IOP and Tonometry

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Article initiated by:
Jody Piltz-Seymour, M.D., Tak Yee Tania Tai, MD
All contributors:
Sarwat Salim MD, FACS, Jody Piltz-Seymour, M.D., Ahmad A. Aref, MD, MBA, Oscar D. Albis-Donado, MD, Qi N. Cui, MD PhD, Vivian Qin, Érico B. P. P. Sant'Anna, MD
Assigned editor:
Qi N. Cui, MD PhD
Review:
Assigned status Update Pending
 by Qi N. Cui, MD PhD on June 22, 2022.


Intraocular Pressure

Definition

The intraocular pressure (IOP) of the eye is the fluid pressure inside the anterior chamber. It 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 modified Goldmann equation is a mathematical formula that seeks to explain the relationship between the IOP and the dynamics of aqueous humor production and outflow. It states:

P₀ = [(Fₐ - Fᵤ)/Cₜ] + EVP

P₀ is the IOP in millimeters of mercury (mmHg), Fₐ is the rate of aqueous flow, Fᵤ is the rate of uveoscleral flow, Cₜ is the facility of outflow, and EVP is the episcleral venous pressure.[1][2]

Clinical Significance and Demographics

An association between increased IOP and the loss of sight in glaucoma has been noted for many centuries. In the 17th century, English physician Richard Bannister noticed the hardness of eyes in cases where cataract operations did not improve vision. In the 19th century, English ophthalmologist William Bowman developed a method of estimating the tension, or hardness, of the eye by palpating it with his fingers through the closed eyelid (see #Tactile Tension). 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.[3]

Although IOP has been the primary focus in the diagnosis and treatment of glaucoma for many years, there is no definite cut-off point which defines a IOP high enough to cause glaucoma. A large 1958 population study by Leydhecker et al deduced a mean IOP in individuals of European ancestry of 15.5 mmHg, with a standard deviation of 2.57 mmHg. Later studies have generally agreed with the findings of this study.[4] Traditionally, a pressure above 21 mmHg was used to separate patients at risk (roughly 2% of the population or 2 standard deviations above the mean).[5]

The approach of considering all IOPs above 21 mmHg as abnormal has since been questioned to take into account the fact that glaucoma is a complex, multifactorial disease. There are patients with high IOP but no nerve damage nor the visual field loss associated with glaucoma, a state known as Ocular Hypertension;[6] there too are patients with normal IOP yet also all the clinical features of glaucoma, in what is known as Normal Tension Glaucoma.[7]

The Ocular Hypertension Treatment Study 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.[8]

As the risk factors for glaucoma continued to be explored further, IOP remains at present the only significantly modifiable risk factor in the treatment of glaucoma. Treatment is initiated in eyes that have developed glaucomatous optic nerve damage and/or visual field loss, or in eyes at significant risk for developing glaucoma. IOP is then lowered to a "target level" determined by many factors including baseline level of IOP, extent of damage, rate of prior change, other relatively static risk factors, life expectancy, medical history, and family history. Target IOP should be constantly re-evaluated to ensure stability of the optic nerve and visual field and to ultimately preserve patient’s visual function.[7]

Production of Aqueous Humor

Aqueous humor is produced by nonpigmented epithelial cells in the ciliary processes at a rate of 2–3 µL/min when one is awake and roughly half that amount during sleep.

Aqueous humor production is made possible by three processes:

  • active secretion (energy-spending movement of ions against gradient),
  • ultrafiltration (hydrostatic pressure being greater than the oncotic gradient favoring flow into the eye), and
  • simple diffusion (passive movement of ions).[2]

Trauma, surgery, inflammation, and various classes of systemic and topical drugs (such as carbonic anhydrase inhibitors and β₂-receptor blockers) may suppress the production of aqueous humor. Carotid occlusive disease may also play a role in reducing aqueous humor production. Increased body temperatures raise the production of aqueous humor.[2]

Aqueous Humor Outflow

There are two main means of aqueous humor outflow: through the trabecular meshwork or through the uveoscleral pathway. Aqueous humor outflow can be measured through fluorophotometry, a visual exam which measures the rate of decrease in fluorescein concentration in the eye after topical application. This is useful as a tool to infer a measurement of aqueous humor production (equal to the rate of outflow), as direct non-invasive measurement of the rate of production is impossible.[9]

The outflow resistance, the mathematical inverse of the facility of outflow (Cₜ), averages between 0.22 and 0.30 µL/min/mmHg in healthy eyes. It may be measured through tonography, in which a weighted indentation tonometer is used to acutely increase IOP and the rate at which pressure declines over time afterwards is measured. Tonography is a rarely used clinical tool, being usually restricted to research.[10]

Trabecular Meshwork Outflow

The iridial angle is important in trabecular meshwork outflow. Drawing from the AAO Image Collection.

