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Use of an Electrotactile Vestibular Substitution System to Facilitate Balance and Gait of an Individual with Gentamicin-Induced Bilateral Vestibular Hypofunction and Bilateral Transtibial Amputation

Robinson, Barbara Susan PT, DPT; Cook, Jeanne L. PT, MS; Richburg, Cynthia McCormick PhD, CCC-A; Price, Stephen E. PT, MPT

Journal of Neurologic Physical Therapy: September 2009 - Volume 33 - Issue 3 - p 150-159
doi: 10.1097/NPT.0b013e3181a79373
Case Reports

Background and Purpose: This case report describes the use of an electrotactile vestibular substitution system (ETVSS; BrainPort Balance Device, Wicab, Inc., Middleton, WI) to facilitate balance and gait of an individual with bilateral vestibular hypofunction and bilateral transtibial amputation.

Case Description: A 69-year-old man with a 2.5-year history of bilateral vestibular hypofunction, due to gentamicin toxicity, participated in a rehabilitation program using an ETVSS. Because of lower extremity infection, the patient had bilateral prosthetic legs after bilateral transtibial amputation.

Intervention: Focused on the following three phases of training with the ETVSS during 12 months: orientation, clinical training, and in-home training. The patient was periodically assessed with balance and gait tests, in addition to surveys of patient confidence and perception of handicap. All testing was performed without ETVSS.

Outcomes: Improvements were demonstrated in all outcome measures used with this patient. Sensory Organization Test composite scores increased from 23 to 48, Dynamic Gait Index scores increased from 11/24 to 21/24, and distance walked during six minutes increased from 212 to 363 m. Standing balance with eyes-closed improved from less than two seconds to more than 20 minutes. The patient reported improved confidence and lower perception of handicap with fewer functional limitations.

Discussion: For this patient, an intervention program of sensory substitution using the ETVSS improved outcome measures beyond those previously achieved with vestibular rehabilitation therapy and balance training. The feedback provided by the ETVSS may have facilitated the patient’s ability to use proprioception, thus allowing better balance control.


Department of Physical Therapy (B.S.R., J.L.C.), Missouri State University, Springfield, Missouri; Department of Special Education and Clinical Services (C.M.R.), Indiana University of Pennsylvania, Indiana, Pennsylvania; and Hearing and Balance Centers at the Elks (S.E.P.), Idaho Elks Rehabilitation Hospital, Meridian, Idaho.

Address correspondence to: Barbara Susan Robinson, E-mail:

Supplemental digital content is available for this article. Direct URL citations appear in the printed text and are provided in the HTML and PDF versions of this article on the journal’s Web site (

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Bilateral vestibular hypofunction (BVH), a type of sensory loss that results from damage to the vestibular system, can be extremely debilitating. This loss can significantly alter an individual’s functional ability, thereby affecting the overall quality of life (QOL).1 BVH causes an individual to rely more heavily on vision and proprioception for balance.2,3 The primary symptoms and physical findings of BVH include dizziness, disequilibrium, gait ataxia, decreased gait velocity, inability to walk in the dark, and oscillopsia.4–10

BVH has several causes; however, as many as 50% of BVH cases are the result of the potent side effects of aminoglycoside antibiotics, particularly gentamicin.11–17 Aminoglycosides, such as gentamicin, are widely used because of their effectiveness and economy, especially in situations of severe bacterial infection.18 Approximately 3% to 4% of patients who receive gentamicin experience bilateral vestibular damage.19 Although aminoglycosides are harmful to the cochlea, gentamicin has an even more destructive effect on the cellular layer of the peripheral vestibular system. Thus, toxicity may occur without noticeable damage to the auditory system.20 Excessive use or overdosing of gentamicin has been shown to cause permanent damage to hair cells within the otolith organs and semicircular canals.14,21,22 However, damaging effects on these structures have been reported even in the absence of toxic gentamicin levels.12,15 The vestibulotoxic effects of gentamicin are often delayed in onset, resulting in symptoms that are overlooked by medical professionals.12,23 Oscillopsia, disequilibrium, and dizziness are early symptoms associated with gentamicin toxicity and are frequently reported among individuals with BVH.3,24,25

