Tonometry

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Tonometry

• Dr. Sudhir Tambile.

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Tonometry

• It is noninvasive measurement of intraocular
  pressure
• Tonometer is an instrument that exploits the
  physical properties of the eye to permit
  measurement of pressure without the need to
  cannulate the eye
• The physical properties of the normal cornea
  determine the limits of accuracy of tonometry.

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• The intraocular pressure immediately prior to the
  application of a tonometer is symbolically
  represented as Po.
• The intraocular pressure during tonometry is
  symbolically represented as Pt.
• Po and Pt are never exactly equal for any
  tonometer.
• Spontaneous intraocular pressure without
  reference to tonometry is symbolically
  represented as Pi.

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Kinds properties of tonometer

• Tonometers in which the intraocular pressure is
  negligibly raised during intraocular pressure
  measurements, for example less than 5, are
  termed low-displacement tonometers.
• Tonometers that displace a large volume of fluid
  and consequently raise intraocular pressure
  significantly are termed high-displacement
  tonometers.

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Classification of Tonometers

• I. Invasive instruments
• A. Needles and cannulas (manometry)
• B. Transensors
• II. Noninvasive instruments (tonometers)
• A. Instruments that touch the eye
• 1. Static instruments
• a. Applanation tonometers
• (1) Constant area (Goldmann, Mackay-Marg)
• (2) Constant force (Maklakoff)

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• b. Indentation tonometers
• (1) Constant indentation
• (2) Constant force (Schiøtz)
• 2. Dynamic instruments
• a. Ballistic tonometers
• (1) Impact acceleration
• (2) Impact duration
• (3) Rebound velocity
• b. Vibration tonometers (Krakau)
• B. Instruments that do not touch the eye
  (noncontact tonometers) (Grolman). 

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Indentation tonometers

• With the Schiotz tonometer a series of known,
  standard weights are applied to the cornea via a
  plunger .
• The plunger indents the cornea, and a scale
  records the deformation of the globe.
• These two values are then used to determine the
  IOP.
• The plunger moves in a vertical fashion in the
  center of the instrument and passes through a
  curved footplate that sits on top of the cornea
  with the patient in the supine position.
• A holder fixes the footplate on the cornea but
  allows free movement of the plunger and the
  attached weights in the vertical direction

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• A movement of the plunger from its 0 position
  into the cornea can be correlated with the
  deformation of the cornea.
• Since the excursion of the plunger is relatively
  small and would be difficult to read, a lever
  magnifies the excursion along a more readable
  calibrated scale.
• The greater the scale reading with a given
  weight, the greater will be the excursion of the
  plunger and the deformation of the globe. The IOP
  will therefore be lower.

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Theoretical Basis

• When the tonometer is placed on the eye, the
  indentation of the cornea results in distention
  of the globe.
• Thus the scale reading, along with indicating the
  indentation of the cornea, also reflects this
  same distention.
• Friedenwald's formula relates this distention to
  the IOP. The formula required a constant K or
  the coefficient of ocular rigidity, which is a
  measure of the resistance of the eye to the
  distending forces of the tonometer.

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Theoretical Basis

• Friedenwald determined that the value of K for an
  individual eye could be calculated from two
  tonometric scale readings using different
  weights.
• Friedenwald's nomogram allows one to
  graphically determine K from these two values.
• Presently, simplified tables exist that obviate
  the need for calculation and provide both the Po
  and K values from the paired scale readings on
  the involved eye

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Clinical Technique

• The patient is supine with the cornea
  anesthetized.
• The fingers of the examiner spread the lids
  carefully to avoid putting pressure on the globe.
  
• The patient is asked to fixate while the
  tonometer footplate is applied to the cornea, and
  the handle is positioned to keep the tonometer
  vertical and to allow free movement of the
  plunger to indent the cornea.
• The needle will oscillate with the ocular pulse,
  and the midpoint of the excursion is used as the
  scale reading.
• If the value is not greater than 4 units, an
  additional weight is added.
• Then IOP read from appropriate table.

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Limitations

• If the true K of the eye is higher than the
  average K, the table will overestimate the true
  IOP.
• Similarly, a false-low IOP will result if the
  true K is less than the average K.
• High ocular rigidity has been reported in
• high hyperopia, 
• extreme myopia,
• chronic glaucoma,  and
• vasoconstrictor therapy.

