Goldmann and Perkin Tonometer- An Overview


What is Tonometer & what is it used for? 

A tonometer is a medical device that is used to measure the intraocular pressure (IOP) of a patient's eye. The device typically consists of a small probe that is inserted into the patient's eye, and a handheld unit that displays the measured pressure. The probe uses the applanation tonometer which is a non-invasive instrument that measures how much the cornea deforms when it is flattened against a glass plate.

The applanation tonometer employs two different techniques, which are explained below:

  1. Goldmann Tonometer 
  2. Perkins Tonometer

Let us understand more about the Goldmann tonometer and the Perkin tonometer.

Goldmann Applanation Tonometer

The Goldmann applanation tonometer measures the force required to flatten a 3.06mm corneal region. The capillary attraction of the tear film meniscus for the tonometer head balances the cornea's resistance to flattening at this diameter. The flattening force (in kg) multiplied by 10 equals the IOP (in mm Hg). To highlight the tear film, fluorescein dye is applied to the patient's eye. The image of the tear meniscus is divided into a superior and inferior arc using a split-image prism. When these arcs are lined so that their inner borders just touch, the intraocular pressure is measured. Goldmann Tonometry is the most extensively used approach.

Perkins Applanation Tonometer

The Perkins tonometer is a portable Goldmann applanation tonometer that can be used with the patient standing or lying down. This is a handheld variant of the Goldmann that does  not require the use of a slit lamp.

How the intraocular pressure is assessed using Goldmann Tonometer?

The cornea is flattened by an acrylic measuring prism on a ring support at the end of the Tonometer sensor arm assembly. To avoid corneal injury, it is flat with smooth or rounded borders. By sliding the slit lamp forward, the measuring prism is brought into contact with the patient's eye. The measurement drum is then spun to increase the pressure on the eye until it achieves a continuous,  uniform applanated surface with a diameter of 3.06 mm and an area of 7,354 mm2. The doubling prism  divides the image into two semicircular parts and displays them at 3.06mm apart.

Here are some significant advantages of a Goldmann Applanation Tonometer:

• The Slit Lamp can be used to measure intraocular pressure during a regular examination.
• The standard deviation of individual measurements is about 0,5 mmHg.
• The value is read immediately on the instrument and is expressed in millimeters of mercury.
• Scleral stiffness does not need to be considered because the modest volume moved (0.56 mm3) only 
  increases intra-ocular pressure by roughly 2.5 percent.
• There are no problems with standardization or calibration.

Who needs a tonometry test?

If your eye doctor suspects you're at risk for glaucoma, they may perform the Goldman applanation tonometry test. It is the most common test performed to detect the eye disorders. According to the American Academy of Ophthalmology, you may be at risk for glaucoma if you:

  • have a family history of glaucoma 
  • are over 40 years old
  • have diabetes, high blood pressure, or poor blood circulation
  • have additional chronic eye disorders and are nearsighted or farsighted
  • have been using corticosteroid drugs for a prolonged period of time

All of these symptoms could indicate that you have glaucoma:

  • gradual loss of peripheral vision
  • tunnel vision
  • severe eye pain
  • blurred vision
  • halos around lights
  • reddening of your eye

The following are some helpful DO's and DONT's to keep in mind when measuring a patient's IOP by Goldmann Ophthalmology:

  1. Prior to checking Intraocular Pressure , consider the patient's position, comfort, and clothes.
  2. Do not push down on the globe. Do not allow patients to close their eyes.
  3. Make sure you use the right amount of fluorescein.
  4. Make sure there are no eyelashes in the contact region and that the patient isn't squeezing his eyelids shut. To prevent putting pressure on the patient's eye, gently open the lids.
  5. Make certain your instrument is properly calibrated.
  6. Do not modify your treatment based on a single IOP reading.

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