1 day ago

david 4.3k

@david_fe

New Podcast Episode

In this episode, we explore the evolving understanding of intraocular pressure (IOP) and its critical relationship to glaucoma, drawing insights from the 2024 academic article by Dr. Sanjay Asrani and colleagues, "The Relationship Between Intraocular Pressure and Glaucoma: An Evolving Concept."

We discuss the key factors influencing IOP, innovations in measurement technologies, and the limitations of current monitoring approaches. The conversation highlights the significance of IOP fluctuations—especially those occurring outside clinic hours—and their independent role in glaucoma progression. Advocating for home IOP monitoring, we examine how this approach could revolutionize glaucoma management by improving patient outcomes and addressing health disparities.

Finally, we emphasize the need for further research to refine IOP monitoring protocols and deepen our understanding of its dynamic role in glaucoma. This episode offers a forward-looking perspective on how advancements in IOP monitoring could transform care for glaucoma patients worldwide.

Find other glaucoma and eye health podcast episodes here: https://rss.com/podcasts/fiteyes-eye-health-nutrition-glaucoma/

Note: This discussion is AI-generated based on peer-reviewed research studies. This version enhances clarity, readability, and engagement while maintaining a professional tone. It also emphasizes key themes and provides a clear structure for the audience.

Disclaimer:

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The opinions expressed by the hosts and guests are their own and do not necessarily reflect the views of the podcast producers or this channel. While we strive to provide accurate and up-to-date information, we cannot guarantee the completeness or accuracy of the content discussed. We provided peer reviewed research to the podcast producers, but we have no control over what the hosts say.

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1 day ago

david 4.3k

@david_fe

Intraocular Pressure and Glaucoma: Glossary of Key Terms

• Aqueous Humor: The clear fluid that fills the anterior and posterior chambers of the eye, providing nutrients and maintaining intraocular pressure.
• Applanation Tonometry: A method of measuring intraocular pressure by flattening a known area of the cornea using a measured force.
• Central Corneal Thickness (CCT): The thickness of the cornea at its center. CCT influences IOP readings; thicker corneas lead to overestimations, while thinner corneas result in underestimations.
• Circadian Rhythm: The body's natural 24-hour cycle, including physiological changes in hormone levels, metabolism, and intraocular pressure.
• Corneal Hysteresis: A measure of the cornea’s viscous damping behavior, or its ability to absorb and dissipate energy, which affects IOP readings.
• Corneal Resistance Factor (CRF): A measure of corneal resistance to deformation, also impacting IOP readings.
• Cupping/Excavation: The posterior deformation and enlargement of the optic nerve head, characteristic of glaucoma.
• Diurnal Variation: Fluctuations in IOP that occur over the course of a day.
• Episcleral Venous Pressure (EVP): The pressure in the veins of the sclera, which affects outflow and contributes to intraocular pressure.
• Glaucoma: A progressive neurodegenerative optic neuropathy leading to retinal ganglion cell death and vision loss.
• Goldmann Applanation Tonometry (GAT): A standard applanation tonometry method used for measuring IOP and generally considered to be the gold standard.
• Imbert-Fick Principle: The underlying principle of applanation tonometry, stating that the pressure within a sphere is equivalent to the counterpressure needed to flatten the sphere's surface.
• Indentation Tonometry: A method of measuring intraocular pressure by measuring the depth to which a plunger can indent the globe with a set force.
• Intraocular Pressure (IOP): The pressure inside the eye, determined by the balance between aqueous humor production and outflow, and a key risk factor for glaucoma.
• Juxtacanalicular Tissue: The region of the trabecular meshwork located near Schlemm’s canal, considered the main site of outflow resistance.
• Lamina Cribrosa: A mesh-like structure in the scleral portion of the optic nerve head through which the axons of the retinal ganglion cells pass.
• Neuroretinal Rim: The border of neural tissue in the optic nerve head, often reduced or damaged in glaucoma.
• Notching: Focal loss of the neuroretinal rim tissue in the optic nerve head, commonly seen in glaucoma.
• Ocular Hypertension (OHT): Elevated intraocular pressure without visual field loss or optic nerve damage.
• Ocular Response Analyzer (ORA): A non-contact tonometer that measures corneal hysteresis and calculates a corneal-compensated IOP.
• Optic Neuropathy: Damage to the optic nerve that can result in vision loss, as in glaucoma.
• Pneumotonometry: A method of measuring intraocular pressure by using an air-powered probe to flatten the cornea.
• Retinal Ganglion Cells (RGCs): Neurons in the retina that transmit visual information to the brain; these cells are damaged in glaucoma.
• Rebound Tonometry: A method of measuring IOP by bouncing a probe off the cornea and measuring deceleration.
• Schiotz Tonometry: An indentation tonometry method using a weighted plunger to depress the cornea.
• Schlemm’s Canal: A lymphatic-like channel in the sclera that drains aqueous humor from the eye.
• Steroid Response: The tendency of some individuals to have increased IOP when taking steroid medications.
• Trabecular Meshwork (TM): A sieve-like network of tissue responsible for draining aqueous humor from the eye.
• Tono-Pen: A handheld tonometer that uses a small plunger to measure IOP using principles of both applanation and indentation tonometry.
• Valsalva Maneuver: Exhaling forcefully against a closed airway (e.g., breath-holding), which increases intrathoracic pressure and temporarily raises IOP.
• Water Drinking Test: A provocative test in which drinking a large volume of water is used to evaluate the eye's aqueous outflow by observing a change in IOP.

