1) Indentation (Schiotz)

2) Applanation (Goldmann, Perkins ahnd help, tonopen, pneumotonometer, mackay marg, NCT and Keeler pulsair)

3) Dynamic Contour Tonometry (PASCAL by Zeimer)

Schiotz (rarely used anymore):

• measures the amount of indentation created by a fixed force
• as the patient is lying down, the plunger of a set weight will indent the cornea by an amount
• the amount of movement of the plunger indicaes the IOP by a reference chart
• the method is similar to pushing your finger into a balloon
• since this decreases the internal volume of the eye, it may give artificially high IOP

Measures the force necessary to flatten a standard area

• Goldmann - pressure needed to applanate a standard area
• Perkens hand held - standard area
• Tonopen - standard area
• Pneumotonometer - standard area
• Mackay Marg - standard area
• NCT and Keeler Pulsair - amount of applanation with a standard force

provides a direct trans-corneal IOP measurement

• PASCAL by Zeimer:
  • claims to eliminate the errors with corneal thickness and rigidity
  • can also detect the ocular pulse amplitude (OPA) due to the patient's heartbeat
  • mounted on the slit lamp
  • single use tip cover is needed

the background theory for Goldmann tonometry:

the pressure inside any sphere can be found by: W = P x A

W = applanation force

P = measured pressure

A = applanation area

Considering this formula, the amount of pressure may be found by stnadardizing either the area of applanation (fixed area) or the aount of force (fixed force)

the law only works if the membrane of the sphere is:

• infinitely thin
• perfectly flexible
• perfectly elastic
• dry

thus, the equation is modified for the cornea to:

W + S = (P x Ai) +b

S = force due to tears which pull the applanating surface towards the cornea

b = force requred to bend the cornea due to its rigidity and thickness

Ai - area of the inner corneal surface which is flattenered, since this is the surface against which the IOP act

NOTE: S anb b cancel each other out when teh applanation is performed with a 3.06mm diameter circle.  the Goldmann probes applanate a 3.06mm circle, therefore the simpler Imbert-Fick law is applicable to Goldmann tonometry

• all patients during a routine eye exam for baseline data
• applanation tonometry should be performed on all cooperative patients regardless of age
• all patients before dilation/cycloplegia
• glaucoma suspects/patients
• usually done at the end of the slit lamp exam and BEFORE dilation.  Hoever, man practitioners have NCT done as a pretest

The following conditions relate to possibilities of infection or inaccuracy in measurement and may contraindicate tonometry:

• recent ocular injury (perforation, corneal abrasion or chemical burn)
• ocular infection or discharge
• herpes on teh cornea or lids
• marked corneal edema: increass the chance of epithelial displacement and causes inaccurate readings
• marked nystagmus
• corneal distortion, thickening, stromal loss or major scarring
• significant apprehension, blepharospasm, or strong Bell's phenomenon as avoidance response
• constant uncontrollable coughing
• endemic out breaking of ocular infections such as epidemic keratoconjunctivitis (adenovirus type 8)
• use your common sense for certain situations

• ocular hypertension
• low tension and normotensive glaucoma
• upper limits of normal depends on the instrument
• wide diurnal variations
• instrument variations

unless IOPs are very high (40mmHg), glaucoma cannot be diagnosed based on IOP's alone

Expected Values: Normal

1. Normal:
2. Most:
3. Dirunal variations:
4. Difference between OD and OS:

1. Normal: 8-21 mmHg
2. Most: 10-22 mmHg
3. Dirunal variations: 4-6 mmHg
4. Difference between OD and OS: 2-3 mmHg

Expected Values: Borderline

1. Borderline:
2. Dirunal variations:
3. Difference between OD and OS:

1. Borderline: 21-24 mmHg
2. Dirunal variations: 6-9 mmHg
3. Difference between OD and OS: 4-6 mmHg

Expected Values: Abnormal

1. Abnormal:
2. Dirunal variations:
3. Difference between OD and OS:

1. Abnormal: >24 mmHg
2. Dirunal variations: >9 mmHg
3. Difference between OD and OS: >6 mmHg

• method of measurement
• diurnal variations
• corneal thickness
• refractive surgery
• patient apprehension and tight lids
• accommodation
• fluid intake
• pharmaceutical agents
• corneal hydration
• corneal biomechanics

• tonometer must be clean, including all contact surfaces
• instructions to patients and a brief description of what you will be doing

• patient's name
• doctor's name
• date
• DPA's used and time instilled
• time of tonometry
• type of tonometry
• each eye's recordings in mmHg (not an average)

Diagnostic Pharmaceutical Agents (DPA's):

What are they used for?