The conventional pathway of aqueous humor drainage is through the trabecular meshwork, located in the iridial angle near the ciliary body. This is a pressure-sensitive pathway, meaning alterations in IOP change the dynamic of the pathway.[2]

After leaving the anterior chamber through the trabecular meshwork, the aqueous humor reaches Schlemm's canal, a lymphatic-like vessel with a complex system of collector channels. These drain into the aqueous veins (or first into the intrascleral plexus, which then leads to the aqueous veins). From there, the aqueous humor returns to the bloodstream through the episcleral venous system and the superior ophthalmic veins, which drain into the cavernous sinus.[2]

A healthy eye has 50% of its outflow directed to the trabecular meshwork. This rate is reduced when there is outflow resistance, most of which is associated with the juxtacanalicular meshwork, a layer adjacent to Schlemm's canal. Older adults have less trabecular meshwork cells and a thicker basal membrane, which may lead to higher IOP over time.[2]

The pressure in the episcleral veins, known as episcleral venous pressure (EVP), represents the lowest possible IOP in a healthy and intact eye, averaging around 6 to 9 mmHg in healthy eyes. It can be measured using venomanometry, an exam in which a transparent membrane is placed over the sclera and pressure increased until the first episcleral vein is seen to collapse.[2]

Numerous physiological and pathological factors, as well as medical interventions, alter EVP. EVP increases with vascular malformations and obstructions, such as in Sturge–Weber Syndrome, carotid-cavernous sinus fistulas, cavernous sinus thrombosis. Per the Goldmann equation, a greater EVP leads to a greater IOP.[2]

Uveoscleral Outflow

Any other means of physiological outflow are termed uveoscleral or unconventional outflow. This outflow is pressure-insensitive, meaning changes in pressure do not alter its rate, although its maintenance still requires a constant gradient of pressure in the eye. The rate of uveoscleral outflow cannot be non-invasively measured. It can, however, be inferred through the Goldmann equation.[11]

The uveoscleral pathway leads from the anterior chamber through interstitial spaces between ciliary muscle bundles into the supraciliary and suprachoroidal spaces. It is theorized that the pathway may continue through the sclera, the vortex veins, or lymphatic vessels.[11]

Cycloplegics, adrenergic agents, prostaglandin analogues, and some surgical interventions like suprachoroidal stents and cyclodialysis clefts increase uveoscleral outflow rate. Miotics are associated with its reduction.[11][2]

Physiological Factors Affecting IOP

Genetics is known to be correlated with IOP, as glaucoma has a strong hereditary influence (see The Genetics of Glaucoma).[4]

Increases in the EVP acutely increase the IOP, as predicted by the Goldmann equation. The EVP is increased by changes in body position, particularly when an individual is recumbent rather than upright. Straining and Valsalva maneuvers also increase EVP, a relevant factor when examining anxious patients, who must be comfortable during examination and encouraged to breathe for accurate IOP measuring. Other factors that may increase eye pressure include repeated eyelid squeezing, blinking, or rubbing, which can raise IOP to as high as 90 mmHg.[12]

Aerobic exercises are known to lower IOP, as is pregnancy.[2] Exposure to cold air reduces IOP (apparently through EVP reduction).[4]

Non-glaucomatous individuals have a 2 to 6 mmHg circadian fluctuation in IOP. Most people have peaks in pressure during sleep or in the early morning, especially just after waking up. This physiological difference in IOP along the day, along with the established association between IOP fluctuation and glaucoma progression, has led to interest in measuring IOP outside of usual office hours as a means to investigate glaucoma in patients with apparently normal IOP (see #Continuous IOP Monitoring).[7]

IOP Alteration as a Side Effect of Systemic Drugs

Anticholinergics and topiramate must be avoided in glaucoma patients, as they are known to induce angle closure.[13] Prolonged use of systemic corticosteroids is also known to increase IOP, causing a condition known as steroid-induced glaucoma.[14]

Exposure to tobacco smoke briefly increases IOP, although a link between chronic smoking and glaucoma is not evident. Lysergic acid diethylamide (LSD) is also responsible for increases in IOP.[4]

