PachymetryEdit
Pachymetry is the measurement of corneal thickness, with particular emphasis on central corneal thickness (CCT). The cornea’s thickness is a fundamental parameter in ocular anatomy and biomechanics, influencing refractive properties, intraocular pressure (IOP) assessment, and the planning of corneal surgery. In a typical adult, CCT falls in the range of roughly 500–570 μm, though values vary with age, ethnicity, and measurement technique. Modern pachymetry has evolved from simple contact methods to sophisticated noncontact imaging that can map thickness across the corneal surface, providing a detailed pachymetric profile in addition to a single central value. These measurements are routinely integrated into glaucoma workups, keratoconus screening, cataract and refractive surgery planning, and the management of corneal edema.
Pachymetry sits at the intersection of anatomy and clinical decision-making. The thickness of the cornea affects its rigidity, how it responds to surgical alterations, and how clinicians interpret intraocular pressure readings. Because different devices can yield systematically different results, consistent use of a single method for a patient over time is often favored to detect true change rather than device drift. Across populations, normative data are helpful, but device-specific and population-specific differences mean clinicians must interpret results in the context of the particular instrument and patient demographics.
Measurement modalities
Ultrasound pachymetry
Ultrasound pachymetry uses a probe that contacts the corneal surface to emit high-frequency sound waves and measure the time required for the echo to return from the posterior corneal boundary. The measured distance corresponds to CCT. This method is highly accurate when performed by trained operators, but because it is a contact technique it requires topical anesthesia and careful attention to probe placement and corneal surface conditions. Ultrasonic measurements can be influenced by corneal curvature, edema, or scarring, and there can be inter-operator variability. See ultrasound pachymetry for a dedicated overview.
Optical pachymetry
Anterior segment optical coherence tomography (AS-OCT): This noncontact imaging modality uses light waves to capture cross-sectional images of the anterior segment, including the cornea. By segmenting the anterior and posterior corneal surfaces, AS-OCT yields CCT and, with additional scans, pachymetric maps that show thickness distribution across the cornea. This technique offers high resolution and repeatability, is well suited for serial follow-up, and reduces the risk of infection or epithelial disruption associated with contact methods. See anterior segment optical coherence tomography.
Scheimpflug imaging and rotating camera systems: Devices in this category capture multiple cross-sectional images as the camera rotates around the eye to reconstruct a three-dimensional model of the anterior segment. These systems provide pachymetric maps and can assess corneal thickness variation across the central and peripheral zones. See Scheimpflug imaging.
Scanning-slit topography and optical biometers: Some platforms combine topographic data with pachymetry to produce thickness maps or to estimate CCT from optical reflections. Examples include systems that integrate with routine refractive measurements. See pachymeter and refractive surgery for related concepts.
Other optical devices and biometry platforms: A variety of instruments—including optical biometers used for biometry in cataract surgery—also report CCT measurements in routine practice. See intraocular pressure and glaucoma for contexts in which these measurements are used clinically.
Device comparability and mapping
Different pachymetry devices can yield slightly different CCT values for the same eye due to differences in measurement principle (sound vs light), segmentation algorithms, and calibration. Therefore, longitudinal monitoring typically uses the same instrument, or at minimum, practitioners interpret changes with an understanding of the device-specific baseline and variability. Mapping-based pachymetry (pachymetric maps) is increasingly common and helps identify focal thinning or thickening associated with diseases such as keratoconus or corneal edema. See pachymetric map where relevant.
Clinical relevance
Glaucoma risk assessment and intraocular pressure
Central corneal thickness influences the interpretation of intraocular pressure measurements. Thicker corneas can yield higher readings on applanation tonometry, potentially masking true pressure elevation, while thinner corneas can produce lower readings and may underestimate risk. Consequently, CCT is included in risk calculators and helps guide decisions about monitoring frequency and treatment initiation. See glaucoma and intraocular pressure for broader context, and Goldmann applanation tonometry for the common method whose readings are affected by corneal thickness.
Refractive surgery planning
In refractive procedures such as LASIK and PRK, CCT is a critical limiting factor. Before surgery, surgeons evaluate the total corneal thickness and the residual stromal bed to avoid ectasia or postoperative complications. Pachymetric maps also aid in planning ablation profiles and assessing the stability of corneal thickness over time. See LASIK and refractive surgery for related topics.
Keratoconus and corneal disease
Keratoconus and other corneal ectatic disorders are often accompanied by focal thinning that may escape detection if only a central value is considered. Pachymetric maps reveal regional thinning and help distinguish normal variation from pathology. In edema, pachymetry tracks the development and resolution of thickening associated with epithelial or endothelial dysfunction. See keratoconus and corneal edema.
Pediatric and orbital considerations
Pachymetry is useful in pediatric ophthalmology for assessing anterior segment development and in evaluating congenital corneal conditions. In addition, corneal thickness can inform contact lens fitting and keratoconus screening in younger patients. See pediatric ophthalmology for general context.
Variability and standardization
Device-dependent variability: Because measurement principles differ between ultrasound and optical methods, CCT values can differ by device. Consistent use of a single device for a patient’s follow-up minimizes misinterpretation of changes. See pachymeter and anterior segment optical coherence tomography for device-focused discussions.
Population and racial considerations: While genetics and development influence corneal structure, robust normative databases require careful interpretation. Some studies have reported differences in average CCT across populations, but results are not universally consistent, and the clinical significance varies. Clinicians should consult device-specific normative data and consider patient demographics when evaluating CCT. See keratoconus and glaucoma for how population variations can intersect with disease risk.
Calibration and maintenance: Regular calibration and adherence to manufacturer guidelines improve accuracy. When devices are upgraded or when longitudinal data are re-baselined, clinicians should be cautious about apparent changes that reflect instrumentation rather than biology. See medical device calibration for general context.