The near point of convergence (NPC) is an important measurement in optometry, often overlooked by those both inside and outside the field. It’s a measure of how well the eyes can converge, or turn inward, to maintain single vision when looking at a close object. This simple yet effective test can diagnose a range of vision problems, from convergence insufficiency to more complex binocular vision issues.
Understanding and accurately measuring the near point of convergence is essential for anyone studying optometry or vision sciences. Whether you’re a seasoned professional looking to refresh your knowledge or a student new to the field, this guide will provide you with practical insights and step-by-step instructions on how to measure the near point of convergence.
- What is the Near Point of Convergence?
- Equipment Needed for Measuring the Near Point of Convergence
- A Step-by-Step Guide to Measuring the Near Point of Convergence
- Recording the Results
- Interpreting the Results
- Skill Activity
- Further Reading
What is the Near Point of Convergence?
The near point of convergence (NPC) is a measurement in optometry that assesses how well the eyes can converge (turn inward towards each other), when focusing on a close object. This point is the closest distance at which the eyes can maintain single vision before experiencing double vision. Essentially, the NPC tells us about the efficiency and effectiveness of the eye muscles involved in convergence.
When the eyes cannot converge properly, it can lead to symptoms such as eyestrain, headaches, and difficulty with close-up tasks such as reading. Convergence insufficiency is a common issue linked to a reduced NPC. This means that the eyes struggle to converge making maintaining single vision at a closer distance difficult, causing visual discomfort and double vision.
Understanding and measuring the near point of convergence is vital in diagnosing and managing conditions like convergence insufficiency. As optometrists, we often perform NPC assessments during routine eye exams to ensure that any potential issues are identified and managed. Doing this as part of a regular, routine eye exam means we can identify and manage it before it becomes a problem, affecting the daily lives of our patients.
Equipment Needed for Measuring the Near Point of Convergence
To accurately measure the near point of convergence (NPC), it’s essential to have the right tools at hand. While the test itself is straightforward, using the appropriate equipment ensures precise and reliable results.
Fixation Targets:
There are multiple targets that you may consider using when measuring the near point of convergence and these targets may have an impact on the measured near point of convergence. It is important to keep in mind that there are pros and cons to each target and you should be aware of the underlying mechanisms that explain this.
There is often debate on which to use during the measurement of the near point of convergence and different university lecturers and supervisors may urge you to use one method or another. Please ensure you know which your assessor wishes you to use, as this guide is for information purposes only.
Pen or Pencil: A common and readily available target for NPC tests. Ideally you will have access to a pen or something similar in shape and size in most clinics
Finger or Fingertip: Another easy option, especially when you don’t have other tools available.
Budgie Stick: A favourite among optometrists, this stick often features small letters or characters to engage patients, particularly children.
RAF Rule: This versatile tool, also known as the Royal Air Force rule, is specifically designed for near point of convergence and for accommodation measurements. It includes various accommodative targets on a rotating four-sided cubical drum, making it ideal for detailed assessments (see photo below).
Why the Debate on the Target Used?
Essentially the debate arises due to the complex relationship that accommodation has on convergence. As the eyes converge, it triggers accommodation and as the eye accommodate, it triggers convergence.
If you use an accommodative target (such as small characters or letters on the budgie stick or RAF Rule), the eyes will also accommodate and this will increase the eyes ability to converge (likely giving a better reading). This could mean it is more sensitive to picking up convergence insufficiency.
However, with a smaller, accommodative target, patients without accommodation (such as presbyopes or those with other accommodative dysfunction) may notice the blurring of the target as it comes closer, either resulting in not being able to see it clearly enough to maintain fusion or falsely report their blur as double vision – potentially leading to a false diagnosis of accommodative insufficiency.
Larger, non-accommodative targets, such as your finger or a pen, limit the effect of accommodation and generally result in a more isolated assessment of convergence (there may still be some accommodation, but it is less likely to be engaged to the same extent). As the target is larger, it will also be less easy to notice when it blurs, meaning the near point of convergence is likely to be closer to the eye in these cases – as the patient is less likely wrongly to note the blurring as diplopia.
However, it may be less sensitive at detecting subtle convergence issues, as the larger target may be more tolerant to slight misalignments.
Studies, such as that by Siderov et al. (2001), have found that clinically the target size is likely insignificant in measuring near point of convergence in routine primary care.
Ruler or Tape Measure:
An essential piece of equipment for measuring the distance from the fixation target to the patient’s eyes. A standard ruler, tape measure or a specially designed one for NPC testing, such as the one marked upon the RAF rule, generally works well.
