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 (breakpoint). 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 breakpoint, that is when the vergence movement stops.
Base-out prism is used to force the eyes to converge. The prism bends incident light 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 anymore to line up the fovea with the image. This is the breakpoint. 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 in that the prism bends incident light to the base of the prism but this time the retinal image moves 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 breakpoint. 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).
These figures come from research studies.
A colleague, who is a very experienced binocular vision practitioner, tells me that in her experience it would be unusual for any patient to have a base-in breakpoint of 21. Her experience is that even patients with good base-in reserves at near would struggle to get beyond 16. This is one of those examples where there is a disconnect between research findings and clinical experience.