Anesthetic drugs, especially depolarizing muscle relaxants such as succinylcholine and inhalational anesthetics, decrease IOP. Alcohol and cannabis (see also Cannabinoids for Glaucoma) both lead to a transitory decrease in IOP, but this has not been successfully explored clinically due the short duration of action and undesirable side effects.[15]

Tonometry

Tonometry is the non-invasive measurement of IOP. There are numerous methods of tonometry, although none are able to achieve consistent and accurate results in all patients. For an in-depth analysis of the drawbacks of each type of tonometer, see The Reliability of Intraocular Pressure Measurements.

Tactile Tension

Palpation of the eyelid using the examiner's fingers was the earliest developed method of estimating IOP, based off previous observations that hardened eyes were correlated to the progression of glaucoma.[16] In 1862, William Bowman first described the technique, in which a patient looking down or with eyes closed "as if asleep" has their eyelid gently pressed by two of the examiner's fingers, who will then subjectively attempt to ascertain the degree of tension. Bowman further distinguished various degrees of tension, ranging from T3 (third degree, or extreme tension) to negative degrees of tension, which he commented were less easy to define.[3]

Studies comparing tactile assessment of tension to Goldmann and Tono-Pen tonometers have suggested general inaccuracy even in experienced examiners, but it has been shown to be a useful warning tool in identifying large excursions from the norm (particularly IOPs over 30 mmHg) when proper equipment is not readily available.[17] Tactile tension remains in use to assess IOP in eyes with keratoprostheses, as examination both applanation and indentation instruments is impossible.[18]

Schiøtz Indentation Tonometry

Indentation tonometry attempts to measure the IOP by evaluating the size of the indentation on the cornea produced by a fixed weight. Its principle is that the weight will sink into a soft eye further than into a hard eye. The Schiøtz tonometer, introduced in 1905 by Norwegian ophthalmologist Hjalmar Schiøtz, 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.[19]

The main sources of error in indentation tonometry are the false assumption that all eyes respond the same way to an external force and the expulsion of intraocular blood during examination. Nevertheless, Schiøtz's tonometer is relatively cheap and portable, which made it widely adopted before the development of the Goldmann tonometer. It still remains in use in many regions, being manufactured in China, Pakistan, and India.[19] A 2016 study has shown a relatively accurate correlation between Schiøtz and other "gold standard" tonometers.[20]

Applanation Tonometry

Applanation tonometry is a type of exam 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. The measured IOP is not substantially affected by ocular rigidity, an advantage over indentation methods.[2]

Goldmann Applanation Tonometer

The Goldmann applanation tonometer as seen from the side.

The Goldmann applanation tonometer (GAT), created in 1954 by Austrian-Swiss ophthalmologist Hans Goldmann, is the most commonly used type of tonometer, as it is safe and relatively accurate in most situations.

Goldmann's device consists of a small, plastic, optically clear truncated cone (prism) with its larger base attached to a metal holder, itself held upright by a metal arm. The rod extends inside the tonometer's housing, where a spring weight applies variable forward force to the tonometer's arm. The amount of force can be altered by a adjustment knob on the sides of the housing, with numbers corresponding to weight in grams. The entire apparatus is usually mounted to a Slit Lamp Biomicroscope, allowing for the examination of an upright patient.[21][22]

The prism in this tonometer is used to flatten a corneal area with a diameter of 3.06mm. This area was chosen by Goldmann due to its property of canceling out the opposing forces of the tear surface tension (pulling the tonometer outwards) and corneal elasticity (pulling the tonometer inwards), as well as conveniently allowing for the flattening force in grams multiplied by 10 being equal to the IOP in mmHg.[22]

Examination Technique

Fluorescein dye and topical anesthetic are placed in the patient's eye before examination. The tonometer is moved so that the prism is facing straight towards the patient. The tip of the prism is illuminated by a wide cobalt blue light beam to highlight the tear film. The patient must be relaxed, free of tight collars and neckties (which may increase IOP) and ideally seated in an upright position 5 minutes before starting the measurement. Once the patient is positioned in the slit-lamp, they are asked to look at a fixed distant target.[22][12]

The examiner will then slowly move the joystick forwards in order for the prism to slightly touch the cornea. If it is necessary to manually hold the eyelids open, this must be done without applying pressure to the globe. The split-image prism divides the image of the highlighted tear meniscus into superior and inferior semicircles or mires. The intraocular pressure is taken through the reading on the knob when these semicircles are of the same size and shape and aligned so that their inner margins just touch.[22][12]

