pachometer

pachometer.

See pachymeter.

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pachometer 

A device, mounted on a slit-lamp, that is used for measuring corneal thickness (or the depth of the anterior chamber). It consists of an optical system that provides two half-fields by means of two glass plates with parallel sides placed in front of one objective of the microscope, the other being occluded. These plates rest one on top of the other with the junction between them situated so as to horizontally bisect the objective. The top plate can be rotated while the bottom one is fixed. The observer viewing through the microscope sees two corneal optical sections and adjusts the top plate until the outer surface of the epithelium appears aligned with the inner surface of the endothelium (Fig. P1). The corneal thickness is then read directly from a scale attached to the pachometer and calibrated in millimetres. To increase the accuracy of the measurement a special eyepiece is used with the microscope. It has a magnification of ✕10 and has two additional components: a horizontal slit and a bi-prism. The role of the eyepiece is to remove from the field of view half of the two optical sections. The measurement of the depth of the anterior chamber is made with a similar device but with a different scale. Note: also spelt pachymeter.Pachometry can also be carried out using slit-scanning topography in which a computerized system integrates a series of slit-beam images to produce a map of the curvature and elevation of the anterior and posterior surfaces as well as corneal thickness. An instrument with greater magnification (called a micropachometer) has been devised, principally for research purposes, using a projection system which incorporates variable doubling plates and forms two slit images on the cornea in conjunction with the viewing system of a slit-lamp and a magnification of up to ✕100, mounted on another arm. This instrument allows the measurement of the thickness of the corneal epithelium alone with a precision that can reach 61 μm. The above pachometers are referred to as optical pachometers to differentiate them from ultrasonic pachometers, which use high-frequency ultrasound waves, reflected from the anterior and posterior corneal surfaces and a transducer probe placed against the cornea. Ultrasound and slit-scanning pachometers have higher reproducibility and less interobserver variation than subjective optical pachometers. Usage of pachometers (pachymeters) includes evaluation of contact lens wear, pre- and post-refractive surgery (e.g. PRK, LASEK, LASIK), glaucoma detection and monitoring corneal oedema. See ultrasonography.

Fig. P1 View through a pachometer after having rotated the top corneal section to align its back with the front of the bottom section