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?
Highlights
When an object is viewed during normal binocular viewing conditions, the image falls upon the same fixation point of both eyes
When a base in aligning prism (Exo FD) was present at near the size of the aligning prism was a good correlate of opposing fusional amplitude to blur, break and recovery, in both the symptomatic group of patients and the entire group (p,0.001)
This finding is unsurprising in light of earlier research which documents that an aligning prism of $1 dioptres in pre-presbyopes and $2 dioptres in presbyopes at near is likely to be associated with symptoms attributed to decompensating heterophorias [7]
Summary
When an object is viewed during normal binocular viewing conditions, the image falls upon the same fixation point (fovea) of both eyes. Percival’s Criterion states that for the heterophoria to remain compensated the larger horizontal fusional reserve should be no more than twice the size of the smaller [3]. If the fusional reserve is insufficient the heterophoria will decompensate into a heterotropia giving rise to symptoms and/or a reduction in the quality of that patient’s binocular vision. The 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 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
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