Care must be taken during examination to avoid large semicircles in the tear film (excessive fluid), as the larger meniscus lowers the surface tension and leads to a falsely high measured IOP. In this case, the tonometer should be withdrawn and the prism dried with a cotton wool swab. An opposite situation, with a falsely low IOP, occurs when there are very thin semicircles (insufficient fluorescein and/or drying of the lacrimal film). Goldmann's original description included fluorescein rings of approximately one-third the radius of the fluorescein-free central zone, a size noticeably larger than the one suggested by current manufacturer manuals.[12][23]

Image seen during examination[23] Interpretation Correction
GoldmannCorrectFinalPosition.jpg Correct final position. No correction required.
GoldmannRingTooThick.jpg Fluorescein ring too thick. Withdraw tonometer and dry prism with cotton wool swab.
GoldmannRingTooThin.jpg Fluorescein ring too thin. Ask the patient to blink and repeat; if necessary, apply more fluorescein.
GoldmannImproperAlignment.jpg Incorrect vertical and horizontal alignment.
The tip is shifted down and to the right of the expected position.		 Use the slit-lamp joystick to center the tonometer (move up and to the left).
GoldmannTooMuchPressure.jpg Too much pressure applied. Use the dial to decrease the pressure.
GoldmannTooLittlePressure.jpg Too little pressure applied. Use the dial to increase the pressure.

Eyes with marked corneal astigmatism (more than three diopters) may produce elliptical images when observing the image through the prism. In these cases, to avoid inaccurate readings, the prism can be rotated so as to align the red mark on the metal holder along the least-curved axis of the cornea.[12]

Repeated applanation over a short period, such as by inexperienced examiners, may artificially reduce IOP, in what is known as the "tonographic effect". This has been variously ascribed to the mechanical expulsion of aqueous humor from the anterior chamber, a reflectory change in aqueous formation, and the use of topical anesthetics.[24]

Corneal Biomechanics and Measurement Error

Changes in corneal biomechanics lead to measurement errors during applanation tonometry. Central corneal thickness (CCT) is an important source of error, as Goldmann designed his tonometer for corneas with a thickness of 520 microns. 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 (such as after keratorefractive surgery) can give an artificially low reading. Corneal ectasia or other irregularities in curvature tend to result in a higher measurement. Corneal scars may falsely increase the measured IOP, while corneas with decreased rigidity of the cornea, such as in corneal edema, will produce artificially low readings. Any measurements performed over a contact lens will falsely lower the IOP.[22]

Cleaning and Calibration

A damaged Goldmann tonometer prism as seen under the slit-lamp microscope. Cracks in the prism can harbor microorganisms and cause damage to the patient's cornea during examination.

A 2017 literature review found reusable tonometer tips were most commonly disinfected with alcohol wipes, chlorine, and hydrogen peroxide, all of which damage the prism over time, producing cracks which may harbor microorganisms or produce corneal abrasions. The review ultimately suggested soaking the prism in 1:10 dilute bleach for 5 minutes then rinsing with water and drying for high-level disinfection against the most epidemiologically relevant microbes. This must be coupled with examination of the tip under the slit-lamp to check for damage before every use.[25]

Tonometer manufacturer Haag-Streit suggests discarding reusable tips 2 years after the first use. There have been no reported cases of transmission of prion disease such as Creuzfeldt-Jakob through applanation tonometry; when examining these rare cases, disposable tonometer covers or single-use tonometers should be used.[26]

Guidelines from the Asia-Pacific Glaucoma Society[27] and the World Glaucoma Association[28] suggest calibrating Goldmann tonometers at least twice yearly. A 2016 study found that older tonometers were prone to shorter intervals between the appearance of significant calibration error, which led the authors to suggest checking GATs older than a year for error at least monthly.[29]

Goldmann tonometers are checked for calibration error at dial positions for 0, 2, and 6 grams (respectively 0, 20, and 60 mmHg) using an adjusted check weight or calibration rod usually provided by the manufacturer.[30] 85% of ophthalmology residents reported never checking their Goldmann tonometers for calibration error.[26]

Perkins Tonometer

The Perkins tonometer is a portable version of the Goldmann tonometer that does not require mounting to a slit-lamp. It has the abililty to examine the patient in either the upright or supine positions, although the IOP is increased in supine positions (see #Physiological Factors Affecting IOP), as well as any patients unable to be examined under the slit-lamp, such as small children. It includes a forehead rest which can be used to minimize movement during the examination.[31][23]

Tono-Pen

Tono-Pen in use.

