Is the Aligning Prism Measured with the Mallett Unit Correlated with Fusional Vergence Reserves?

Abstract

BackgroundThe Mallett Unit is a clinical test designed to detect the fixation disparity that is most likely to occur in the presence of a decompensated heterophoria. It measures the associated phoria, which is the “aligning prism” needed to nullify the subjective disparity. The technique has gained widespread acceptance within professions such as optometry, for investigating suspected cases of decompensating heterophoria; it is, however, rarely used by orthoptists and ophthalmologists. The aim of this study was to investigate whether fusional vergence reserves, measured routinely by both orthoptists and ophthalmologists to detect heterophoria decompensation, were correlated with aligning prism (associated phoria) in a normal clinical population.Methodology/Principal FindingsAligning prism (using the Mallett Unit) and fusional vergence reserves (using a prism bar) were measured in 500 participants (mean 41.63 years; standard deviation 11.86 years) at 40 cm and 6 m. At 40 cm a strong correlation (p<0.001) between base in aligning prism (Exo FD) and positive fusional reserves was found. Of the participants with zero aligning prism 30% had reduced fusional reserves. At 6 m a weak correlation between base out aligning prism (Eso FD) and negative fusional reserves was found to break (p = 0.01) and to recovery (p = 0.048). Of the participants with zero aligning prism 12% reported reduced fusional reserves.Conclusions/SignificanceFor near vision testing, the strong inverse correlation between base in aligning prism (Exo FD) and fusional vergence reserves supports the notion that both measures are indicators of decompensation of heterophoria. For distance vision testing and for those patients reporting zero aligning prism further research is required to determine why the relationship appears to be weak/non-existent?

Fusional stamina: An alternative to Sheard’s criterion

“…The concept of converting Sheard's criterion into a continuous variable is not new. Conway et al (2012) defined "Sheard's value" as "the fusional reserve opposing the heterophoria to blur point (or, if no blur to break point) divided by the heterophoria" (p. 2). This implies that Sheard's criterion is unmet if the value is below two.…”

“…Fixation disparity, which normally represents a fraction of the measured phoria in the same direction, can be corrected with aligning prisms, and the patient will see the effect while watching the Mallet unit. Conway, Thomas, and Subramanian (2012) compared results from this unit with measures of fusional vergence reserves in 500 adults, and found a strong inverse correlation between positive fusional vergence (PFV) and reported (exo) aligning prism from the Mallett unit for near evaluations (40 cm). At near, 299/500 (≈60%) participants required no aligning prism, however, of these, 107/299 (≈36%) complained of visual symptoms, and 89/299 (≈30%) failed to meet Sheard's criterion (see below).…”

“…Additionally, there is no evidence that symptoms from a phoria increase at the moment the criterion is not fulfilled or that all symptoms disappear when it is barely met. Attempts to convert Sheard's criterion to a continuous variable have been made previously, but primarily for statistical reasons (Conway et al, 2012;Evans, Busby, Jeanes, & Wilkins, 1995). Moreover, norms and repeatability of measurements have not been published.…”

Effects of Prism Eyeglasses on Objective and Subjective Fixation Disparity

“…First, there is considerable evidence from application of the Mallett unit [ 47 , 48 ] that an exo subjective fixation disparity in near vision (or a base-in aligning prism required to compensate a subjective fixation disparity) is associated with asthenopic symptoms [ 49 – 52 ]. A base-in aligning prism at near is associated with reduced fusional vergence reserves in the opposing direction [ 53 ]. Second, studies based on the Mallett-unit showed that an artificially induced fixation disparity (by means of prisms) can reduce binocular acuity [ 54 ].…”

Visual Binocular Disorders and Their Relationship with Baropodometric Parameters: A Cross‐Association Study

“…Binocular vision uses both eyes that move simultaneously due to the extraocular muscles, which control convergence and divergence and are responsible for binocular vision [24,41]. Binocular vision disorders can generate postural adaptations to maintain binocularity and achieve visual comfort.…”