What are fusional reserves? What are normal base-in and base-out values at near?
Fusional reserves are a measure of how much vergence a person has in reserve that can be used to overcome a heterophoria.
The patient is instructed to fixate on a near target (usually at 33 cm) binocularly and a horizontal prism bar is placed before one eye starting with the weakest prism strength.
The prism strength is increased until the patient reports blur (blur point) and then diplopia (break point). The bar is held in place and the prism strength slowly reduced until the patient reports single vision again (recovery point). The prism strength at the blur, break and recovery points are recorded. It can be useful to watch the patient’s eyes to confirm the break point, that is, when the vergence movement stops.
Base-out prism is used to force the eyes to converge. The prism moves the image towards its base and this moves the image on the retina temporal to the fovea. The fovea ‘chases’ the image by the eyes converging. Even though the prism is only in front of one eye both eyes converge. Once the eyes have exhausted all their ability to converge the accommodation system provides more convergence (through the accommodative convergence/accommodation relationship).
This over accommodation for the distance the target is at provides more convergence but causes the near target to become blurred. This is the blur point. As the prism strength is increased further the eyes exhaust their ability to converge (convergence and accommodative convergence) and the target becomes double. The image is temporally off the fovea in one eye and the eye cannot converge any more to line up the fovea with the image. This is the break point. When the prism strength is reduced the eyes have enough convergence to line the fovea up with the image and binocular single vision is restored.
It is similar for base-in prism except that the image is moved to the base of the prism which this time moves it nasally off the fovea. The eye has to diverge to chase the image. The blur point is when the eyes have exhausted their ability to diverge and accommodation relaxes in order to find a bit more divergence. This under accommodation for the distance of the near target gives the base-in blur point. As the base-in prism strength is further increased the eyes have exhausted all their ability to diverge, the fovea can no longer line up with the nasally displaced image and diplopia occurs.
This is the break point. When the prism strength is reduced the eyes have enough divergence to line the fovea up with the image and binocular single vision resumes. The eyes naturally have more ability to converge than diverge and therefore base-out values should be higher than base-in values for patients with normal binocular vision.
Normal convergent (base-out) reserves for near are blur 17 (standard deviation [sd] 5), break 21 (sd 6) and recovery 11 (sd 7).
Normal divergent (base-in) reserves for near are blur 13 (standard deviation [sd] 4), break 21 (sd 4) and recovery 13 (sd 3).
Hi, if a patient has a poor recovery (eg: BO break 14, recovery 2), how can we train to improve the recovery?
Thank you for your question. When the patient has a reasonable break point but a poor recovery it shows that their binocular vision is working at a lower level than it could be. If they are doing close work they are likely to have symptoms. If they are symptomatic then you could discuss the options of eye exercises or of prism in their glasses. Some people like the idea of using exercises and have a ‘natural’ fix to their symptoms. Others don’t want to invest the time, money and effort in eye exercises and opt for prism. When I advise exercises I give at least 3 different types, usually pen to lid (as the most basic), plus a dot card, plus jump and push-up using small letters close up and larger letters at around 2 m. I’ll be writing more in the coming weeks about when and how to use eye exercises and what types their are. I recommend them for 15 mins each day, ask the patient to keep a diary of which days they were done and review in 2 weeks. If I go for prism I use a near Mallett unit to determine the aligning prism and then prescribe that in their glasses. If you don’t have access to a near Mallett unit start with a 1 prism base-in in front of one eye and increase the prism value until you see a smooth and rapid recovery of any exophoria present on the alternating cover test at near. If there is already smooth and rapid recovery on alternate cover testing without a prism then try using increasing trial frame base-in prism with the patient looking at small print at their habitual working distance until they tell you that the print looks clearer an/or that their eyes feel better. Hope this helps.
Unable to measure Phorias at distance in exam room ?
Maddox rod at near ortho horizontally and vertically .
Had refractive surgery March18th 2023 ou
Esophoria
Thank you for being part of the EyeTools Community and for your post. I’m not sure what your question is. If you what to rephrase it I will try to answer. Thanks.
hello you stated that distance convergent fusional reserves blur point is x. should accommodation not play a part here, i thought its around 9.
Thank you for your question. Accommodation is not active at distance for an emmetrope or for a person whose refractive error is fully corrected.
Hi,
what are the management options for reduced fusional vergence? If it’s exercises which type specifically? and is the PFR recored as the recovery value?-when it goes back to bsv.
Thanks for your question. I find the Institute Free-space Stereogram a very useful combined exercise for managing reduced fusional reserves. You can get further information here http://www.ioosales.co.uk/html/practice/eye04A.html
I would expect an improvement in the blur/break/recovery aspects of the fusional reserves and would record all three values. The blur point occurs when the person tries to use accommodative vergence to keep a single image. It’s the stimulation of accommodation (for extra convergence) or the relaxation of accommodation (for extra divergence) that causes the blur. Then there is the break point when the person notices diplopia and the recovery point when the magnitude of the test prism is reduced allowing binocular single vision again. Hope this helps.
Splendid questions but the responses are even more thought provoking and fantastic. I was super excited going through them all . Much thanks Dearest Mr Frank.
Thank you for your positive comments and for being part of the EyeTools Community.
Hi a few questions:
what are the norms for distance vision?
Do the values differ in a child below 8 years old?
If assessing reserves in a patient who has an esophoria would you measure with base out or base in?
If the break point is less than 21 indicating weak fusional reserves does the value between the measure and the norm of 21 have any relationship to the size of the deviation?
What size of deviation and break point in fusional reserves, in your experience, lead to a decompensated phoria or a complete break down into a tropia?
Thank You in Advance
What are the norms for distance vision?
Distance convergent fusional reserves
Mean Standard deviation
Blur – –
Break 7 3
Recovery 4 2
Distance divergent fusional reserves
Mean Standard deviation
Blur 13 4
Break 21 4
Recovery 13 3
Do the values differ in a child below 8 years old?
These values are okay for children older than 6 years. For children younger than 6 years they should be able to converge through a 20 prism base out.
If assessing reserves in a patient who has an esophoria would you measure with base out or base in?
Best to measure both but measure the base in fusional reserves first.
If the breakpoint is less than 21 indicating weak fusional reserves does the value between the measure and the norm of 21 have any relationship to the size of the deviation?
I don’t think it does. If there is a relationship I can’t think how this would be clinically useful
What size of deviation and break point in fusional reserves, in your experience, lead to a decompensated phoria or a complete break down into a tropia?
Some people with large phorias don’t have symptoms because they have enough fusional reserves to prevent the phoria from causing symptoms.
Some peeple with small phorias do have symptoms because they don’t have enough fusional reserves to prevent symptoms.
Symptoms or a complete break down depend on the relationship between the size of the phoria and the amount of fusional reserves.
Hope this helps.
Thanks for your questions.
Doesn’t the image get moved toward the apex of a prism?
Thank you for your comment. As light passes through a prism it is bent towards the base of the prism. The patient behind the prism perceives that the image they see moves towards the apex of the prism. In terms of understanding how prisms work with heterophorias and heterotropias I find it easier to think in which direction the light passing through the prism moves. Hope this helps.
the eye moves towards the Apex of the prism but image is moving towards the Base of prism
Thank you for your comment. As light passes through a prism it is bent towards the base of the prism. The patient behind the prism perceives that the image they see moves towards the apex of the prism. In terms of understanding how prisms work with heterophorias and heterotropias I find it easier to think in which direction the light passing through the prism moves. Hope this helps.