2 days ago

david 4.3k

@david_fe

New Podcast Episode: Season 1, Episode 2

In this episode, we continue the discussion we started in episode #1. 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.

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.

References:

1. The relationship between intraocular pressure and glaucoma: An evolving concept - PubMed

2. Self-tonometry in glaucoma management--past, present and future - PubMed

3. FitEyes Podcast, Season 1, Episde 2

Disclaimer:

The information provided in this podcast is for informational and educational purposes only and is not intended as a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician, pharmacist, or other qualified healthcare provider with any questions you may have regarding a medical condition, dietary supplements, or any health-related decisions.

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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2 days ago

david 4.3k

@david_fe

Briefing Document: Intraocular Pressure (IOP) and Glaucoma Management

1. Introduction: The Central Role of IOP in Glaucoma

• Glaucoma Defined: Glaucoma is a progressive neurodegenerative optic neuropathy, characterized by the death of retinal ganglion cells (RGCs). It's a multifactorial disease, but "Intraocular pressure (IOP) is the most important identifiable and, presently, the only modifiable risk factor for glaucoma."
• IOP and Optic Nerve Damage: Elevated IOP initiates RGC axonal injury at the optic nerve head. Glaucoma causes biomechanical remodeling of the lamina cribrosa leading to characteristic optic nerve head appearance such as cupping, notching and progressive thinning of retinal nerve fiber layer, RGC layer, and inner plexiform layer.
• Historical Perspective: Glaucoma was described in the 10th century by Ibn Isa and was initially thought to be due to hardening of the lens. IOP measurement via palpation, while lacking precision, has been historically used to identify significant deviations from the norm.

2. IOP Measurement Techniques: From Palpation to Advanced Technology

• Palpation: Tactile IOP assessment by ophthalmologists deviates from Goldmann applanation tonometry (GAT) by more than 30% in 31% of measurements. While unreliable, may identify large excursions from norm where ophthalmic equipment is unavailable.
• Schiotz Tonometry: This indentation tonometer, invented in 1905, measures IOP by measuring the amount of corneal deformation when a standard force is applied. It was the "first gold standard in tonometry prior to the advent of applanation techniques". However, Schiotz tonometry is subject to flaws based on lid squeezing and corneal properties.
• Goldmann Applanation Tonometry (GAT): Based on the Imbert-Fick principle, GAT is still considered the gold standard for IOP measurement due to its accuracy, precision and low variability. "IOP = contact force / area of contact" assumes the cornea is an infinitely thin membrane with perfect elasticity. A truncated cone is pressed on the cornea with a blue light and doubling prism that creates two arcs to be aligned.
• GAT Variability and Limitations: Though accurate under ideal conditions, GAT has intra- and inter-observer 95% confidence limits on variability of 2.5 and 4.0 mmHg, respectively. Corneal biomechanical properties (thickness, astigmatism, hydration), can impact the accuracy of GAT, as well as fluorescein concentrations, patient factors like Valsalva maneuver and shifts in gaze. A thin cornea can cause an underestimation of IOP, and a thick cornea causes overestimation.
• Tono-Pen: A portable, lightweight device that uses both applanation and indentation principles based on the Mackay-Marg principle. The plunger contacts the cornea and measures the strain when the plunger and footplate contact the cornea. Multiple readings are averaged. Repeatability coefficients are ±4.3 mmHg, higher than GAT. Tono-Pen IOP values correlate well with GAT over 11-20 mmHg, but not interchangeable and often underestimates IOP values above 20 mmHg and unreliable over 30 mmHg. It is less influenced by corneal thickness, but this still effects accuracy. The Tono-Pen has increased utility in patients with corneal irregularities.
• Pneumotonometry: Uses a slightly convex silicone probe with a floating sensor. The pressure increases to match the IOP. Pneumotonometry measures IOP continuously over several seconds and calculates the ocular pulse pressure. Correlation with GAT is between 0.77 and 0.95, but tends to overestimate. The probe is placed perpendicularly after anesthesia.
• Ocular Response Analyzer (ORA): A newer form of air puff tonometry that considers corneal thickness and biomechanics. It delivers an air pulse and measures applanation as the cornea flattens, forms concavity, and rebounds. It calculates “Goldmann-corrected IOP” (IOPg) and corneal-compensated IOP (IOPcc), incorporating corneal elasticity and viscosity. This allows an evaluation of the biomechanical properties of the cornea
• Diaton Tonometer: A portable, handheld device measuring IOP through the upper eyelid using rebound tonometry. It measures the deceleration of a metal rod bouncing off the eyelid and sclera to estimate IOP.
• Triggerfish: A contact lens based sensor which measures IOP-induced changes in the radius of curvature of the cornea via a strain gauge. It provides IOP as millivolt equivalents rather than mmHg and is best for monitoring trends rather than exact values. A change of 3 um corresponds to a 1 mmHg change in IOP. It contains a microprocessor that transmit IOP data via wireless antenna in continuous fashion.
• Implantable Sensors (EYEMATE-IO, EYEMATE-SC): The EYEMATE-IO is an implantable sensor in the ciliary sulcus using pressure and temperature sensors. The EYEMATE-SC is a suprachoroidal pressure transducer for telemetric monitoring. Both use a micro-electromechanical system that measure IOP and data transfer.

