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Corneal Pachymetry
Corneal Pachymetry
Joseph J. K. Ma
Anthony P. Adamis
The term pachymetry is a contraction of two Greek words, pachos (“thick”) and metry (“measure”). Traditionally, the primary role of pachymetry in clinical practice was to act as a gauge of corneal endothelial cell layer function. The emergence of refractive surgery and the identification of corneal thickness as an important clinical variable in the assessment of intraocular pressure in glaucoma and glaucomasuspect patients, has and will continue to redefine the importance of pachymetry not only for corneal and refractive surgeons, but in routine ophthalmic care.
Although optical slit-lamp pachymetry and specular microscopy have been available for decades, most clinicians rely on traditional ultrasonic pachymetry methods for determining corneal thickness primarily because of its simplicity and shallow learning curve for paramedical staff. However, the technological advances of the last decade have given birth to a cornucopia of new methodologies, as well as improved versions of existing techniques. The techniques can be divided into methodologies that are based on optical principles, such as traditional optical pachymetry, optical coherence tomography (OCT), optical low-coherence reflectometry (OLCR), confocal microscopy through-focusing (CMTF), specular microscopy, and laser Doppler interferometry, and on ultrasonic principles. We briefly describe the essentials of each, and Table 7-1 summarizes what we believe are the essential elements published at the time of writing. We refer the reader to the primary references listed at the end of the chapter for more detailed descriptions.
Although each technique is idiosyncratic in its own way, most methods have been touted as reliable in their own right. There are, however systematic differences in the values obtained by different methods. Because true corneal thickness cannot be verified, the accuracy of any given method is uncertain. From a practical standpoint, this means that although most methods are reliable enough for the longitudinal follow-up of patients, the values obtained cannot simply be substituted between modalities. Although the systematic deviation of pachymetry measurements is not consistent between studies and varies between model types, it is generally observed that the order of measurements, from the modality that provides the highest measurements on down, is as follows: contact specular, Orbscan, ultrasound, noncontact specular, OCT, OLCR, and optical pachymetry (1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18). Furthermore, in applications that require accurate corneal thickness measurements such as with lamellar or refractive techniques, a method-dependent safety margin of error must be factored into estimations of corneal thickness.
METHODS FOR PACHYMETRY
Ultrasound
Traditional ultrasound pachymetry is a simple dry contact technique that measures corneal thickness at 10 to 20 MHz with an estimated of velocity of sound through cornea of 1630 m/sec and has become the standard method used by most clinicians (19). It is generally thought to underestimate corneal thickness in edematous corneas (20,21). Ultrasound biomicroscopy (50 MHz) and very-high-frequency ultrasound (70 MHz) require a water bath and are able to discern sublayer detail and pachymetry (22, 23, 24, 25, 26, 27).
Optical Pachymetry
Optical pachymetry is measured using a slit-lamp-mounted device that allows the observer to align the front and back surface of the cornea through image doubling, to attain an estimation of corneal thickness on a Vernier scale. This is an observer-dependent technique based on assumptions of the refractive index of the cornea and anterior radius of curvature (28, 29, 30, 31).
Specular Microscopy
Scanning Slit Based (Orbscan)
Orbscan is a noncontact technique that incorporates a scanning-slit-based method of pachymetry in the process of topography and elevation data acquisition. There has been concern that this method underestimates corneal thickness in hazy corneas and corneas that have undergone refractive surgery (7,11,34).
