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BLOG: What's that red line for, anyway?
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Today in optometry, much emphasis is put on correctly measuring a patient’s IOP to the exact mm Hg. With attention being placed on corneal pachymetry and hysteresis, there is another simple – yet important – step you can do to ensure proper IOP reading. It has to do with that red line on your Goldmann tonometer.
When Dr. Goldmann developed his tonometer, he calibrated it to produce a specific area of flattened cornea: 7.354 mm2. This is derived from the equation to find the area of a circle: area = 3.14 x radius2. In our situation, the radius of the Goldmann probe is 1.53 mm. When measuring spherical corneas, measurements can be made on any meridian. We use 0 and 180 for convenience.
But when eyes have corneal astigmatism, the flattened areas during applanation are not circular but elliptic. This yields an area not equal to 7.354 mm2. Dr. Goldmann found that in very toric corneas, placing the prism 43o to the meridian of the lower power ensures a flattened area of very close to 7.354 mm2, which yields correct IOP measurements. Haag-Streit recommends doing this in eyes with greater than 3 D of corneal astigmatism.
The mires were stretched vertically with the first measurement.
Images: Rett D
This is where our red line comes into play. The red line is separated from the 0o white line by 43o. The red line should be placed at the prism degree mark corresponding to the flattest meridian (greatest radius). In other words, in minus cylinder form, put the patient’s axis on the red line.
The mires were correctly positioned when the prism was properly placed.
For example, consider a recent case in my clinic: a 24-year-old white female presented with a refraction of -7.50- D 4.00 x 030 OD, and keratometry revealed 3.50 D of corneal cylinder. Goldmann tonometry was performed with the prism placed in the usual horizontal plane, at 180o. The resultant IOP was 20 mm Hg, with the mires appearing stretched vertically, as shown in the accompanying figure.
The mires are not aligned when the prism is rotated.
The prism was then rotated to the correct placement, with the 30o mark of the prism in line with the red line of the prism holder. The resultant IOP was 17 mm Hg, with the mires in the correct (albeit tilted) final position, as shown in the second figure. The third figure represents the appearance of the mires when the measuring drum is left at 17 mm Hg, and the prism is rotated back to 180o. As you can see, the mires are not in alignment, and the IOP appears to be higher than it actually is.
This patient’s IOP lowered 3 mm Hg when measured the correct way. Taking into account high corneal astigmatism when measuring IOP is an important and easy step you can do every day when taking care of your own patients.