2 days ago

david 4.3k

@david_fe

Briefing Document: Intraocular Pressure and Glaucoma

1. Introduction: Glaucoma and IOP

• Definition of Glaucoma: Glaucoma is defined as a progressive neurodegenerative optic neuropathy of multifactorial etiology, leading to the death of retinal ganglion cells (RGCs).
• "Glaucoma is a progressive, neurodegenerative optic neuropathy of multifactorial etiology which results in the death of retinal ganglion cells (RGCs)."
• IOP as a Risk Factor: Intraocular pressure (IOP) is identified as the most important modifiable risk factor for glaucoma.
• "Intraocular pressure (IOP) is the most important identifi-able and, presently, the only modifiable risk factor for glaucoma."
• Pathophysiology: Elevated IOP causes axonal injury to RGCs at the optic nerve head. Glaucoma causes biomechanical remodeling of the lamina cribrosa, leading to optic nerve head changes such as cupping, excavation and notching.
• "IOP stress initiates an RGC axonal injury at the level of the optic nerve head."
• "Glaucoma differentiates itself from other optic neuropathies by inducing biomechanical remodeling of the lamina cribrosa leading to a characteristic optic nerve head appearance."
• Structural Changes: Glaucoma is associated with thinning of the retinal nerve fiber layer (RNFL), RGC layer, and inner plexiform layer, detectable by optical coherence tomography (OCT). These structural changes occur alongside characteristic patterns of vision loss.

2. Importance of IOP Reduction

• Landmark Clinical Trials: Several clinical trials have shown that reducing IOP can prevent or delay glaucomatous damage.
• Ocular Hypertension Treatment Study (OHTS): Demonstrated that a 20% reduction in IOP lowered the risk of developing glaucoma by over half (from 9.5% to 4.4%).
• "Over a period of 5 years, lowering of intraocular pressure by 20% from baseline reduced the risk of developing glaucoma by more than half, from 9.5% to 4.4% (Kass et al., 2002)."
• Early Manifest Glaucoma Trial (EMGT): Showed a significant delay in glaucoma progression with an average 25% IOP reduction from pretreatment levels.
• "a significant delay in progression was noted in patients who were treated and who experienced an average of 25% reduction in pretreatment IOP compared (Heijl et al., 2002)."
• Collaborative Initial Glaucoma Treatment Study (CIGTS): Found that aggressive IOP lowering (≥35%) reduced the likelihood of further visual field loss in patients with more advanced disease.
• "aggressive IOP lowering (≥35%) reduced the likelihood of further visual field loss in patients with more advanced disease (Musch et al., 2009a)."
• Advanced Glaucoma Intervention Study (AGIS): Eyes with average IOP ≤ 14 mmHg experienced less visual field progression compared to eyes with higher average IOPs.
• "Eyes with an average IOP ≤14 mmHg experienced less visual field progression compared with eyes with an average IOP ≥18 mmHg."
• United Kingdom Glaucoma Treatment Study (UKGTS): Treatment with a prostaglandin analog lowered IOP and prevented visual field worsening in newly diagnosed glaucoma patients.

