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    4 Ocular Tonometry

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    majd dan

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    4 Ocular Tonometry

    Uploaded by

    majd dan
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    4 Ocular tonometry

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    4 Ocular Tonometry

    Uploaded by

    majd dan

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    © All Rights Reserved
    Download as PDF, TXT or read online from Scribd
    Download as pdf or txt
    of 3

    Ocular tonometry

    Goldmann tonometry

    General principles: Tonometry is an objective method of measurement of the intraocular


    pressure based on the necessary force of corneal flattening . Aplanation tonometry is based on the
    Imbert-fick principle (P=F/A) that says for an ideal sphere, dry with thin walls, the pressure inside the
    sphere(P) is equal to the force necessary for flattening it (F) divided at the surface of flattening (A).
    Intraocular pressure is proportional with the pressure applied on the eyeball and with the thickness of
    the walls of the globe(thickness of the cornea, which is variable). The human eye is not an ideal sphere,
    the cornea is rigid and inflexible. Capillary attraction of the tear film tends to attract the tonometer on
    the cornea. Corneal rigidity and capillary attraction cancel one another when the flattened surface has a
    diameter of 3.06 like in Goldmann tonometry. The Goldmann tonometer is a tonometer with variable
    force very precise and consists from tho prisms.

    Procedure

    1. Topical anesthetic and fluorescein is instilled in the conjunctival bag.


    2. At the biomicroscope, the Goldmann prism is applied axial on the corneal surface.
    3. Will appear two semicircles, one above and one beneath the middle horizontal line.
    4. Will rotate the grading of the tonometer, until we align the interior margins of the semicircles.
    5. Intraocular pressure is equal with = the grading *10

    Potential errors

    1 Improper model either due to excess of fluorescein( the semicircles are to thick, distance is to
    small) or a small quantity of fluorescein(the semicircles are to thin, distance is to big)

    2 Pressure upon the eyeball- either from the fingers of the examinator, or because of
    thightening the eyelids it can result a bigger value, artificial.

    3 Incorect calibrating of the tonometer can result in incorrect results. That is why is important to
    verify the calibration at certain regular periods of time.

    4 Corneal pathology (edema, abnormal thickness, deformation) can result in incorrect values.

    5 Prolonged contact of the prism with the cornea can result in lesions that alter the result. In
    case of big astigmatism, it should be measured the pressure on both meridians than the media is done.
    Schiotz tonometer

    Based on the principle of indentation tonometry, in which a piston with predetermined


    weight is indenting the cornea. The value of indentation is measured on a scale and converted
    in mmHG with a special table. The tonometer is cheap, easy to use, without the need of a
    biomicroscope, but nowadays is rarely used. From the exterior it applies a constant force upon
    the cornea and it measures the deformation that it produces. Schiotz tonometer has a vertical
    hollow inside cylinder which at the distal extremity has a leg with the concavity in interior, and
    at the proximal extremity comes in contact with an indicator that moves in front of a grading
    scale. In the cylinders axe is a metal rod which will indent the central corneal area.Cylinder has
    weights of 5,5 g/ 7,5 g/ 10 g/ 15 g(for higher intraocular tensions you raise the force)

    Procedure

    1 Pacient is in dorsal decubitus and is fixing a fixed point

    2 Anesthetic is intilled

    3 The tonometer is applied on the center of the cornea

    4 The examiner notes the position of the indicator and the weights used

    Advantages: simple procedure

    Disadvantages: The calibration of the tonometer is exactly for medium ocular rigidity(k=0,0215)

    A High rigidity = pressure is overestimated( hypermetropia)

    B Low rigidity= pressure is underestimated(myopia)

    If the rigidity is modified it can appear differences of 5mmhg

    Average corneal radius is 7.8mm

    A Radius bigger than the average=low apparent rigidity

    B Radius smaller than the average= higher apparent rigidity. The method gives errors in case of
    buftalmia or microoftalmia.

    Other tonometers:

    1 Perkins tonometer is an aplanation tonometer that is hold in hand, that uses a Goldmann
    prism adapted at a small source of light. Is small, without the need of a biomicroscope, that is
    why is used for patients that cannot move or are under anesthetic.
    2. Non-contact tonometer with flush air is based on the principle of aplanation, but without
    using a prisms , the central part of the cornea is flattened from a flushed air. The time necessary
    for flattening the cornea is direct linked with the values of the intraocular pressure. The tool is
    easy to use and doesn’t need topical anesthetic. Mostly is used for screening. Biggest
    disadvantage is the accuracy. The flush air can scare the patient by sound and apparent force.

    3 Pulsair Keeler is a noncontact tonometer to hold in the hand, which incorporates a


    mechanism that reduces errors made by the “reader”. The tool doesn’t make noise. Offers
    values comparable with The Goldmann tonometry. It is necessary a recalibration after certain
    amount of time.

    4 Tono-Pen is a contact tonometer, portable, works with batteries. The top of the probe
    consists in a transducer that measures the applicable force. A microprocessor measures the
    curve force/time generated by the transducer in time of the indentation of the cornea for
    calculating the pressure. The tool correlates well the Goldmann tonometry but it overestimates
    a small pressure and it underestimates a higher pressure. The biggest advantage is measuring
    the pressure on an eye with corneal edema or deformation, also through contact lens.

    5 Maklakoff is a tonometer with variable surface of aplanation and with constant force. Is a
    dumbbell with the surface of 10mm, that can have a weight of 5/ 7,5/ 10 or 15 g. PAcient stays in
    dorsal decubitus. Results are read on a scale.

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