Vestibular function Tests of the functional integrity of the vestibular system are designed to incorporate movements that stimulate vestibular system activity in order to elicit the VOR. As the motor output of the vestibular system is reflected in the oculomotor system, measuring VOR responses will give information about vestibular system function. Initial assessment of VOR function is to have the patient fix the vision on a target while moving the head side to side, stimulating the horizontal semicircular canals to elicit a VOR response through the MLF to maintain gaze stabilization. The examiner slowly moves the patient’s head side to side approximately 30° in both directions. The patient can look at the examiner’s nose as the target, while the examiner assesses the patient’s ability to maintain visual fixation on that target. The examiner then assesses the vestibulo-ocular reflex cancellation (VORc). In this test, the patient follows a moving target with the head and eyes, maintaining ocular position in the center of the orbits. This requires input from higher cortical centers to override the VOR, with abnormal findings indicative of a centrally mediated lesion. A more quantitative assessment of VOR is the Dynamic Visual Acuity (DVA) test. In this test, the examiner uses a Snellen eye chart to assess the patient’s baseline visual acuity. The patient can be either sitting or standing, as appropriate, the proper distance from the eye chart, and should wear his or her glasses if needed for distance visual correction. The patient recites the lowest line (smallest font size) they can read clearly on the chart. The examiner stands behind the patient, firmly grasping the head with both hands on either side of the head. The patient’s head is then moved side to side in a smooth rotational trajectory at a frequency of 2 Hz (two full side-to-side cycles per second) while the patient recites the lowest line on the Snellen chart he or she can read clearly. If the VOR is functioning normally, the patient’s eyes will move in the opposite direction of head movement at the same frequency, allowing for visual fixation on the target (eye chart). Normally, a patient’s visual acuity will be somewhat reduced under these conditions as head- eye movement is not a perfect 1:1 relationship, and he or she can demonstrate up to a two- line degradation (increase in font size, moving up on the chart) over the baseline visual acuity line. DVA is abnormal if there is a three-or-more-line degradation, indicating difficulty with gaze stabilization (Herdman & Clendaniel, 2014). It is important for the examiner to maintain a smooth horizontal trajectory of head movement to ensure that any degradation of visual acuity can be attributed to diminished VOR function and not to distortion of the target. Using a metronome, which can be accessed online, will ensure that the test is performed at the proper testing frequency of 2 Hz. It is important to ensure that the frequency is at or above 2 Hz to limit the use of pursuit eye movements, which would be able to track the target at lower frequencies. The Head Thrust Test (HTT), also called the Head-Impulse Test (HIT), examines VOR function and can identify unilateral or bilateral vestibular loss. In this test, the patient fixes his or her gaze on the examiner’s nose while the examiner quickly Positional testing The previous section presented tests that assess the functional integrity of the vestibular system. Positional testing assesses for vestibular system dysfunction due to BPPV (distorted function). The gold standard for diagnosing BPPV of the posterior canal is the Hallpike-Dix test. In this test, the patient is seated on a treatment table with the examiner holding the patient’s head in both hands, positioning the head in 45° of cervical rotation toward the affected ear, and 20° of cervical extension. The patient is then rapidly brought to a supine position with the examiner supporting the head hanging off the end of the table, maintaining the 45° of rotation and 20° of cervical extension. In this position, the affected posterior semicircular canal is placed
rotates the head to one side through a small amplitude of movement, maintaining the end point position while observing for the patient’s ability to maintain visual fixation (see Resources section). Noting the need for a corrective saccade to refixate vision on the target is indicative of loss of vestibular function on the side to which the head is being turned. Before performing this test, it is important to “clear” the neck by assessing passive range of neck motion, determining any restrictions due to loss of cervical spine mobility, pain, or guarding. This test has been found to have a 35% sensitivity and 95% specificity for detecting unilateral vestibular loss (Harvey et al., 1997). Thus, we can interpret a positive finding on the HTT as the patient having loss of vestibular function with 95% confidence. However, it also incurs a high percentage of false negative results (sensitivity). In other words, there is only a 35% chance that the patient does not have loss of vestibular function if the test is negative. The degree of loss has been determined to be a factor in the sensitivity of this test, with sensitivity being as much as 87% in patients with severe loss of vestibular function (Beynon et al., 1998). Positioning the patient’s head in a 30° downward tilt to place the horizontal canals in the plane of motion and making the timing and direction of the head thrust motion unpredictable further improves the sensitivity of the test (Schubert et al., 2004). The Head-Shaking Nystagmus Test also assesses imbalance in bilateral vestibular function as seen in the presence of nystagmus after rapidly oscillating the patient’s head in a small amplitude to stimulate the VOR. In this test, the patient’s head is tipped down 30° to position the horizontal semicircular canals in the horizontal plane, the patient’s eyes are closed, and the examiner oscillates the patient’s head 20 times. The patient is then asked to open the eyes immediately upon cessation of the movement while the examiner observes the presence of nystagmus, which indicates a vestibular imbalance. A negative test (no nystagmus) will occur in a normally functioning vestibular system, or when bilateral loss is present; there will be no imbalance between each side to generate the nystagmus. The patient is wearing Frenzel lenses or infrared goggles during this test to obliterate visual fixation, while the examiner can see the patient’s eye movement. Visual fixation would suppress any nystagmus that may be present; thereby the test result would be falsely interpreted as negative or normal. The sensitivity and specificity of the test is similar to the HTT for determining loss of vestibular function (Harvey et al., 1997). The patient with acute vestibular syndrome will typically present to the emergency department due to the acute onset and severity of the symptoms. This vestibular presentation must be differentiated from stroke in order for appropriate and timely intervention to take place. The acronym “ HINTS ” helps in the differential assessment process, using the H ead I mpulse test, assessing N ystagmus, and T est of S kew. Findings of a negative HIT, the presence of direction-changing nystagmus, and skew deviation has been found to have 100% sensitivity and 98% specificity in identifying a central cause (Kattah et al., 2009). in a gravity-dependent position, creating movement of the otoliths in the canal (see Figure 6). If the test is positive for posterior canal BPPV, the examiner will see an upward beating and torsional nystagmus toward the ear in the dependent position within 10 seconds, extinguishing within 60 seconds. In conjunction with these findings, the patient must also experience the familiar symptoms of vertigo to confirm a positive test. Symptoms persisting for as long as 2 minutes before fatiguing suggest cupulolithiasis as the basis of the BPPV. The patient may experience a brief episode of symptoms upon being brought up to the seated position at the end of the test.
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