• used to aid in teh examination or assist in making a diagnosis
• not used directly for treatment
• anesthetics for tonometry

1. Case History: Ocular History!
   1. previous history with eye drops (any previous problems with drops? anesthetics?)
   2. pregnancy category C
2. VA's (ALWAYS!!!)
3. history of reacting to anesthetic at the dentist
   1. consider alternative procedures which avoid use of anesthetics
   2. this is very rare!

• tonometry
• gonioscopy
• examining corneal injury if patient is unable to cooperate due to pain

Local Anesthetics:

1. Onset:
2. Duration:
3. Effects on Cornea:

1. Onset: 15 seconds
2. Duration: 15-30 minutes
3. Effects on Cornea:

• epithelium will remain "soft" for about one hour
• cornea is more permeable for up to 2 hours (make it more penetrable to teh mydriatic drops)

Local Anesthetics:

Side Effects

• delayed wound healing
• corneal sloughing if used repeatedly
• decrease fluorescence of dyes
• sting upon application
• redness
• frontal headache

Local Anesthetics:

What are the two most commonly used?

Which is the DOC?  Why?

Proparacaine 0.5%

• DOC
• least discomfort
• rare allergies

Tetracaine 0.5%

• more corneal toxicity than proparacaine
• stings more than proparacain
• allergic reaction more common than with proparacaine

AO NCT:

• standardized air blast is directed at the cornea
• corneal deformity is measured by photoelectric cells
• this is transformed into a reading in mmHg

NCT:

Advantages

• no anesthesia
• relatively easy (?)
• less patient apprehension (??)
• little side effects
• easy to train technicians to use

NCT:

Disadvantages

• large
• less accurate
• some patients hate it
• a dinosaur

Keeler Pulsair:

Advantages

• gentler puff (about 1/4 of AO NCT)
• great for kids!

Keeler Pulsair:

Disadvantages

• expensive (~$6,000-7,000)
• steep learning curve (tricky alignment until experienced)
• need about 5 readings to get a good average
• makes a noise like a vacuum cleaner which can be distracting to patients

Keeler Pulsair:

Instrument Parts

• the size of a large briefcase
• main "case" has all teh pressure generating machinery
• cord to connect the housing and the hand held portion of the instrument
• the hand held portion looks like a staple gun

Keeler Pulsair:

Procedure

• educate the patient about the procedure
• switch the machine on
• remove the hand piece from the cradle and press the set-reset switch to zero the machine
• hold the jet nozzle about 20mm from the patient's cornea
• hold the tonometer plane to the patient's face and directly along the visual axis
• have the patient look between the two red lights and open wide
• steady the tonometer by placing your hand on both the instrument and the patient's forehead
• align the red corneal reflexes with the center of the eyepiece
• the tonometer will activate automatically once the alignment is achieved
• press the set-reset button and repeat the measurement
• record as with any other tonometry, stating the type of tonometry used

Hand Held NTC:

(other than Keeler Pulsair)

Advantages

• portability
• cordless
• easy to use
• "softest puff available today"
• measures automatically upon alignment

Hand Held NTC:

(other than Keeler Pulsair)

Procedure

• doctor views patient's eye through the eye piece and moves the instrument towards the patient's eye
• once the instrument has a view of an eye, an alignment system is activated automatically which guides the operator
• it fires automatically once alignment is reached

Tonopen:

Advantages

• small
• good for screenings and ambulatory patients
• good for patient's with corneal scarring

Tonopen:

Disadvantages

• expensive (~$3,500)
• requires anesthetic
• requires tip covers

Tonopen:

Readings

• 1.0mm transducer tip
• displays the average of 10 independent readings
• displays statistical confidence of average readings

