Introduction
Understanding the Jackson Cross Cylinder test is crucial for any budding optometrist. This technique is a cornerstone in refining astigmatic corrections, a common necessity in many eye exams.
The Jackson Cross Cylinder test not only identifies the presence of astigmatism but also fine-tunes the correction to ensure optimal visual acuity. Whether you’re new to the field or looking to deepen your knowledge, this article will guide you through the essentials of the Jackson Cross Cylinder, its history, application, and common pitfalls.
The History of the Jackson Cross Cylinder
The Jackson Cross Cylinder (JCC) has its roots in the late 19th century. In 1887, Edward Jackson introduced a method to detect astigmatism using modified Stokes lenses1. This was a significant advancement in the field of optometry. Fast forward to 1907, Jackson refined his invention, developing what we now know as the Jackson Cross Cylinder. This tool was designed to accurately determine the axis and power of astigmatism.
Jackson’s contribution revolutionised the way optometrists approached astigmatism. Before his invention, detecting and correcting astigmatism was a challenging task. The JCC provided a systematic way to refine astigmatic correction, making it an essential tool in eye exams.
This cross-cylinder lens, which is characterised by its two cylindrical lenses of equal but opposite power, allowed optometrists to flip between lenses to find the best correction for the patient.
Over the years, the Jackson Cross Cylinder has remained a staple in optometric practice, thanks to its simplicity and effectiveness. It’s a testament to Jackson’s ingenuity that his invention, now over a century old, is still in regular use today.
Why We Do the Jackson Cross Cylinder Test
Astigmatism is a common refractive error, causing blurred vision due to an irregular curvature of the cornea or lens. This irregularity prevents light from focusing properly on the retina, leading to multiple focal points instead of a single one2,3. The Jackson Cross Cylinder test is essential in diagnosing and refining the correction for astigmatism, ensuring patients achieve optimal visual clarity.
The Conoid of Sturm is key to understanding how the Jackson Cross Cylinder works. It describes the light focus in an astigmatic eye, where light forms two focal lines at different distances from the retina2,3, creating an “interval of Sturm”2. At the midpoint of this interval is the circle of least confusion, which represents the point of minimal blur2,3. This can be seen in more detail in the image below:
Using the Jackson Cross Cylinder, optometrists manipulate these focal lines to refine astigmatic correction3. By flipping the JCC in front of the eye, the optometrist determines which orientation provides a clearer image, adjusting the axis and power to position the circle of least confusion on the retina. This minimises the interval of Sturm, reducing blur and enhancing clarity.
Accurately determining the axis and power of astigmatism is crucial, as even minor errors can cause significant discomfort and suboptimal vision. The Jackson Cross Cylinder allows for precise corrections, ensuring the best possible visual outcomes for patients.
How to Perform the Jackson Cross Cylinder Test
Conducting the Jackson Cross Cylinder test follows the determination of the Best Vision Sphere (BVS)4. Ensuring that the circle of least confusion sits on the inner limiting membrane is critical for accurate results. This guide is written for those working in minus cylinder (the generally accepted standard in UK optometry), however if you do work in plus cylinder, you would work towards the positive cylinder markings instead. Here is a step-by-step guide:
1. Selecting the Appropriate JCC Lens
Choose a Jackson Cross Cylinder lens based on the patient’s visual acuity (VA) after the BVS and duochrome test. For VA of 6/12 or better, use a ±0.25DC JCC lens; for VA of 6/18 or worse, use a ±0.50DC JCC lens. Ensuring that duochrome is equal or slightly on the green helps confirm the circle of least confusion is appropriately positioned.
2. Choosing the Right Target
Opt for a target like Verhoeff’s circles4 or concentric rings. The thickness and diameter of the inner ring should align with 6/6 vision, and the outer ring with 6/15. Dotted rings are also a suitable choice.
For poorer visual acuities, selecting an “O” or a “C” on the Snellen/LogMAR chart that is at least one line larger than the patient’s current acuity4 will also act as a suitable target.