Typically, individuals with a diagnosis of vestibular hypofunction are treated with vestibular rehabilitation therapy (VRT).7,14,26–29 Cawthorne27 and Cooksey28 were the first to design therapy specifically for individuals with vestibular pathology. Individuals with unilateral vestibular hypofunction (UVH) demonstrate greater improvement with VRT than those with bilateral lesions, because of the body’s unique ability to compensate and adapt by using contralateral vestibular structures.29 Individuals with BVH are unable to fully draw on adaptive mechanisms and must rely on substitutive methods of recovery, using visual and proprioceptive cues.7,29 These substitution principles are manifested in the potentiation of the cervico-ocular reflex and central preprogramming of eye movements to manage symptoms of BVH, particularly oscillopsia.29 Despite compensation strategies, deficits in posture and balance are expected.12 Functional outcomes vary widely for individuals with BVH and may be due to the severity of vestibular hypofunction and/or the presence of multiple medical problems.1,9

Historically, individuals with BVH have faced a difficult recovery because fewer, and less successful, therapeutic options have been available to them.30 However, Wall et al31 suggest that current data demonstrate the need to develop balance prostheses for individuals with balance impairments, including BVH. For example, use of a vibrotactile balance prosthesis was found to reduce the anterior/posterior sway of individuals with UVH and BVH during selected conditions of the Sensory Organization Test (SOT).32,33 Additionally, Hegeman et al34 investigated the use of an auditory prosthetic feedback device with individuals with long-term BVH and reported improved balance, as long as balance tasks were performed with the eyes open. Other studies indicate that individuals with BVH experienced decreased sway while using an audio-biofeedback system, with the sensor applied to the torso, during quiet stance on foam.35,36

An electrotactile vestibular substitution system (ETVSS; BrainPort Balance Device; Wicab, Inc., Middleton, WI) transmits head position through tactile sensation of the tongue. Preliminary data suggest that use of this ETVSS is an effective intervention for individuals with BVH who have had limited success with VRT.10,37–40 In a published study, 28 subjects with peripheral or central vestibular loss participated in an uncontrolled study using the ETVSS and demonstrated statistically significant improvements in balance, posture, and gait.40 Murphy et al41 investigated the effects of the ETVSS in a pilot study with four subjects with UVH who had previously completed a program of vestibular rehabilitation. After six weeks of in-home use of the device, statistically significant changes were observed in SOT composite scores and Dynamic Gait Index (DGI) scores.41

The use of ETVSS has not been investigated in individuals who have impairment of both the vestibular and proprioceptive systems. This case report examined the use of this device with an individual with BVH and lower extremity proprioceptive loss due to bilateral transtibial amputation (TTA). This study was approved by the Protection of Human Subjects Institutional Review Board at the Missouri State University and the patient gave informed consent.

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Patient History

A 69 year-old man with BVH and bilateral TTA was the subject of this study. He reported the following comorbidities: peripheral vascular disease, polymyositis rheumatica, osteoporosis secondary to long-term prednisone use, high cholesterol, history of deep venous thrombosis, and myelodysplasia (since resolved). Sporadic episodes of phantom limb pain since undergoing bilateral TTA were also reported. The patient experienced symptoms of dizziness (described as lightheadedness and unsteadiness), disequilibrium, oscillopsia, periodic nausea, headaches, occasional sleep disturbances, and moderate to severe pain and stiffness in the muscles of the shoulders, upper arms, and neck due to polymyositis rheumatica. Additionally, he noted difficulties with functional tasks, such as transfers and activities of daily living. Medications included Fosamax (alendronate sodium), Zocor (simvastatin), prednisone, warfarin, naproxen, baby aspirin, calcium supplements, multivitamins, and flaxseed oil.

The patient reported a history of bilateral ingrown toenails that were surgically removed approximately 33 months before the initial examination for this investigation. Lower extremity infection developed in both feet, and the infection was treated unsuccessfully with cephalexin and hyperbaric oxygen therapy. He was hospitalized for this infection four weeks after removal of ingrown toenails. Intravenous gentamicin was administered for 10 days. During this time, the patient reported experiencing several episodes of dizziness; however, he was unaware of the potential connection to gentamicin use. Because of further spread of infection, the patient underwent bilateral TTA three months after the ingrown toenail surgery. Approximately three months after bilateral TTA, the patient was diagnosed with BVH by his physician. Electronystagmography, including bithermal caloric testing, was the physician’s primary means for making this diagnosis. Ice water calorics were not part of this assessment. Although the electronystagmography results indicated that the patient’s vestibular system was unresponsive to bithermal caloric testing, this form of testing evaluates only the horizontal canals at nonphysiologic frequencies. Rotary chair, vestibular-evoked myogenic potentials, head thrust test (HTT), and dynamic visual acuity assessments may have provided additional information regarding the extent of damage to the patient’s vestibular system; however, these forms of evaluation were not completed or requested by the physician at the time of diagnosis of BVH. Furthermore, the patient was not referred for further testing or rehabilitation at that time. This lack of referral for testing or rehabilitation may have been due to the perception that multiple impairments made his rehabilitation potential low.

