover it works in such complicated clinical cases when it is impossible to use classical tonometry methods.

• Diaton is a helpful tool for ophthalmologists faced with pediatric patients or who have corneal abnormalities such as corneal edema, erosions or keratoprosthesis.

for doctors:• Accurate results• Wide range of medical opportunities• Quick & efficient procedure of measuring (IOP measurement takes several

seconds)• Low dispensable materials' expenses

• for patients:• No contact to the cornea• No risk of infection• No anaesthesia drops• No pain• No need to take out contacts

Transpalpebral "diaton" tonometer is effective and irreplaceable in complicated medical cases:

• Can measure IOP even in the presence of viral infections, allergic reactions, dry eye syndrome, contraindications for corneal tonometry• Can serve as non-invasive day monitoring tool while selecting the adequate

hypotensive medical treatment• Can measure IOP on patients after corneal surgeries• Сan measure IOP with contact lenses .• Can measure IOP on immobilized patients• Screening tool for elevated IOP by PrCarePhys.• Trained family members can monitor IOPs of glaucoma patients at home.

THANK YOU B.JYOTHISWAROOP