If you don’t have one as part of your kit, small, portable ones are cheap enough to buy on Amazon. For other budget additions for your optometry kit, please check out the post on Learning Clinical Skills at Home: Budget Equipment for Student Optometrists.
Specialised NPC Devices:
Bernell Vergel™ Device: A battery-operated device featuring a 9-point font target. It’s designed for precise NPC measurements and is particularly useful in clinical settings. You can read more about this item here: Bernell Vergel™ Device. Whilst technology can advance your practice, this test can be done with just your finger and ruler – so don’t feel you need to break the bank by reading that this device exists!
Each of these tools has its own advantages and can be chosen based on the specific needs of the patient and the clinical setting. Try and keep your techniques consistent between appointments as to reduce the risk of different targets affecting the near point of convergence between baseline and subsequent visits.
A Step-by-Step Guide to Measuring the Near Point of Convergence
Measuring the near point of convergence is a relatively quick and simple test that can be done within a minute, making it a useful tool to include in your routine eye examination entry tests. Personally, I do this technique immediately after near cover testing – as I already have a near target in my hand and perform this prior to recording my cover test results to make best use of my time.
For this case, I will use the technique performed without the RAF Rule, as not everyone will have access to one – but a pen or finger is usually more accessible!
1. Ensure the room is well illuminated and ensure the patient is wearing their near prescription (if required). Explain to the patient that you are going to look at how well their eyes come together when things get closer to them.
2. Sit directly in front of your patient and ask them to keep their head straight and position their eyes in a slight downwards gaze.
3. Position your target (pen/fingertip/letter on the budgie stick) approximately 50 cm in front of the patient, but slightly below eye level.
4. Ask your patient (with both eyes) to watch the target, confirm that they can only see one of them and tell them you will move the target closer to them. They need to inform you when the target becomes doubled/breaks into two images, but not blurred. You will need to inform your patients that it is likely to go blurry before it goes double.
5. Smoothly, but slowly, move the target closer to the bridge of your patient’s nose. The process should take about 10 seconds, so aim to be moving at a speed of 4-5 cm per second.
6. Watch the patient’s eyes as you do this. You can make an objective measurement of the near point of convergence when one of the patient’s eyes flick outwards. This is often met with the patient informing you that they now have double vision (subjective near point of convergence).
7. When the eyes break convergence, ask the patient to apply more effort to see if they can re-establish single vision again. If this can be done, continue to move forward towards the nose. At the break point you can measure the “break point” from the ruler or tape measure. If the patient can converge on the target as it touches the bridge of their nose, the near point of convergence can be recorded “to the nose”.
8. Once the patient cannot make the target single again, move the target back and see if they can make it single again. When they do, you can record this as the “recovery point”. If the near point of convergence was to the nose, no recovery point is required.
9. Repeat the test to confirm your findings, before recording them. It is repeated to ensure that you can assess for test repeatability with the patient. Remember, each time to inform them to let you know when it goes double and not blurred.
Recording the Results
It is important to record your findings – even if the near point of convergence is normal. Ideally, record “near point of convergence” or “NPC”, followed by the distance of the break point and then the recovery point (Barrett and Elliott, 2007).
When watching the eyes, I find making a note of the eye that loses fixation can be useful in determining if that eye suppresses. If they note double vision, then suppression is less likely. However, if they do not note diplopia and you objectively see a separation of the eyes then the deviating eye is potentially suppressing. Therefore make a note of this finding.
Interpreting the Results
After measuring the near point of convergence, understanding the results is key to understand if the patient’s convergence is normal or abnormal. Please do take time to pay attention to these findings and do not just measure the near point of convergence and then move on without considering what the results mean.
Normal Values
Rouse et al. (1999) found that both adults and children should be able to converge to 7.5 cm before breaking, with a recovery point of approximately 10.5 cm. However, Hayes et al. (1998) found that the majority younger children had convergence of less than 6 cm, so do consider convergence issues if a child is symptomatic and has a near point of convergence of less than 6 cm.
Figures for a near point of convergence that is more than the 7.5 cm break and 10.5 cm recovery does hint at convergence insufficiency and and further investigation into the patient’s accommodation and convergence should be performed. One would also suggest starting by assessing for distance and near heterophoria via the cover test and the patient’s fixation disparity, before considering jump convergence, accommodative functions and fusional reserves.
Skill Activity
Near Point of Convergence Skill Activity
Further Reading
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