TONOMETER

Intoduction Definition: A tool used to measure the pressure of the eye. A tonometer is used to help detect glaucoma. A tonometer measures the production of aqueous humor, the liquid found inside the eye, and the rate at which it drains in to the tissue surrounding the cornea. Tonometry is the procedure performed to determine the intraocular pressure (IOP)

History: 1826:  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. These instruments were all of the indentation type and rested on the sclera (no anaesthetic was used until 1884). 1885: Maklakov designed an applanation tonometer. This was refined in 1892. Used for a number of years in Russia and Eastern Europe. This was used till 1959. 1905:  Hjalmar Schiotz produced his indentation tonometer. This made tonometry a simple and routine clinical test.

Albrecht von Grafe tonometer Donders tonometer

What is IOP ? increased risk to develop glaucoma: >21mm Hg

Intraocular pressure (IOP) is the fluid pressure inside the eye Intraocular pressure (IOP) is the fluid pressure inside the eye. Tonometry is the method eye care professionals use to determine this. IOP is an important aspect in the evaluation of patients at risk from glaucoma. Most tonometers are calibrated to measure pressure in millimeters of mercury (mmHg). Eye pressure is measured in millimeters of mercury (mm Hg). Normal eye pressure ranges from 12-22 mm Hg, and eye pressure of greater than 22 mm Hg is considered higher than normal. When the IOP is higher than normal but the person does not show signs of glaucoma, this is referred to as ocular hypertension. High eye pressure alone does not cause glaucoma. However, it is a significant risk factor. Individuals diagnosed with high eye pressure should have regular comprehensive eye examinations by an eye care professional to check for signs of the onset of glaucoma.

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

Factors modifying IOP with respiration and heart beat Physiological variations : the IOP normally fluctuates 2-5mmHg throughtout the day : with respiration and heart beat with time of the day with the venous pressure with the arterial pressure with the osmotic pressure of blood

Continue dilatation of the pupil 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 trabeculum, 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

Measurement of IOP Manometry : it is measured by inserting a cannula, directly into the anterior chamber which is connected with a manometer.

Continue Digital tonometry

continue Instrumental tonometry contact non contact tonometer tonometer indentation tonometer Schiotz, Macky-Marg Applanation tonometer Goldmann,Parkin’s,Tono-pen

Commercially available instrument Goldmann tonometer Parkines hand held tonometer McKay-Marg tonometer Schiotz tonometer Air puff tono-pen

Instruments applanation tonometer

Continue Parkines Tono-pen Schiotz

Continue Air puff

Applanation tonometry Goldmann applanation tonometery The force necessary to flatten a given area of cornea is measured Concept introduced by Goldmann in 1954 Consists of double prism mounted on a standard slit lamp Standard area of cornea of 3.06 mm diameter has been flattened

continue It based on Imbert-Fick law - it states that the pressure in a sphere filled with fluid and surrounded by an infinitely thin,flexible membrane, may be measured by the force that just flatten the membrane to a plane surface P = W/ A

Procedure patients counseling dry, clean tonometer probe should be inserted slit lamp should be adjusted and the patient comfortably positioned topical anesthetic place a fluorescein strip in the lower fornix room light should be dimmed illumination system should be at approximately 60 degree to the lateral side of the eye to be measured

continue keep both eyes open cornea and biprism is illuminated by cobalt blue light from slit lamp biprism is then advanced until it just touches the apex of cornea at this point 2 semi circles are viewed

Errors in Measurement The fluorescein ring is too wide or too narrow thin corneas produces underestimate thick cornea d/t increased collagen gives overestimate, if d/t edema gives underestimate. inadequate vertical alignment of semicircles leads to overstimate of IOP. distortion d/t irregular cornea influences accuracy, less useful with corneal scarring. squeezing of eyelids repeated tonometry may induce decline in estimated IOP. increase of 1 mmHg for every 3D increase in corneal power tear film of cornea, anesthetic drops.

contraindication Head trauma suspicion of ruptured globe narrow angle

Correction for astigmatism With semicircles displaced horizontally, IOP underestimated by 1 mmHg for every 4D of WTR astigmatism, vice versa for ATR astigmatism.

Tono-pen fully portable held in any position operated by index finger including its power source weight only 57g 185 mm long 25mm wide fully portable held in any position operated by index finger reading displayed as adigital readout located on the handle are not affected by corneal astigmatism

perkin’s hand held same type of goldmann applanation probe applies force by counterbalanced arm applanated corneal surface viewed by a magnifying lens situated behind the lens illumination buit into the instrument

continue used with patient in any position portable

INDENTATION TONOMETRY Schiotz tonometer free floating barrel with a footplateof 10.1 mm slightly flattents the cornea Total weight varies from 5.5g

continue Principal : based on the principal, that a plunger will indent a soft eye more than a hard eye. indication : all types of glaucoma,uveitis,hyphaema contraindication : corneal ulcer, conjunctivities,panophthalmitis are in infectious condition and corneal abrasion,severe dry eye in non-infectious condition

procedure sterilization by boiling or wiping with alcohol Patients counseling use short acting topical anesthetic supine or in semi supine position fixation target separate the eyelid place tonometer directly onto the cornea vertically note scale reading antibiotic

Advantage and disadvantage Advantage : cheap , portable, easy use Disadvantage : false reading with abnormal scleral rigidity, corneal abrasion

What is ocular rigidity? The resistance of the coats of the eye to indentation factors affecting rigidity : Refective error : high in hyperopes and lower in myopes drugs : miotics reduce , vasoconstrictor rise surgery : lowers

Non-contact tonometer Air-puff tonometer

continue non-contact only 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.

Continue Mean IOP readings compare favorably with Goldmann tonometry Advantage :The instrument is beneficial in mass glaucoma screenings because it does not require topical anesthetic and, with proper use, there is no risk of injuring the cornea.