Tonometry PDF

Tonometry: the measurement of the tension of the eye IOP Combined resistance to deformity Uses: ○ All routine examinations ○ Glaucoma diagnosis and management ○ Trauma ○ Uveitis ○ Medications side effects Digital Tonometry (Palpation)...

Tonometry: the measurement of the tension of the eye IOP Combined resistance to deformity Uses: ○ All routine examinations ○ Glaucoma diagnosis and management ○ Trauma ○ Uveitis ○ Medications side effects Digital Tonometry (Palpation) Standard of care in late 19th century and beginning of 20th century Procedure: ○ Use both index fingers over the superior eyelid ○ Use one fighter to make pressure and the other to feel if the eyeball is too stiff or too soft ○ Record: soft, moderate, or hard Tender to touch, hard to touch Advantages: easy/quick, no equipment needed, no anesthesia Disadvantages: not a proper measurement, depends on examiner, easily over or under estimation Tonometry by Indentation (Schiøtz) Uses a plunger to indent the cornea IOP is determined by measuring how much indentation is produced by a given weight ○ 2.5 g, 5.5 g, 7.5 g, 10.5 g Less accurate than applanation tonometry ○ Humanitarian missions or ER By measuring the depth impression caused by the plunger and accounting for the weight used, IOP can be determined  Procedure: ○ Test the tonometer using the spherical mold in the box and the 5.5g weight The pointer should immediately mark “0” ○ Clean the plunger and the disc, wipe dry Make sure no disinfecting solution is left ○ Lie the patient is a supine position, with the head support ◦ Stand behind the patient, hands leveled with the patient's head ○ Instill topical anesthetic ○ Give the patient a fixation point and ask them to be still ○ With the thumb and index finger hold open the patient's eyelid without making pressure on the globe ○ With the other hand, hold the tonometer between the thumb and index finger and place the plunger on the central cornea ○ Allow the disc to lower gently onto the cornea and note the reading on the scale If the scale reading is less than “2”, replace the 5.5g weight with the 7.5g weight and repeat the procedure ○ This is an indication of elevated IOP After noting the scale reading convert the number on the scale with the conversion table and record the IOP ○ Tonometry by Applanation (Contact) Goldmann Applanation Tonometry (GAT) ○ Introduced by Goldmann in the 1950s and has since then become the “Gold” standard of care ○ Based on the Imbert-Fick Law ○ Based on this principle GAT measures IOP by the force required to applanate a certain area of the central cornea 3.06 mm ○ Considering the cornea’s true condition, 2 additional forces are also considered Surface tension force: caused by the tear film Bending resistance of the cornea: resistance to flattening (rigidity) ○ Taking into account these 2 variables, Imber-Ficks Law had to be modified F = PA was transformed into F + T = PA + C P = F + T - C/ A ○ Where: T: capillary attraction of the tear meniscus (surface tension) C: Corneal rigidity ○ ○ The GAT uses a biprism tip mounted on the tonometer’s head and positioned against the cornea ○ The force causing the cone of the biprism to applanate the central cornea is increased gradually until the required area of applanation is achieved Required area of applanation is 3.06 mm ○ The applanation pressure is read from the pressure application mechanism ○ This is recorded and considered equal to the IOP ○ When performing GAT, the goal is to perfectly applanate 3.06 mm of corneal area If the applanation area is larger than 3.06 mm, the force inside the globe is less than the pressure being applied If the implantation area is smaller than 3.06 mm, the force inside the globe is more than the pressure being applied ○ When performing GAT, essentially we are adjusting the size of the fluorescein ring to fit an area of 3.06 mm In practice, we are “aligning” 2 semicircles ○ Procedure:  Educate the patient on procedure “Voy a tomar la presion de su ojo” Dim room illumination Adjust patient and slit lamp Evaluate the cornea Should have been evaluated w/ NaFl during SLE/TBUT Nothing should be compromising the cornea Anesthetize the cornea Proparacaine 0.5% 1 gtt OU, 1 drop NaFl OU ○ Onset 13-15 sec, effect last for 15 min Fluress: Benoxinate Hydrochloride 4 mg (0.4%) + NaFl 2.5 mg (0.25%) Contraindication if patient is allergic to Novocain (used in dental procedures, that is how patient knows they’re allergic is if they have undergone a dental procedure) ○ Disinfecting Hydrogen peroxide 3% can ruin the probes If not properly dried can cause marked pain and a secondary anterior uveitis Place it