The BVH combined with altered proprioception and biomechanical changes associated with the use of prosthetic devices had a devastating impact on the patient’s balance, gait, and QOL. Twenty-one months before participation in this study, the patient was treated with VRT (consisting of gaze stabilization exercises to improve the remaining vestibular function and central preprogramming, exercises to improve postural stability, and compensatory strategies) and balance training, two to three times per week for three months (27 visits). Intervention continued with a home program of VRT and balance training (consisting of weight shifting, limits of stability, and static and dynamic balance activities over variable surfaces). Rehabilitation also included balance activities related to prosthetic use, including re-establishment of center of gravity and weight shifting. Additionally, monochromatic infrared light was applied to the residual limbs in an attempt to decrease phantom limb pain. The patient was periodically reassessed, and his home VRT and balance programs were adjusted to ensure that he continually worked at a maximum level. The patient was highly motivated and demonstrated excellent compliance with his home VRT and balance program, as indicated by daily journal entries. Initial testing with the SOT component of dynamic posturography (NeuroCom SMART Balance Master, NeuroCom International, Inc., Clackamas, OR) resulted in a composite score of 0 from a maximum possible of 100; however, after three months of VRT and balance training, his composite score improved to 17/100. Before intervention with VRT and balance training, the patient walked slowly with the assistance of two standard canes. He was unable to walk in a straight line and frequently needed to sidestep to maintain balance. After six months of VRT, he was able to progress to walking with one standard cane; however, the patient’s walking speed remained slow. His ability to walk in a straight line improved, although frequent sidestepping persisted. When the patient was screened for use of the ETVSS, he was performing daily in-home VRT and balance exercises and used a standard cane when walking.

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During the patient’s physical therapy examination completed just before intervention with the ETVSS (approximately 32 months after administration of gentamicin), the following tests and surveys were administered: SOT, Activities-specific Balance Confidence (ABC) scale, Dizziness Handicap Inventory (DHI), DGI, 6-minute walk test using the Biodex Gait Trainer treadmill, and HTT. An audiologist completed videonystagmography (VNG), including bithermal caloric testing. The patient’s gait was videotaped for scoring of the DGI and for qualitative assessment. Examination of residual limb integument revealed no gross tissue irritation or breakdown. Two physical therapists, one physical therapy student, and an audiologist participated in all aspects of examination and treatment for this patient. Retesting was completed after one week, two weeks, one month, two months, three months, six months, nine months, and one year after the start of the intervention. The patient kept a journal, documenting training sessions and symptoms associated with BVH.

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The SOT component of computerized dynamic posturography was used to measure deficits in postural control by determining the patient’s ability to use vestibular, visual, and proprioceptive information. The test uses principles of sway referencing during which the support surface and visual surround are manipulated to challenge an individual’s spatial orientation feedback. The SOT uses six conditions for the evaluation of sensorimotor ability. Comprehensive, norm-referenced results are calculated after assessment.42 The clinical usefulness,43 in addition to the reliability and validity of the SOT44 and computerized dynamic posturography in older adults45 have been reported. A composite equilibrium score (0-100) is automatically computed with higher SOT composite scores indicating greater stability.

The use of the SOT with individuals with BVH and bilateral TTA has not been previously investigated. The SOT has been used to measure standing balance of individuals with unilateral transfemoral amputation46; however, the reliability and validity of its use have not been reported for patients who have amputations.

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ABC Scale

The ABC scale was used to determine this individual’s self-perceived level of confidence when performing 16 functional activities, such as walking in different environments, navigating stairs, and reaching for something above the head.47 A score of 0% on the ABC scale indicates no confidence, whereas a score of 100% indicates complete confidence. Answers are summed to generate an overall mean score.47,48 Myers et al49 indicated that older adults who score >80% are high functioning. Older adults with scores between 50% and 80% have a moderate level of function, and those who score <50% share characteristics that are similar to individuals who are homebound.49 Strong intraclass correlation coefficients have been reported after multiple-week test-retest periods with the ABC scale.47,50 The scale has also demonstrated moderate validity in individuals with BVH51,52 and those with unilateral lower limb amputation.50 The ABC scale has been compared with both the DHI and DGI. A moderate correlation was reported between the ABC and DGI; their concurrent use was recommended when evaluating individuals with vestibular dysfunction.52 Similar research found an analogous correlation between the ABC scale and DHI among individuals with vestibular dysfunction.51