Tonopen:

Calibration

• place latex cover over the probe prior to calibration
• hold the switch down until a beep is heard, instrument will show = = =
• release teh switch and the display will be _ _ _ followed by another beep
• hold the instrument vertically with the probe down and press the switch twice quickly.  Two beeps will sound and there will be a "CAL" display
• another beep will sound and the display with read "UP"
• now turn the tonometer so that the probe is up and the instrument is vertical.  A beep should sound followed by a readout of "Good".  You are now ready to start
• if "Bad" is displayed, repeate the above until "Good" is displayed

Tonopen:

Procedure

• calibrate the instrument daily
• give adequate patient instructions
• instill anesthetic
• place latex glove on the tonometer head
• fixation target for the patient
• hold instrument like a pen, perpendicular to the patient's central cornea
• anchor ring finger on patient's cheek
• lightly tap on the cornea several times listening for the chirp sound
• a beep will sound when enough valid readings have been taken
• an average IOP estimate will be displayed
• record your findings, indicating teh type of tonometry used

Proview Phosphene Tonometer:

Describe how it works

• uses pressure phosphene phenomenon to determine intraocular pressure
• pressure required to produce a phosphene correlates with IOP

Proview Phosphene Tonometer:

Advantages

• inexpensive
• well tolerated
• non-invasive
• self monitor/travel capabilities

Proview Phosphene Tonometer:

Disadvantages

• not well normed
• over estimates at higher IOPs
• false sense of security
• not everyone notices the phosphenes

Proview Phosphene Tonometer:

Procedure

• patient looks down and temporally (eye open)
• a small plunger is placed on the nasal lid and pressed until the patient notes the phosphene
• pressure is determined by how hard the plunger had to be presed

Icare Tonometer:

Advantages

• no anesthesia or dye are necessary
• hand held
• easy to use
• portable
• words for difficult patients

Icare Tonometer:

Disadvantages

• need a new probe for every patient
• expensive (~$3,500)

Icare Tonometer:

Describe how it works

• uses a new measuring principle (Rebound Technology)
• a very light blunt tipped probe is projected onto teh cornea
• the probe rebounds off the cornea and sensors measure the IOP (average of 6 readings)
• the measurement is barely noticed by the patient (may not cause corneal reflex)

Diaton Tonometer:

Advantages

• measures IOP through the eyelid
• no anesthesia required
• no contact with teh cornea
• hand held and portable
• BUT, expensive (~$2,500)

PASCAL DCT:

What method does it use?

contour matching instead of applanation

(minimizes influencing factors, such as CCT, curvature, rigidity and elasticity)

PASCAL DCT:

Describe how the instrument works

• instrument uses a miniature pressure sensor embedded in the tip
• the tip matches corneal curvature with minimal corneal deformation and constant oppositional force
• measures IOP directly using the build-in pressure sensory
• when the sensor is subjected to a chance in pressure:
  • the electrical resistance is altered, and
  • the PASCAL's computer calculates a change in pressure similar to the change in resistance
  • **this is the KEY to the PASCAL's ability to neutralize the effect of intra-individual variation in corneal properties
• once the portion of the central cornea has taken up the shape of the tip, the integrated pressure sensor begins to acquire data
• measures IOP 100 times per second (a complete measurement cycle requires about 8 seconds of contact time(
• during the measurement cycle, adio feedback is generated, which helps the clinician insure proper contact with the cornea

PASCAL DCT:

What is the key that allows PASCAL to neutralize the effect of intra-individual variation in corneal properties?

when the sensor is subjected to a change in pressure:

• the electrical resistance is altered, and
• the PASCAL's computer calculates a change in pressure similar to teh change in resistance

PASCAL DCT:

What is the design of the tip?