in the solution for a minimum 5 minutes but no more than 10 minutes. Rinse well!!!!!!! Opticide: disinfecting solution Same precautions for the cornea as before Does not damage the probe Place 3 minutes in the solution Isopropyl alcohol 70%: can also destroy the probes Same precautions for cornea as before Not as effective for Hep B and C Rinse and tap dry ○ Procedure Place the probe on the tonometer The patient needs to keep their head tightly against the forehead and keep the chin on the chinrest Give the patient a fixation target One target for OD (i.e. Dr’s right ear but looking to the left) One target for OS (i.e. Dr’s left ear but looking to the right) Just before moving the slit lamp, ask the patient to blink 3-4 times and then keep their eyes wide open Set the measuring drum on 1 Place the light source at 60-65° temporal to the probe The view of the probe and mires are seen only w/ one eye Position the tonometer arm so the applanation prism is aligned in front of the left ocular Use wide, diffuse, high illumination  45-60° temporal Cobalt blue filter and low to medium (10-16X) magnification Direct the probe w/ the slit lamp towards the patient’s cornea Observing from the outside bring the tonometer close to the cornea until ~1-3 mm from the cornea Be careful to not make sudden/rough movements Now look through the oculars (without moving the SL) Before the probe touches the cornea make sure: Probe is centralized, but not touching the cornea No lashes or lids are in the middle If lids or lashes are in the way and you need to open the patient’s eyes DO NOT APPLY PRESSURE Cotton tip applicator with pressure to the orbital roof, not the globe or else pressure will go up Looking through the ocular, find the ghost mires (not indicative of touching cornea) Pale blue/gray semicircles Make sure they are equal If unequal, follow the larger mire until both are equal and centered Using the ghost mires is the safest way to avoid disrupting the epithelium of the cornea After aligning the ghost mires, bring the SL closer to the cornea using the joystick Mires should be aligned (adjust the SL to keep the mires aligned) Once the cornea is touched by the probe the fluorescein mires appear If aligned, start applanating until the inner ring of the superior semicircle meets the inner ring of the inferior semicircle  When the probe just touches the cornea you will see a fluorescein glow Once you see this STOP, you have advanced far enough Look for the mires and align them as indicated If you advance the probe too forward or are using more than 16x magnification, you may not be able to see anything ○ THIS MAY LEAD TO A CORNEAL ABRASION If the mires are too wide the readings will not be accurate ○ Pull the slit lamp back, dry off the end of the probe and start over ○ Causes: lashes in contact with the probe, too much fluorescein or excessive tearing ○ Mires Ideally, the mires should measure 0.25-0.30 mm If the mires are too thick, the reading will be falsely high If the mires are too thin, the reading will be falsely low Mires have to be aligned and centered If not centered, follow the larger one If one is missing, following the missing one ○ Practice  Imbert-Fick Law ○ States that “the pressure in a sphere filled w/ fluid and surrounded by an infinitely thin and flexible membrane is measure by the counter-pressure which just flattens the membrane to a plane.” ○ Pressure inside an ideal sphere (P) is equal to the force (F) necessary to flatten the sphere’s surface, divided by the area (A) that was flattened. 𝑃 =F/A −−−→ 𝐹 = 𝑃𝐴 ○ Pressure (P) can be determined if the force (F) is fixed or if the area (A) is fixed ○ Cornea is not a perfect sphere and it is not infinitely thin and perfectly elastic Variable to the Imbert-Fick Law Goldmann Tonometry Errors ○ Amount of fluorescein ○ Valsalva maneuver: anxiety from pt which will lead for them to hold their breath high IOP ○ Eye movement ○ Eyelids ○ Deformity of the eyeball or cornea ○ Surface attraction of the probe ○ High astigmatism (>3 D) ○ Successive measurements ○ Contact lens epithelial edema ○ Increased venous pressure ○ Inter-observer variability (+ 3 mmHg) ○ Accuracy of Goldmann tonometry-calibration ○ Pressure on the patient’s globe by examiner (falsely high IOP) ○ Eyelashes between cornea and biprism) Corneal thickness effect on GAT  ○ Goldmann and Schmidt suggested that GAT is more precise on average thickness corneas (550 μm) ○ Thinner corneas = less force to applanate = underestimation Keratoconus = as it becomes more “pointy”, area is becoming more thinner so falsely low IOP ○ Thicker corneas = more force to applanate = overestimation ○ Conversion tables (perform Pach. & determine corneal thickness) but not generally used because it is not a consensus of conversion factors GAT-uses Has to be performed before dilation/cycloplegic Standard of care for glaucoma patients ○ Not a diagnostic of glaucoma, but important factor to consider ○ Ocular HTN: IOPs over 21 mmHg w/o optic nerve head damage/VFD Is recommended to finalize the patients refraction before performed GAT since the anesthesia may cause blurry vision Hints If the fluorescein pattern in pulsating, this is caused by the venous pulsation w/in the eye and the cardiac cycle. Take several measurements. ○ Make sure the patient is not too far away or breathing heavily Contact lens wearers: ○ Flush all the NaFl out of the eye first ○ Advise not to use CL for 1 hr., since they could stain yellow for SCL ○ Also for RGP/SCL, the anesthesia will numb the cornea which may lead to not feeling pain secondary to a corneal abrasion Astigmatism (can also be one of our sources of error) IOP is underestimated by 1 mmHg for every 4 DC of WTR astigmatism ○ Overestimated for ATR astigmatism over 3DC To minimize, the prism should be rotated to the axis of the flattest K reading ○ The red line should be placed at the prisms axis mark corresponding to the flattest meridian ○ Ex: 41.00 @ 030/ 45.00 @ 120. Place the red line (or A) on the probe at the 030 mark. Pearls Eyelashes b/w the cornea and the probe will produce inaccurate readings and may leave linear epitheliopathy (SPK) After the procedure, examine the cornea for keratitis or abrasions ○ Evaluate w/ cobalt blue filter using the remaining NaFl If SPK is present, treat w/ Artificial Tears PF Corneal edema may present if the probe was not rinsed well If the patient has anterior segment conditions (previous abrasion or trauma), it is preferred to use NCT over GAT Applanation Epitheliopathy: Goldmann Keratitis “stamping” the cornea IOP varies over 24 hrs by ~3-5mmHg (diurnal fluctuations) o Highest readings early in the morning (6:00am-8:00am) This is important for glaucoma patients and suspects ○ Recommended to take a diurnal curve, with several measurements ○ Spikes with variations greater than 5mmHg are suspicious ○ Ex: 21mmHg OU @8:30am and 14mmHg OU @ 3:30pm is suspicious IOP differences between eyes of more than 4mmHg must be questioned ○ Consider conditions such as: glaucoma, retinal detachments, uveitis-iritis, poor technique or poor instrument calibration. After LASIK and PRK, the IOPs are underestimated ○ ~10 microns of tissue removed per diopter of RE ○ Ex: 6 D of myopia corrected = 6 microns of tissue removed Recording Expected Values: Between 10mmHg and 21mmHg Record ○ Tonometry method (GAT, iCare, NCT) ○ Eye (OD, OS) ○ Pressure (mmHg) ○ Time Example: GAT: OD 12mmHg OS 11mmHg @ 1:00pm Perkins Tonometer Goldmann tonometry hand-held Combined systems principles Electronic indentation tonometry (MacKay-Marg, Tonopen) Electronic devices that convert the eye transmitted pressure to electrical pulse signals and then calculates to mmHg Useful for children, screenings, non-cooperative patients, and veterinary MacKay-Marg ○ In 1959, McKay and Marg introduced their tonometer based on a combination of indentation and applanation processes ○ The tonometer has a 3.06 mm diameter applanating surface which is obtained by the footplate ○ The small notch observed in the electrical waveform helps identify the force of application ○ Since the area of applanation is known, the IOP can be calculated ○ Less dependent on corneal factors Performs well in irregular or scarred corneas ○ Tonopen ○ The tonopen incorporates the same principle as MacKay-Marg but in a small, handheld, battery-powered body ○ Useful in screenings, bedside or in the operating room Can be measured through a BSCL if cannot be removed  ○ Uses disposable latex covers, which have to be changed for each patient ○ Takes an average of 4-10 readings (depending on the model) and gives a stats indicator of reliability ○ Pascal Tonometer The Pascal-Dynamic Tonometer (DCT) uses the principle of contour marching instead of applanation to eliminated the errors that applanation has Uses a miniature piezoresistive pressure sensor embedded within a tonometer tip that is contour-matched to the shape of the cornea. The tonometer tip rests on the cornea with a constant appositional force of 1g ○ The probe force is variable When the sensor is subjected to a change in pressure, the electrical resistance is altered and the DCT’s computer calculates a change in pressure according to the change in resistance and calculates IOP It measures the ocular systolic and diastolic pressure and