3. Aqueous Humor Dynamics and IOP

• Balance: IOP is determined by a balance between aqueous production and outflow.
• Outflow Pathway: Aqueous humor drains through the trabecular meshwork (TM), Schlemm’s canal, and collector channels.
• Resistance to Outflow: Most resistance to outflow (approximately 75%) occurs at the intersection of the juxtacanalicular tissue and Schlemm’s canal.
• "Most (approximately 75%) of the resistance to outflow lies in the region where the juxtacanalicular and Schlemm’s canal intersect. (Ethier, 2002; Johnson, 2006)."
• Schlemm's Canal: The inner wall of Schlemm’s canal is critical for fluid transport via cellular outpouchings or giant vacuoles and pores.
• "The inner wall of Schlemm’s canal has significant hydraulic conductivity, with the mechanical force causing the cells to push off of the basal lamina to form cellular outpouchings or giant vacuoles and pores—a process critical to fluid transport (Johnson et al., 2000)."

4. Evolution of Tonometry

• Initial Methods: IOP measurement began with digital palpation.
• "Estimation of IOP was first achieved by digital palpation of the globe"
• "The first recognition of elevated IOP as measured by palpation, was described in the 10th century by oculist Ibn Isa (Jesu Hali)"
• Palpation Limitations: Palpation is unreliable and imprecise but can be used to detect large deviations from normal, when advanced equipment isn't available.
• "Measurement of IOP via palpation lacks precision but may have a limited role in iden-tifying large excursions from the norm in situations where ophthalmic equipment is not readily available."
• Schiotz Tonometry: An indentation method using a weighted plunger, was the first gold standard prior to applanation techniques.
• Goldmann Applanation Tonometry (GAT): Based on the Imbert-Fick principle, GAT is the current gold standard due to its accuracy, precision, and low variability.
• "GAT was first invented in 1948 and is still considered the gold standard for IOP measurement due to its proven accuracy, precision, and low intra- and inter-observer variability"
• GAT Limitations:Corneal Factors: Corneal thickness, astigmatism, and hydration affect GAT measurements. Thicker corneas may overestimate IOP, and thinner corneas may underestimate IOP. A range of 0.11-0.71 mmHg increase in measured IOP for every 10μm increase in corneal thickness has been reported.
• "In human eyes with average corneal thickness, tonometer and hydrostatic pressure in the eye coincide, however in thicker corneas the readings are higher and in thinner corneas they are lower than the actual hydrostatic pressure in the eye."
• "A range of 0.11–0.71 mmHg increase in measured IOP for every 10 μm increase in corneal thickness has been reported"
• Patient Factors: Fluorescein concentration, elevated episcleral venous pressure (EVP), and patient factors like accommodation and large shifts in vertical gaze can influence GAT readings.
• Disease States: Corneal transplantation, keratoconus, and other corneal disorders impact GAT accuracy.
• Tono-Pen: A portable device using applanation and indentation, useful in patients with corneal irregularities, though less reliable at high IOP.
• "The Tono-Pen often measures lower than GAT for IOP values > 20 mmHg, and demonstrates significant error with IOP >30 mmHg, making it a less reliable and precise tool in patients with elevated IOP"
• Pneumotonometry: A form of tonometry using a probe and air pressure, that correlates well with GAT within normal ranges of IOP but may overestimates IOP.
• Non-Contact Tonometry: Uses air puff to assess IOP without anesthesia.
• Air-Puff Tonometry: Tends to report higher IOP values than GAT, is influenced by corneal hysteresis and corneal resistance factor, and should be confirmed with GAT before referral.
• "Measurements of IOP with an air puff tonometer are usually reported as higher than those obtained by GAT and rebound tonometry"
• Ocular Response Analyzer (ORA): A newer form of air-puff tonometry that accounts for corneal biomechanics, providing "Goldmann-corrected IOP" (IOPg) and "corneal-compensated IOP" (IOPcc).
• "The ORA also calculates a corneal-compensated IOP, IOPcc, which is a value of IOP calculated by incorporating the biomechanical properties of the cornea (elasticity and viscosity)."
• Corvis ST: A Scheimpflug-based non-contact tonometer that measures corneal deformation in real-time and provides a biomechanical IOP (bIOP).
• Diaton: A tonometer based on rebound principles, using a metal rod to measure IOP through the eyelid; however, this device does not correlate well with GAT and is not recommended as a replacement.
• Contact Lens Sensors (Triggerfish): Measures changes in corneal curvature to estimate IOP, primarily useful for monitoring trends and diurnal rhythms, rather than providing exact IOP values.
• "This technology is best utilized for monitoring IOP trends or diurnal/circadian rhythms rather than deriving exact IOP values"
• Implantable IOP Sensors (EYEMATE-IO, EYEMATE-SC): These sensors, placed in the ciliary sulcus or suprachoroidal space, respectively, offer continuous IOP monitoring.

