Extension of the eXpanded Nystagmus Acuity Function to Vertical and Multiplanar Data

Abstract

Purpose:: To update and extend the functionality of the eXpanded Nystagmus Acuity Function (NAFX), allowing for its application under more circumstances and improving its predictive value in clinical evaluations.

Methods:: The original NAFX calculations were based upon minimum-necessary foveation times taken from a "tau surface," an array whose values were individually calculated for each discrete combination of position and velocity limits. For the updated NAFX, the tau surface was replaced by an idealized mathematical function that generally matched the original tau surface, but which accounted for irregularities in its surface due to idiosyncracies in the waveforms of the subjects that were used for the initial calculations. To extend applicability of the NAFX to multiplanar data, a simple method was developed to combine multiple channels (e.g., horizontal and vertical) eye-movement data into a single waveform by use of vector summation; previously, this had to be done manually. Torsional data were ignored because it has been shown that torsional motion has little effect on visual acuity. To more accurately relate the NAFX value to visual acuity of individual patients, age-related relationships were incorporated. These relationships were derived from visual acuity population data.

Results:: Using the same patient fixation data that established the original NAF and NAFX functions, we verified that the updated NAFX yielded equivalent results for uniplanar data. For biplanar data, the NAFX values were also equivalent to those from uniplanar data of the same magnitude.

Conclusions:: While still requiring a degree of expert knowledge to apply, the new, more automated version of the NAFX allows greater accuracy in prediction of potential visual acuity for subjects of all ages, for both uniplanar and multiplanar nystagmus. This will allow researchers and clinicians to select the best therapy (or combination of therapies) for a wider range of their nystagmus patients.

Keywords: eye movements • nystagmus • visual acuity