The Tono-Pen, an evolution of the older Mackay-Marg tonometer, involves both applanation and indentation processes. It is a small, handheld, battery-powered electronic device. The tonometer has an applanating footplate with a tiny plunger protruding minimally from the center. After application of topical anesthetic, the tonometer is used to repeatedly and gently tap a small area of the cornea, receiving resistance from it and 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, which is then recorded electronically. Multiple readings are averaged and, because the area of applanation is known, the IOP can then be calculated.[4]

The readings correlate well with Goldmann tonometry within normal IOP ranges.[22][32][33] Tono-Pen has the advantage of having a contact surface small enough to avoid areas of corneal scarring or edema. A main disadvantage, however, is that it tends to overestimate low IOPs and underestimate high IOPs.[34]

Pneumotonometry

The pneumatic tonometer or 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 mmHg. The readings correlate well with Goldmann applanation tonometry within normal IOP ranges.[32]

Dynamic Contour Tonometry

The PASCAL DCT in use. Image by Dr. Anton C. Wirthlin Laubisruetistr.

The PASCAL Dynamic Contour Tonometer (PDCT; Zeimer Ophthalmic Systems AG, Port, Switzerland) is a marketed version of a device developed in 2005 by Kanngiesser et al. as an attempt to reduce the bias to the biomechanical variations of the cornea associated with the Goldmann tonometer. The PDCT is so-called as the curvature of the tip closely resembles that of the cornea rather than being flat. It uses a piezoelectric sensor embedded in the tip of the tonometer to measure the dynamic pulsatile fluctuations in the tear film in order to estimate IOP. It seems to be the most accurate method to measure 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.[35]

Noncontact Tonometry

In noncontact or air puff tonometry, the indentating 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, being usually used in large-scale public health programs. A minimum of 3 readings should be averaged to estimate the mean IOP as IOP varies during the cardiac cycle.[24]

Ocular Response Analyzer

The Ocular Response Analyzer (ORA) is a type of non-contact tonometer that uses mathematical algorithms to attempt to correct the applanation point for high or low corneal hysteresis (elasticity). The ORA 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 "corrected" IOP, called corneal compensated IOP (IOPcc), is thought to be less dependent on corneal thickness than other forms of applanated pressures and to more strongly correlate to glaucoma progression than Goldmann or rebound tonometry. Reduced corneal hysteresis has also been associated with glaucoma progression.[2][22]

Corvis ST

Corneal Visualisation Scheimpflug Technology or Corvis ST (Oculus) is a non-contact tonometer that includes a ultra-high speed Scheimpflug camera to record the minute, dynamic deformations of the cornea as it is indented. This allows for the assessment of various biomechanical properties of the cornea and pachymetry at the same time as the IOP, producing a biomechanical corrected IOP (bIOP).[31][36]

Rebound Tonometry (iCare)

Rebound tonometry measures the deceleration of a small probe after being thrown into the cornea as a means to estimate IOP. The iCare device (created in 1997 in Helsinki, Finland) is a handheld battery-powered rebound tonometer in which a 1.8mm diameter plastic ball on a stainless steel wire is held in place by an electromagnetic field. 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. This process is quickly repeated 6 times for an average IOP.[22]

No anesthetic is necessary, as the ball touches the cornea only for microseconds. It shows good agreement with Goldmann and Tono-Pen readings, is comfortable, and very easy to use.[37] iCare IC100, a 2nd generation model, has a light at the probe base to assist in correct positioning of the device (red indicating incorrect alignment, green indicating correct alignment). The iCare pins are meant to be single-use, which adds to the overall cost of examination but makes disinfection a non-issue.[22]

IOP measurements obtained with this tonometer are also influenced by CCT, with an even greater tendency of overestimation in thicker corneas when compared with Goldmann and Tono-Pen. Other biomechanical properties of the cornea also alter measurements, including corneal hysteresis and corneal resistance factor.[38][39][40]

Continuous IOP Monitoring

Advances have been made to develop techniques that can monitor IOP beyond the in-office measurements, ensuring adequate tracking of IOP fluctuations. Early animal studies have investigated technologies for permanent IOP monitoring, including the surgical implantation of a pressure transducer system (implantable pressure-sensing bioMEMS; biomechanical microeletromechanical system), as well as the implantation of an intraocular sensor into the lens capsule. The main drawbacks of these strategies are the surgical risks.[31]