5. IOP Fluctuations

• Dynamic Nature: IOP is dynamic and fluctuates over various timescales.
• Types of Fluctuations: Sources of fluctuation include interobserver differences, device calibration, biological factors, and more.
• Variability vs. Fluctuation: Variability refers to differences across a population, while fluctuation refers to changes within individuals over time.
• "Whereas the term variability has a precise statistical definition and generally refers to differences in measurements obtained across a range of independent samples within a population, the term fluctuation refers to differences in a measured parameter within individuals over time"
• Timescales:Hyperacute Fluctuations: Transitory spikes (≥20 mmHg) during blinking and eye movements, not considered clinically relevant.
• Short-Term Fluctuations: Occur over minutes to hours, are considered to be clinically relevant.
• Circadian Fluctuations: Diurnal and nocturnal patterns, may vary significantly between individuals and are influenced by several factors.
• Metrics of IOP Fluctuation:Standard Deviation: Measures dispersion of data from the mean.
• Range: Difference between maximum and minimum IOP values.
• Mean Amplitude: Correlates with glaucomatous damage.
• "An alternative metric, the mean amplitude of IOP excursions, has been shown to correlate with glaucomatous damage in a cross-sectional dataset better than other IOP metrics"
• Monitoring Duration: Trends in IOP fluctuation can change day to day; single diurnal or 24-hour sessions may not fully capture an individual's IOP exposure.

6. Factors Affecting IOP Fluctuation

• Hormones: Melatonin, cortisol, and other hormones influence IOP fluctuations.
• Lipids: Endogenous lipids in the aqueous humor and TM may affect IOP regulation. Transgenic mice lacking a certain enzyme have altered IOP rhythms.
• Physical Activity: Activities inducing Valsalva maneuvers (breath-holding exercises, weightlifting, and wind instrument playing) cause temporary IOP elevations.
• "Weightlifting while breath holding induced a mean IOP rise of 4.3 mm Hg whereas weightlifting with normal exhalation induced a mean rise of 2.2 mmHg."
• Eyelid Movements: Eyelid squeezing and rubbing can cause significant transient IOP spikes.
• "Eyelid rubbing induced an average peak IOP change of 59 mmHg from baseline, squeezing induced a peak IOP increase of 42.2 mmHg, relaxed lid closure induced a peak change of 3.8 mmHg, and voluntary blinking induced a peak change of 11.6 mmHg."
• Tonometer Calibration: Regular calibration is crucial; however, studies have shown that calibration errors are frequent.
• Observer Variation: Inter-observer agreement can vary significantly.
• "IOP measurements differed by > 2 mmHg between physicians 17% of the time and between physicians and ophthalmic technicians 25% of the time."
• Water Drinking Test: This provocative test can induce an IOP increase, thought to be related to changes in outflow facility, though the utility for glaucoma management is still being assessed.
• "The water-drinking test is a form of stress test which enables an indirect assessment of the outflow facility of the eye (Kronfeld, 1975)."
• Steroid Trial: Steroids can cause IOP elevation, with glaucomatous eyes being more sensitive. This is generally not used as a diagnostic/prognostic tool because of risk of glaucoma damage.