3. Setting Up the Room and Patient
Ensure room lighting is adequate. Occlude one eye (left) and test the other eye (right), and then switch. Select the target and explain the test to the patient. An example of what to say is given below, but as long as all of the relevant information is conveyed, you may alter the wording accordingly.
“During this test, I will hold this lens in two positions in front of your eye to find the side that gives you the best vision. Don’t worry about giving a wrong answer; everything is double-checked. Compare the roundness and clarity of the circles/dots while I hold this lens in different positions. The circles/dots may look slightly blurred in both positions, but tell me which position makes them rounder and clearer or whether they both look the same.”
4. Refining Cylinder Axis
Start with refining the cylinder axis first. Align the Jackson Cross Cylinder handle with the axis of the cylindrical lens in the trial frame. Flip the JCC and ask, “Are the rings rounder and clearer in position 1 or 2?” Adjust the axis towards the minus mark of the JCC, closer to the handle. Continue until there’s no difference detected between the two positions or the point where the patient’s responses see-saw back and forward over a given axis.
If there is any doubt on the final axis found, it is important to note this end-point and then readjust the axis away from this point to see if the axis returns to this same point4. This could be incorporated as part of your routine as it acts as good practice to ensure you have the correct final axis.
Below is an animation on how to refine the cylinder axis with the Jackson Cross Cylinder.
Doing this step allows for the correct orientation of the interval of Sturm, allowing for the shape projected on to the retina to become more circular
5. Adjusting Cylinder Power
Set the JCC with one of its axes parallel to the trial cylindrical lens axis. Flip the JCC and ask, “Are the rings rounder and clearer in position 1 or 2?” Increase the cylinder power if the patient prefers the minus axis (side 1); decrease if the plus axis (side 2) is preferred. Repeat until there is see-sawing of reversal on presentation (and opt for the lowest powered cyl). It may be that the patient reports that they appear equally blurry, and this too can be considered an end point.
Doing this step will help change the length of the interval of Sturm, allowing the size of the circle of least confusion to become smaller and allow for the projected image on the retina to become sharper.
6. Fine-Tuning Sphere Power Adjustments
For each 0.50DC change in cylinder power, adjust the sphere power by 0.25DS in the opposite direction. For instance, if the cylinder is increased by -0.50DC, adjust the sphere power by +0.25DS4. This ensures the circle of least confusion remains in the correct position.
I have nicknamed this the “2-for-1 rule” to remind myself that for every two 0.25 cylinder power changes that I have to change the sphere power by one 0.25 in the opposite direction.
Following these steps ensures a precise and effective Jackson Cross Cylinder test, leading to accurate astigmatic correction for optimal visual acuity.
Common Errors
Performing the Jackson Cross Cylinder test can be tricky, and even seasoned optometrists can run into a few common errors:
Misalignment of Axes
Ensure that the Jackson Cross Cylinder is aligned correctly with the trial frame axes. Misalignment can lead to incorrect axis and power determination4, skewing the results.
Incorrect Presentation Times
The timing for presenting each lens position is crucial. Move too quickly, and the patient may not have enough time to see the differences4; move too slowly, and their responses may become inconsistent. Find a balance to ensure accurate responses.
Ignoring Sphere Power Adjustments
After a significant change in cylinder power (0.50DC or more), remember to adjust the sphere power accordingly. Neglecting this step can disrupt the position of the circle of least confusion and cause their vision to start to blur. This may cause answers obtained via Jackson Cross Cylinder to become less reliable.
Handling Unreliable Patient Responses
Patients may provide inconsistent responses due to indecision or misunderstanding. Clearly explain the test and ensure your instructions are precise. If inconsistency persists, consider using alternative methods like the fan and block test.
Assuming Axis is Correct Prematurely
A patient stating “both look the same” doesn’t necessarily mean the axis is correct. It could indicate no astigmatism or an axis that’s significantly off. Double-check these findings for accuracy – especially if the visual acuity is less than expected (or worse than prior to Jackson Cross Cylinder).
Inadequate Axis Adjustment
Small adjustments on smaller cylinders can significantly increase your testing time, so remember to bracket your adjustments and narrow them down to the final alignment.
