Bilateral vestibular hypofunction: An interesting problem

Although Pathways primarily focuses on neuroaudiology and CAPD, we will occasionally have articles on closely related issues such as the one below reviewing bilateral vestibular hypofunction, which is not only a peripheral, but also a central vestibular problem.

Bilateral vestibular hypofunction: An interesting problem

Stephanie A. Waryasz, B.S.
University of Connecticut

Bilateral vestibular hypofunction (BVH) is a disorder that creates reduced or absent function on both sides of the vestibular system, as its name implies. The disorder is characterized by general imbalance, especially during movement, movement-induced dizziness and movement-induced vision instability, also known as oscillopsia. Such symptoms are usually debilitating and greatly affects one’s ability to participate in activities daily living such as walking, driving, and reading, which can consequently impact a person’s employability, social life, and emotional well-being (Braswell & Rine, 2006; Herdman, Hall, Schubert, Das, & Tusa, 2007; Guinand, Pijnenburg, Janssen, & Kingma, 2012; Ward, Agrawal, Hoffman, Carey, & Della Santina, 2013).

BVH has also been associated with certain cognitive deficits such as spatial learning and spatial memory deficits due to a 17% reduction in hippocampal volume and decreased metabolic activity within the anterior hippocampus (McCall & Yates, 2011). From data analysis of the 2008 US National Health Interview Survey, it is believed that severe to profound BVH affects upwards of 28 in 100,000 adults in the United States or 64,046 Americans (Ward et al., 2013). These prevalence estimates are considered to be conservative, as people with confounding neurological and visual conditions, which could comorbidly exist with BVH, were excluded from the criteria used by Ward et al. (2013). Furthermore, Ward et al. (2013) used strict criteria to estimate the prevalence of BVH based on affirmations to case history questions that are known symptoms of severe to profound BVH. By extrapolating United States estimates, Ward et al. (2013) suggest that 1.8 million people worldwide could be affected by BVH, with the caveat that prevalence of BVH may vary geographically based on medical treatments such as the use of vestibulotoxic drugs.

Etiology

The most common etiology of BVH is systemic vestibulotoxicity from aminoglycoside antibiotics, specifically gentamicin and streptomycin (Schubert & Minor, 2004; Ward et al., 2013). These types of antibiotics are known for more selectively damaging vestibular hair cells while oftentimes leaving auditory hair cells unharmed, depending on the medication dosage administered (Schubert & Minor, 2004). The incidence of BVH ranges between 3%-4% when a patient is treated with gentamicin (Schubert & Minor, 2004). The incidence of BVH increases to between 12.5% and 30% for patients treated with both gentamicin and renal dialysis simultaneously (Schubert & Minor, 2004). These figures further emphasize the vulnerability and susceptibility of vestibular hair cells to drug-induced toxicity effects.

Less common etiologies of BVH include meningitis, head trauma, transient ischemic episodes of vessels supplying the vestibular system, bilateral tumors on cranial nerve VIII including vestibular schwannoma tumors, and sequential cases of unilateral vestibular neuronitis (Schubert & Minor, 2004). Ménière’s disease, encephalitis, labyrinthitis, autoimmune disease, and iatrogenic damage due to surgical procedures such as cochlear implantation have also been cited as potential etiologies of BVH (McCall & Yates, 2011; Ward et al., 2013;).

Pathological Process

The pathological process of BVH suggests a deficit in the vestibular system that results in inadequate compensatory eye movements during head movement that creates a slip of visual targets across the retina (McCall & Yates, 2011; Schubert & Minor, 2004), which consequently yields oscillopsia, one of the critical symptoms of BVH for differential diagnosis (Ward et al., 2013). This deficit makes it impossible for the vestibulo-ocular reflex (VOR) to respond appropriately. The VOR is responsible for generating compensatory eye movements that correspond to opposite head movements, ultimately stabilizing vision and gaze (Guinand et al., 2012; Vital et al., 2010). VOR deficits contribute to issues with gait, postural instability, and disequilibrium upon movement (Schubert & Minor, 2004). Such deficits are enhanced with an increase the velocity of head movements (Guinand et al., 2012; Vital et al., 2010).