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The DHI was used to rate the patient’s perceived handicap resulting from dizziness. To separate dizziness from impairments associated with bilateral TTA, the patient was asked to consider only the problems he experienced because of BVH (eg, dizziness and oscillopsia). The DHI requires an individual to answer either “yes,” “no,” or “sometimes” in response to 25 questions related to the emotional, physical, and functional components of vestibular dysfunction. Scores range from 0 to 100, with higher overall scores suggesting greater perceived handicap.53 The DHI has high internal consistency and test/retest reliability.53,54 A moderate statistical correlation exists between the SOT and the DHI, a finding suggestive of the ability of the SOT to confirm the presence of handicap or disability.55,56 Higher DHI scores were found to correlate with decreased confidence, as measured by the ABC scale.57 Whitney et al57 reported that patients with vestibular disorders who have DHI scores >60 are functionally impaired based on the DGI and reported more falls.

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The DGI is a functional outcome measure that was used to assess the patient’s ability to maintain gait during eight different tasks: (1) walking at normal speeds, (2) walking at fast and slow speeds, (3) walking with horizontal head movement, (4) walking with vertical head movement, (5) turning quickly, (6) walking around objects, (7) stepping over objects, and (8) climbing stairs. All DGI assessments were videotaped for further review and observational gait analysis. DGI scores are assigned on a 0 to 3 scale based on whether the individual demonstrates severe, moderate, mild, or no impairment during the performance of each task. Total scores of less than 19 of a possible 24 indicate greater risk for falls.58–60 Good interrater reliability has been found in patients with peripheral vestibular disorders.61 Test-retest reliability of the total DGI score was found to be excellent in patients with peripheral vestibular disorders.62 Among community-dwelling older adults, the DGI has excellent test-retest and interrater reliability.52 Use of the DGI with individuals with lower extremity amputations has not been studied.

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Six-Minute Walk Test

The six-minute walk test is used as a functional measure of exercise capacity among individuals with chronic heart failure or lung disease.63,64 However, the test is also useful in the clinical setting as a determinant of mobility and functional status of older adults.65 The Gait Trainer Treadmill (Biodex Medical Systems, Shirley, NY) was used to measure total distance (meters), average walking speed (m/sec), average step length (left and right), and time distribution (percentage of time spent in stance for each leg) during the six-minute walk test. The ambulation index (scaled from 0 to 100), generated as part of the Gait Trainer’s test results, measures average step cycle determined by the foot-to-foot time distribution ratio.66 Gait characteristics of individuals with bilateral TTA have been previously described.67 The six-minute walk test has been shown to be both valid and reliable in assessing physical function in previous studies.65,68 It has been used with individuals with unilateral TTA amputation and found to be a reliable measure of functional capacity with this patient population.69

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VNG and Additional Vestibular Tests

Oculomotor, positional, Dix-Hallpike, and caloric test results were assessed during a standard VNG battery after VRT and again after one year of intervention with the ETVSS. During both testing sessions, oculomotor, positional, and Dix-Hallpike test results indicated no abnormalities. Bithermal caloric testing revealed no horizontal canal function. It is not known whether ice water calorics would have been capable of stimulating a response.

The patient had a positive HTT in both directions. The HTT was found to have a sensitivity of 84% in subjects with BVH and an overall specificity of 82% to rule out vestibular hypofunction.70 Although rotary chair testing is considered the gold standard for diagnosing BVH,71 it was not available for this patient. Thus, residual function of the vestibular system was not quantified by rotational testing.

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The ETVSS used in this case report (BrainPort Balance Device) has two principle components: the controller and the intraoral device (IOD; Fig. 1). The controller contains an electronics package that includes a 32-bit 120 MHz microprocessor, drive electronics, safety circuits, user controls, and rechargeable battery power supply. The IOD uses a two-axis accelerometer (to sense head tilt and motion in an anteroposterior or left-right direction) mounted to the back of a 100-point electrotactile array (10 × 10-mm matrix of 1.5-mm diameter gold-plated electrodes). The IOD is attached to the controller by a tether (a polyimide-based strip, 12 mm wide × 2 mm thick), which uses flexible circuit technology.



A mild electrotactile stimulus is generated by the ETVSS on the electrotactile array of the IOD, which is held on the tongue against the roof of the mouth. The position and movement of the stimulus correspond to head position and movement, as detected by the accelerometer (Fig. 2). The ETVSS has hardware and software controls to ensure both the safety and comfort of tactile stimulation and will shut down if tongue stimulation current exceeds 6.0 mA.