3. The Dynamic Nature of IOP: Fluctuations and Influencing Factors

• IOP is Eye-Specific: IOP fluctuates significantly over multiple timescales in a chronological, biological, and intraindividual manner, much like blood pressure and blood glucose.
• Circadian Rhythms: "IOP may exhibit unique phenotypes with different patients consistently exhibiting a nocturnal acrophase, a diurnal acrophase, variable timing to the acrophase, no acrophase at all (i.e., a relatively invariable IOP), and/or varying degrees of phase amplitude". A normal range of fluctuation over 24 hrs is 2-6 mmHg.
• Hormonal Influence: Hormones may affect IOP fluctuations.
• Corneal Biomechanics: While corneal thickness and hysteresis affect tonometry, they generally don't affect circadian changes in IOP. Corneal thickness also fluctuates diurnally.
• Endogenous Lipids: Ocular lipid synthesis plays a role in IOP regulation, influencing cell shape and motility in the trabecular meshwork (TM) which alters flow resistance. Altered lipid levels have been seen in POAG.
• Physical Activity: Valsalva maneuvers, induced by static exercises, breath holding during weight lifting, and wind instrument use, increase IOP. Eyelid squeezing and rubbing can also produce large excursions in IOP. "Eyelid rubbing induced an average peak IOP change of 59 mmHg from baseline, squeezing induced a peak IOP increase of 42.2 mmHg".
• Tonometer Calibration: Regular use of tonometers leads to calibration errors. Clinical error in IOP correlates with calibration error. It may be clinically acceptable to use tonometers with calibration errors of up to 3 mmHg.
• Water Drinking Test: This test can elicit a peak IOP response which stresses the normal physiologic system, but may not be easily exercised in the clinical setting. A response of >5mmHg is associated with a 6-fold increase in the risk of glaucoma compared with individuals with <1mmHg response. Peak IOP during the water-drinking test correlates with the peak 24-hour IOP.
• Steroid Trial: Glaucoma patients are at higher risk for steroid response, however, challenging patients with steroids is uncommon due to the risk of glaucoma damage. Steroid use may lead to an elevated IOP.

4. IOP Fluctuation as a Glaucoma Risk Factor

• Greater Variability in Glaucoma Patients: Studies show that IOP variability is greater in glaucoma eyes than healthy control eyes.
• Independent Risk Factor: Fluctuations in IOP represent an independent glaucoma risk factor, independent of mean IOP.
• 24h vs Office Hours: IOP range during office hours does not correlate with longer 24h IOP range.
• IOP Fluctuation and Disease Progression: Higher 24h IOP range is associated with greater visual field loss. Specific phenotypes of IOP fluctuation may be associated with specific types of optic disc damage and field loss. "Ultimately, the role and impact of IOP fluctuation in glaucoma pathophysiology may be patient-, pathology-, and context-specific".

5. Self-Tonometry: A Paradigm Shift in IOP Monitoring

• Limitations of Office-Based Measurements: Contemporary office-based measurements are not sufficient to discover diurnal changes and spikes, nor do they demonstrate the effect of medication and compliance.
• Benefits of Self-Tonometry: Patient-directed self-tonometry can be taken throughout the day, is the subject of much discussion and research, and allows clinicians a better understanding of fluctuations of IOP.
• Historical Context: The idea of self-tonometry dates back to 1958 with Maurice's concept of a continuous recording tonometer.
• Pressure Phosphene Tonometer: The Proview Eye Pressure Monitor works by applying pressure to the superonasal orbit, creating a visual sensation that indicates IOP.
• Ocuton S: A hand-held electronic automatic applanation tonometer that is based on GAT principles. Studies suggest it may read slightly higher than GAT, and the results of studies vary.
• Tono-Pen for Self-Tonometry: Though widely used clinically, it has only been investigated once for self-tonometry. One case study demonstrated a patient successfully using it over 4 years to manage IOP.
• Implantable Sensor in an IOL: An early concept was using an intraocular sensor embedded into an artificial IOL (intraocular lens).
• Potential for Self-Tonometer Adaption: Many modern tonometers can be automated for home self-tonometry but cannot be used during sleep or vigorous activity. Examples include the Reichert AT555, Reichert Ocular Response Analyzer, and Nidek NT 4000.
• Ambulatory, Telemetric Self-Measurement: "Ambulatory, telemetric self-measurement of IOP is a feasible, reliable, and clinically informative test." The iCare HOME device has been validated for accuracy, reproducibility and user acceptability and is FDA cleared. Correlation between self-IOP measurement and office based GAT is good. Patient reliability has been demonstrated when certain requirements have been met.
• iCare HOME and iCare HOME 2: The iCare HOME device is a portable, rebound-based tonometer that is well validated. The iCare HOME 2 has decreased training times and provides reliability checkmarks on the device.
• Future of Self-Tonometry: The ideal device needs to be safe, reproducible, reliable, and easy to use, accurate, minimally invasive, removable, and require minimal patient/doctor training. Long-term devices should be implantable, biocompatible, low maintenance and durable. "A single device is not capable of meeting all these criteria, and thus it is envisaged a range of self-tonometers will need to be developed."

6. Conclusion

• Shifting Focus from Single IOP Measurement to Continuous Monitoring: The focus of glaucoma management is shifting towards understanding the dynamic nature of IOP and away from single, static measurements.
• Self-tonometry offers a unique approach to obtain a complete IOP profile, assess the effects of medications, and assist with glaucoma management.

This briefing document provides a comprehensive overview of the complex relationship between intraocular pressure and glaucoma, highlighting the importance of understanding IOP fluctuations and the emergence of self-tonometry as a valuable tool for disease management. It also highlights the evolution of IOP measurement from rudimentary techniques to state-of-the-art technologies.