Schoolvision tests – Brock String

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Schoolvision tests – Brock String

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February 12, 2015Damaris Anderson

As part of a proper Schoolvision assessment, there are 8 tests necessary to complete the child’s eye examination. With thanks to Geraint Griffiths, the founder of the schoolvision technique, in the following 8 blogs we will describe each test separately through series of questions and answers.

Here is a list of the eight tests that we will cover in the upcoming blogs:

  1. Brock string
  2. Dynamic fixation
  3. Accommodation facility
  4. Measuring Muscle Balance
  5. CREST
  6. Trial Frame Challenges
  7. Retinoscopy
  8. Follow – up

 

Brock String

To start, here is a video overview of the Brock String test.

Now, to help you understand the test a little better, here is a series of questions and answers about the Brock String test.

Q: If the R string is higher and in front of the bead (i.e. the one seen by the L eye Ö ), does this mean a decompensated L hypo phoria? Or equally can this be called a R hyper?

A: String up, eye down just like the Nonius bars in the Mallet test. The base of the prism goes in the direction of the bar or the string, that is, in the opposite direction to the eye. There seems to be a complicating eso decompensation here as well (strings cross in front of the bead).

A very important point is that left hypo-phoria is not the same as right hyper-phoria. The prism whenever possible needs to be placed in front of the eye, which is showing the slip. This is the eye with the paraetic muscle. Mr Mallet has always said this (prism in front of the eye with the slip), and we now know this is absolutely key to resolving unstable eye dominance and dyslexia.

Splitting the prism for example should be avoided if possible, unless the required effect can be achieved by putting some prism in front of the other eye

Q: How do you know which eye is decompensating if both strings equally clear? I assume that if one eye is suppressing, this is the eye, which is decompensating?

A: Sometimes (rarely) the strings appear to cross at different places horizontally. Usually you have to defer back to the Mallet test to determine which eye is decompensating.

The suppressing eye is not always the one that is decompensating, suppression might be secondary to paresis in the other eye.

You can see that if this is the case and the suppressing eye is the dominant eye, prism in non-dominant eye could be the solution to the problem. The rule is prescribe for what you see, keep it simple and don’t try and second-guess the answer (until you’re really good and confident and can find no other way out; then expect to be wrong from time to time)

Q: If one string is coming and going, indicating (intermittent) suppression in that eye, does this mean that if prism is prescribed it should be given in the suppressing eye?

A: This may be true but doesn’t always follow (See above)

Q: I have really only been using the Brock in the primary position – should I be using it in all positions of gaze?

A: Yes you can, this would be like doing an amplified motility test you will immediately see what is going on in all directions of gaze.

Incommitant decompensation, however may be secondary to the muscle balance problem in the primary direction of gaze. This is why Dynamic Fixation is particularly good at picking up dyslexic tendencies. It is a motility test and a tracking test at the same time with the advantage over the Brock String, of being able to measure the effect with a stopwatch.

The reality is however, even though it is nice to know what is happening in different directions of gaze we are unable to vary the prism. A prism has to be prescribed for the primary direction. Fortunately this improves the situation is in all the other directions as testified by better performance on Dynamic Fixation.

The implication is that if the fixation disparity is corrected in the primary position the eyes will be better controlled in other directions of gaze, especially in the position where the affected muscle has it primary action

Q: Maybe it’s the way I’m describing the test or asking the questions, maybe it’s the age of some of my Schoolvision kids, but children find it difficult to understand how to do the test. They find it difficult to continue looking at the bead while I am asking them about the strings. Any tips?

A: A difficulty understanding the test is in itself a sign.

The main thing is to keep the questions simple and in a logical order e.g. With the string in the primary horizontal position and the bead at a distance of 40 cm:

  • Can you see the red bead
  • How many beads do you see
  • As you look at the red bead how many strings to see you going towards it
  • Do the strings cross
    • At the bead
    • In front of the bead
    • Behind the bead
  • Are the strings equally clear
  • Are they level

A measure of the binocular difficulty is how far the patient gets down this list of questions without error. If you’re not getting anywhere with the Brock then you need to move back to other tests

This entry was posted in Trivex for Kids

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