The contact lens-based sensor (CLBS) is the main minimally invasive approach to continuous IOP monitoring. The CLBS measures changes in ocular dimensions over a 24-hour period, which has shown acceptable correlation with true IOP.[41] 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.[31]

Additional Resources

References

  1. ↑ Brubaker RF. Goldmann’s equation and clinical measures of aqueous dynamics. Experimental Eye Research. 2003;78(3):633-637. doi:10.1016/j.exer.2003.07.002
  2. ↑ 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 2.11 Chapter 2: Intraocular Pressure and Aqueous Humor Dynamics. In: Boland MV, Giaconi JA, Lee RK, Lin SC, Muir KW, Sood S, Welsbie DS, eds. 2025-2026 Basic and Clinical Science Course, Section 10: Glaucoma. American Academy of Ophthalmology; 2025:21-39.
  3. ↑ 3.0 3.1 Bowman W. On Glaucomatous Affections, and Their Treatment by Iridectomy. Lecture presented at the 30th annual meeting of the British Medical Association, London, 1862. In: The Collected Papers of Sir William Bowman, Bart., F.R.S. Vol 2. Harrison and Sons; 1892:291-300.
  4. ↑ 4.0 4.1 4.2 4.3 4.4 Chapter 2: Intraocular Pressure and Tonometry. In: Allingham RR. Shields Textbook of Glaucoma. 7th ed. Wolters Kluwer; 2021.
  5. ↑ Alimuddin M. Normal Intra-ocuar pressure. Br J Ophthalmol 1956; 40(6): 366-72.
  6. ↑ Armaly MF. On the distribution of applanation pressure and arcuate scotoma. In: Patterson G, Miller SJ, Patterson GD, eds. Drug Mechanisms in Glaucoma. Boston, MA: Little, Brown; 1966.
  7. ↑ 7.0 7.1 7.2 Asrani SG, McGlumphy EJ, Al-Aswad LA, et al. The relationship between intraocular pressure and glaucoma: An evolving concept. Progress in Retinal and Eye Research. 2024;103:101303. https://pmc.ncbi.nlm.nih.gov/articles/PMC12556014/ Accessed May 30, 2026.
  8. ↑ Kass MA, Heuer DK, Higgenbotham EJ, et al. The Ocular Hypertension Treatment Study, a randomized trial determines that topical hypotensive medication delays or prevents the onset of primary open-angle glaucoma. Arch Ophthalmol 2002; 120(6): 701-13.
  9. ↑ Toris CB. Uveoscleral Outflow: Current understanding and methods of measurement. Glaucoma Today. 2013;Sep-Oct:36-37. https://assets.bmctoday.net/glaucomatoday/pdfs/gt09113_F_toris.pdf Accessed May 30, 2026.
  10. ↑ Kazemi A et al. Comparison of Aqueous Outflow Facility Measurement by Pneumatonography and Digital Schiøtz Tonography. Investigative Ophthalmology & Visual Science. 2017;58(1):204-210. https://iovs.arvojournals.org/article.aspx?articleid=2598503 Accessed May 30, 2026.
  11. ↑ 11.0 11.1 11.2 Johnson M, McLaren JW, Overby DR. Unconventional aqueous humor outflow: A review. Experimental Eye Research. 2017;158:94-111. https://pmc.ncbi.nlm.nih.gov/articles/PMC4970980/ Accessed May 31, 2026.
  12. ↑ 12.0 12.1 12.2 12.3 12.4 Whitacre MM, Stein R. Sources of error with use of Goldmann-type tonometers. Survey of Ophthalmology. 1993;38(1):1-30. https://doi.org/10.1016/0039-6257(93)90053-A Accessed May 29, 2026.
  13. ↑ Pinto-Gouveia C, Bernardes L, Renca S. Topiramate-Associated Acute Bilateral Angle-Closure Glaucoma. Journal of Clinical Psychopharmacology. 2022;42(3):320-321. https://journals.lww.com/psychopharmacology/citation/2022/05000/topiramate_associated_acute_bilateral.16.aspx Accessed May 30, 2026.
  14. ↑ Roberti G, Oddone F, Agnifili L, et al. Steroid-induced glaucoma: Epidemiology, pathophysiology, and clinical management. Survey of Ophthalmology. 2020;65(4):458-472. https://www.surveyophthalmol.com/article/S0039-6257(20)30024-2/abstract Accessed May 30, 2026.
