Applanation tonometer

Applanation tonometer

Eye care is very important as we age, especially if there is a family history of glaucoma.  Eye screening is also as important as screening for breast cancer, testicular cancer or prostate cancer. The worst part of neglecting eye screening can cost your eyesight due to glaucoma advancement.  Applanation Tonometer is an instrument used to measure the ocular or eye pressure and which is an important clue for the diagnosis of glaucoma.

Glaucoma is the increased pressure within the eye which has the direct effect on the optic nerve, which is responsible for our sight. The persistence of high intraocular pressure for a longer duration can lead to severe damage and in no time can cause blindness. The screening for glaucoma with Application Tonometer can detect the increased eye pressure very easily. The “Goldmann method” is used as a main principle in the Applanation Tonometer. Before knowing how to measure the intraocular pressure, we need to know about the parts of Applanation Tonometer.

Parts of Applanation Tonometer

  • Tonometer prism – This is the piece which directly contacts with the cornea of the eye and thus enabling the Applanation Tonometer to measure the intraocular pressure. There are two more extra prisms also comes with Applanation Tonometer.
  • Tonometer prism holder – The prism should be fixed tightly in order to get an accurate measure of the intraocular pressure. The Tonometer prism holder fixes the prism tightly so that the prism is fixed at one position and the movement of the prism would not cause any changes in the reading.
  • Slit lamp attachment – This is the holder for the slit lamp. The slit lamp gives enough light to visualise the eye and to measure the intraocular pressure. In addition, this holder helps to adjust the position of the slit lamp if needed and to provide a constant amount of light.
  • Measurement box – The Tonometer prism holder is attached to the measurement box and the measurement box helps to find the exact reading.
  • Reading adjustment knobs – The adjustment knobs helps the attached Tonometer prism to move back and forth in order to produce an accurate reading. These knobs are designed to produce fine movements in order to increase the efficacy of the tonometer.

Applanation Tonometer

How do we measure the intraocular pressure?

The forehead and the chin rest of the slit lamp should be disinfected. Moreover, the tonometer probe should be disinfected according to the guideline from CDC. The prism should be rinsed under running cold water in order to remove tears and fluorescein. Moreover, the tip should be soaked in 3% H2O2 (hydrogen peroxide) for at least 10 minutes. After soaking in the hydrogen peroxide, once again wash thoroughly with running cold water. The wet tip and prism should be pat dry with the help of tissues or let the prism dry on its own.

After disinfecting the tonometer prism and its tip, the pat dried or air dried prism should be installed, in order to begin the process of measuring intraocular pressure. The height of the slit lamp holder or table should be adjusted to a comfortable position between the examiner and the patient. In addition, the chin rest should be adjusted in order to align properly.

After aligning the applanation tonometer set the magnification either on the lowest degree or at the maximum power in order to evaluate the condition of the anterior segment that needs to be examined, before measuring the intraocular pressure. The anterior segment or more specifically, cornea should be free from any infection or trauma at the time of intraocular pressure measurement.  Later, the cornea should be made ready for examination by adding one drop of anaesthetic with liquid fluorescein. After preparing the patient’s cornea for examination, insert cobalt filter to the tonometer and scan the cornea for already available stain, before taking the reading.

After that, the tonometer arm should be positioned, so the applanation prism is aligned in front of the left eye. After aligning it properly, the tonometer prism should be rotated in order to align the “O“ with the white mark on the holder of the prism. However, this method might be altered if a person has astigmatism that is greater than 3dp. In order to fix the problem, rotate the prism to align with the prism holder to the red marking on the prism holder meet with the axis mark, and the reading goes to the negative axis. The slit lamp is opened to its brightest and the illumination arm at 450, in order for the tip of the tonometer prism to be lighted with bright blue light. This allows a better visibility of the cornea so that the measurement of the intraocular pressure is absolutely right.

The probe of the tonometer is set a bit lower than the visual axis, and the probe is moved closer to the cornea. When the probe reaches the cornea, and with a distance apart of 2 mm, the adjustment knobs are rotated carefully to touch the prism to the cornea. When this contact is made, the limbus part will show an excellent glow. The glow can be seen externally through the ocular piece of the applanation tonometer near the slit lamp. This allows for the better visualisation of the eye. Once the prism made the contact and the limbus is glowing, you need to look through the left ocular and align the semicircles transversely and vertically. This is because proper aligning of these semicircles facilitates the accurate reading of the intraocular pressure.

