Convergence ability can be tested with 2 different methods:
- Near point of convergence (NPC) - is the point where the visual axes intersect under the maximum effort of the convergence. It is a measure of pursuit convergence.
- Jump convergence - the convergence level being increased by rapid jump to a closer point. A qualitative assessment of the convergence quality of fixation jumps from distant to the near target point.
The convergence ability should be a part of routine examinations. Patients with a poor convergence while reading something can suffer from headache and diplopia. People also can complain of blurred words after reading. Sometimes children are shy and do not complain about this symptoms. Surprisingly patients with poor convergence can also be asymptomatic. If a child is diagnosed with convergence weakness he should be re-tested and it is useful to obtaining a full history of complains. In general asymptomatic convergence weakness don't need any specific treatment.
It is not appropriate to test convergence ability for heterotropia patients
Near point of convergence
This is a standard test for convergence ability. It is easy to perform and does not require special equipment.
Equipment: A fixation target and a ruler.
The fixation target is a pen or pencil. To get the child's attention a small toy with common character can be used. It should be with small details that are clear to see. Also there is special ruler for convergence and accommodation test.
If there is a high level myopia or hypermetropia test should be done with needed correction in advance.
- Patient should sit down in a comfortable chair with looking directly at the fixation object that is placed at the nose level approximately 50 cm from it.
- Room should be well illuminated so the examiner could notice minimal changes in eye movements.
- Examiner will move the fixation object slowly and smoothly close to the patient's nose. Moving speed should be proximately 40 cm in 10 seconds.
- Ask the patient to note when he sees the fixation object in double. We should explain to the patient that object can become blurry, but still need to continue to fixate on it.
- While performing the test, the examiner should observe the patient's both eyes because some patients do not notice diplopia - when one of the eyes is not fixating to the object. As a result this point is the near point of the convergence .
- Move the fixation object far front from both eyes of the patient until they start to fixate on the object. Keep in mind that there is a recovery point when we are moving from the diplopia point to the point where the convergence will act again.
The test should be performed more than once to be sure that the patient understood and will follow the test. For the patients with convergence insufficiency repeated tests will move the near convergence point far from the previous ones.
Normal near point of convergence is about 6-10 centimetre for normal eyes but the convergence recovery point (CRP) is until 15 centimetre. If the near point of convergence (NPC) is more than 10 centimetre there is sign of poor convergence.
In addition, long hours and intense reading in people with convergence problem can cause asthenopia (diplopia and headaches). In this situation the recovery near point is more than 15 centimetre.
The results of the test should be noted for example, NPC 7cm, CRP 12 cm
This test is more realistic to typical near viewing situation. As additional test it can be used for the patients with signs of the convergence insufficiency.
Equipment: The Dot Card or Brock String. The length of the card can be vary but must not be shorter than 20 centimetre. There are some black points at the midline of the card that are placed 1-2 centimetre from each other.
If the patient is high myopic or hypermetropic then the test should be performed with needed correction.
- Patient should sit down in a comfortable chair and the card is placed at the patient's nose level.
- Room should be well illuminated, so any changes in eye movements can be noticed easily.
- Patient will be asked to fixate at the farrest point at the card and should be asked:
- How many lines are there? - Are these lines crossing at the fixation point?
- If the patient can fixate to this point then move it to the closer point until the patient sees two parallel lines or just one line (in this case the other eye is suppressed)
- The closest point where the patient can see these two lines crossing at the fixation point is the near point of convergence (NPC).
The test should be performed more than once to be get sure that the patient understood and will follow the procedure. For patient who has convergence insufficiency repeated tests will move the near convergence point far from the previous ones.
Test results should be written down for example convergence 8 centimetre (Brock String)
In an individual with a good convergence ability both tests should show the similar results.
Most commune errors
- Relying on subjective near point convergence (NPC) measures and not objective observation of the eye fixation.
- Doing the test only once.
- Moving the target too fast that can over estimate the convergence ability or moving the target too slow so the child can lose the interest of doing the test.
- Not complying patient or where the examiner is not encouraging the patient (most of the time a child) to follow the NPC target. Using different objects at each time.
- Not testing the patient in non standard gaze for convergence studies which is slight downward gaze for example testing in upward gaze.
- David B. Elliott PhD, Clinical procedures in Primary eye care Fourth edition. Elsevier limited, 2014
- Brad Bowling Kanski's Clinical ophthalmology Eight edition. Elsevier limited, 2016
- Frank Eperjesi PhD, Hannah Bartlett PhD, Mark Dunne PhD Ophthalmic Clinical Procedures Elsevier limited, 2007
- Darcy A.Umphred PhD, Gordon U.Burton PhD, Rolando T. Lazaro PhD, Margaret L. Roller Umphred's neurological rehabilitation Six edition. Mosby, 2013.