Please note, this article of fixation disparity is long and as such use of the table of contents below, along with the “back to top” buttons is advised.

Introduction

Fixation disparity is a subtle ocular misalignment that occurs when both eyes attempt to fuse a single image^{1,2,3,4,5,6,7}. Even when a person sees one image, the eyes may not be perfectly aligned, and this small misalignment can lead to symptoms like eye strain, headaches, and poor concentration⁵.

Detecting and correcting fixation disparity is essential for improving visual comfort and performance, making it a critical area in optometric practice³. This guide, aimed at optometry students and pre-registration optometrists, aims to cover fixation disparity and aid those who find the topic complex.

Please note, binocular vision is a vast and complex field and as such some concepts are only briefly discussed (or otherwise purposefully omitted) as to not distract away from the purpose of this article (which is to perform and interpret fixation disparity within an optometric clinical setting).

Disclaimer: This article is for educational purposes only and not for self diagnosis. If you have any of the symptoms described within this article, it is imperative that you discuss them with your optometrist or general health practitioner.

The History of Fixation Disparity

The concept of fixation disparity has intrigued eye care professionals for decades (and terrified optometry students for just as long!). Early explorations into binocular vision and the subtle misalignments that can occur paved the way for the development of various testing methods.

The Mallett unit, introduced by Robert Mallett in the 1960s³, revolutionised how optometrists and ophthalmologists approached the detection and correction of fixation disparity. Designed to create natural viewing conditions, the Mallett unit uses a central fixation target alongside monocular markers, providing a practical and efficient way to measure and correct these small ocular misalignments.

The Mallett unit is particularly valued in the UK^3,5,6, where it has become a standard tool in many practices. Its ability to determine the minimum prism power needed to correct fixation disparity has made it indispensable for eye care professionals seeking to alleviate symptoms such as eye strain, headaches, and poor concentration in their patients⁵.

Parallel to the development of the Mallett unit, other methods like the Wesson card⁷, nonius lines⁷, and disparometer⁷ were also introduced, each offering unique benefits for measuring fixation disparity. The Thomson Test Chart (and other derivatives of electronic testing charts) are another significant advancement, providing a digital solution for fixation disparity testing. With its polarised mask and versatile testing options, it allows for simultaneous detection of horizontal and vertical disparities.

These tools and methods have not only enhanced our understanding of binocular vision but also improved the accuracy and efficiency of clinical assessments. The evolution of fixation disparity testing reflects the ongoing commitment to advancing eye care, ensuring patients receive the most effective treatments for their visual needs. As such, a robust understanding of fixation disparity is vital to ensure it is correctly assessed and managed by optometrists.

Understanding Fixation Disparity

As mentioned in our introduction, fixation disparity occurs when the eyes aren’t perfectly aligned while focusing on a single point. Even though a person sees one image, their eyes may be slightly misaligned⁶. This subtle misalignment can lead to eye strain, headaches, and difficulty concentrating⁵.

In an ideal situation, the visual axes of both eyes would converge precisely on whatever is being looked at, requiring no effort. However, some people have a slight tendency for their eyes to drift inward (esophoria) or outward (exophoria) when not actively maintained⁶. This tendency is known as heterophoria^1,5,6. To keep vision single and clear, the visual system employs fusional reserves—muscular efforts that compensate for these misalignments.

When these fusional reserves are under stress or become insufficient, they may not fully correct the misalignment, resulting in fixation disparity. Despite this, the brain still fuses the images from both eyes into one. This is possible because of Panum’s fusional area, a zone where slight differences in the images from each eye are combined seamlessly. The figures below help to explain this.

Understanding fixation disparity is important because even small misalignments can cause significant discomfort over time. Activities that require prolonged focus, like reading or computer work, can exacerbate symptoms^1,5. Identifying and addressing fixation disparity can greatly improve visual comfort and reduce associated symptoms. By recognising how the eyes work together and the factors that lead to misalignment, effective solutions can be implemented to enhance overall visual performance.

Fixation Disparity Testing Methods

There are several ways to test for fixation disparity. Each method has its own approach and benefits. Understanding these can help in choosing the right test for each patient.

Mallett Unit Fixation Disparity Test

The Mallet unit is a popular tool, especially in the UK^5,6. It uses a central fixation target with two monocular markers. These markers are seen by each eye separately when the patient wears polarised filters or lenses^1,3,5,6. The test creates natural viewing conditions, which is important for accurate results. The patient looks at the target and reports if the markers are aligned. If they’re misaligned, prisms are added until alignment is achieved^1,5. The minimum prism power (aligning prism) needed indicates the degree of fixation disparity¹.