Applanation tonometry

After setting up the semicircles properly, check the thickness of the mire. The optimal thickness of the mire is 1/10th the diameter of the aligned semicircle. The increase in thickness of the mire can be a sign that there is an immense amount of fluorescein or it can be a sign of the very large amount of tearing. Either way, too thick mire can cause improper reading of the intraocular pressure. The reading for the intraocular pressure should only be taken, when the mire is in the right amount of thickness and also the semicircles are aligned properly to the centre. When these conditions are favourable, you need to turn on the pressure dial. After making the reading, the tonometer tip should be carefully removed without damaging the cornea. As soon the tonometer tip is removed from the cornea, the tip should be wiped with a tissue or washed to get rid of the fluorescein dye.

More often, it is highly advisable to take at least 3 readings and to obtain the average as the final reading. This process can improve the accuracy of the results obtained. The measurement of IOP or intraocular pressure is measured in mmHg. More commonly, the reading obtained is multiplied by 10 to get the exact reading of the intraocular pressure.

Disinfecting the tonometer after usage

After obtaining the reading, the tip of tonometer prism, as well as the tonometer, should be disinfected thoroughly before next use. The prism should be washed thoroughly under running cold water and then soaked in hydrogen peroxide and once again washed with running cold water before next usage.

However, the body of the applanation tonometer also need to be cleaned. The external surface of the body of applanation tonometer is wiped with a tissue dipped in water or a solution of 2% detergent or 70% alcohol solution can be used. After wiping with a wet cloth, the surface should be manually dried with a help of another dry cloth. The tonometer should not be packed before the tonometer completely become dry.

Errors and solutions while using applanation tonometer

One of the major problem when measuring with applanation tonometer is the presence of thick mires. This can be corrected by asking the patient to blink in order to remove excessive fluorescein and also cleaning the tip of the prism with a soft and dry cloth. After doing this, the measurement should be repeated. This allows measuring accurate reading of IOP.

Another important problem encounter by most of the people who measure IOP is thin mires. The thin mire is usually the result of very less staining with fluorescein. The amount of fluorescein used should be increased and the measurement should be repeated to have a reliable reading of IOP.

Another problem experienced when using applanation tonometer is the visibility of one-half of the mire. This is usually as a result of an enormous amount of pressure on the cornea. This pressure needs to relive by removing the tip from the cornea and also removing the slit lamp. After some time, the reading should be repeated.

One of the less common problems experienced by applanation tonometer usage is non-semicircle mire. The mires are not arranged in a semicircle shape, instead of that, the mires are either higher or lower in location than each other. The solution to this problem can be achieved by pulling slit lamp from its location and adjusting the position of the slit lamp and the prism is centred in a vertical direction on the cornea. If the problem is that the mires are not aligned horizontally centred. This can be sorted out by adjusting the slit lamp height so that the prism can centre its location in the horizontal direction.

Maintenance of the applanation tonometer

The usage of the applanation tonometer depends on how does the applanation tonometer is taken care of.  Therefore, extreme care should be taken. The applanation tonometer is always calibrated beforehand during the manufacturing process. Therefore, disassembling of the applanation tonometer should not be done, as removing the tonometer into parts need calibrations of the tonometer again.

The care should be given to the tonometer prism. The prism should be cleaned and washed after each usage. The 3% hydrogen peroxide solution should only be used other than water in order to disinfect the prism. The usage of alcohol products can damage the quality of prism and in long run produce severe damage to the prism. In addition, the use of ultraviolet rays for sterilisation also should be avoided. The prism can withstand temperature below 60C and therefore, the very hot temperatures should be avoided.

Moreover, the body of the applanation tonometer should be cleaned only with water or 2% detergent solution. The tonometer never should be immersed in water or any other solution as this can cause permanent damage to the apparatus.

Warnings and precautions

The applanation tonometer only should be used by the person who have professionally trained or healthcare workers. This is because the use of applanation tonometer requires swift hand and very good idea on what is happening or how to do it. The unauthorised use of applanation tonometer can result in severe damage to the eye as well as to severe damage of the tonometer.

In addition, if there is any damage to the apparatus, and then the usage of the instrument should not be encouraged. The damaged apparatus can produce wrong reading and also endanger the eyesight by damaging the eye during its direct contact.

Moreover, the use of the tonometer in different people would give different results as the corneal thickness determine the accuracy of the measured intraocular pressure.

The use of applanation tonometer should be limited to the indoor usage. This is due to the fact that the applanation tonometer cannot withstand too much of moisture, temperature above 60C or very low temperature. These factors can damage the tonometer and the result it produces. Therefore, the applanation tonometer should be protected from harsh weather conditions by using it indoor.

More importantly, the use of the applanation tonometer should be used when there is a rich supply of oxygen, combustible liquid or gas.

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