Clinical Assessment

Clinical tests often used to diagnose BVH include the head thrust test to examine the VOR (Minor, 1998; McCall & Yates, 2011), various Romberg stances and tandem walks to view static imbalance (Minor, 1998; McCall & Yates, 2011), rapid full-body turns or external perturbations such as light shoves can help to assess dynamic imbalance (Minor, 1998; McCall & Yates, 2011), optokinetic testing presents with a severe reduction in nystagmus or no nystagmus with BVH (McCall & Yates, 2011), and caloric testing, which can identify reduced gaze stabilization of the VOR in the low frequency range (Vital et al., 2010). Due to the symptomology of BVH indicating severe deficits in gaze stabilization especially with movement, perhaps one of the best tests to assess BVH is the dynamic visual acuity test (Guinand et al., 2012; Herdman et al., 2007; Herdman et al., 1998; Schubert & Minor, 2004; Vital et al., 2010).

The dynamic visual acuity test assesses visual acuity while the head is in motion, which is typically self-generated (Schubert & Minor, 2004). Additionally, it examines the capacity of the VOR to maintain gaze stability during such head movement (Minor, 1998). Many different versions of this test exist, ranging from bedside to computerized to using scleral search coils to performing functional assessment on a treadmill (Guinand et al., 2012; Herdman et al., 2007; Herdman et al., 1998; Minor, 1998; Schubert & Minor, 2004; Vital et al., 2010). The bedside test can be performed informally by having the patient read a newspaper with the head remaining still, then having the patient try to continue reading while moving their head from side to side at a rate of about 2 cycles per second (Minor, 1998).

A more formal option would be to utilize computerized dynamic visual acuity testing. Herdman and colleagues (1998 & 2007) researched a method of calculating dynamic visual acuity by counting the total number of errors in identifying the orientation of the computerized optotype “E” while the subject’s head was moved predictability to the right or to the left. This quantity was then subtracted from the subject’s static visual acuity score to yield their dynamic visual acuity score. Herdman et al. (1998) reported 96.2% sensitivity, 100% specificity, and 97.5% overall accuracy in using this test to identify subjects with BVH in comparison to normal control subjects. The capacity of this test to distinguish between unilateral vestibular hypofunction (UVH) and BVH was also noted.

Dynamic Visual Acuity Testing Research

Vital et al. (2010) created a different computerized dynamic visual acuity test to assess peripheral vestibular function. This study assessed the subject’s ability to compute differences between visual acuity in static and dynamic conditions during active and passive horizontal head rotations at velocities exceeding 100°/sec and exceeding 150°/sec, respectively. Velocities were measured with a Sparkfun velocity sensory fixed to a headset worn by each subject. An active head rotation was generated by the subject’s participation in active movements. The passive head rotations were delivered by the examiner at random intervals by holding the head laterally on both sides outside of the subject’s visual field. To obtain the visual acuity level for each condition, the subject needed to identify the orientation of Landolt rings, a common optotype used for vision tests that looks like the letter “C”. Landolt rings were presented in sets of five for each visual acuity level.

The test terminated after the subject was unable to correctly identify at least three Landolt ring orientations at a given visual acuity level. To help with fixation during head rotation, a small dot was placed on the center of the monitor, which was only extinguished immediately before a Landolt ring was presented.

Vital and colleagues (2010) also verified their computerized results with quantitative horizontal head impulse testing (qHIT) with scleral search coils around the cornea of the right eye. The researchers noted that this test was clinically useful in identifying semicircular canal function in the high frequency range, which was reported to be more important in the assessment of gaze stabilization of the VOR.

The benefit of this test is that it can be easily administered within the office excluding the scleral search coil verification. However, the researchers warn that the examiner should be experienced with this test before utilizing it and the patient should be trained to minimize learning curve effects. The sensitivity of this test for identifying BVH was reported to be 100% and the specificity was reported to be 94% when performing passive rotation with the head velocity exceeding 150°/sec.