Nonius Lines Test

This test uses a pair of lines, called nonius lines⁷. One line is seen by one eye, and the other line by the other eye. For horizontal fixation disparity, vertical lines are used. For vertical disparity, horizontal lines are used. The patient adjusts the lines until they appear aligned. The amount of adjustment needed shows the fixation disparity. This method measures both horizontal and vertical disparities effectively.

An example of how fixation disparity can be subjectively experienced can be seen in the figures below. The first figure will show the nonius lines that you can use to demonstrate fixation disparity yourself (follow the instructions). The second figure is an animated example to demonstrate what is visible to those who may not be able to get the test to work through lack of binocularity or suppression.

The animation illustrates perception after fusion. Each central vertical line is seen by one eye only and does not fuse, whereas the central circles and ‘Xs’ are viewed binocularly. The fusion of the central circle indicates approximate alignment of the eyes. Any misalignment or movement of the vertical lines relative to each other reveals the fixation disparity.

Whilst this example does not quantify the amount of fixation disparity, it does help demonstrate the subject view of nonius lines and fixation disparity – which may aid in your understanding.

Wesson Card

The Wesson Card is a simple and effective tool used to measure fixation disparity. It has a central fixation target and several lines offset by known amounts⁷. The patient wears polarisation glasses and reports which coloured line appears aligned with the target, whilst maintaining the clarity of the words that surround it⁷.

The Wesson Card is useful for detecting small disparities and is easy to use in a clinical setting.

Disparometer

The Sheedy Disparometer measures fixation disparity using polarised lines. It presents two lines with opposite polarisation. The patient wears polarised glasses, so each eye sees only one line. The lines are adjusted until they appear aligned to the patient⁷. This instrument provides precise measurements and is useful for detailed assessments.

Choosing the appropriate testing method depends on the patient’s needs and the clinician’s preference, however in many cases clinicians may be limited to what is available to them in their testing room. Several studies mention that there is poor agreement between the results of some of these tests^8,9,10, it is therefore vital to use the same testing technique when subsequently following-up patients.

Distance Fixation Disparity Testing with the Mallett Unit

Testing for distance fixation disparity using the Mallett Unit is essential for assessing binocular vision. This guide has included the Mallett Unit as it is present in many UK optometry testing rooms and, despite some differences in the testing method, the principles of the test are very similar.

The test should take place with the patient’s optimal optical prescription¹, following subjective refraction.

1. Preparation

Ensure the Mallett unit is correctly illuminated and calibrated for the correct distance. Confirm that the polarising visor matches the unit’s polarisation angles. Check that all equipment is functioning properly. Additionally ensure the patient has their optimal optical correction in place¹ and their eyes are viewing through the optical centres of the trial lenses to prevent prism being induced by lens decentration¹.

The bars should be aligned vertically to begin with, as so to test horizontal fixation disparity prior to vertical fixation disparity^1,5. An example of how the Mallett unit should be orientated and how the bars should look aligned can be seen below:

You can personally check the polarisation angles (i.e. what nonius bars are seen with each eye). Don’t do this via the patient, as occluding the eyes will interfere with their binocular vision and interrupt their performance on the test.

2. Explain the Test to the Patient

“This test will help us determine if your symptoms might be due to your eye muscles not working properly. Sometimes, the eyes can be slightly misaligned, which can cause issues like eye strain, headaches, or difficulty concentrating. By measuring this misalignment, we can better understand your symptoms and find the right solution to improve your comfort and vision.”

Ask the patient to look at the Mallett unit and ask them if they can see the two red lines. Additionally ask them if the two lines are in alignment as to let them know what alignment looks like⁵, whilst noting any perceived misalignment the patient may subjectively see prior to the test. This is important for when you come to interpret the results¹.

3. Applying the Polarising Filters

Place the polarising filters over the patient’s eyes. Ensure it fits comfortably and is properly aligned. The filters allows each eye to see one horizontal line separately while still viewing the central cross binocularly¹.

4. Assessing Alignment with Filters

Instruct the patient to look at the central cross again. Ask “can you still see two red lines”. If they cannot then deep central suppression may be present in the eye that should see the missing line and, as such, no further measurements from this test will be possible to obtain¹.

An example of this may be seen in the illustration below. With the top line being seen with the left eye and the bottom line being seen with the right, if the bottom line missing when the polarising filters are in place, then the patient is suppressing the right eye¹.