Intervention with the ETVSS was carried out over one year and had three phases: orientation, clinical training, and in-home training. During the orientation phase, baseline data were obtained. The patient was familiarized with the ETVSS and taught to use feedback from the device to facilitate balance. The patient was asked to discontinue use of any medications that could affect or contribute to symptoms of dizziness or disequilibrium during the 48 hours before testing. He was instructed to refrain from alcohol, nicotine, and caffeine within the 24 hours before testing because these substances can affect tactile sensitivity and alter test results.

The clinical training phase consisted of two training sessions per day, separated by at least four hours, with each session lasting approximately 60-90 minutes. These sessions occurred during a two-week period with a total of 21 sessions. Before each training session with the ETVSS, the patient performed joint mobility exercises, including chin tucks, forward and backward shoulder rolls in the scapular plane, pelvic tilts, and hip hikes. Ten to 15 repetitions of each movement were completed. This helped to facilitate isolation of joints, which may have been maintained in a guarded position due to relative postural stiffness. Postural stiffness is a strategy used by some individuals with vestibular loss to compensate for impaired balance.72 Nine three-minute trials using the ETVSS were completed with approximately one minute of rest between each trial. Trials were performed with the patient standing with his eyes closed and the IOD contacting his tongue. The patient was periodically exposed to the following different standing conditions and surfaces during trials: feet apart, feet together, tandem Romberg modified with the front foot slightly to the side, 5-inch thick, high-density (5.5 lb) viscoelastic memory foam (Sleep Innovations, Inc., West Long Branch, NJ), and Airex foam (Airex/Alusuisse Composites, St Louis, MO). A five-minute break followed the ninth trial. The patient then completed a 20-minute trial under similar standing conditions. Because of fatigue and pain secondary to bilateral TTA, the patient occasionally performed a portion of the 20-minute trial in a dynamic sitting position with his eyes closed on a physioball. The patient was frequently given verbal cues for relaxation and deep breathing before and during trials. In addition, he wore a gait belt and was monitored at all times to ensure safety. The patient wore the same prosthetic legs and shoes during all sessions.

During the in-home training phase, the safety of the patient’s training environment was assessed by a home visit. The patient was instructed to complete at least two 20-minute self-training sessions each day, separated by a minimum of four hours. Preliminary investigations using the device showed that 20-minute training sessions were more effective than 10-minute sessions when measuring aftereffects on postural sway.37 Forty-minute training sessions were no more effective than 20-minute sessions.37 The patient was allowed to use either a standing or dynamic sitting position for training, depending on the level of fatigue or pain present in his lower extremities. The patient logged his experiences in a journal, and weekly contact was maintained through telephone, e-mail, and journal entries.

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All tests were performed in the absence of ETVSS. The patient demonstrated improvements in all objective measures used (Table 1).



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The SOT composite score improved from 23/100 to 48/100 after two months and was 46/100 at the time of the six- and 12-month retests. The patient demonstrated normal equilibrium scores on only three of 12 trials when assessed before intervention with the ETVSS. After two months of treatment, the patient’s equilibrium scores were normal for his age group on 10 of the 12 trials in SOT conditions 1 to 4. The number of falls (loss of balance) recorded during the 18 trials of the SOT decreased from 12 to 6 after 12 months of intervention.

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ABC Scale and DHI

Increased confidence in balance resulted in higher ABC scale scores (increasing from 49% to 61%). Lower perception of handicap was indicated by decreased DHI scores (decreasing from 80 to 66). The DHI score is divided into three subscales: physical, emotional, and functional. The patient improved in all three subscales with a decrease of 4 points in the physical and emotional scales (22/28 to 18/28 and 28/36 to 24/36, respectively). The largest decrease was in the functional scale (30/36 to 24/36), indicating the patient perceived that his symptoms had less influence on his ability to function.

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DGI and Biodex Six-Minute Walk Test

Higher scores on the DGI indicated improved gait (11/24 to 21/24), with less evidence of imbalance and fewer gait deviations noted during testing. Although the investigators were not blinded to the intervention, they did not review previous DGI scores before reassessment.

During the six-minute walk test, distance walked increased from 212 to 363 m and ambulation index increased from 55/100 to 91/100. The patient demonstrated improved gait characteristics with increased velocity, improved gait symmetry with smaller coefficients of variation on right/left time distribution, and similar right/left stride lengths (Table 2).