• tip radius: 10.5mm
• the tip rests on the cornea with a constant appositional force of one gram
  • this is an important difference from all forms of applanation tonometry in which the probe force is variable

PASCAL DCT:

Details of measurements:

• allows for continuous reading of IOP waveform
• records the dynamic pressure fluctuations of the eye
• when the piezoresistive pressure sensor detects pressure, it is digitized and stored in teh device's memory
  • built-in microprocessor
• determines the intro-ocular pressure and its pulsatile fluctuations
  • OPA = ocular pulse amplitude
  • caused by cardiac activity

PASCAL DCT:

Procedure

• place new probe cover on sensor tip
• patient needs to be anesthetized
• activate it with a small turn of the blue knob
• advance the probe to touch the eye
  • need to have enough pressure to get adequate readings
  • will see a circular tear film area - center this
  • acquire proper sound
  • need to leave the tip on the cornea for at least 8 seconds
• when the PASCAL sensor tip touches the corneal surface, a piezoresistive pressure sensor directly measures IOP

PASCAL DCT:

Digital Display

• IOP: average minimum
• OPA: ocular pulse amplitude
• Q-Values: quality score of the reading (1-5)
  • 1-2 = excellent
  • 3 = acceptable
  • 4-5 = unreliable

PASCAL DCT:

Potential Use of OPA

• normal OPA range found to be 3.0 +/- 1.2
• positive correlation between higher IOP and higher OPA
• link between lower OPA and NTG (normotensive glaucoma)
• softer eye gives smaller OPA

PASCAL DCT:

OPA Correlations in Hispanic Population:

• OPA decreases with:
  • increasing axial length
  • increasing myopia
• OPA increases with:
  • increasing IOP
• In this study, OPA:
  • range from 0.7-4.7 mmHg
  • mean OPA was 2.1 mmHg
• Four subjects had OPA < 1.0 - all were myopic

PASCAL DCT:

Price of Instrument

~$6,000 range, which discount as you buy more of them

$0.75 per tip cover

Issues with GAT:

Corneal Thickness

• OHTS study found:
  • thicker corneas have higher measured IOP (false high) that doesn't respond well to meds
  • thinner corneas have lower measured IOP (false low) and have a greater chance of converting from OHTN to glaucoma
• GAT measures based on 520 micrometers, but:
  • Caucasians: ave 555
  • African-Americans: ave 535
  • Mean K thickness: 545-550
• Need to find corrective factor for K thickness
  • studies have found anywhere from 1.5-4.5 mmHg for every 50 microns of corneal thickness
  • error factor: 2.5 mmHg for every 50 microns away from 550 (subtract if above, add if below)
• LASIK patients:
  • 1D of correction = 12 microns reduction in K
  • 5D correction = 60 microns reductin in K
  • depending on corrective factor, need to adjust IOP anywhere from 1.5-4.5 mmHg
• adding pachymetry to the equation became the standard of care in the past few years
• recent research is showing that this is an unreliable way to compensate for this problem

PASCAL DCT:

Reasons for PASCAL

• problems with GAT revealed by the OHTS study (adding pachymetry to equation of IOP is proving to be an unreliable way to compensate for varying corneal thicknesses)
• developed to accurately measure the IOP without the influence of corneal thickness and rigidity
• the name is after Blaise Pascal, the seventeeth century mathematician and physicist

PASCAL DCT:

Studies

• show that DCT IOP to closely compare to manometric values in cadaver eyes
• sho that pre- and post-LASIK IOP reveals no significant change in IOP with the instrument

PASCAL DCT:

Corneal properties that affect IOP measurement:

• biomechanical material properties:
  • collagen and supporting matrix
  • hydration
  • age (stiffer corneas and reduced tear film with age)
  • effects of corneal biomechanical properties alone:
    • stiffer cornea = higher IOP
    • softer cornea = lower IOP
    • potential error is huge (>10 mmHg)
• corneal thickness (CCT)
  • thicker cornea = higher measured IOP
  • thinner cornea = lower measured IOP
  • potential error is moderate
• corneal curvature:
  • flatter cornea = lower measured IOP
  • steeper cornea = higher measured IOP
  • potential error is low

PASCAL DCT:

Summary of corneal biomechanics effects from Liu and Roberts:

• corneal radius of curvature: very small
• central corneal thickeness: small/moderate
• corneal material properties: large

PASCAL DCT:

Conclusions

• DCT is significantly less susceptible to the effects of CCT than GAT
• GAT overestimates DCT in older subjects
  • suggests an age-related corneal biomechanical change that may induce error
• where does that leave us? MORE RESEARCH!