Vestibular Consequences of Blast-related Mild Traumatic Brain Injury (TBI)

Air-conducted cervical vestibular evoked potential (cVEMP) inter-ear amplitude asymmetry ratio was used as a measure of otolith organ function (saccular-collic pathway). Inter-ear amplitude asymmetry ratio was calculated as: \[(\|L\_P1-N1\| - \|R\_P1-N1\|)/ (\|L\_P1-N1\| + \|R\_P1-N1\|)\] x100, where L\_P1-N1 = peak-to-peak cVEMP amplitude of the left side and R\_P1-N1 = peak-to-peak cVEMP amplitude of the right side. The amplitudes were calculated from cVEMP responses at a stimulus intensity of 120 dB peakSPL. The criterion for abnormal cVEMP was defined as an absent cVEMP or a corrected cVEMP amplitude asymmetry ratio greater than or equal to 40%, either of which would indicate a unilateral vestibular loss. A bilateral vestibular loss was indicated by absent cVEMPs bilaterally.

Time frame: Up to 30 minutes

Population: Cervical VEMP could not be evaluated on 2 participants in the TBI \& Blast group and 3 participants in the Blast Only group.

Bone-conducted ocular vestibular evoked potential (oVEMP) inter-ear amplitude asymmetry ratio was used as a measure of otolith organ function (utricular-ocular pathway). Inter-ear amplitude asymmetry ratio was calculated as: \[(\|L\_N1-P1\| - \|R\_N1-P1\|)/ (\|L\_N1-P1\| + \|R\_N1-P1\|)\] x100, where L\_N1-P1 = peak-to-peak oVEMP amplitude of the left eye/right ear and R\_N1-P1 = peak-to-peak oVEMP amplitude of the right eye/left ear. The oVEMP is a contralateral response; therefore, recordings from the left eye reflect the response of the right ear and vice versa. The amplitudes were calculated from oVEMP responses at a stimulus intensity of 155 dB peakFL. The criterion for abnormal oVEMP was defined as an absent oVEMP or a corrected oVEMP amplitude asymmetry ratio greater than or equal to 40%, either of which would indicate a unilateral vestibular loss. A bilateral vestibular loss was indicated by absent oVEMPs bilaterally.

Time frame: Up to 20 minutes

Population: Ocular VEMP could not be obtained from 4 participants from the TBI \& Blast group.

The caloric weakness was determined using monothermal warm inter-ear difference (MWIED) which was calculated as: (\|RW\| - \|LW\| )/( \|RW\| + \|LW\|) x 100, where RW = the maximum slow phase velocity (SPV) of nystagmus induced by warm water irrigation in the right ear and LW = the maximum SPV of nystagmus induced by warm water irrigation in the left ear. For participants with MWIED \> 10, then cool caloric irrigation was also performed and caloric weakness was determined using a bithermal inter-ear difference (BIED) calculated as: (\|RW\| + \|RC\|) - (\|LW\| + \|LC\|) / (\|RW\| + \|RC\| + \|LW\| + \|LC\|) x 100, where RC = maximum SPV of nystagmus induced by cool water irrigation in the right ear and LC = maximum SPV of nystagmus induced by cool water irrigation in the left ear.

Time frame: up to 30 minutes

Population: Caloric weakness could not be obtained from 1 participant in the Blast Only group and 2 participants in the Healthy Control group.

Rotary chair slow harmonic acceleration (SHA) vestibulo-ocular reflex (VOR) gain at 0.01 Hz was used as a measure of peripheral vestibular function (VOR/horizontal semicircular canal). VOR gain is defined as the ratio of the slow component velocity eye movement (output) to the velocity of the head movement (input).

Time frame: up to 30 minutes

Population: Rotary chair slow harmonic acceleration (SHA) VOR gain at 0.01 Hz was not evaluated in 1 participant in the TBI \& Blast group.

Rotary chair slow harmonic acceleration (SHA) vestibulo-ocular reflex (VOR) phase at 0.01 Hz was used as a measure of peripheral vestibular function (VOR/horizontal semicircular canal). The phase is the timing difference between the velocity of head movement and the slow-phase eye velocity. This parameter is normalized for a full cycle of a sinusoid (360 degrees) and presented in an angular unit of degrees rather than a unit of time. For perfectly compensatory eye movements the phase is 0 degrees, meaning there is no difference between the actual eye velocity and the ideal VOR (by convention, degrees is added to the phase so that the comparison is based on the ideal VOR responses instead of the actual head motion).

Time frame: Up to 30 minutes (SHA phase is obtained simultaneously with SHA gain)

Population: Rotary chair slow harmonic acceleration (SHA) phase at 0.01 Hz was not calculated for 2 participants in the TBI \& Blast group and 1 participant in the Blast Only group. Phase at 0.01 Hz could not be calculated for individuals with bilateral vestibular loss.

Visual fixation suppression was used as a measure of central vestibular/CNS function. Visual fixation suppression is a measure of vestibulo-ocular reflex (VOR) gain obtained during visual fixation at 0.16 Hz slow harmonic acceleration on the rotary chair. VOR gain was defined as the ratio of the slow component velocity eye movement (output) to the velocity of the head movement (input). Visual fixation suppression was considered normal if VOR gain is suppressed \> 50% with visual fixation compared to no fixation.

Time frame: 1 minute

Population: VOR gain during a visual fixation task and 0.16 Hz could not be obtained on 2 participants in the TBI \& Blast group and 3 participants in the Blast Only group.

The Dizziness Handicap Inventory (DHI) was used as a quality of life measure.The DHI measures the subject's self-perceived dizziness. The scale has 25 questions with 3 possible answers each: Yes = 4 points, Sometimes = 2 points, and No = 0 points. The minimum number of points that a subject can score is 0 and the maximum number of points is 100. The subject's self-perceived dizziness is reported as a percentage with a range of 0-100%, and is calculated by: subject's total number of points/maximum number of points (100) x 100%. The higher the score on the DHI, the worse a patient's self-perceived dizziness.

Time frame: Up to 10 minutes

This measure is the composite equilibrium score from six conditions of the sensory organization test obtained with the Neurocom Equitest. Results of the SOT were calculated based on maximum peak-to-peak anterior-posterior sway expressed as an equilibrium score ranging from 0 to 100, with 0 indicating loss of balance (i.e., required support of harness, took a step, touched walls for support or opened eyes in eyes closed conditions) and 100 indicating perfect stability. The outcome measure was the equilibrium composite score and was calculated by the software as the weighted average of the equilibrium scores for the six conditions. For ages 18-59 years, the normative value (mean - 1.67 SD) for the composite score is at least 70 (NeuroCom, 2011).

Time frame: Up to 20 minutes

Population: The composite equilibrium score of the sensory organization test was not obtained for 4 participants in the TBI \& Blast group, 2 participants in the Blast Only group and 2 participants in the Healthy Control group.