Cross Cylinder Technique
The cross cylinder technique to measure astigmatism was described by Edward Jackson in 1887. In Jackson’s words, the cross cylinder lens is probably “far more useful, and far more used” than any other lens in clinical refraction. Every ophthalmologist should be familiar with the principles involved in its use. Although the cross cylinder is usually used to refine the cylinder axis and power of a refraction that has already been obtained, it can also be used for the entire astigmatic refraction.
A cross cylinder is a lens with a spheroequivalent power of zero but with mixed astigmatism of equal amounts. Common cross cylinders are: −0.50 +1.00 × 090 or −0.25 +0.50 × 090. They are mounted so that they can be rotated about their axis (90° or 180°) or at a point halfway between the axis (45° or 135°).
The first step in cross cylinder refraction is to adjust the sphere to yield the best visual acuity with accommodation relaxed. Begin by selecting a starting point: this may be the current prescription, the retinoscopy, or autorefraction findings. Dial this into a trial frame or phoropter. Fog the eye to be examined with plus sphere while the patient views a visual acuity chart; then decrease the fog until the best visual acuity is obtained. If astigmatism is present, decreasing the fog places the circle of least confusion on the retina, creating a mixed astigmatism. Then use test figures that are 1 or 2 lines larger than the patient’s best visual acuity. At this point, introduce the cross cylinder, first for refinement of cylinder axis and then for refinement of cylinder power.
If the starting point refraction contains no cylindrical component, the cross cylinder may be used to check for the presence of astigmatism in the following manner. The cross cylinder is first placed at 90° and 180°. If a preferred flip position is found, cylinder is added with the axis parallel to the respective plus or minus axis of the cross cylinder until the 2 flip choices are equal. If no preference is found with the cross cylinder axes at 90° and 180°, then check the axes at 45° and 135° before assuming that no astigmatism is present. Once any cylinder power is found, axis and power should be refined in the usual manner.
Another method of determining the presence of astigmatism is to dial 0.50 D of cylinder into the phoropter (while preserving the spherical equivalent with a compensatory 0.25 D change in the sphere). Ask the patient to slowly rotate the cylinder axis once around using the knob on the phoropter. If the patient finds a preferred position, it becomes the starting point for the cross cylinder refinement. If doing so has no effect, there may be no clinically significant astigmatism. Keratometry or corneal topography may be helpful if the examiner suspects that astigmatism may still be present.
Always refine cylinder axis before refining cylinder power. This sequence is necessary because the correct axis may be found in the presence of an incorrect power, but the full cylinder power is found only in the presence of the correct axis. The axis may be rechecked after the power is determined and is more important in higher levels of astigmatism power.
Refinement of cylinder axis involves the combination of cylinders at oblique axes. When the axis of the correcting cylinder is not aligned with that of the astigmatic eye’s cylinder, the combined cylinders produce residual astigmatism with a meridian roughly 45° away from the principal meridians of the 2 cylinders. To refine the axis, position the principal meridians of the cross cylinder 45° away from those of the correcting cylinder (if using a handheld Jackson cross cylinder, the stem of the lens handle will be parallel to the axis of the correcting cylinder). Present the patient with alternative flip choices, and select the choice that is the blackest and sharpest to the patient. Then rotate the axis of the correcting cylinder toward the corresponding plus or minus axis of the cross cylinder (plus cylinder axis is rotated toward the plus cylinder axis of the cross cylinder, and minus cylinder axis is rotated toward the minus cylinder axis of the cross cylinder). Low-power cylinders are rotated in increments of 15°; high-power cylinders are rotated by smaller amounts, usually 5°, but can be as small as 1° for very high astigmatism. Repeat this procedure until the flip choices appear equal.
To refine cylinder power, align the cross cylinder axes with the principal meridians of the correcting lens (Fig 4-17). The examiner should change cylinder power according to the patient’s responses; the spherical equivalent of the refractive correction should remain constant to keep the circle of least confusion on the retina. Ensure that the correction remains constant by changing the sphere half as much and in the opposite direction as the cylinder power is changed. In other words, for every 0.50 D of cylinder power change, change the sphere by 0.25 D in the opposite direction. Periodically, the sphere power should be adjusted for the best visual acuity.
Figure 4-17 Jackson cross cylinder. A, In the phoropter. B, Manual trial lens. C, Rotating manual trial lens.
(Parts A and C courtesy of Thomas F. Mauger, MD. Part B courtesy of Tommy Korn, MD.)
Continue to refine cylinder power until the patient reports that both flip choices appear equal. At that point, the 2 flip choices produce equal and opposite mixed astigmatism, blurring the visual acuity chart equally.
If the examiner is planning on prescribing for astigmatism at an axis different from that measured with the cross cylinder (ie, 90° or 180° instead of an oblique axis) the cross cylinder can be used to measure the cylinder power at the new axis.
Remember to use the proper-power cross cylinder for the patient’s visual acuity level. For example, a ±0.25 D cross cylinder is commonly used with visual acuity levels of 20/30 and better. A high-power cross cylinder (±0.50 D or ±1.00 D) allows a patient with poorer vision to recognize differences in the flip choices.
The patient may be confused with prior choices during cross cylinder refinement. Giving different numbers to subsequent choices avoids this problem: “Which is better, 1 or 2, 3 or 4?” and so forth. If the patient persists in choosing either the first or second number, reverse the order of presentation to check for consistency.
Table 4-2 summarizes the cross cylinder refraction technique. Also see Part 2 of the Quick-Start Guide.
Guyton DL. Retinoscopy: Minus Cylinder Technique, 1986; Retinoscopy: Plus Cylinder Technique, 1986; Subjective Refraction: Cross-Cylinder Technique, 1987. Reviewed for currency, 2007. Clinical Skills DVD Series [DVD]. San Francisco: American Academy of Ophthalmology.
Jackson E. A trial set of small lenses and a modified trial size frame. Trans Am Ophthalmol Soc. 1887;4:595–598.
Wunsh SE. The cross cylinder. In: Tasman W, Jaeger EA, eds. Duane’s Clinical Ophthalmology on CD-ROM. Vol 1. Philadelphia: Lippincott Williams & Wilkins; 2006.
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.