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Limitations

• Low ocular rigidity may occur with
• high myopia,  
• miotic therapy (especially cholinesterase
  inhibitors)  
• after retinal detachment surgery,
• intravitreal injection of gas,  and
• vasodilator therapy
• False-high IOP readings may be obtained with
• thick corneas or very steep corneas. 
• With significant corneal pathology, and on an
  irregular surface.

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• Applanation tonometry

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Theoretical Basis

• Applanation tonometry is based on the Inbert-Fick
  principle.
• Which states that for an ideal sphere the
  pressure (P) inside the sphere is equal to the
  force (F) required to applanate (flatten) its
  surface, divided by the area (A) of flattening
• P F/A or F PA.
• The ideal sphere is dry, thin-walled, and readily
  flexible.
• The cornea, which is not even a true sphere, is
  none of these three. Because of this, there are
  two other significant forces at work.

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• The force of capillary attraction (T) between the
  tonometer head and the tear film is additive to
  the external force.
• In addition, a force (C), independent of IOP, is
  required to flatten the relatively inflexible
  cornea. Thus,
• F PA , becomes
• F T PA C , or
• P ( F T - C) / A

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• The A, is actually on the interior surface of the
  cornea.
• The Goldmann applanator is designed so that A is
  equal to 7.35 mm 2.
• To achieve this, the diameter of flattening of
  the cornea is 3.06 mm.
• With this value for A, the opposing forces of
  capillary attraction and corneal inflexibility
  cancel out.
• P F / 7.35 mm 2

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• In addition, with this value for A the IOP in
  millimeters of mercury (mmHg) is equal to ten
  times the force applied to the cornea in grams,
  which is a convenient conversion.
• Since only 0.5 mmu is displaced from the eye
  and the additional increase in pressure induced
  in the eye from its steady state by the tonometer
  tip is negligible, applanation tonometery is not
  significantly affected by ocular rigidity.

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Goldmann Applanator

• The tonometer tip, a tapered plastic cylinder
  containing a biprism, is the contact point with
  the cornea.
• The tip is connected via a rod to the body of the
  tonometer which contains an adjustable spring
  that provides the appropriate applanating force.
• The force is adjusted manually via a knob that
  contains a scale indicating the force applied in
  grams.

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• When the end-point is reached, the reading in
  grams is multiplied by 10 to convert to
  millimeters of mercury.
• The biprism splits the image of the circle of
  contact into two semicircles.
• When the inner margin of these semicircles just
  touch, a 3.06-mm diameter circle of cornea is
  applanated.
• The instrument is attached to the slit lamp,
  aligning the axis of the tip with the ocular and
  allowing visualization of the semicircles or
  mires.

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Clinical measurement

• The patient is positioned at the slit lamp in the
  usual fashion after instilling topical anesthetic
  and sodium fluorescein into the tear film.
• The patient is instructed to fixate in the
  distance, to relax, and to breathe normally.
• If necessary, the lids are separated (without
  pressure).
• As the tip is advanced toward the cornea, gross
  horizontal and vertical adjustments are made by
  the examiner without using the oculars as the
  instrument approximates the cornea.

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• The cobalt-blue filter is inserted into the
  slit-lamp illuminator, and maximal illumination
  is used.
• When contact is imminent, the examiner uses the
  ocular to observe the mires, which will appear
  green against a blue background.
• If the mires are of unequal size, vertical
  adjustment is made.
• The tonometer knob is rotated until the end-point
  is achieved.
• Ocular pulsations are noted, and the mid-point of
  the excursion of the internal margin of each
  semicircle is aligned

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Precautions

• Valsalva maneuvers, or breath holding by the
  patient, must be avoided.
• The semicircles should be clear with distinct
  margins.
• Wider, blurred semicircles result in false-high
  readings as does vertical misalignment. 
• Measurements without the use of fluorescein
  underestimate the true IOP
• Corneal astigmatism may result in false pressure
  readings. The error has been calculated at 1 mm
  for every 4 diopters (underestimated for
  with-the-rule overestimates for against-the-rule

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• Corneal curvature appears to influence
  applanation tonometry readings.
• In 200 patients examined for routine eye
  examination, whose vital statistics and mean IOP
  demonstrated the group to be a representative
  sample of the general population, there was a
  positive correlation between corneal curvature
  and tonometer readings.
• For each 3-diopter increase in corneal power in
  this sample, the average intraocular pressure
  increased 1 mmHg

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• Thin corneas produce false-low readings, as does
  a thick cornea secondary to edema.
• A thick cornea secondary to increased collagen
  results in a false-high reading. 
• Prolonged contact of the applanator to the cornea
  should be avoided.
• Damage to the cornea may occur with fluorescein
  staining and distortion of the mires.

