The technician should recommend an ultrasonic pachymeter. Pachymetry, sometimes spelled pachometry, is the measurement of corneal thickness. Ultrasonic pachymeters use reflected sound waves to measure that thickness, and they are the type used more commonly. Some probes are hard-wired to a tabletop instrument. Others are pen-like and portable, so they can be carried from room to room. Either style gives the doctor the same number.
The probe tip is placed gently on the cornea and the machine reads the thickness in micrometers, written µm. A normal adult cornea measures about 550 µm, or 0.55 mm, at the center. Many ultrasonic units add corneal mapping display programs that give information about planned and finished refractive surgery. The older optical pachymeter attaches to the slit lamp and works on optical principles. It measures the distance between the epithelium and the endothelium, the outer and innermost corneal cell layers. Both designs answer one question, which is how thick this cornea is at the spot measured.
Sort the corneal instruments by what each one reports, and the answer choices sort themselves. Thickness comes from a pachymeter, and the ultrasonic model is the one most offices own and the one you will be handed. Knowing which ultrasound-based instrument does which measurement is part of the diagnostic ultrasound content area of the COA exam.
Incorrect Answers
- A. An optical pachymeter does measure thickness, but it is the less commonly used of the two designs. It works on optical principles and cannot leave the slit lamp.
- B. A Placido disk topographer projects lighted rings and maps the corneal curve. Curve and thickness are different measurements, and this camera never reports thickness.
- C. A specular microscope photographs and counts endothelial cells. It reports a cell count, not a thickness in micrometers.
Testing Pearls
- Ultrasonic pachymeters use reflected sound waves and are the type used more often.
- Probes may be tabletop-wired or pen-like and portable.
- Optical pachymeters mount on the slit lamp and read from epithelium to endothelium.
- Normal central corneal thickness is about 550 µm (0.55 mm).
- Pachymetry is ordered for corneal dystrophies, glaucoma suspects, and surgical planning.
References
- American Academy of Ophthalmology. Ophthalmic Medical Assisting: An Independent Study Course. Chapter 10, Adjunctive Tests and Procedures.
