Window to an Unusual Vestibular Disorder By Mark Parker

Transcription

1 WELCOME BACK to an ongoing series that challenges the audiologist to identify a diagnosis for a case study based on a listing and explanation of the nonaudiology and audiology test battery. It is important to recognize that a hearing loss or a vestibular issue may be a manifestation of a systemic illness. Being part of the diagnostic and treatment team is a crucial role of the audiologist. Securing the definitive diagnosis is rewarding for the audiologist and enhances patient hearing and balance health care and, often, quality of life. Window to an Unusual Vestibular Disorder By Mark Parker History A 52-year-old male referred to the clinic for a vestibular evaluation. The patient reported a long history of near constant lightheadedness accompanied by attacks of a feeling that stationary objects directly in front of him appear to slowly rotate to one side or another, usually to his left. He denied hearing loss but reported that at times he heard a pulsing in his ear and his own voice sounded distorted. Audiological Testing The patient exhibited air and bone conduction thresholds within normal limits. Immittance audiometry Hillary Snapp, Investigator-in-Chief CSI Reference Guide: Visit and search keywords CSI Reference Guide. FIGURE 1. Results from initial audiometric evaluation revealing normal immittance and hearing sensitivity. 50

2 FIGURE 2. Saccade test results demonstrating normal function. revealed bilaterally normal tympanograms; however, the patient became dizzy during testing on the right side. Acoustic reflexes were not tested due to reports of dizziness. The patient had excellent speech discrimination at normal conversation levels bilaterally. All results were determined to be within normal limits. Findings are shown in FIGURE 1. Vestibular Testing Rotary Chair Analysis Like many vestibular assessments, there are advantages and disadvantages to rotary chair analysis. Advantages include the ability to obtain a majority of the subtests in a standard video nystagmography (VNG) analysis with a high degree of accuracy, and the ability to collect additional data to help determine the site of lesion, including tests of utricular dysfunction. Rotational chair is valuable in confirming bilateral vestibular loss as indicated on calorics, assessing compensation of the peripheral system, and identifying signs of central involvement. Additionally, rotary chair analysis can be conducted in lieu of caloric analysis for patients with middle ear disorder, for children, or for difficult-to-test patients. Disadvantages include the inability to test vertical pursuit and calorics, the high cost of the equipment, and the fact that no peer-reviewed standardized norms exist for interpreting the data (each clinic collects its own normative data). A holistic approach is required when interpreting rotary chair data. A single abnormal test result may not necessarily point to a specific etiology; rather, the data must be interpreted in aggregate to reach a conclusive diagnosis. Tests of Oculomotor Function The examination revealed no spontaneous or gaze-evoked nystagmus and unremarkable oculomotor testing (FIGURES 2 4). In addition to testing oculomotor function, saccades also verify that recordings are properly calibrated. Normal oculomotor findings require a minimum of 70 percent normal values in peak velocity, latency, and accuracy FIGURE 3. Horizontal smooth pursuit test results demonstrating normal function. represents normal responses FIGURE 4. Oculomotor trapezoidal test results demonstrating normal function. Vol 27 No 1 Jan/Feb 2015 AUDIOLOGY TODAY 51

3 according to our clinical normative data. All results were determined to be within normal limits. Chair Sinusoidal Harmonic Rotation Test (SHA) SHA test assesses balance function by measuring the vestibular ocular reflex (VOR) produced in response to back-and-forth oscillations generated by the rotational chair from 0.01 to 0.64 Hz. The patient is seated upright with the head positioned at a 30 degree downward angle to ensure maximal stimulation of the horizontal canal as the chair rotates in a right-left sinusoidal motion. Phase, gain, and symmetry of the VOR response are analyzed. As the chair rotates, the eye will move in the opposite direction of the head approximately 180 degrees out of phase, and the gain (speed) of the eye movement should approximate that of the head. If the eye leads the head in phase, a peripheral disorder is suggested whereas when the eye lags, a central disorder is suspected (Li et al, 1991). When gain of the eye movement is reduced, a peripheral disorder is suggested whereas increased gain of a response suggests a more central site of lesion (Li et al, 1991). Symmetry compares the magnitude of right versus left responses. Symmetry in the presence of abnormal gain and phase suggests This case underscores the importance of listening to the patient. compensation of a peripheral disorder. All results are within normal limits (FIGURE 5). Rotary Chair Velocity-Step Test The velocity-step test measures the decay rate of nystagmus following an abrupt angular acceleration or deceleration. This decay is defined as the time required for the nystagmus to reduce to 37 percent of its maximum value (Stockwell and Bojrab, 1997). The chair rotates clockwise or counterclockwise at 100 degrees per second and maintains this velocity until the VOR response decays, the chair is then abruptly stopped, and the decay time of the resulting VOR response is measured again. The VOR response occurs in response to rapid head movement generating a lag in fluid movement in the horizontal canal, resulting in deflection of the cupula. Once the movement of the fluid equals that of the head, the cupula is no longer deflected and the VOR response ceases. Likewise, when the chair is abruptly stopped, the head is no longer moving, but the fluid continues to move. This results in deflection of the cupula in the opposite direction and generation of nystagmus in the opposite direction. Decay times that are shorter than normal indicate an abnormality in the velocity storage mechanism, which is most consistent with peripheral site of lesion. Abnormally FIGURE 5. Sinusoidal harmonic acceleration test results demonstrating normal gain, phase, and symmetry. For this figure the white area and the shaded area represents FIGURE 6. Velocity-step test results demonstrating abnormal peek eye velocity. 52

