Optician India

IOP (Intraocular Pressure) is the measure of pressure inside the eye created by the balance between aquous humor production and its drainage.  

IOP measurement is performed as a part of routine clinical eye examination to screen glaucoma and other ocular diseases. Elevated or fluctuating IOP can damage the optic nerve and leads to progressive visual field loss. For both early diagnosis and long-term illness management, precise IOP measurement and monitoring are still essential. Frequent IOP monitoring aids in guiding treatment choices, evaluating therapeutic response, and halting more optic nerve injury. (1) The average normal intraocular pressure (IOP) in the general population ranges between 10 and 21 mmHg. (2) Tonometers are based on many physics ideas and principles that specify how IOP levels are measured and what variables may potentially affect these results. Depending upon the technique of IOP measurement being used, variety of factors can affect the IOP measurement. (3) Goldmann Applanation Tonometry is considered as the gold standard for the IOP measurement among the several techniques available. GAT measure the IOP by measuring the force required to flatten the fixed area of the cornea. Measurement of IOP with Goldmann Applanation Tonometer is influenced by various factors such as the corneal hysteresis, fluorescein staining, corneal thickness, corneal curvature, corneal scarring, patient cooperation, scleral rigidity and eye position.

Keratometry and Its Role in Intraocular Pressure Assessment

Keratometry measures the curvature of cornea, which indicates how flat or steep cornea is This measurement is a standard component of eye exams and is often expressed in dioptres (D). In addition to its conventional optical uses, including contact lens fitting, intraocular lens power calculation for cataract surgery, and refractive surgery, keratometry also contributes to the evaluation of intraocular pressure (IOP). Goldmann Applanation Tonometer (GAT) is calibrated for average corneal curvature of approximately 43D. (4) The diagnosis and monitoring of glaucoma depend on accurate IOP measurement, however keratometry-related mistakes might distort the results. The corneal shape of each eye is assumed to be standard in the GAT calibration, but in practice, there is a great deal of diversity. While the cornea resists flattening, it takes more energy to achieve applanation when cornea is steeper than usual. (5) The IOP is unnaturally raised as a result of this additional resistance. For instance, even when an optic nerve is healthy, a patient with naturally steep corneas may appear to have high IOP, leading to a false suspicion of glaucoma. Conversely, a flatter cornea is easier to applanate and provides less resistance, which may lead to an artificially lowered IOP reading. (5) Patients who have flat cornea post corneal refractive surgery procedures may exhibit falsely low IOP readings, which could conceal dangerously high actual pressure readings. If keratometry is not taken into consideration, these differences may result in incorrect diagnoses or treatment choices.

Clinicians are increasingly using correction algorithms and integrating keratometry measurements with central corneal thickness to reduce these inaccuracies. Adjusted IOP values that are closer to the actual physiological pressure are provided by formulas like the Kohlhaas formula and the Ehlers formula.

Ehlers Formula- One of the first attempts to increase the precision of intraocular pressure (IOP) readings by taking central corneal thickness into consideration was the Ehlers nomogram. Ehlers et al proposed a nomogram that is used to date in clinics to correct the IOP measured using GAT for the errors induced due to variation in CCT. (6) The basic idea is that GAT assumes a normal corneal thickness of about 520 µm. (7) IOP measurements are higher in thicker corneas and lower in thinner corneas. In order to more accurately estimate the actual intraocular pressure and minimize mistakes in glaucoma assessment, the Ehlers nomogram applies a correction factor based on central corneal thickness, which is represented in mmHg. (8)

Kohlhaas Formula- The Kohlhaas formula considers both corneal curvature and central corneal thickness, offering a more accurate way to correct Goldmann Applanation Tonometry (GAT) measurements. Inaccurate IOP readings may result from variations from the average CCT of 520 µm and mean curvature of 43 D, which are assumed by GAT. In particular, IOP tends to be underestimated by thinner corneas or flatter curvatures, and overestimated by thicker corneas or steeper curvatures. (9)

The formula, expressed as

IOP corrected?=IOPGAT?+0.07×(CCT−520) +0.3×(K−43)

By quantitatively combining both corneal characteristics, this formula offers a useful modification that produces a more precise estimation of the genuine intraocular pressure. (10)

These formulas are a useful tool for improving glaucoma care, despite the fact that they are not perfect because corneal biomechanics like elasticity and viscoelastic qualities also play a part. (10)

Conclusion

Goldmann Applanation Tonometry overestimate or underestimate real IOP, even though it is still the clinical standard which is impacted by a number of corneal characteristics, such as thickness, curvature, and biomechanical characteristics. Clinicians can reduce cornea-related mistakes by combining keratometry, central corneal thickness, and correction formulae like Ehlers and Kohlhaas with cutting-edge tools like PASCAL dynamic contour tonometry and biomechanical analyzers. (11) Eye care providers can obtain more accurate IOP measurements by taking into account the entire range of corneal features. This will help them make better clinical judgments and provide better care for patients with glaucoma.

References:

1. Weinreb, R. N., Aung, T., & Medeiros, F. A. (2014). The pathophysiology and treatment of glaucoma: A review. JAMA, 311(18), 1901–1911. https://doi.org/10.1001/jama.2014.3192

2. Alimuddin, M. (1956). Normal intra-ocular pressure. British Journal of Ophthalmology, 40(6), 366–372. https://doi.org/10.1136/bjo.40.6.366

3. Zeppieri, M., & Gurnani, B. (2023, June 11). Applanation tonometry. National Library of Medicine.

4. Shimmyo, M., Ross, A. J., Moy, A., & Mostafavi, R. (2003). Intraocular pressure, Goldmann applanation tension, corneal thickness, and corneal curvature in Caucasians, Asians, Hispanics, and African Americans. American Journal of Ophthalmology, 136(4), 603–613. https://doi.org/10.1016/s0002-9394(03)00424-0

5. Maverick, K. J., Conners, M., & Huang, A. (2004). Effect of intraocular pressure on keratometry in porcine eyes. Investigative Ophthalmology & Visual Science, 45, 2876.

6. Ehlers, N., Bramsen, T., & Sperling, S. (1975). Applanation tonometry and central corneal thickness. Acta Ophthalmologica (Copenhagen), 53(1), 34–43. https://doi.org/10.1111/j.1755-3768.1975.tb01135.x

7. Whitacre, M. M., & Stein, R. (1993). Sources of error with use of Goldmann-type tonometers. Survey of Ophthalmology, 38(1), 1–30. https://doi.org/10.1016/0039-6257(93)90053-A

8. Gunvant, P., Newcomb, R. D., Kirstein, E. M., Malinovsky, V. E., Madonna, R. J., & Meetz, R. E. (2010). Measuring accurate IOPs: Does correction factor help or hurt? Clinical Ophthalmology, 4, 611–616. https://doi.org/10.2147/opth.s11105

9. Kohlhaas, M., Boehm, A. G., Spoerl, E., et al. (2006). Effect of central corneal thickness, corneal curvature, and axial length on applanation tonometry. Investigative Ophthalmology & Visual Science, 47(5), 2367–2372.

10. Komninou, M. A., Seiler, T. G., & Enzmann, V. (2024). Corneal biomechanics and diagnostics: A review. International Ophthalmology, 44(1), 132. https://doi.org/10.1007/s10792-024-03057-1

11. Solomon, K. D., et al. (2000). Comparison of dynamic contour tonometry with Goldmann applanation tonometry and manometry. Ophthalmology, 107(3), 463–468.