Pachymetry

A corneal pachymeter measures corneal thickness, a sensitive indicator of endothelial physiology that correlates well with functional measurements. Optical pachymetry, which is performed using a special device attached to the slit-lamp biomicroscope, is somewhat imprecise and is rarely used today. Ultrasonic pachymetry, which is based on the speed of sound in the normal cornea (1640 m/sec), is both easier to perform and more accurate. The applanating tip of the pachymeter must be perpendicular to the ocular surface because errors are induced by tilting. Scanning-slit technology, Scheimpflug-based anterior segment imaging, OCT, and high-resolution ultrasonography are newer techniques that can be used to produce precise maps of the entire corneal thickness, including curvature (see Fig 2-19).

The thinnest zone of the cornea is usually about 1.5 mm temporal to the geographic center, and the cornea becomes thicker in the paracentral and peripheral zones. The average central thickness of the normal human cornea is between 540 and 550 μm. In the Ocular Hypertension Treatment Study, the average central corneal thickness was higher, at 573 ± 39 μm, but it was acknowledged that central corneal thickness was probably higher in the study population than in the general population. Corneal thickness affects the measurement of intraocular pressure (IOP), with thicker corneas producing falsely higher IOP readings and thinner corneas producing falsely lower readings. However, Liu and Roberts demonstrated that the biomechanical properties of the cornea, particularly stiffness, may have a greater impact on IOP measurement errors than does corneal thickness or corneal curvature. Adjustment for corneal biomechanical properties may lead to a more accurate measurement of the IOP. Despite these adjustments, low pachymetry measurements have been shown to be an independent risk factor for glaucoma even when the artificial lowering of IOP is accounted for.

Pachymetry can also be used to assess corneal hydration and the function of the corneal endothelium in its dual role as a barrier to aqueous humor and as a metabolic pump. When functioning normally, the endothelial pump balances the leak rate to maintain the corneal stromal water content at 78% and the central corneal thickness at about 540 μm. Acute corneal edema is often the result of an altered barrier effect of the endothelium or epithelium. Chronic corneal edema is usually caused by an inadequate endothelial pump. Early signs of corneal edema evident on slit-lamp examination include patchy or diffuse haze of the epithelium, mild stromal thickening, faint but deep stromal wrinkles (Waite-Beetham lines), Descemet membrane folds, and a patchy or diffuse posterior collagenous layer. Folds in the Descemet membrane are first seen when corneal thickness increases by 10% or more; epithelial edema occurs when corneal thickness exceeds 700 μm. Stromal edema alters corneal transparency, but vision loss is most severe when epithelial microcysts or bullae occur. A central corneal thickness greater than 640 μm, corneal thickening that is asymmetric between the 2 eyes, or a thicker central cornea compared with inferior corneal measurements within the same eye may indicate a higher risk for symptomatic corneal edema after intraocular surgery.

Brandt JD, Beiser JA, Kass MA, Gordon MO. Central corneal thickness in the Ocular Hypertension Treatment Study (OHTS). Ophthalmology. 2001;108(10):1779–1788.

Khaja WA, Grover S, Kelmenson AT, Ferguson LR, Sambhav K, Chalam KV. Comparison of central corneal thickness: ultrasound pachymetry versus slit-lamp optical coherence tomography, specular microscopy, and Orbscan. Clin Ophthalmol. 2015;12(9):1065–1070.

Liu J, Roberts CJ. Influence of corneal biomechanical properties on intraocular pressure measurement: quantitative analysis. J Cataract Refract Surg. 2005;31(1):146–155.

Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.