GAZE STABILIZATION TEST (GST)
Description
GST Report
Functional Implications
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Description
In contrast to the DVA that examines changes in visual acuity with fixed velocity head movements, the GST quantifies the head movement velocities over which the patient is able to maintain an acceptable level of visual acuity.
Like the DVA, the GST can be used to test impairments in gaze stabilization for each axis of head movement: yaw, pitch, and roll. The first step of the test is to determine the patient's static visual acuity using the same methods described for the DVA. This is accomplished by seating the patient a prescribed distance before the computer monitor and displaying sequences of the optotype "E" of predetermined size and in one of four possible random orientations. When the patient correctly identifies the orientation of at least 3 of 5 successive "E" presentations of a given size, the optotype size is reduced and the process repeated until the orientation of the optotype can no longer be reliably determined. Static visual acuity is then based on the smallest "E" that can be identified accurately, and is used to set the visual acuity reference level for the dynamic phase of testing. When the DVA and GST are both performed, the results of a single static visual acuity test can be used for both.
The operator selects among the three head-movement axes for the second (dynamic) phase of testing. Using the sensor to measure head movements, the patient is instructed to perform the required head movement at a predetermined velocity and to identify the orientation of the "E", which is presented only while the head is moving at the prescribed velocity on the selected axis. If the patient successfully identifies the orientation of at least three of five presentations, the head velocity needed to trigger the presentation of the "E" is increased and the process is repeated until the patient fails to achieve the minimum number of correct responses. If the patient fails to identify correctly three of the five orientations during the initial trials, the velocity required to trigger the display is reduced and the process is repeated until three correct orientations are identified.
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GST Comprehensive Report
- Maximum Gaze Velocity Separate graphs of GST results are provided for each axis of head movement. The maximum head movement velocities at which the patient can maintain the visual acuity reference level are displayed for each direction. Velocities are shown in units of degrees per second.
- % Left/Right Symmetry Differences in maximum gaze velocity between the two directions of a given axis are expressed as a percentage of the sum of the two velocities.
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Functional Implications
The GST provides impairment information related to the ability of the VOR system to maintain gaze stability during active head movement. Like the DVA, the GST provides impairment information specific to the axis and direction of head movement. In addition, the GST quantifies the velocity range for effective VOR compensation as a function of axis and direction. As the severity of a vestibular system deficit increases, the maximum movement velocities over which the VOR system provides effective compensation decrease.
The impairment information provided by the GST is of value during treatment design. GST results help target treatment for retraining specific axes and directions of head movement, and suggest the velocities of head movement activity most likely to achieve the maximum benefit.
GST information is also of significant value in disability rating. The extent of visual acuity loss during daily life activities, such as walking and driving a car, can be predicted accurately by comparing the GST velocities to those encountered during daily life tasks in question.
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