Should they still see the two vertical lines, ask, “Do the two lines now appear to be in line with each other?” If they report alignment, fixation disparity is not present horizontally. Where misalignment is present, proceed to the next step to determine the specifics of the misalignment.

5. Determining the Direction of Misalignment

To identify the direction of misalignment, ask the patient which line appears out of alignment. Use straightforward questions like, “Is the top line shifted to the left or right of the bottom line?” This helps pinpoint the direction of the shift, crucial for proper correction.

Several results can be reported if there is no suppression and fixation disparity is present. In these cases, assume that the right eye sees the bottom line and the left eye is seeing the top line.

a) If the lower line is to the left of the ‘X’ and the upper strip is to the right, the disparity is crossed and as such an exo disparity exists (see below)¹.

b) If the lower strip remains below the ‘X’ but the upper moves to the right, exo fixation disparity is present in the left eye only. Unilateral disparity is typically seen in the non-dominant eye and is more common in vertical misalignments (see below)¹.

c) The lower bar is to the right of the ‘X’ and the upper bar to the left of the ‘X’ and lower strip. This indicates there is uncrossed disparity and therefore the disparity is eso in nature (see below)¹.

It is important to note that if the top bar was being seen by the right eye and the bottom bar was being seen by the left eye, the inverse of crossed and uncrossed disparity from the above examples would be true. The interpretation can be extrapolated from these examples.

6. Neutralising Fixation Disparity

To neutralise fixation disparity, use the lowest prism power that corrects the misalignment. In cases of esophoria, the weakest positive spherical lens may be used instead.

When dealing with unilateral fixation disparity, apply the minimum prism to the eye showing the slip. If both eyes exhibit disparity, you only need to correct the disparity in one eye. It is not necessary to place prisms before both eyes¹.

Allow time for the eye to adapt to the new prism. Whenever you adjust the prism, have the patient read a few lines of the Snellen chart. Afterward, return to the Mallett unit to recheck for fixation disparity¹.

It is important to remember the base of the prism matters:

For horizontal slips¹:

Base OUT will correct for ESO misalignments
Base IN will correct for EXO misalignments

For vertical slips (see sections below on measuring vertical slip):

Base DOWN will correct for HYPER misalignments
Base UP will correct for HYPO misalignments

7. Verifying Consistency

Remove the correction and then reapply it to confirm consistent results. Ensure the patient consistently perceives alignment with the correction and misalignment without it. This step verifies the accuracy of the prescribed correction.

8. Measuring Vertical Fixation Disparity

Following on from the procedure described above, if there is horizontal fixation disparity present, you will need to correct this before measuring for vertical fixation disparity^1,5.

For measuring vertical fixation disparity, either rotate the OXO by 90 degrees (if using a rotating Mallett unit) to make the OXO appear vertical, providing horizontal nonius bars, or direct the patient to observe the horizontal bars on the full Mallett Unit¹.

Repeat the assessment as described previously for horizontal fixation disparity, adjusting the instructions to focus on the vertical alignment of the nonius bars¹.

9. Recording and Interpreting Results

When documenting the results of fixation disparity tests, several important notes should be considered. Always note the method used to obtain the measurements, in this case, the Mallett unit.

Suppression: If the patient could only see one strip and not both, one eye is likely suppressing. Record which eye is suppressing the image¹. In cases where suppression is intermittent, note whether the suppression is constant or intermittent, as some patients may see the lines coming into view intermittently.

No Fixation Disparity: If the lines are aligned in both meridians and there is no evidence of fixation disparity, document that there was no fixation disparity at the distance tested. For example, you might write, “Mallett unit: No FD in distance” or simply “Distance: no slip.”¹

Presence of Fixation Disparity: If fixation disparity is detected, record the lowest amount of prism or spherical lens required to align the bars¹. If the disparity is present in one eye only, specify which eye demonstrated the disparity¹.

Near Fixation Disparity Testing with the Mallett Unit

Assessing fixation disparity at near distances is essential, especially for patients who experience discomfort during close work. The Mallett unit offers an effective way to detect and correct these misalignments.

The near Mallett unit follows the same steps as for the distance Mallett unit, but with the patient holding the unit at their habitual working distance with the most optimal near visual correction in place^1,5. Remember to correct the patient’s near pupillary distance on the trial frame to ensure there is no induced prismatic effect caused by decentration of the lenses¹.

The patient should be asked to describe the alignment of the nonius lines prior to adding the polarising filter to assess for their understanding of alignment and once adding the filter, a paragraph of small text must be worn to ensure the patient is accommodating/focused at the correct viewing distance¹.