7. IOP Fluctuation and Glaucoma Risk

• Independent Risk Factor: IOP fluctuation is an independent risk factor for glaucoma, beyond just mean IOP.
• Increased Fluctuation: Glaucoma patients often exhibit greater IOP fluctuations than healthy controls.
• "In general, studies which have assessed IOP variability in patients with glaucoma, with or without comparison to healthy control groups, have documented greater variability and fluctuations in the glaucoma eyes compared to the healthy control eyes."
• Visual Field Loss: Greater fluctuations are associated with greater visual field loss in some studies.
• "Sakata et al. (2013) evaluated 33 patients with POAG and found that higher 24h max and range in IOP was associated with greater visual field loss."
• Patient and Context Specificity: The impact of IOP fluctuation may be patient-, pathology-, and context-specific.

8. Ambulatory, Telemetric Self-Measurement

• Feasible and Reliable: Ambulatory devices like iCare HOME enable patients to self-measure IOP, providing clinically informative data.
• "Ambulatory, telemetric self-measurement of IOP is a feasible, reliable and clinically informative test"
• Validation: These devices have been validated for accuracy, reproducibility, and user acceptability.
• Training: Patients can be trained to use these devices effectively, with high success rates.
• Advantages: Home tonometry allows for greater understanding of IOP trends, outside of a clinic setting.

9. Conclusion

• The document suggests that a better understanding of IOP, beyond just mean readings, will improve diagnosis, treatment, and management of glaucoma. It notes that ambulatory tonometry is a feasible option to improve that understanding.

This briefing document provides a comprehensive overview of the themes, facts, and ideas presented in the provided source.