In larger cylinders, large adjustments can significantly degrade the visual acuity and prevent your patient from distinguishing between the options4. In cases where the astigmatism is in excess of 2.00 DC, consider 5 degree steps4 of adjustment – especially if your starting acuity is 6/7.5 or better.
By being mindful of these common errors, you can refine your technique and achieve more accurate results with the Jackson Cross Cylinder test.
Summary
The Jackson Cross Cylinder test is an invaluable tool in the optometrist’s arsenal for accurately diagnosing and refining astigmatic corrections. By following the step-by-step procedure, selecting the appropriate JCC lens, and making necessary adjustments to the cylinder and sphere powers, you can achieve precise astigmatic correction.
Common errors, such as misalignment of axes and incorrect presentation times, can be mitigated by careful technique and patient communication. Understanding these pitfalls and how to avoid them will enhance the accuracy of your testing. Never be afraid of double-checking the results, especially if your patient’s responses are inconsistent.
The Jackson Cross Cylinder test, with its rich history and proven effectiveness, remains a cornerstone in the field of optometry. Mastering this technique not only improves patient outcomes but also strengthens your expertise as an optometrist.
Frequently Asked Questions
What is the Jackson Cross Cylinder test?
The Jackson Cross Cylinder test is a technique used in optometry to refine astigmatic correction. It helps in determining the axis and power of astigmatism by flipping between two cylindrical lenses of opposite power.
How does the Jackson Cross Cylinder work?
The Jackson Cross Cylinder consists of two cylindrical lenses with equal but opposite powers, placed at 90 degrees to each other. By presenting each side to the patient and asking which view is clearer, the optometrist can accurately adjust the axis and power of the astigmatic correction.
When should the Jackson Cross Cylinder be used?
The Jackson Cross Cylinder test is used after the best vision sphere (BVS) determination and duochrome test. It is crucial in cases where astigmatism is suspected, and a precise correction is needed.
How do I handle inconsistent patient responses during the Jackson Cross Cylinder technique?
Clearly explain the test procedure to the patient, and ensure your technique is consistent. If inconsistent responses persist, consider alternative methods like the fan and block test for assessing astigmatism.
Remember, there is no harm in rechecking responses should you doubt the reliability of the responses given by your patient.
How do you choose the most appropriate Jackson Cross Cylinder power?
The choice depends on the patient’s visual acuity. For VA of 6/12 or better, use a ±0.25DC lens; for VA of 6/18 or worse, use a ±0.50DC lens.
Skill Activity
Take part in this skill activity to test your knowledge on the Jackson Cross Cylinder. Please take the time to engage with these multiple choice questions as cementing your understanding in the Jackson Cross Cylinder technique is key to performing accurate refractions.
Further Reading
- Tsakiris KA, and Moschonas K (2024). Cross-cylinder technique for subjective refraction. EyeWiki [Online.] Available at: https://eyewiki.org/Cross-Cylinder_Technique_for_Subjective_Refraction#:~:text=In%201887%2C%20Edward%20Jackson%20described,JCC)%20(see%20image). [Accessed: 3rd November 2024].
- Askovitz SI (1956). The circle of least confusion on Sturm’s conoid of astigmatism. AMA Archives of Ophthalmology 56(5): 691-697.
- Parthasarathi P, and Stockkermans TJ (2024). The conoid of Sturm. StatPearls. Treasure Island: StatPearls Publishing. [Online]. Available at: https://www.ncbi.nlm.nih.gov/books/NBK587355/#:~:text=The%20conoid%20of%20Sturm%20is%20the%20configuration%20of%20rays%20refracted,called%20the%20interval%20of%20Sturm. [Accessed: 3rd November 2024].
- Elliot DB (2007). Determination of the refractive correction. Primary Procedures in Primary Eye Care: 3rd Edition. Philadelphia: Butterworth Heineman pp. 112-114.
If you have found this article useful, please recommend it to your friends by sharing via email or social media. The continued effort to create content such as this is reliant on people accessing and viewing The Eye Care Advocate. Be sure to sign up to our updates by subscribing below. We aim to release an article every week, which aims to help us upgrade eye care and eye care advocacy across the globe.
Leave a Reply