IOP measurements

IOP measurements

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  INTRAOCULAR PRESSURE MEASUREMENTS OFIOP MARYAM FIDA OPTOMETRY AND ORTHOPTICS
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  Intraocular pressure Intraocular pressure(IOP) is the fluid pressure inside the eye. . IOP is an important aspect in the evaluation of patients at risk of glaucoma. Tonometry is the method eye care professionals use to determine this. Most tonometers are calibrated to measure pressure in millimeters of mercury (mmHg). Physiology  Intraocular pressure is determined by the production and drainage of aqueous humour by the ciliary body and its drainage via the trabecular meshwork and uveoscleral outflow. The reason for this is because the vitreous humour in the posterior segment has a relatively fixed volume and thus does not affect intraocular pressure regulation.  The intraocular pressure (IOP) of the eye is determined by the balance between the amount of aqueous humor - that the eye makes and the ease with which it leaves the eye. The Goldmann equation states: Po = (F/C) + Pv  Po is the IOP in millimeters of mercury (mmHg),  F is the rate of aqueous formation,  C is the facility of outflow,  Pv is the episcleral venous pressure. Measurements Intraocular pressure is measured with a tonometer as part of a comprehensive eye examination. Types of Tonometry 1. Applanation tonometry Applanation tonometry is based on the Imbert-Fick principle, which states that; ‘’The pressure inside an ideal dry, thin-walled sphere equals the force necessary to flatten its surface divided by the area of flattening’’ P = F/A where P = pressure, F = force and A = area
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  In applanation tonometry,the cornea is flattened and the IOP is determined by varying the applanating force or the area flattened. Goldmann and Perkins applanation tonometry Equipment  Tonometer, either Goldmann (used on slit lamps) or Perkins (hand-held)  Applanation prism  Local anaesthetic drops  Fluorescein strips  Clean cotton wool or gauze swabs. Figure 1 Applanation tonometry Method  The Goldmann applanation tonometer measures the force necessary to flatten an area of the cornea of 3.06mm diameter. At this diameter, the resistance of the cornea to flattening is counterbalanced by the capillary attraction of the tear filmmeniscus for the tonometer head.
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   The IOP(in mm Hg) equals the flattening force (in grams) multiplied by 10. Fluorescein dye is placed in the patient’s eye to highlight the tear film. A split-image prism is used such that the image of the tear meniscus is divided into a superior and inferior arc. The intraocular pressure is taken when these arcs are aligned such that their inner margins just touch.  Applanation tonometry measurements are affected by the central corneal thickness (CCT). When Goldmann designed his tonometer, he estimated an average corneal thickness of 520 microns to cancel the opposing forces of surface tension and corneal rigidity to allow indentation. It is now known that a wide variation exists in corneal thickness among individuals. Thicker CCT may give an artificially high IOP measurement, whereas thinner CCT can give an artificially low reading.  Other errors that may affect the accuracy of readings from a Goldmann tonometer include excessive or insufficient fluorescein in the tear film, high astigmatism, irregular or scarred cornea, pressure from a finger on the eyelid while taking the measurement, and breath holding and Valsalva maneuver by the patient during measurement.  The Perkins tonometer is essentially a portable Goldmann applanation tonometer that can be used with the patient in either upright or supine position. Non-Contact Tonometry  In air puff tonometry, the applanating force is a column of air which is emitted with gradually increasing intensity.  At the point of corneal flattening, the air column is shut off and the force at that moment is recorded and converted into mmHg. Readings from these machines may
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  underestimate IOP athigh ranges and overestimate IOP at low ranges as compared to the Goldmann applanation tonometer.  A minimum of 3 readings should be averaged to estimate the mean IOP as IOP varies during the cardiac cycle. Figure 2 Non-Contact Tonometry Ocular Response Analyzer  The ocular response analyzer is a newer type of non-contact tonometer.  This device also uses a column of air of increasing intensity as the applanating force. The ocular response analyzer notes the moment of applanation, but the air column continues to emit with increasing intensity until the cornea is indented.  The force of the air column then decreases until the cornea is once again at a point of applanation. The difference in the pressures at the two applanation points is a measure of the corneal elasticity (hysteresis).  Mathematical equations can be used to “correct” the applanation point for high or low elasticity. This “corrected” IOP is thought to be less dependent on corneal thickness than other forms of applanated pressures.
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  2. Indentation tonometry Theprinciple of indentation tonometry is that a force or a weight will indent or sink into a soft eye further than into a hard eye. Schiotz Tonometer  The Schiotz tonometer consists of a curved footplate which is placed on the cornea of a supine subject. A weighted plunger attached to the footplate sinks into the cornea in an amount that is indirectly proportional to the pressure in the eye.  The plunger will sink into the cornea of a soft eye further than it will in a harder eye. A scale at the top of the plunger gives a reading depending on how much the plunger sinks into the cornea, and a conversion table converts the scale reading into IOP measured in mm Hg. Figure 3 Ocular Response Analyzer
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  Figure 4 SchiotzTonometer Pneumotonometer  The pneumotonometer is an applanation tonometer with some aspects of indentation tonometry. It consists of a 5mm diameter, slightly convex, silicone tip on the end of a piston that rides on a stream of air.  The cornea is indented by the silicone tip. When the cornea and the tip are flat, the pressure pushing forward on the tip is equal to the IOP. The device measures the pressure within the system at this point and the pressure in mm Hg is displayed. The readings correlate well with Goldmann applanation tonometry within normal IOP ranges. Figure 5 Pneumotonometer
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  Tono-Pen  The Tono-Peninvolves both applanation and indentation processes. It is a small, handheld, battery-powered device. The tonometer has an applanating surface with a tiny plunger protruding microscopically from the center.  As the tonometer -makes contact with the eye, the plunger gets resistance from the cornea and IOP producing a rising record of force by a strain gauge.  At the moment of applanation, the force is shared by the foot plate and the plunger resulting in a momentary small decrease from the steadily increasing force. This is the point of applanation which is read electronically.  Multiple readings are averaged. Because the area of applanation is known, the IOP can be calculated. The readings correlate well with Goldmann tonometry within normal IOP ranges. Figure 6 Tono-Pen 3. Rebound tonometry  The newest version of the rebound tonometer is the ICare device (Helsinki, Finland). A 1.8mm diameter plastic ball on a stainless-steel wire is held in place by an electromagnetic field in a handheld battery-powered device. When a button is pushed, a spring drives the wire and ball forward rapidly.  When the ball 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 and is converted by the device into IOP. No anesthetic is necessary. It shows good agreement with Goldmann and Tono-pen readings.  IOP measurements obtained with this tonometer have also shown to be influenced by central corneal thickness, with higher IOP readings with thicker corneas.
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   This tonometerhas been shown to be affected by other biomechanical properties of the cornea, including corneal hysteresis and corneal resistance factor. Figure 7Rebound tonometer 4. Pascal Dynamic Contour Tonometer  The Pascal Dynamic Tonometer (Zeimer Ophthalmic systems AG, Port, Switzerland) utilizes a piezoelectric sensor embedded in the tip of the tonometer to measure the dynamic pulsatile fluctuations in IOP. In contrast to the Goldmann tonometer, measurements with the DCT are reported to be influenced less by corneal thickness, and perhaps corneal curvature and rigidity.  These claims are supported by in vitro and in vivo manometric studies. DCT can also be used to measure the ocular pulse amplitude. Disposable covers are used for each measurement and the digital display provides a Q-value which assesses the quality of the measurements.
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  Figure 8 PascalDynamic Contour Tonometer