2 days ago

david 4.3k

@david_fe

Timeline of Main Events & Discoveries

• 10th Century: Ibn Isa (Jesu Hali) first describes elevated intraocular pressure (IOP) through digital palpation, initially attributing it to lens hardening.
• 1905: Halmar Schiotz invents the Schiotz tonometer, the first standard for tonometry. This indentation tonometer measures IOP by the degree of deformation of the eye when a known weight is applied.
• 1948: Goldmann Applanation Tonometer (GAT) is invented, based on the Imbert-Fick principle, using a prism to flatten a standard area of the cornea. It becomes the new gold standard due to its accuracy and low variability.
• 1957: Goldmann & Schmidt publish their work on applanation tonometry, noting that corneal thickness influences IOP readings.
• 1959: Mackay and Marg develop the Mackay-Marg tonometer, which uses a circular ring to gently flatten a small area of the cornea, precursor to the Tono-Pen.
• 1960s: Stephen Drance publishes a study on diurnal IOP fluctuations using Schiotz tonometry, noting most people have a range of less than 5mmHg, but some healthy individuals can have much higher ranges.
• 1963: Armaly and Becker and Mills demonstrate that people with glaucoma have a greater IOP response to steroids.
• 1963: Grant publishes work showing that the porous corneoscleral and uveal meshwork provide little resistance to aqueous outflow.
• 1965: Durham et al. introduces the pneumatic applanation tonometer (pneumotonometer) which employs a floating sensor to measure pressure.
• 1972: Grolman develops the first air-puff tonometer, a non-contact method of measuring IOP by flattening the cornea with a puff of air.
• 1973: Jensen and Maumenee describe the first home tonometry.
• 1975: Kronfeld conceives the water-drinking test as a way to stress the system and assess the outflow of the eye.
• 1983: Johnson & Kamm determine that most resistance to aqueous outflow is located at the intersection of the juxtacanalicular region of the trabecular meshwork and Schlemm’s canal.
• Late 1980s: Perkins tonometer, a portable version of the Goldmann applanation tonometer, becomes available.
• 1993: Whitacre et al. investigate the influence of corneal thickness on IOP measurements using applanation tonometry, showing that thinner corneas result in underestimation, and thicker corneas in overestimation of IOP.
• 1990s: The Tono-Pen is introduced as a portable, lightweight tonometer using principles of both applanation and indentation tonometry based on the Mackay-Marg principle. Studies on the link between low blood pressure and progression of glaucoma begin.
• Early 2000s: Landmark clinical trials like OHTS, EMGT, CIGTS, AGIS, and UKGTS show that IOP reduction delays/prevents glaucoma and that aggressive lowering of IOP reduces further visual field loss.
• 2005: The Ocular Response Analyzer (ORA) is developed, which measures IOP and provides corneal biomechanical data.
• 2009: Triggerfish contact lens sensor is cleared for use in Europe, and later in 2016 in the US, and enables continuous telemetric IOP monitoring.
• 2011: The Oculus Corvis ST is developed and uses a jet of air to deform the cornea and capture the biomechanical responses in real time to calculate IOP.
• 2016: The EYEMATE-SC suprachoroidal pressure transducer is developed for telemetric monitoring of IOP in the suprachoroidal space.
• 2017: The iCare HOME tonometer, a rebound tonometer, is cleared for self-measurement of IOP by patients. The EYEMATE-IO (ARGOS) ciliary sulcus pressure transducer is also developed for telemetric monitoring of IOP.
• Present: Ongoing research continues to explore the role of IOP fluctuation in glaucoma pathophysiology, with increasing emphasis on the use of continuous IOP monitoring devices.

Cast of Characters

• Sanjay G. Asrani: Ophthalmologist at Duke University Medical Center. Co-author of the review, with primary roles in writing, supervision, methodology, analysis, data curation and conceptualization.
• Elyse J. McGlumphy: Ophthalmologist at Delaware Ophthalmology Consultants. Co-author of the review, with primary roles in writing, analysis and conceptualization.
• Lama A. Al-Aswad: Ophthalmologist at Scheie Eye Institute, University of Pennsylvania. Co-author of the review, with a role in writing and formal analysis.
• Craig J. Chaya: Ophthalmologist at John A. Moran Eye Center, University of Utah. Co-author of the review, with roles in writing, formal analysis, and data curation.
• Shan Lin: Ophthalmologist at Glaucoma Center of San Francisco. Co-author of the review, with roles in writing, formal analysis, and data curation.
• David C. Musch: Ophthalmologist and epidemiologist at Kellogg Eye Center, University of Michigan. Co-author of the review, with roles in writing, formal analysis, and data curation.
• Ian Pitha: Ophthalmologist at Wilmer Eye Institute, Johns Hopkins University School of Medicine. Co-author of the review, with roles in writing, formal analysis, and data curation.
• Alan L. Robin: Ophthalmologist at Wilmer Eye Institute, Johns Hopkins University School of Medicine. Co-author of the review, with roles in writing, formal analysis, and data curation.
• Barbara Wirostko: Ophthalmologist at John A. Moran Eye Center, University of Utah. Co-author of the review, with primary roles in writing, supervision, project administration, methodology, analysis, data curation and conceptualization.
• Thomas V. Johnson: Ophthalmologist at Wilmer Eye Institute, Johns Hopkins University School of Medicine. Co-author of the review, with primary roles in writing, visualization, validation, supervision, project administration, methodology, analysis, data curation and conceptualization.
• Ibn Isa (Jesu Hali): 10th-century oculist who first described elevated IOP via palpation.
• Halmar Schiotz: Invented the Schiotz tonometer in 1905.
• Goldmann: Developed the Goldmann Applanation Tonometer (GAT) in 1948 and his work contributed to the basic understanding of applanation tonometry.
• Schmidt: Collaborated with Goldmann on the development of the GAT.
• Mackay: Co-developer of the Mackay-Marg tonometer.
• Marg: Co-developer of the Mackay-Marg tonometer.
• Stephen Drance: Published early studies on diurnal IOP fluctuations in the 1960s.
• Armaly: Studied the steroid response in relation to IOP.
• Becker and Mills: Demonstrated that patients with glaucoma show a greater IOP response to steroids than healthy individuals.
• Grant: Researched the outflow of aqueous through the trabecular meshwork.
• Durham: Co-developed the pneumatic applanation tonometer.
• Grolman: Developed the air puff tonometer.
• Jensen: Co-author on an early paper on home tonometry.
• Maumenee: Co-author on an early paper on home tonometry.
• Kronfeld: Conceived the water-drinking test as a way to assess the outflow facility of the eye.
• Johnson & Kamm: Researched the location of resistance to outflow at the intersection of the juxtacanalicular region and Schlemm’s canal.
• Whitacre: Researched and wrote extensively on the sources of error in Goldmann-type tonometry, notably the role of corneal thickness.
• Luce: Developed the Ocular Response Analyzer (ORA).
• Reznicek: Developed the Corvis ST tonometer.
• Mansouri: Involved in research and development of telemetric IOP measuring technologies, especially the contact lens sensor.