Before intervention with the ETVSS, the patient used one cane to walk. His wide-based gait observed during the first item of the DGI was characterized by an immobile trunk with decreased arm swing (see Video, Supplemental Digital Content 1, After intervention with the ETVSS, the patient was able to walk without an assistive device, although he continued to use a cane when walking in public. Arm swing improved, with increased neck and trunk mobility observed on qualitative assessment of the video record (see Video, Supplemental Digital Content 2,

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Investigating the effects of an ETVSS on individuals with BVH is relatively new. In this case report, intervention with an ETVSS was provided to a patient with BVH and bilateral TTA. Much of the success of this case report must be credited to the patient, who was fully dedicated to every aspect of intervention with the ETVSS. His commitment to the project may have enhanced his ability to experience many of the reported outcomes.

Apart from improved test scores, functional improvements were observed and reported by the patient. During the testing session preceding the intervention with the ETVSS, the patient was able to stand with his eyes closed for less than two seconds. Midway through week 2, he stood for 48 seconds, eyes closed, without the ETVSS. The following day, the patient was able to stand for eight minutes with eyes closed, using no device. He later videotaped himself standing for 20 minutes without the device (eyes closed) during the in-home training period. The patient reported that the ability to stand and maintain balance with his eyes closed for extended durations without the device gave him the confidence to function in situations with limited lighting: he could turn off his office light and walk to the bedroom in the dark, and work in his garden at dusk without the fear of being unable to return to the house. The patient also reported improvements in gaze stability and visual tracking, longer periods of uninterrupted sleep, and less frequent symptoms of headache and nausea. The effect of this intervention on his QOL was indicated in the following comment that he made during the treatment: “My improvements are slow and small. For example, I have had to lean against the wall to slip my shirts over my head in the morning, but this morning I fell into the routine I have used all my life by slipping the shirt over my head while I was free standing. After I got the shirt on, I realized what I had done. You can’t imagine how exhilarating these small moments are.”

Participation in an intervention program using the ETVSS allowed the patient to experience rapid improvement of balance and gait, as indicated by the degree of improvement experienced during the first 2 months of treatment and maintained for a period of one year. In comparison, Badke et al73 reviewed results of VRT (8-54 weeks duration) for 20 patients with vestibular dysfunction (central or peripheral vestibular dysfunction) and noted a mean change in SOT composite scores of 8.41 ± 4.5 points and mean change for DGI scores of 4.4 ± 3.3 points. In a more recent review of 12 patients with peripheral vestibular dysfunction, improvements on the SOT composite score (mean = 10.3 ± 16.1 points) were reported after treatment intervention ranging from four to 18 weeks.74 Additionally, Brown et al75 noted a significant change in mean SOT composite scores from 35/100 to 48/100 for eight patients with BVH after VRT. Although the findings of these three studies show impressive gains after VRT, our patient’s SOT composite score increased 25 points, and his DGI score improved by 11 points after only two months of participation in the intervention program with the ETVSS. Even though the patient started with relatively low baseline scores as a consequence of BVH and bilateral TTA and had greater room for improvement than those with only vestibular disorders, the gains he experienced are noteworthy. Whitney et al76 reported that an SOT composite score <38/100 had the highest sensitivity and specificity for identifying individuals who were recurrent fallers. Our patient’s change in SOT and DGI scores clearly altered his risk of future falls.

Changes in median DHI scores of 14 points, with a median improvement in the functional subscale of 6 points, were reported for 16 subjects with chronic vestibular disease after intervention with VRT.54 In a retrospective review of 12 patients with BVH who were treated with VRT, Brown et al75 noted a mean change of 2 points on the functional subscale of the DHI. The bilateral TTA in addition to BVH in our patient requires that any comparison of DHI scores with those for subjects with only vestibular disorders should be made with caution. However, a decrease of 6 points on the functional subscale after intervention with the ETVSS indicated that he perceived less interference with social activities, job, or household responsibilities and less restriction of travel.

Gait velocity during the six-minute walk test increased after three months of intervention from 0.59 to 0.93 m/sec (a 57.6% increase) and after one year to 1.00 m/sec (363 m) (a 69.5% increase relative to initial speed) with improved gait symmetry, as noted by more equal step lengths (right = 0.56 m, left = 0.60 m). Improvements exhibited in gait velocity and distance allowed the patient to become a proficient community ambulator who could walk at the speed required for safe crossing of intersections (0.74-1.06 m/sec).77 The coefficient of variation for step length and time decreased from 19% to 6% on the right and from 51% to 8% on the left, indicating a more consistent step length. Krebs et al7 noted a significant improvement in free gait velocity for individuals receiving VRT for eight weeks (7.28% increase) and 16 weeks (14.8% increase). A 17% increase in gait velocity was noted after an average treatment period of 3.8 months for 13 subjects with BVH who participated in a customized treatment program that included one or more of the following: balance and gait training, general strengthening, flexibility exercises, and adaptation or substitution exercises.75

