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).


  1. Richard Michael Higson

    Doesn’t the image get moved toward the apex of a prism?

    • Frank Eperjesi

      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.


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