The clinical examination of the vestibulo-ocular reflex
(VOR)with the use of video-based tool that measures the eye velocityin response
to a fast, high acceleration and unpredictable head impulses in horizontal and
vertical planes, also called video head-impulse test (vHIT) is becoming a
standard for the evaluation of the patients with dizziness. The vHIT allows for
the evaluation of the VOR evoked induced after stimulation of each of the
re-ceptors in the different semicircular ampullae of the canals [1].In normal
subjects a fast head thrust or impulse to one side will evoke an eye movement
that matches the former in velocity; the eye movement occurs in the opposite
direction in such a waythat at the end of the head movement the eye remains in
placebecause both displacements were similar.
In the case when the head
movement is incorrectly detected, as occurs in patients with vestibulopathy,
the eye movement will be under-compensatoryand an eye-position error ensues
which provokes visual blurring.In order to resolve this situation, fast eye
movements will be pro-duced to move the eye to the expected end point: these
are called refixation saccades [2]. With the use of a head mounted camera to
register the eye movement and a sensor for head velocity, the plots of the head
and eye velocity are generated in a computer (Figure 1) as a main part of the
results of the vHIT’s test. Most of vHIT’s medical devices only output the
results of the head and eye movements based on velocity data and graphs, for
this research we consider that the position and acceleration data and graphs
could have some clinical interest, enhancing the velocity-based results of thevHIT.
In theory, applying the classical mechanics
principles, the position and acceleration data can be calculated with the
veloc-ity data of the same movement. In this article we will describe a
standardized method for the head and eye position and accelera-tion calculation
from the velocity data given by some vHIT com-mercial devices. As example of
the clinical interest and possibleapplication of this developed methodology we
use it to measure and compare the head and eye displacement occurred on a small
sample of vHITs. Materials and Methods The position and acceleration data
calculation methodology was developed using MATLAB (MATLAB Release 2014a, The
Math Works, Inc., Natick, Massachusetts, United States). First step was to
access the original vHIT (ICS impulse, GN Otomet-rics, Denmark) results
database (ICS Data base version 1.2.23).We used the vHIT´s exported results in
XML (extensible MarkupLanguage) file format. The access method for the XML file
recog-nition was written in MATLAB code; this method goes over the XML file and
stores all impulse data with some metadata informa-tion. With the velocity data
of each impulse we calculated the po-sition and acceleration data. The integral
and differential calculus principles were applied to obtain the acceleration
and positiongraphs according the principles of classical mechanics [3].
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