Su et al67 examined gait characteristics of individuals with bilateral TTA and found that the freely selected walking speed of their subjects (0.9 m/sec) was slower than that of nondisabled control subjects (1.2 m/sec). After the intervention, our patient’s gait velocity (1.0 m/sec) was similar to that of these subjects even with the presence of BVH. The presence of multiple comorbidities, as well as the frequency, duration, and intensity of the intervention, makes it difficult to compare results from this case study with those of previous studies or to attribute the changes solely to intervention with the ETVSS. As our patient’s activity level increased, in addition to completing his daily training sessions with the ETVSS, he began walking for exercise, progressing to one mile on sidewalks or one-half mile on uneven terrain without an assistive device. This patient was highly motivated to engage in activities on his own, which could have facilitated improvements in gait velocity and symmetry.

For the brain to correctly interpret information from a sensory substitution device, studies indicate that it is not necessary for the information to be presented in the same form as the natural sensory system.30,78 With this ETVSS device, the tongue provides a useful human-machine interface due to its sensitivity, the protected environment of the mouth, and the presence of saliva to ensure good electrical contact.78 Perhaps with training, the brain learns to appropriately interpret information from the device, using the additional feedback to supplement the remaining sensory information for better control of balance.

Although the patient was previously treated with VRT and balance training, his improvements were slow and reached a plateau after six months. The rapid improvement that he demonstrated during intervention with the ETVSS is comparable to improvements observed in other patients with UVH or BVH who have undergone training with this device.30,40 Subjects with gentamicin toxicity (n = 6) demonstrated a mean change in SOT composite score of 17.2 points after an average of 5.3 hours of training during the course of three to four days.40 Scores for the same subjects (n = 4) who were evaluated with the DGI, ABC, and DHI changed by 4.8, 19, and 60.8 points, respectively.40 Other vestibular prosthetic devices have been developed and tested; however, it should be noted that improvements were measured in subjects only when the devices were being used and in place.32–36 In contrast, all outcome measures used in this case study were completed without the ETVSS in use during testing. This implies a carryover effect of training with the ETVSS that has not yet been demonstrated with other vestibular prosthetic devices. A previous investigation using this ETVSS with 39 subjects with vestibular dysfunction found aftereffects with improved stability lasting from four to 12 hours and persistent effects that lasted for eight weeks in one of the subjects who completed 40 training sessions with the device.30,40

The patient’s bilateral TTA presented an additional rehabilitative challenge. The mechanical integrity of residual limb tissue and the fit or comfort of prostheses can affect an individual’s weight-bearing, walking velocity, and pain threshold.79,80 Ehde et al81 found that approximately 75% of individuals with amputations experience residual limb pain, 60% of whom report moderate to severe discomfort. These findings are of particular significance to our study because fatigue and pain were the greatest limiting factors to this patient’s tolerance of intervention with the ETVSS. Additionally, the fit of the prostheses and pain may have influenced the patient’s balance, gait, and degree of improvement. Postural strategies used by the patient for equilibrium were also affected by bilateral TTA. Ankle strategy is typically drawn on when the body experiences subtle perturbations,82 but is unavailable to individuals with bilateral TTA.

Given the reliance of SOT conditions 5 and 6 on vestibular function,83 the patient’s poor performance on these conditions was expected. Horak et al84 noted that subjects (n = 4) with bilateral vestibular loss fell within seconds of the onset of SOT conditions 5 and 6, whereas the postural sway of healthy control subjects with somatosensory loss due to hypoxic anesthesia of the feet and ankles was not significantly affected. Individuals with unilateral transfemoral amputation have demonstrated the ability to maintain standing under these SOT conditions.46 The patient’s inability to maintain balance for the duration of each 20-second trial in SOT conditions 5 and 6 may be partially attributed to the absence of ankle joints and loss of normal cutaneous and somatosensory information from the feet and ankles, as well as BVH. However, during the course of the investigational period, SOT raw data from these conditions showed small, progressive increases in this patient’s stance time.