Guinand et al. (2012) created a unique type of dynamic visual acuity test that assessed functional dynamic visual acuity while on a treadmill. The difference between static and dynamic visual acuity was computed to determine the subject’s total dynamic visual acuity. Subjects were asked to read Sloan letters placed 2.8 meters away starting at the 20/25 visual acuity line. The visual acuity value of the most finite visual acuity with at least three correct responses was recorded as the acuity level for that test. Dynamic visual acuity testing was performed at three different velocities on a treadmill: 2 km/hr, 4 km/hr, and 6 km/hr. Researchers reported 97% sensitivity to identifying BVH when the visual acuity was measured at all three velocities. The sensitivity decreased to 76% if only 2 km/hr was recorded in the dynamic condition and 84% sensitivity if only 4 km/hr was recorded. If the dynamic testing was conducted at both 2 and 4 km/hr the sensitivity increased to 95%. Specificity was not reported by Guinand and colleagues (2012).

The benefits of this type of testing include its simple and cost-effective procedure that could be performed in less than 10 minutes without prior training for the patient or the test administrator. The main drawback to this test is that it requires a treadmill, which may not be accessible to all clinicians who would like to assess dynamic visual acuity. The researchers also noted that this test may be clinically useful in assessing functional outcomes of a vestibular training protocol. Additionally, this test could be used to assess candidacy for treatment options such as vestibular prosthesis implantation and post-surgical functional assessment. A summary of the three previously reviewed types of dynamic visual assessment tests (Guinand et al., 2012; Herdman et al., 2007; Herdman et al., 1998; Vital et al., 2010) can be seen in the Table below.

Table: A review of different dynamic visual acuity (DVA) assessment measures to identify bilateral vestibular hypofunction (BVH) with reported benefits, drawbacks, and clinical utility of each protocol from supporting research.

Due to the disease process of BVH, spontaneous recovery is rare (McCall & Yates, 2011). Therefore, techniques are necessary to provide relief to patients dealing with the debilitating effects of this disorder. There is currently no standardized treatment for BVH; however, research has shown potential for therapies and procedures to help manage BVH.

A review by McCall and Yates (2011) discusses vestibular physical therapy as a treatment option for BVH that has been shown to improve dynamic gait stability in addition to dynamic visual acuity in approximately 50% of patients. This figure suggests that half of the BVH patient population may already have innate compensatory processes that render such treatment fruitless. Sensory substitution has been highlighted as an alternative treatment method for BVH. This technique offers information typically processed by the vestibular system in an alternate modality such as visual, auditory, or tactile. The use of visual cues, auditory cues, head-mounted vibrotactile devices, or electrotactile feedback to the tongue are examples of alternate means of providing sensory feedback for the patient. Current research shows promise of a vestibular prosthesis in humans, which will function similarly to a cochlear implant in that an electrode array will be inserted into the semicircular canal. Conversely, the electrical stimulation for the vestibular prosthesis would be controlled by an accelerometer to detect head motion. It is believed that this stimulation will elicit appropriate eye movement compensation.

In summation, BVH is a debilitating disorder affecting both peripheral and central vestibular systems that most commonly yields postural instability, gait deficits, and oscillopsia due to a slip of the visual target across the retina interfering with the VOR (Herdman et al., 2007; Guinand et al., 2012; Ward et al., 2013). Central basis for BVH would likely also have central auditory findings, as these pathways are very close in proximity to each other, especially in the brainstem. The most commonly reported etiology of BVH is vestibulotoxicity due to use of aminoglycoside antibiotics (Schubert & Minor, 2004; Ward et al., 2013). This disorder can be identified through numerous tests, with dynamic visual acuity testing being one of the most clinically useful (Guinand et al., 2012; Herdman et al., 2007; Herdman et al., 1998; Schubert & Minor, 2004; Vital et al., 2010). Treatment options are widely varied for this disorder and should be individualized to meet the specific needs of the patient.

Stephanie Waryasz is a third year doctoral student of Audiology at the University of Connecticut.  She completed her undergraduate Bachelor of Science degree in Communication Disorders at Southern Connecticut State University graduating with departmental honors.  At Southern Connecticut State University, she completed an undergraduate thesis on the topic of mild traumatic brain injury and the treatment of blast victims in VA facilities returning from the Iraq and Afghanistan wars.  Currently, Stephanie is examining the effects of sports-related concussion on auditory processing in university athletes for her Au.D. capstone research project at U Conn.  Stephanie also enjoys volunteering for the Healthy Hearing Initiative at the Special Olympics in New Haven, CT and for Hear Here Hartford in Wethersfield, CT – an organization that helps to empower teens and young adults with hearing loss.

References

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