Pneumotonometer Principle

[Karlyn Hemmerling]

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An association between increased IOP and the loss of sight in glaucoma has been noted for many centuries. In the 17th century, Richard Bannister (English physician) noticed the hardness of eyes in cases where cataract operations did not improve vision. In the 19th century, William Bowman ( English ophthalmologist) developed a method of estimating the tension, or hardness, of the eye by palpating it with his fingers through the closed eyelid. Bowman and others noticed that there was a definite relationship between the level of IOP and the likelihood that the eye would lose sight; the higher the IOP, the greater the chance that the eye would become blind. Therefore, IOP remained the primary focus in the diagnosis and treatment of glaucoma for many years.[2]

The Ocular Hypertension Treatment Study also investigated ocular hypertensive patients and addressed whether treatment of elevated IOP prevented or delayed the onset of glaucomatous damage. Half of the participants were randomized to treatment to lower their IOP by 20%, and half were randomized to observation. All subjects were followed closely with visual field exams and optic nerve photos. After 5 years of follow up, 9.5% of the observation group developed glaucoma while 4.4% of the medication group developed glaucoma, defined as optic disc or visual field deterioration. Decreasing the IOP reduced the risk of progression to glaucoma;- however, the majority of ocular hypertensive patients did not develop damage within 5 years.[5]

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). In applanation tonometry, the cornea is flattened, and the IOP is determined by varying the applanating force or the area flattened. [1]

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 affecting the thickness of the overlapping arcs, high astigmatism, irregular or scarred cornea, pressure from a finger on the eyelid while taking the measurement, and breath holding or Valsalva maneuver by the patient during measurement.

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 underestimate IOP at high 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.

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 into 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.

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 is displayed in mm Hg. The readings correlate well with Goldmann applanation tonometry within normal IOP ranges.[7]

The Tono-Pen involves both applanation and indentation processes. It is a small, handheld, battery-powered portable device. The tonometer has an applanating footplate with a tiny plunger protruding minimally from the center. As the tonometer makes contact with the eye, the plunger receives resistance from the cornea and the IOP producing a record of rising 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 recorded electronically. Multiple readings are averaged. Because the area of applanation is known, the IOP can then be calculated. The readings correlate well with Goldmann tonometry within normal IOP ranges. [6][7][8]

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.[6] IOP measurements obtained with this tonometer have also shown to be influenced by central corneal thickness, with higher IOP readings with thicker corneas.[9][10] This tonometer has been shown to be affected by other biomechanical properties of the cornea, including corneal hysteresis and corneal resistance factor.[11][12]

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. [13]

IOP is a dynamic parameter that can fluctuate 4-5 mmHg in healthy individuals and even more variably in glaucoma patients. Advances have been made to develop techniques that can monitor IOP beyond the in-office measurements.Early animal studies have investigated technologies for permanent IOP monitoring, including the surgical implantation of a pressure transducer system, as well as the implantation of an intraocular sensor into the lens capsule.[14] The main drawbacks of these strategies include the surgical risks. The main device developed for temporary IOP monitoring is the soft contact lens sensor (CLS) that measures changes in ocular dimensions over a 24-hour period, which has shown good correlation with true IOP in in vitro manometry studies. This device is currently approved in Europe for clinical use. Main drawbacks of this technology include difficulty interpreting the volume of collected data, as well as the inability of the output signal to be directly translated into clinically used mmHg scale.[14]

To compare the effectiveness of transpalpebral scleral tonometry (TPST) and corneal pneumotonometry in children, and assess the discomfort level when measuring intraocular pressure (IOP) by these methods.

TPST provides broader possibilities for IOP control in pediatric practice, yielding more reliable and accurate results than pneumotonometry, eliminating the influence of corneal thickness and irregularity on the measurement result, and ensuring a calmer behavior and more comfort of children during the procedure.

In a number of clinical situations, frequent and constant monitoring of IOP in children is required, such as congenital glaucoma or suspicion of it [11], high degree of myopia, or prolonged instillations of atropine solution to stop its progression [12, 13], prolonged instillations of steroids [14] or systemic treatment with certain medications [15], etc.

However, even the most commonly used non-contact method of corneal IOP measurement in pediatric practice, pneumotonometry, cannot always ensure the child's calm behavior during the examination and exclude his/her reaction to the measurement process (reflex blepharospasm, extraocular muscle tension, blinking, etc.), which can lead to deviation of the obtained values from the real IOP level, i.e. contribute to an increase in the measurement error [3].

Besides, using pneumotonometry in the current epidemiological situation is associated with the risk of spreading the infection [16]. Recent publications convincingly indicate an increased risk of spreading the viral disease when performing non-contact pneumotonometry: particles of tears that contain the virus easily enter the environment in the form of aerosol bubbles. This effect is cumulative and increases with higher IOP values and/or in the case of instillations of any eye drops shortly before the examination [17]. When performing the non-contact tonometry, the tear film breaks, and tear particles are released under the influence of a powerful stream of air. These aerosol bubbles can remain in the air for a long time and gradually settle on surrounding objects, including medical equipment [18].

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