This timeline and cast should give you a comprehensive overview of the key events, people, and concepts discussed in this episode.

2 days ago

david 4.3k

@david_fe

Frequently Asked Questions on Intraocular Pressure (IOP) and Glaucoma

1. What is the relationship between intraocular pressure (IOP) and glaucoma? IOP is currently considered the only modifiable risk factor for glaucoma, a progressive neurodegenerative optic neuropathy. Elevated IOP causes stress on retinal ganglion cell (RGC) axons at the optic nerve head leading to their injury. While it’s not the sole cause of glaucoma, IOP is crucial because reducing it has been proven to prevent or delay the onset and progression of glaucomatous damage. Glaucoma is also characterized by biomechanical remodeling of the lamina cribrosa, leading to optic nerve head changes like cupping and notching.
2. How do different methods of measuring IOP work, and what are their limitations?

• Palpation: The earliest method, involving feeling the globe with a finger, lacks precision and can only detect large deviations from normal IOP.
• Schiotz Tonometry: This indentation method measures the deformation of the globe when a weighted plunger is applied to the cornea. It’s an older technique and is affected by lid squeezing, corneal thickness, and curvature.
• Goldmann Applanation Tonometry (GAT): Considered the gold standard, GAT measures the force required to flatten a specific area of the cornea. It assumes a thin, perfectly elastic cornea, which is not always the case. Factors like corneal thickness, astigmatism, and hydration levels can cause measurement errors.
• Tono-Pen: A portable device using applanation and indentation principles. It is less affected by corneal thickness than GAT, but may overestimate lower IOPs and is less reliable at very high IOPs.
• Pneumotonometry: Uses a probe to measure IOP through a column of air and also measures pulse pressure. It correlates well with GAT within normal ranges, but can overestimate IOP.
• Non-Contact Tonometry (Air Puff): Uses a puff of air to flatten the cornea. Some models tend to report higher IOP readings than GAT, but newer models are highly accurate.
• Ocular Response Analyzer (ORA): Also uses air-puff, but measures two applanation points, factoring in corneal biomechanical properties and providing a “Goldmann-corrected IOP” and a corneal-compensated IOP.
• Corvis ST: A non-contact tonometer that measures the biomechanical response of the cornea to an air puff using a Scheimpflug camera to generate multiple measurements, including IOP and corneal biomechanical data.
• Diaton: A portable tonometer that measures IOP through the upper eyelid, using rebound tonometry. It shows very poor correlation with GAT and low accuracy, but may be useful in certain cases when other methods aren't. It is not suitable for self-monitoring (home tonometry).
• Contact Lens Sensors (e.g. Triggerfish): Measures IOP-induced changes in the cornea's radius of curvature over extended periods. These are useful for tracking trends in IOP rather than obtaining precise IOP measurements. Unfortunately, the Triggerfish device is difficult to obtain (available through just a few doctors in the USA) and it can typically be worn for only 1 day per month.
• Implantable Sensors (e.g. EYEMATE-IO and EYEMATE-SC): Provide continuous IOP monitoring via sensors placed in the ciliary sulcus or suprachoroidal space. They offer accurate and reliable data, but are surgically implanted.

