HEAD SHAKE-SENSORY ORGANIZATION TEST (HS-SOT)
Description
HS-SOT Report
Functional Implications
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Description
The HS-SOT is a two-condition enhancement to the standard SOT that is appropriate for patients who perform within the normal range on the standard SOT, yet remain symptomatic. The HS-SOT identifies impairments in the patient's ability to effectively use vestibular inputs for balance while simultaneously moving the head, as well as isolate impairments to left-right (yaw), up-down (pitch), or side to side (roll) movement axes.
The test consists of repeating SOT condition 2 (eyes closed on a firm surface) and condition 6 (eyes closed on a sway-referenced support surface) while the patient wears a head movement monitor and performs a continuous rhythmic head movement about a specified yaw, pitch, or roll axis. The patient is instructed to maintain the frequency (approximately one turn per second) and amplitude (approximately 30 degrees in each direction for the yaw axis) of movement so that the average velocity of movement is maintained at or above a set minimum (xx degrees per second yaw; yy degrees per second pitch, and zz degrees per second roll).
For each condition, the patient is given one (unscored) practice trial, followed by up to five scored trials.
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HS-SOT Comprehensive Report
- Equilibrium Score Ratio The equilibrium score ratio is a number between 0 and 1.0 that compares the three-trial average equilibrium score of each head-shake condition to the average score achieved of the comparable condition performed with head fixed. This score is provided only when the standard SOT and HS-SOT are both performed on the same day.
- Equilibrium Score This graph displays the individual raw equilibrium scores for the condition 2 and condition 5 trials.
- (Movement Axis) Velocity This graph displays the average head movement velocity scores for the selected head movement axis (yaw, pitch, or roll). A horizontal reference line compares the velocity scores to the required minimum velocity. Scores falling below the minimum are highlighted in red, whereas those meeting the criteria are shown in green.
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Functional Implications
The HS-SOT is more difficult than the standard SOT, and the equilibrium scores of normal subjects are slightly lower. The HS-SOT provides additional challenge to the sensory organizational mechanism and can quantify problems in patients with subtle problems that perform within normal limits on the standard SOT. Thus, the HS-SOT is a useful adjunct to the standard SOT in symptomatic patients with normal results.
Patients may perform abnormally on the HS-SOT for either one or both of two reasons. First, their stability is reduced under one or both of the two conditions. Second, they maintain stability, but do so only by moving the head more slowly than the required minimum velocity.
Head movements challenge the system by generating a vestibular stimulus in addition to that generated by the patient's sway. To maintain balance in the absence of alternative visual and somatosensory inputs while moving the head, the brain must differentiate the sway and head-shake stimuli. Degradations in the sensitivity and accuracy of the vestibular receptors, however, can interfere with the process of signal differentiation and reduce stability during head shaking. Because the vestibular system is composed of multiple, direction specific sense organs, these degradations may also be axis specific, creating instability only when head movements occur about the involved axis.
In addition to the above effects on vestibular control of balance, head-shaking places an additional task demand on the patient. Some patients with subtle sensory problems successfully perform the standard SOT by exerting conscious effort to augment their impaired automatic reactions. In these cases, the additional task demands of the HS-SOT can abnormally reduce stability by interfering with the patient's ability to consciously augment balance responses.
Abnormal HS-SOT performance limited to a specific axis is most likely caused by reduced sensitivity and accuracy of receptors acting about that axis. Whereas, HS-SOT problems caused by attentional demands would most likely be independent of the movement axis.
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