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• Contaminated tonometers are well-established
  vectors of infection.
• Along with the more common bacteria and viruses
  that cause ocular infection, hepatitis-B surface
  antigen can be isolated from the tonometer tip
  after applanation of infected patients.
• Human immunodeficiency virus (HIV) (AIDS) has
  been isolated from human tears, although no cases
  of transmission from contaminated tonometers has
  been reported.

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Sterilization

• Meticulous sterilization may reduce the risk of
  these pathogens.
• The Centers for Disease Control suggests a 5- to
  10-min soaking in 3 percent hydrogen peroxide or
  70 percent ethanol or isopropanol.
• The tip should then be washed under running water
  and dried thoroughly before reuse

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Other applanation tonometers

• Perkins' applanation tonometer
• MacKay-Marg tonometer
• Pneumatonometer
• The Tono-Pen
• Non contact tonometer

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Perkins' applanation tonometer

• It uses the same biprism as the Goldmann
  applanator.
• The light source is powered by battery, and a
  counter balance enables the instrument to be used
  in both the vertical and horizontal positions.
• The readings are consistent and compare quite
  well with the Goldmann applanator.
• It is especially useful in the operating room for
  examinations under anesthesia and for invalid
  patients, infants, or children who cannot sit at
  the slit lamp. 

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MacKay-Marg tonometer

• It applanates the cornea via a plunger that moves
  within a sleeve, similar in fashion to a Schiøtz
  tonometer.
• The excursion of the plunger is electronically
  coupled to a transducer and graphically records
  the movement of the plunger on a moving strip of
  paper.
• The plunger first indents the cornea recording on
  the graph paper, the sum of the force required to
  flatten the cornea and the IOP.

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• As the tonometer advances, the sleeve abutts the
  cornea, transferring the force required to
  flatten the cornea to the sleeve.
• The pressure tracing then decreases to a level
  that represents the IOP.
• Because the tonometer records instantaneously,
  multiple readings should be averaged in order to
  adjust for fluctuation in pressure due to the
  ocular pulsation.  
• It is especially useful in edematous or irregular
  corneas.

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Pneumatonometer

• The principle of the Pneumatonometer is similar
  to that of the MacKay-Marg tonometer.
• Corneal contact of the pencil-like tip records
  both the IOP and the force required to bend the
  cornea.
• Further advancement of the tip transfers the
  latter force to the surrounding collar.
• In this case, the plunger is replaced by a
  column of air and the contact surface is a
  Silastic membrane.

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Pneumatonometer

• The air column is continually vented via a port.
• Changes in pressure in the column resulting from
  the applanation via a transducer records the
  measurement on a moving strip of paper.
• Similar to the MacKay-Marg unit, this instrument
  is especially useful with edematous and irregular
  corneas.

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The Tono-Pen

• The Tono-Pen, which is a miniature, hand-held
  tonometer, works on a similar principle as the
  MacKay-Marg tonometer.
• The instrument is 18 cm in length and weighs only
  60 g.
• The MacKay-Marg wave form is internally analyzed
  by a microprocessor.
• Three to six estimations of the pressure are then
  averaged, and a digital readout displays the IOP
  with the range of the coefficient of variance.

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The Tono-Pen

• For pressures from 6 to 24 mmHg, the Tono-Pen
  measured an average of 1.7 mm higher than the
  Goldmann tonometer.
• Above 24 mmHg, the readings were similar.
• Large discrepancies (greater than 6 mmHg) were
  found in only 18 of 270 eyes tested.
• In all except 5, obvious causes such as
  astigmatism or corneal disease could explain the
  discrepancy.

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Non contact tonometer

• 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.

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Non contact tonometer

• Mean IOP readings compare favorably with Goldmann
  tonometry, although relatively large
  discrepancies could be found in some patients.
• 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.

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• TONOGRAPHY

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• In 1950, Grant described tonography, a technique
  to measure the decrease in IOP that occurs when
  an external weight is applied to the eye.
• Schiotz noted that repeated tonography within a
  relatively short period resulted in a lowered IOP
  measurement.
• The rate at which IOP decreased seemed to be
  slower in eyes with glaucoma than in normal eyes.

