Michael R. Feilmeier, MD
Bascom Palmer Eye Institute
Corneal pachymetry is the measurement of corneal thickness. Pachymetry was traditionally used to gauge the
functional status of the corneal endothelial cell layer. More recently,
with the emergence of refractive surgical techniques, corneal pachymetry
is necessary to determine suitable candidates for ablation procedures.
Furthermore, the identification of central corneal thickness (CCT) as an independent indicator of glaucoma risk by the Ocular Hypertensive Treatment Study (OHTS)
has made corneal pachymetry a routine part of the ophthalmic evaluation.
Pachymetry is an important part of the evaluation and management of ocular
hypertension and glaucoma.
Several techniques are available to reliably and reproducibly measure
corneal thickness. The methodologies used in these techniques are based
on either ultrasonic or optical principles. While each of the methods
have a peculiarity of their own, all have been described as reliable.
Keep in mind that systematic differences exist between the different
techniques and result in different values. As a result, the
measurements cannot simply be substituted between the different
modalities.
Traditional ultrasound pachymetry (10-20 MHz) offers the
advantages of portability and relative ease of use. These dry contact
systems are simple, portable, and cost-effective. However, the accuracy
of this technique is dependent on the perpendicularity of the probe's
application to the cornea and the reproducibility relies on precise
probe placement. Ultrasound biomicroscopy (50 MHz) and very-high-frequency ultrasound (70 MHz)
have the disadvantage of requiring a water bath, but allow for determination of
corneal sublayer detail and pachymetry.
Some ultrasound pachymeters contain additional features
beyond simply providing a CCT measurement. For example, the Reichert IOPac
features a built-in glaucoma risk calculator. This tool helps physicians identify a patient’s 5-year risk of
progression in glaucoma, based on the 6 primary risk factors identified by the OHTS study: Age, Race, IOP, Patter Standard Deviation, Cup to Disk ratio, and CCT.
Optical slit lamp techniques measure corneal thickness using a device
mounted to the slit lamp through which the observer aligns the anterior
surface and endothelial surface of the cornea through image doubling.
The corneal thickness is then estimated using an equation based on
assumptions of the refractive index and anterior radius of curvature of
the cornea. These variables, along with the examiner-dependent nature
of the technique are the major disadvantages of this modality.
Specular microscopy is a contact based method that records the
adjustment required in the focal plane of the instrument. The SP-2000P
specular microscope (Topcon Corp.) is a noncontact optical instrument
that provides pachymetry and specular microscopy simultaneously.
Orbscan is a noncontact optical scanning-slit instrument that provides
pachymetry in addition to topographic analysis. It has been reported
that this technique underestimates corneal thickness in both corneas
that have undergone refractive surgery and in corneas with haze.
Optical coherence tomography (OCT) and optical low-coherence
reflectometry (OLCR) are noncontact techniques that acquire pachymetry
measurements based on optical interferometry. These techniques are able
to discern sublayer detail and pachymetry and may be able to perform
measurements on corneas with pathology that do not allow measurement by
other techniques. OCLR is advantageous in that it provides continuous
measurement and is useful in obtaining pachymetry measurements during
corneal ablation procedures.
Confocal microscopy is a contact technique that acquires measurements by
through focusing a confocal microscope through the thickness of the
cornea. It has the advantage of displaying and allowing measurement of
detailed sublayer cellular structure. It is also a useful adjunct in
the identification of corneal microbial pathology. Its use is
significantly limited in the setting of corneal opacities and the
technique is inherently timely in terms of both data acquisition and
interpretation.
The Pentacam Scheimpflug Camera is a noncontact technique that uses a
rotating Scheimpflug camera to rapidly capture images of the anterior
segment of the eye. This technique allows measurement of corneal
thickness from limbus to limbus.
Laser Doppler interferometry is a noncontact technique that uses a
dual-beam infrared laser Doppler interferometry to measure corneal
thickness. There is limited data available in the literature regarding
this technique.
References:
Foster CS, Azar DT, Dohlman CH. Smolin and Thoft's The Cornea:
Scientific Foundations and Clinical Practice 4th Edition. Philadelphia:
Lippincott Williams & Wilkins, 2005.
Krachmer JH, Mannis MJ, Holland EJ. Cornea 2nd Edition. Philadelphia:
Elsevier Mosby, 2005.
Medeiros FA, Weinreb RN, Sample PA, et al. “Validation of a predictive model to estimate the risk of
Conversion from ocular hypertension to glaucoma” Arch Ophthalmology 2005;
123: 1351-1360).