  15. ↑ Daga FB, Almeida I, Prata TS, Paranhos A Jr. Marijuana for glaucoma treatment: a recipe for disaster. Arquivos Brasileiros de Oftalmologia. 2021;84(2):V-VII. doi:10.5935/0004-2749.20210056
  16. ↑ Leffler CT, Schwartz SG, Hadi TM, Salman A, Vasuki V. The early history of glaucoma: the glaucous eye (800 BC to 1050 AD). Clinical Ophthalmology. 2015;9:207-215. https://www.dovepress.com/the-early-history-of-glaucoma-the-glaucous-eyenbsp800-bc-to-1050-ad-peer-reviewed-fulltext-article-OPTH Accessed May 30, 2026.
  17. ↑ Baum J, Chaturvedi N, Netland PA, Dreyer EB. Assessment of Intraocular Pressure by Palpation. American Journal of Ophthalmology. 1995;119(5):650-651. https://www.ajo.com/article/S0002-9394(14)70227-2/abstract Accessed May 30, 2026.
  18. ↑ Chew HF et al. Boston keratoprosthesis outcomes and complications. Cornea. 2009;28(9):989-996. https://journals.lww.com/corneajrnl/Abstract/2009/10000/Boston_Keratoprosthesis_Outcomes_and_Complications.8.aspx Acessed May 30, 2026.
  19. ↑ 19.0 19.1 Albert D, Keeler R. The Pressure: Before and after Schiøtz. Ophthalmology Glaucoma. 2020;3(6):409-413. https://www.ophthalmologyglaucoma.org/article/S2589-4196(20)30110-1/fulltext Accessed May 29, 2026.
  20. ↑ Nagarajan S, Velayutham V, Ezhumalai G. Comparative evaluation of applanation and indentation tonometers in a community ophthalmology setting in Southern India. Saudi Journal of Ophthalmology. 2016;30(2):83–87. https://www.sciencedirect.com/science/article/pii/S1319453415001174 Accessed May 29, 2026.
  21. ↑ Moses RA. The Goldmann Applanation Tonometer. American Journal of Ophthalmology. 1958;46(6):865-869. https://doi.org/10.1016/0002-9394(58)90998-X. Accessed May 29, 2026.
  22. ↑ 22.0 22.1 22.2 22.3 22.4 22.5 22.6 22.7 22.8 Stamper RL. A History of Intraocular Pressure and Its Measurement. Optometry and Vision Science. 2011;88(1):E16-E28. https://onlinelibrary.wiley.com/doi/abs/10.1097/OPX.0b013e318205a4e7. Accessed May 29, 2026.
  23. ↑ 23.0 23.1 23.2 Haag-Streit UK. Perkins MK3 tonometer: instructions for use. 2026. https://haag-streit.com/Country_specific/HS_UK/Quality-Regulatory/IFUs/CCO_Perkins_ifu_5806001A_en.pdf Accessed May 29, 2026.
  24. ↑ 24.0 24.1 Zimmermann M, Pitz S, Schmidtmann I, Pfeiffer N, Wasielica-Poslednik J. Tonographic Effect of Ocular Response Analyzer in Comparison to Goldmann Applanation Tonometry. PLoS One. 2017;12(1):e0169438. https://pmc.ncbi.nlm.nih.gov/articles/PMC5221819/pdf/pone.0169438.pdf Accessed May 30, 2026.
  25. ↑ Junk AK et al. Disinfection of Tonometers: A Report by the American Academy of Ophthalmology. Ophthalmology. 2017;124(12):1867-1875. https://www.sciencedirect.com/science/article/pii/S0161642017316779 Accessed May 29, 2026.
  26. ↑ 26.0 26.1 Griffin JS. American Academy of Ophthalmology. How to Disinfect and Calibrate Your Goldmann Applanation Tonometer. 2018. https://www.aao.org/eyenet/article/how-to-disinfect-and-calibrate-your-tonometer Accessed May 29, 2026.
  27. ↑ Asia-Pacific Glaucoma Society. Intraocular Pressure Measurement. In: Asia Pacific Glaucoma Guidelines. Kugler; 2024:29-36.
  28. ↑ Garway-Heath T et al. Measurement of Intraocular Pressure. In: WGA Consensus Series 4: Intraocular Pressure. Kugler; 2007. https://wga.one/consensus/consensus-4/ Accessed May 29, 2026.
