Measuring the Diameter of the Cornea

An abnormally large or small cornea (megalocornea or microcornea) will be apparent from simple visual inspection. A suspected size anomaly can be easily verified by measuring the cornea with a ruler. Corneal diameter may be determined more accurately with calipers (usually done under general anesthesia, see Fig. 10.21) or with the Wessely keratometer. This is a type of tube with a condensing lens with millimeter graduations at one end. The examiner places this end on the patient's eye and looks through the other end.

Megalocornea in an infant always requires further diagnostic investigation to determine whether buphthalmos is present. Microcornea may be a sign of congenital defects in other ocular tissues that could result in impaired function (microphthalmos).

5.2.7 Corneal Pachymetry

Precise measurement of the thickness of the cornea is crucial in refractive surgery (see radial keratotomy and correction of astigmatism, p. 155). Improving refraction often requires making incisions through 90% of the thickness of the cornea while meticulously avoiding full penetration of the cornea. There are two pachymetry techniques for measuring corneal thickness with the high degree of precision that this surgery requires:

❖ Optical pachymetry with a slit lamp and measuring attachment may be performed on the sitting patient.

❖ Ultrasonic pachymetry; this has the advantage of greater precision and can also be performed with the patient supine.

Recent developments now permit pachymetry by means of specular microscopy (see 5.2.8 and Fig. 5.4).

5.2.8 Confocal Corneal Microscopy

Confocal corneal microscopy is a recently developed examination technique that makes it possible to scan the cornea over a wide area from the outer layer to the inner layer. It differs in this regard from slit lamp examination, which tends to be a focal examination along a shaft of light perpendicular to the eye. Confocal corneal microscopy visualizes cell structures at maximum magnification that cannot be observed in detail with a slit lamp. These include corneal nerves, amebas, and hyphae. Although not yet routinely used in clinical practice, confocal corneal microscopy appears to be a promising examination method for the future.

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