Recovery from BVH occurs more slowly than UVH, and even when treated with VRT, patient response to treatment varies. Telian et al5 treated 22 patients with BVH with VRT and found that 9% noted considerable subjective improvement in symptoms, 64% mild improvement, and 27% reported no improvement. The patient experienced some improvement in balance and gait with VRT; however, he demonstrated marked improvement during the first 2 months of intervention with the ETVSS. It is uncertain whether intervention with this ETVSS alone would show such profound improvements or whether the combination of VRT and the ETVSS is needed. It is possible that completion of joint mobility exercises before training sessions and verbal cues for relaxation and deep breathing during training sessions may also have played a role in his improvement. The intensity of the intervention with the ETVSS during the in-home training phase may have contributed to the outcomes.

As a result of the intervention program with the ETVSS, the patient reported improved QOL and greater ability to function independently at home and in social settings without an assistive device. Although subjective reports correspond to the objective improvements recorded in both the ABC scale and DHI, it should be noted that his scores indicated that he remained impaired. Substantial increases in the DGI and six-minute walk test translated to functional gains demonstrated by the patient because he became a proficient community ambulator. After one year of participation in the study, when he was expected to stop using or purchase the device, the patient opted to continue training (two times per day for 20 minutes) without the device. After three months, he discontinued all training because he believed that it was no longer necessary. He was subsequently retested three months later and had maintained his previous SOT composite score of 46.

It is unclear how an intervention program using the ETVSS facilitates improvement in balance; however, Bach-y-Rita85,86 and others87–89 cited evidence suggestive of sustained neurological function and reorganization with as little as 2% surviving nervous tissue present after lesion. He asserted that recovery from BVH may occur under this framework of neuroplasticity.85 The exact physiological mechanism of action through which the ETVSS transforms tilt perceived by the tongue into improved balance function has not yet been identified, although brain imaging correlates of sensory substitution for vision (from stimulation of the tongue) have been previously described.78,90

Even though raw data from SOT conditions 5 and 6 showed small, progressive increases in the patient’s stance time, we cannot determine whether this was due to improved vestibular function or better use of remaining somatosensory and proprioceptive cues for balance. In a recent pilot study, Murphy et al41 found that, after six weeks of in-home training with this ETVSS, vestibular function measures (subjective visual vertical test, vestibular-evoked myogenic potential test, dynamic visual acuity, and the sinusoidal and step rotary chair tests) for four subjects with UVH did not show significant changes. Similarly, there was no change in our patient’s response to bithermal caloric testing completed preintervention and postintervention with the ETVSS.

Geurts et al91 noted increased dependence on vision for postural control after lower limb amputation. However, after rehabilitation, subjects (n = 10) demonstrated a decrease in visual dependency, indicating central integration of sensory input from the residual limb for postural control.91 The authors stressed the importance of incorporating different sensory conditions into rehabilitation programs after lower limb amputation.91 Rehabilitation for our patient completed before intervention with the ETVSS did not incorporate activities with his eyes closed due to immediate loss of balance. All our patient’s training sessions with the ETVSS were completed with his eyes closed; therefore, it is possible that after intervention, the remaining sensory input from his residual limbs was used more effectively for postural control.

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A patient with bilateral TTA and BVH secondary to gentamicin toxicity participated in an intervention program using an ETVSS. Marked improvements occurred in the SOT, ABC scale, DHI, DGI, and the Biodex six-minute walk test. Appreciable gains were also demonstrated in the patient’s eyes-closed static standing balance, improving from less than two seconds at the onset of this study to more than 20 minutes during the in-home training phase.

Intervention using the ETVSS improved outcome measure results for this patient beyond those previously achieved with VRT and balance training. It is possible that the outcomes could have been attributed to the frequency and intensity of the intervention, although the patient fervently pursued the VRT and ETVSS rehabilitation programs equally (due to his motivation and personal characteristics).The feedback provided by the ETVSS may have facilitated the patient’s ability to use proprioception, thus allowing better balance control.

This is a complex case that illustrates the potential use of a feedback device to facilitate balance and gait. An ETVSS, such as the BrainPort balance device, could enhance treatment outcomes for some patients who have not achieved desired goals after intervention with VRT. Further investigation with a larger population of patients with BVH should be pursued. Additionally, use of the ETVSS may facilitate rehabilitation for individuals with impaired somatosensory systems, as experienced after amputation or in the presence of diabetic peripheral neuropathy, by providing feedback regarding body position.

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We gratefully acknowledge Wicab, Inc., for providing the ETVSS (BrainPort Balance Device) used with this patient with support from the NIDCD under grant #R44 DC004738. The authors thank Yuri Danilov, PhD, and Kim Skinner, MPT, for training in the use of the ETVSS and for technical assistance.

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electrotactile vestibular substitution system; bilateral vestibular hypofunction; transtibial amputation; balance; gentamicin

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