  29. ↑ Choudhari NS et al. How Often the Goldmann Applanation Tonometer Should be Checked for Calibration Error? Journal of Glaucoma. 2016;25(11):908-913. https://journals.lww.com/glaucomajournal/fulltext/2016/11000/how_often_the_goldmann_applanation_tonometer.8.aspx Accessed May 29, 2026.
  30. ↑ Cordero I. How to verify the calibration of Goldmann tonometers. Community Eye Health Journal. 2012;25(79-80):65. https://cehjournal.org/articles/594/files/65f068f7ba0ee.pdf/ Accessed May 29, 2026.
  31. ↑ 31.0 31.1 31.2 31.3 Nuyen B, Mansouri K. Fundamentals and Advances in Tonometry. Asia-Pacific Journal of Ophthalmology. 2015;4(2):66-75. https://www.sciencedirect.com/science/article/pii/S2162098923005595#cesec110 Accessed May 30, 2026.
  32. ↑ 32.0 32.1 Bhan A, Browning AC, Shah S, Hamilton R, Dave Dinesh, Dua HS. Effect of Corneal Thickness on Intraocular Pressure Measurements with the Pneumotonometer, Goldmann Applanation Tonometer, and Tono-Pen. Investigative Ophthalmology & Visual Science. 2002;43(5):1389-1392. https://iovs.arvojournals.org/article.aspx?articleid=2123185 Accessed May 29, 2026.
  33. ↑ Gupta V, Sony P, Agarwal HC, Sihota R, Sharma A. Inter-instrument agreement and influence of central corneal thickness on measurements with Goldmann, pneumotonometer and noncontact tonometer in glaucomatous eyes. Indian Journal of Ophthalmology. 2006;43(5):1389-1392. https://journals.lww.com/ijo/Fulltext/2006/54040/Inter_instrument_agreement_and_influence_of.8.aspx Accessed May 29, 2026.
  34. ↑ Eisenberg DL, Sherman BG, McKeown CA, Schuman JS. Tonometry in adults and ­children. A manometric evaluation of pneumatonometry, applanation, and TonoPen in vitro and in vivo. Ophthalmology. 1998;105(7):1173–1181.
  35. ↑ Kniestedt C, Lin S, Choe J, Bostrom A, Nee M, Stamper RL. Clinical Comparison of Contour and Applanation Tonometry and Their Relationship to Pachymetry. Archives of Ophthalmological Research. 2005;123(11):1532-1537. https://jamanetwork.com/journals/jamaophthalmology/fullarticle/417336 Accessed May 29, 2026.
  36. ↑ Yang K et al. Evaluation of new Corvis ST parameters in normal, Post-LASIK, Post-LASIK keratectasia and keratoconus eyes. Scientific Reports. 2020;10(1):5676. https://www.nature.com/articles/s41598-020-62825-y Accessed May 30, 2026.
  37. ↑ Pakrou N et al. Clinical comparison of the iCare tonometer and Goldmann applanation tonometry. Journal of Glaucoma. 2008;17(1):43-47.
  38. ↑ Chi WS et al. The influence of corneal properties on rebound tonometry. Ophthalmology. 2008;115:80-84.
  39. ↑ Jorge JMM, González-Méijome JM, Queirós A, Fernandes P, Parafita, MA. Correlations Between Corneal Biomechanical Properties Measured With the Ocular Response Analyzer and ICare Rebound Tonometry. Journal of Glaucoma. 2008;17:442-448. https://journals.lww.com/glaucomajournal/fulltext/2008/09000/correlations_between_corneal_biomechanical.4.aspx Accessed May 29, 2026.
  40. ↑ Poostchi A, Mitchell R, Nicholas S, Purdle G, Wells A. The iCare rebound tonometer: comparisons with Goldmann tonometry, and influence of central corneal thickness. Clinical & Experiment Ophthalmology. 2009;37(7):687-691.
  41. ↑ Monsálvez-Romín D, Martínez-Albert N, García-Domene MC, Ortí-Navarro S. Current and Emerging Technologies for Continuous Intraocular Pressure Monitoring in the Control of Glaucoma Progression: A Scoping Review. Journal of Clinical Medicine. 2025;14(24):8795. https://www.mdpi.com/2077-0383/14/24/8795 Accessed May 30, 2026.
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