As per the previous guide on distance fixation disparity, you must correct for the horizontal fixation disparity prior to correcting for the vertical fixation disparity^1,5. That said, sometimes a vertical disparity can destabilise the horizontal, so some advise keeping an open mind when testing⁵.

By thoroughly assessing near fixation disparity and applying suitable corrections, practitioners can greatly enhance a patient’s visual comfort during close work. Addressing misalignments at near distances is vital for those who spend significant time reading or using digital devices.

Common Errors and Corrections in Fixation Disparity Testing

Conducting fixation disparity tests requires precision and attention to detail. Several common errors can affect the accuracy of the results. Being aware of these mistakes helps ensure reliable assessments and effective corrections.

Common Errors

1. Improper Calibration of Equipment and Pre-Test Checks

An incorrectly calibrated Mallett Unit or test chart can lead to misleading results. Regularly check that the equipment is functioning properly and the correct distance for the viewing distance being measured^10,11. Ensure that the polarising filters and visors match the polarisation angles of the test charts. Mismatched components can cause incorrect perceptions of alignment.

2. Inadequate Lighting Conditions

Poor lighting can strain the patient’s eyes and affect their responses^1,10. Ensure the testing room has appropriate illumination. When using polarised visors, the light levels reaching the eyes are reduced. Adjust the room lighting to compensate for this dimming effect.

3. Leading Questions and Communication Errors

The way instructions are given can influence the patient’s responses. Avoid leading questions that might suggest a particular answer^1,5,6. Use neutral language such as, “Do the lines appear aligned?” rather than, “The lines are aligned, aren’t they?” Clear, simple instructions help the patient provide accurate feedback.

4. Excessive Dissociation of the Eyes

Covering one eye or using methods that overly separate the visual input can disrupt binocular vision. The fixation disparity tests are designed to assess alignment under natural viewing conditions^1,3,5,6. Ensure that the testing procedure maintains normal binocular function.

5. Overlooking Pre-existing Misalignments

Some patients may report misalignment even before any filters or prisms are applied. Failing to recognise this can lead to incorrect interpretations. Always ask the patient if they notice any misalignment without the testing apparatus. Whilst the alignment may well be calibrated in alignment, some patients may be overly sensitive or perceive misalignment that may not actually be present⁵ and therefore knowing how the lines appear prior to assessment will allow you to keep this in consideration when interpreting the results.

6. Incorrect Use of Dioptric Corrections

Applying too much prism or spherical adjustment can overcompensate for the disparity. This may create new visual issues or discomfort. Aim to find the minimum correction needed to achieve alignment. This can be done by performing gradual adjustments to the correcting lenses to reduce the chance of overcorrecting¹.

7. Ignoring Patient Fatigue and Discomfort

Testing can be tiring, especially for patients with visual discomfort. Fatigue can affect their responses and the test’s accuracy. Monitor the patient’s comfort level throughout the procedure. If necessary, take short breaks to prevent fatigue from impacting the results. This test can be repeated at another time, prior to dispense of spectacles, if you feel the patient is too fatigued to continue.

Corrections and Best Practices

Regular Equipment Maintenance: Keep all testing equipment in good condition. Replace faulty visors, bulbs, or charts promptly. If the patient chair and/or testing chart can move, make sure that you check that they are set up at the correct working distances.

Proper Training: Ensure that you know what you are doing when conducting the tests. Understanding the nuances of the procedure ensures accuracy – and prevents incorrect prescribing (or non-prescribing) of prismatic correction.

Patient Education: Explain the purpose of the test and what the patient should expect. Informed patients are more likely to provide reliable responses. Remember to not lead them to a particular answer and to allow them time to both understand what they are doing and to provide an answer.

Consistent Testing Environment: Maintain consistent conditions during each test. This includes lighting, distance, and equipment setup. Remember to have the room lighting appropriate to allow for peripheral fusion lock and to account for decreased retinal illuminance from the polarising filters.

By recognising common errors and applying best practices, practitioners can improve the reliability of fixation disparity testing. Accurate assessments lead to better management of binocular vision issues, enhancing patient comfort and visual performance.

Clinical Applications and Interpretation

Fixation disparity testing is a powerful tool in eye care. By detecting subtle misalignments, we can tackle issues that might be causing patients discomfort. For example, someone struggling with eye strain during reading might have a hidden fixation disparity.

Applying the right prism correction can make a huge difference. Even a small adjustment can relieve symptoms like headaches and blurred vision. It’s satisfying to see patients for follow-up appointments no longer experiencing the symptoms they had prior to your intervention.

