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The prism fusion range (PFR) or fusional vergence amplitude is a clinical eye test performed by orthoptists, optometrists, and ophthalmologists to assess motor fusion, specifically the extent to which a patient can maintain binocular single vision (BSV) in the presence of increasing vergence demands. Motor fusion is largely accounted to amplitudes of fusional vergences and relative fusional vergences. Fusional vergence is the maximum vergence movement enabling BSV and the limit is at the point of diplopia (double vision). Relative fusional vergence is the maximum vergence movement enabling a patient to see a comfortable clear image and the limit is represented by the first point of blur.[1] These motor fusion functions should fall within average values so that BSV can be comfortably achieved. Excessive stress on the vergence system or inability to converge or diverge adequately can lead to asthenopic symptoms, which generally result from decompensation of latent deviations (heterophoria) or loss of control of ocular misalignments.[2] Motor anomalies can be managed in various ways, however, in order to commence treatment, motor fusion testing such as the PFR is required.
Equipment required to perform a Prism Fusion Range Test. From left to right: an accommodative fixation stick, a vertical prism bar, and a horizontal prism bar.
The PFR involves placing a prism bar in front of an eye. In a patient with BSV, a natural shift of the eye occurs. When measuring horizontal fusion ranges, base in prisms assess fusional divergence while base out prisms assess fusional convergence.[3] The vertical fusional vergence amplitude can also be measured with base up and base down prisms although the horizontal PFR is typically the main focus when testing. When performing the PFR, prism strengths are increased, placing greater demand on the vergence system, eventually resulting in a break point accompanying diplopia.[4] Break point, recovery and blur are key aspects of this assessment. The break point occurs at the loss of BSV, recovery point when BSV is regained from break and blur point is at the loss of comfortable BSV. These stops can be subjectively indicated when the patient notices a double or blurred image. Both subjective and objective measurements can be considered however the examiner's objective observation is the gold standard.
^Agarwal, A., Agarwal, A., Agarwal, S., Alio, J.L., Apple, D.J., Burrato, L. & Pandey, S.K. (2002). Textbook of Ophthalmology: Basic Sciences, Optics and Refraction, Neuro-ophthalmology, Strabismus (1st ed.). Jaypee Brothers Medical Publishers (P) Ltd
^Cite error: The named reference American was invoked but never defined (see the help page).
^Wright, Kenneth W.; Spiegel, Peter H.; Thompson, Lisa (2006-04-19). Handbook of Pediatric Strabismus and Amblyopia. Springer Science & Business Media. ISBN 9780387279244.
^Codina, C.J. & Ludden, S. M. (2013). Is there an ideal speed for the prism fusion range?. The British and Irish Orthoptic Journal, 10, 51-55. Retrieved from www.bioj.org/index.php/bioj/article/download/103/83
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