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• Grant used a paper strip recorder connected to an
  electronic tonometer to record a continuous
  tracing of the changes in scale units that
  occurred once the tonometer was resting on the
  eye.
• In the normal eye there is a gradual decrease in
  the IOP, resulting in a tracing with a gentle
  downward slope.
• In the glaucomatous eye, which has an increased
  resistance to expression of fluid through the
  outflow channels, there is less of a change in
  the IOP (indicated by a smaller change in the
  Schiotz scale units) with the resultant tracing
  having a flatter slope

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Test performance

• After applanation tonometry has been performed,
  the patient lies supine in a quiet setting.
• Both eyes are anesthetized and the electronic
  Schiotz tonometer, which is previously
  calibrated, is applied to the eye while the
  patient fixates on a ceiling target with the
  uninvolved eye.
• The proper Schiøtz weight used during tonography
  is determined by the initial applanation, and the
  standard tracing of 4 min is obtained.

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• An acceptable tracing has a smooth gradual slope,
  with small oscillations indicating the ocular
  pulse and somewhat less apparent cycles of longer
  duration due to respirations.
• Any Valsalva maneuver, such as coughing or
  sneezing, will invalidate the tracing.
• When an appropriate tracing is obtained, the
  technician draws a line through the tracing,
  approximating the slope that allows one to read
  the scale units at time 0 and at 4 min.
• These are then used to determine C from the
  tonography tables or the Friedenwald nomogram.

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Clinical implications

• Grant's initial paper, involving repeated
  examinations on normal eyes, determined C values
  with a range of 0.15 to 0.34 mmul/min/mmHg,
  with a mean of 0.243.
• Lower C values, some of which were 0.0, occurred
  during attacks of angle-closure glaucoma.
• In chronic angle-closure, C values would decrease
  proportional to the degree of closure of the
  angle.

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• In the glaucomatous eyes, C values did not exceed
  0.11 mmul/min/mmHg.
• It was anticipated that in the glaucoma suspect
  (i.e., patients with normal optic nerves and
  visual fields), patients with C values in the
  glaucomatous range would be especially likely to
  develop optic nerve damage and thus be candidates
  for early intervention.

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• Genetics in Glaucoma

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• Glaucoma is an important and prevalent ophthalmic
  disease.
• A large number of inheritable ophthalmic diseases
  are associated with glaucoma, such as
• aniridia (autosomal-dominant),
• Axenfeld Rieger syndrome (autosomal-dominant),
• neurofibromatosis (autosomal-dominant),
• Lowe's syndrome (X-linked recessive)

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Primary open-angle glaucoma (POAG)

• Primary open-angle glaucoma (POAG) has a strong
  hereditary tendency.
• Anyone who has taken care of POAG patients
  understands the importance in soliciting a
  relevant family history of glaucoma.
• Numerous studies have shown that the prevalence
  of POAG in first-degree relatives of patients
  (2.8 to 13.5) is significantly higher than that
  in the general population (about 1)

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• Attempts have been made to identify genetic
  markers associated with POAG.
• These markers include
• blood group antigens,
• HLA antigen association,
• ability to taste phenylthiourea, and
• association with diabetes mellitus and myopia.
• These studies, however, have not revealed any
  specific association of these genetic markers
  with POAG.

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• At present, POAG is believed to have a polygenic
  or multifactorial inheritance
• In 1993, Sheffield and associates reported
  genetic linkage of one form of familial
  open-angle glaucoma to chromosome 1q21-q31.
• The responsible genes are thought to show
  incomplete penetrance and variable expressivity
• IOP, facility of aqueous outflow and optic disc
  size are also genetically determined.

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Steroid responsiveness

• A mutation of the MYOC gene is strongly
  associated with a subset of juvenile open angle
  glaucoma and is present in 4 of adult of POAG.
• The gene was previously named TIGR (trabecular
  induced glucocorticoid response).
• It resides in the GLCIA region of the long arm of
  chromosome 1 and codes for a protein called
  myocilin.
• The administration of steroids in steroid
  responders induces gene expression and production
  of excessive quantities of myocilin.

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• All gene loci associated with POAG have prefix
  GLC1, with the suffix letter in which the gene is
  identified.
• Genes that have so far been identified in certain
  families with POAG lie on
• chromosome 2 (GLC1B),
• chromosome 3 (GLC1C),
• chromosome 8 (GLC1D),
• chromosome 10 (GLC1E) and
• chromosome 7 (GLC1F).

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Thank U