1. How does corneal thickness affect IOP readings and are there correction methods? Corneal thickness significantly impacts IOP measurements. Thicker corneas can lead to overestimation of IOP, while thinner corneas can cause underestimation when measured with applanation tonometry. While some formulas for correcting IOP based on corneal thickness have been proposed, no single correction method has been universally adopted. Furthermore, corneal biomechanics other than thickness also influence measurements. It’s also important to note that a thin cornea is an independent risk factor for developing glaucoma.
2. Why is intraocular pressure (IOP) not a static value, and what is meant by IOP fluctuation? IOP is not static; it's dynamic and fluctuates over various timescales due to many factors. These include normal physiological processes like the circadian rhythm, biological variations within the individual, asymmetric differences between eyes, and even temporary external factors. IOP fluctuation is about changes in IOP within an individual over time, as opposed to variability which looks at differences in IOP metrics across a group of people. Fluctuation encompasses changes from very short bursts due to blinking to longer cycles of several hours. IOP can exhibit various patterns, including a nocturnal or diurnal peak, or no consistent pattern at all. These fluctuations can be related to specific types of optic disc damage and vision loss.
3. What are the main causes of IOP fluctuation? IOP fluctuations are influenced by multiple factors. These include normal circadian rhythms, hormone levels, posture changes (especially between sitting and sleeping), physical activity (especially activities inducing Valsalva, such as weight lifting or wind instrument playing), and even basic actions like eyelid squeezing and rubbing. Furthermore, measurement factors like tonometer calibration, inter-observer variability, and patient-related factors like fluorescein concentration and breath-holding contribute. Even water intake can cause temporary IOP increases.
4. What is the significance of IOP fluctuations in relation to glaucoma risk and progression? Beyond average IOP levels, IOP fluctuation is emerging as an independent risk factor for glaucoma. Studies show that patients with glaucoma often exhibit greater IOP variability compared to healthy individuals. Higher 24-hour maximum and range in IOP are associated with greater visual field loss. IOP fluctuation, particularly peaks and excursions, may cause additional stress to retinal ganglion cells that is independent of average IOP. This suggests that tracking and managing IOP fluctuations is important for glaucoma management.
5. How can self-measurement of IOP at home benefit glaucoma management? Ambulatory, telemetric self-measurement of IOP, especially with devices like the iCare HOME tonometer, is a feasible and clinically valuable tool. It allows patients to measure IOP multiple times at home, including outside clinic hours, revealing more about their individual IOP patterns. This can provide critical data on the magnitude and timing of IOP fluctuations that are not captured by infrequent office visits, aiding in personalized glaucoma management. For self-tonometry measurements to be considered reliable, patients need to be independent in device setup and use, have their first three measurements in a certain range, and their range of self-tonometry measurements must be below a certain threshold.
6. Beyond IOP, are there other factors influencing glaucoma development and progression? Yes, while IOP is the primary modifiable risk factor, other elements contribute to glaucoma development. These include ocular anatomy, such as the lamina cribrosa, which undergoes biomechanical changes in glaucoma, and characteristics of the aqueous outflow system. Factors like age, race, genetics, and systemic conditions are important to consider. Endogenous lipids that may alter cell shape and motility in the trabecular meshwork can also impact IOP regulation. Additionally, eye movements, accommodation, exercise, and specific medication like steroids can also influence IOP.