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Tonometry involves diagnostic testing to measure the pressure inside the eye or intraocular pressure (IOP). Glaucoma is a silent disease that causes irreversible functional peripheral visual field loss that can ultimately lead to blindness in the very late stages of the disease if not treated. Tonometry should be performed during routine ophthalmic examinations to screen for glaucoma and other ocular diseases. This activity reviews the main instruments based on the applanation principle and highlights the importance of obtaining precise, accurate, and reproducible measurements of IOP by clinicians. This activity shows the various methods, instruments, advantages, and limitations in applanation tonometry. Objectives: Describe the principles of applanation tonometry based on the Imbert-Fick law. Summarize the different types of applanation tonometers used to take IOP readings. Describe each applanation tonometer's correct methodologies, techniques, indications, contraindications, limitations, and advantages. Review the importance of applanation tonometry in the diagnosis, management, and follow-up assessments in managing patients in a routine clinical setting. Access free multiple choice questions on this topic.

Tonometry involves diagnostic testing to measure the pressure inside the eye or intraocular pressure (IOP). Glaucoma is a silent disease that causes irreversible functional peripheral visual field loss that can ultimately lead to blindness in the very late stages of the disease if not treated. Tonometry should be performed during routine ophthalmic examinations to screen for glaucoma and other ocular diseases. IOP must be monitored periodically during the management of patients with glaucoma, ocular hypertension (OHT), and subjects at risk of developing glaucoma. Normal IOP measures in the range of 10 to 21 (mmHg), which is based on average IOP levels in populations surveys in normal subjects, and less than 2 % of normal subjects show IOP greater than 21 mmHg.[1] Possible causes for an IOP under normal rages (hypotonus) include uveitis, ocular traumas, retinal detachment, and post-surgery complications, especially after filtering surgery. Elevated IOP is normally caused by glaucoma. The definition of glaucoma in the past years has evolved from a disease solely defined by IOP>21 mmHg to include the assessments of functional and morphological defects. The concept of determining and re-evaluating a personalized target IOP is currently an important issue in managing patients. These ranges in mmHg are associated with levels of IOP that are thought to cause a minimal likelihood of optic nerve damage or visual field loss, or progression of an existing lesion due to OHT.[2] Treatment of OHT and glaucoma involves lowering IOP using local drop therapy, laser, and/or surgery. It is thus of utmost importance that instruments used to measure IOP are properly calibrated, accurate and precise, considering that the treatment options are based on IOP levels, together with visual field results, clinical evaluations, and morphological assessments of the optic nerve and retinal nerve fiber layer. The true IOP inside the eyeball can be measured by inserting a probe in the anterior chamber to measure the manometric pressure. However, this invasive technique tends to be strictly used in animal models and can surely not be considered in a routine clinical setting.[3]

Treatment of OHT and glaucoma involves lowering IOP using local drop therapy, laser, and/or surgery. It is thus of utmost importance that instruments used to measure IOP are properly calibrated, accurate and precise, considering that the treatment options are based on IOP levels, together with visual field results, clinical evaluations, and morphological assessments of the optic nerve and retinal nerve fiber layer. The true IOP inside the eyeball can be measured by inserting a probe in the anterior chamber to measure the manometric pressure. However, this invasive technique tends to be strictly used in animal models and can surely not be considered in a routine clinical setting.[3] Numerous instruments and tonometers have been created since the 1800s to measure IOP, which have been designed to provide accurate, reliable, precise, and reproducible measurements of IOP. Each method has advantages, disadvantages, and limits and is more or less influenced by ocular factors, rendering some methods clinically acceptable and practical while others are obsolete. Tonometers are based on different concepts and principles of physics that define how IOP levels are measured and what factors can theoretically influence these readings. The force needed to applanate, indent, and/or rebound the surface of the eye is used to estimate and calculate the IOP provided by the numerous tonometers used to date. It is important to note that IOP readings can be influenced by numerous factors based on each tonometer used.[4] These factors can influence accuracy, precision, repeatability, measurement variability, and specificity. The factors that need to be considered include the amount of fluorescein, excessive tear production, corneal astigmatism, scarring, scleral rigidity, corneal edema, central corneal thickness, and arterial perfusion, central venous pressures, eye position, etc.[4]

These factors can influence accuracy, precision, repeatability, measurement variability, and specificity. The factors that need to be considered include the amount of fluorescein, excessive tear production, corneal astigmatism, scarring, scleral rigidity, corneal edema, central corneal thickness, and arterial perfusion, central venous pressures, eye position, etc.[4] Goldmann applanation tonometry (GAT) is currently the most widely accepted method used to measure IOP and is considered the gold standard tonometer in clinics.[5] GAT indirectly measures the IOP by assessing the force needed to flatten a predetermined surface area of the cornea. Taken simplistically, if the eyeball is hard, it takes more force to flatten the surface of the cornea, which is directly influenced by the IOP. GAT is based on the principles of applanation tonometry. Other instruments that have been built using the principles of applanation include the Perkins applanation tonometer, non-contact tonometers, and the Ocular Response tonometer (ORA).[6]

Applanation tonometry, especially contact tonometers like GAT and Perkins that involve direct applanation with the cornea, should be avoided or limited to select cases in the presence of eyes with abrasion, severe trauma and/or globe rupture, signs of infection, and intolerance to fluorescein and/or local anesthesia drops.

Goldmann applanation tonometry (GAT), which is the gold standard method in measuring IOP,  is usually performed by the ophthalmologist and optometrist. Other applanation methods that do not require local anesthesia and fluorescein can also be carried out by nurses, ophthalmic technicians, or emergency medicine. Proper training and communication are needed in tonometry when managing patients, especially considering that accurate IOP measurements at diagnosis and follow-up examinations are of utmost importance in determining therapeutic strategies. GAT needs to be mounted on a slit lamp and tonometry performed on patients in an upright position. Topical anesthesia and fluorescein are used to obtain accurate measurements. Alternative methods can be considered when GAT may prove to be less suitable, which include: open globe wounds; patients that do not tolerate anesthesia drops or fluorescein; eyes with keratoconjunctivitis or central corneal scarring, elevated irregular astigmatism, unhealed corneal abrasions, and/or corneal ulcers. Alternative tonometers may be used in bedridden and non-collaborative. Tonometry readings should be accurate and precise, so choosing the correct instrument to measure IOP when managing patients is crucial. Glaucoma and other ophthalmic conditions can give rise to irreversible vision loss and reduced peripheral vision, which is why IOP needs to be properly assessed at periodic examinations to provide thorough management of patients.[45] [Level 1]

The nursing, allied health, and interprofessional staff help maintain tonometers. The nursing staff is also aware of the calibration and correct method of storage of the tonometers. They also help in the disposal and replacement of the prism of the tonometer.