therefore the ocular pulsatile activity ○ Ocular Pressure Amplitude (OPA)  Ocular Response Analyzer A non-contact tonometer Provides IOP values that are independent of biomechanical corneal properties Using a jet of air lasting ~10 milliseconds ○ Older NCTS applanate the cornea for 1-3 milliseconds ORA records on both onward and backward applanation ○ The difference b/w the two values is corneal hysteresis which is a direct measurement of corneal biomechanical properties ○ Corneal hysteresis: the difference between inward and outward applanation pressures Resistance of cornea, elastic property ○ A measurement of CH is illustrated on a curve, comparing corneal applanation signal and air pressure over time ○  ○ A: Prior to air puff applanation ○ B: First applanation ○ C: Highest concavity ○ D: Second applanation ○ E: Posterior to air puff applanation Corneal hysteresis Corneal hysteresis provides information about the viscous properties of the cornea Corneal Resistance Factor provides information on elastic properties ○ Viscoelasticity of the cornea as a result of the combined effect of the corneal thickness and rigidity ○ The ability to absorb and dissipate energy It is said to be indicative of various disease like Keratoconus, Fuch’s Corneal Dystrophy and Glaucoma Studies reveal a mean value of CH between 10.24 and 10.7 Non-contact tonometer By 1970 optometrist were not allowed to use anesthesia Dr. Bernard Grolman invented the NCT ○ an air blast to the cornea The deformity caused by the air blast is measure by photoelectric cell Good for patients who are allergic to anesthesia, corneal damage or screenings. NCT Measurement Errors Relied on corneal applanation, subject to the same potential measurement errors induced by variations in corneal properties, as in GAT. IOP is taken at a single very brief instant in time. ○ IOP can pulsate considerably since the choroid fills with blood and then empties in concert with the cardiac cycle ○ This can be observed by viewing the pulsation of the mires in GAT and can be taken into account ○ Can account for 5-6 mmHg difference withing 1 seconds while the choroid fills and empties NCT Systems Alignment system: to align the patient Optoelectronic applanation system: a monitoring system consisting of: ○ Transmitter: directs a collimated (parallel) beam of light at the corneal apex ○ Receiver and detector: accepts only parallel coaxial rays of light reflected from the cornea ○ Timer: measures from an internal reference to the point of peak of the light intensity Pneumatic system: generates an air puff directed against the cornea Transpalpebral Tonometer: Diaton On principle: it applies pressure on the eyelid until retinal phosphenes are experienced The pressure that is required to produce phosphenes is proportional to the IOP Useful in children, patients w/ special needs, bed-ridden patients ○ Also useful for home-monitoring IOP Limitations include: scleral rigidity, thickness of the eyelids, tone of the orbicularis muscle and potential intra-palpebral scarring Rebound Tonometer: ICare Initially used by veterinary medicine to avoid general anesthesia or heavy sedation Uses two coils coaxial to a probe shaft that bounces a magnetized probe off the cornea and detects the deceleration of the probe caused by the eye  A moving magnet within a coil induces changes in the voltage at the two ends of the coil generating a magnetic field with a given voltage, which is detected by the tonometer sensor The voltage produced is proportional to the probe speed Of all the variables linked to the probe’s movement, the inverse of its deceleration speed seems to correlate best with IOP The probe is a tiny 1.8mm diameter plastic ball on a stainless-steel wire, held in place by an electromagnetic field in a handheld battery-powered unit When the button on the back is pushed, a spring derives the wire and ball moves forward rapidly When the probe hits the cornea, the ball and wire decelerate ○ The deceleration is more rapid if the IOP is high and slower if the IOP is low ○ The speed of deceleration is measured internally, and a chip calculates the IOP Because the probe makes the contact with the cornea for microseconds, no anesthetic is necessary Rebound tonometry has the least contact time with eyes among all tonometer and may get a reading at any point in the IOP pulse cycle, then its repeatability is compared with GAT The probes are disposable between patients, no disinfection is necessary The disadvantage is the patient has to be sitting straight Accuracy may be an issue especially in patient where accurate IOP measurement are critical for long-term management.