Interpreting the results involves looking at the patient’s symptoms alongside the test findings. Understanding whether the disparity is hyper, hypo, eso or exo helps in selecting the appropriate correction. Customising the approach ensures that the solution fits the patient’s specific needs.

Sometimes, vision therapy exercises may be recommended. These exercises can strengthen the eye muscles and improve coordination. By enhancing the fusional reserves, patients may rely less on optical corrections over time.

It’s important to remember that each person is unique. What works for one patient might not be the best for another. Ongoing assessment and open dialogue help in fine-tuning the treatment plan.

Integrating fixation disparity testing into regular check-ups can uncover issues that might otherwise be missed. This proactive approach enhances patient care and contributes to better visual health overall. By addressing fixation disparity, we’re not just correcting vision—we’re improving quality of life.

Conclusion

Understanding and addressing fixation disparity is crucial for comfortable binocular vision. By using tools like the Mallett Unit, we can detect subtle misalignments that may lead to discomfort.

Correcting these disparities enhances visual performance and reduces symptoms such as eye strain and headaches. Incorporating fixation disparity testing into regular eye examinations allows us to provide comprehensive care. It’s gratifying to help patients find relief from issues they might have endured for years.

Through careful assessment and appropriate corrections, we improve not only vision but overall quality of life. Fixation disparity testing is a valuable part of modern optometric practice, ensuring that patients see the world with clarity and comfort.

Frequently Asked Questions

Skill Activity

Whilst every care has gone into the construction of this skill activity, sometimes errors do occur (especially when we talk about binocular vision!). If you notice something that isn’t quite right, or want to know more on how we reach an outcome, please contact us on our contact page.

Further Reading

The topic of fixation disparity is vast and complex, therefore I urge you to perform some further reading around the subject, as well as performing the skill activity above to test your knowledge.

I include below the list of references that I have used to construct this guide upon fixation disparity and within are many sources that you may find useful in your studies. Do take the time to read them!

1. Barrett B, and Elliott DB (2007). Assessment of binocular vision. In: Elliott DB [Eds] Clinical Procedures in Primary Eyecare 3rd Edition. Philadelphia: Butterworth Heinemann pp. 151-217.
2. Freeman S (2014). Considering ocular motor balance in dispensing. Dispensing Optics 2014: 4-10.
3. Karania R, and Evans B (2006). Binocular vision: the Mallett fixation disparity test. Optician 2006 [Online.] Available at: https://www.opticianonline.net/content/features/binocular-vision-the-mallett-fixation-disparity-test/ [Accessed: November 10th 2024].
4. Jaschinski W (2017). Individual objective and subjective fixation disparity in near vision. PLoS One 12(1): e0170190.
5. Franklin A (2006). A guide to fixation disparity. Optician (No date) [Online.]. Available at: https://www.opticianonline.net/cpd-archive/4557/ [Accessed: November 10th 2024].
6. Karania R, and Evans B (2006). The Mallett fixation disparity test: influence of test instructions and relationship with symptoms. Ophthalmic and Physiological Optics 26(5): 507-522.
7. London R, and Crelier RS (2006). Fixation disparity analysis: sensory and motor approaches. Optometry 77: 590-608.
8. Ngan J, Goss DA, and Despirito J (2005). Comparison of fixation disparity curve parameters obtained with the Wesson and Saladin fixation disparity cards. Optometry & Vision Science 82(1): 69-74.
9. Hashemi H, Mirzaeian M, Narooie F, Nabovati P, Yekta A, Ostadimoghaddam H, Sardari S, and Khabazkhoob M (2021). Agreement of fixation disparity curve between two different instruments. Optometry and Vision Science 98(6): 629-635.
10. Fogt N (2023). Topical review: methodological variables in clinical and laboratory measurements of fixation disparity. Optometry and Vision Science 100(8): 572-594.
11. Jainta S, and Jaschinski W (2002). Fixation disparity: binocular vergence accuracy for a visual display at different positions relative to the eyes. Human Factors: The Journal of the Human Factors and Ergonomics Society 44(3): 443-450.

Recommended Textbooks:

The following textbooks are found on Amazon. Purchasing through these links may generate a commission fee for The Eye Care Advocate.

Pickwell’s Binocular Vision Anomalies

Clinical Procedures in Primary Eyecare: 5th Edition

Clinical Management of Binocular Vision: 5th Edition

Normal Binocular Vision: Theory, Investigation, and Practical Aspects

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