4 increased decay times indicate an issue with the cerebellar clamping mechanism, which suggests central site of lesion. Patients with unilateral peripheral vestibular lesions may present with directional asymmetries on velocity-step tests (Stockwell and Bojrab, 1997). The results (FIGURE 6) show an abnormal peak eye velocity at 100 degrees per second that may suggest peripheral involvement. This was the lone piece of evidence suggesting anything out of the ordinary; however, this result by itself is not enough evidence on which to base a reliable diagnosis. Positional Nystagmus No significant nystagmus was observed during the Dix-Hallpike maneuver to either side, or when lying on either side. These results are normal and suggest that the patient s symptoms are not caused by benign paroxysmal positional vertigo (BPPV). Caloric Irrigations Bithermal caloric irrigation using air resulted in robust and symmetric responses (FIGURE 7). These findings are consistent with normal findings on SHA testing during rotational chair analyses. At this point, the absence of any strong evidence pointing to either a central or peripheral etiology was puzzling. Considering the patient s case history, are there any additional evaluation measures you might consider? VEMPs Based on the patient s report of autophony (abnormal perception of the patient s own voice) and onset of dizziness produced by tympanometry, we wondered if this patent could be experiencing a form of perilymphatic fistula, which are rare cases of a leak appearing in the bony portion of the cochlea or vestibular system. Subtypes of perilymphatic fistulas include leaks around the oval window or round window or superior canal dehiscence (SCD), in which a fistula (hole) appears in the bony portion of the superior semicircular canal. People with these conditions report hearing loss, dizziness, or aural fullness caused by pressure changes. Typically, persons experience membranous perilymphatic fistulas (round window and oval window fistulas) secondary to head trauma, which was negative in our patient. In order to determine whether the patient s symptoms were caused by SCD, we obtained cervical vestibular evoked myogenic potential (C-VEMP) thresholds on each ear. The rationale for using C-VEMPS is that an intact boney labyrinth will need a higher stimulus to elicit the C-VEMP response, and a hole in the boney labyrinth would allow for a C-VEMP response at a lower stimulus intensity when compared to the contralateral ear because less bone is present to attenuate the stimuli. In the practical sense, a normal ear would elicit a robust positive N1 wave with a 95 db nl presentation level, and this response should be absent at 60 db nl presentation level. As seen in FIGURE 8, this is the case for this patient s left side. However, the 60 db nl presentation to the right side elicited an N1 waveform. For this patient, there was no N1 response at 20 db nl presentation. Since the left ear produced responses that were considered normal and the right side produced an abnormal response at FIGURE 7. Bithermal caloric test results demonstrating normal responses. FIGURE 8. C-VEMP demonstrating a normal response for left ear stimulation and abnormal response for right ear stimulation as indicated by positive response at 60 db nl. For this figure, the blue tracings represent left ear responses, and the red tracings represent right ear responses. Vol 27 No 1 Jan/Feb 2015 AUDIOLOGY TODAY 53

5 65 db nl, we hypothesized that this patient was experiencing symptoms due to SCD on the right side. Computed Tomography (X-ray CT) Based on the positive results of the VEMP, the physician ordered a CT scan of the temporal bone. The results of the scan showed a small opening, or dehiscence, on the superior portion of the superior semicircular canal (FIGURE 9). This was diagnosed to be the cause of the patient s symptoms. Surgical plugging of the superior semicircular canal was successful in alleviating the patient s symptoms. Take-Home Lesson This case underscores the importance of listening to the patient. The patient s history and, in particular, observation of his discomfort during tympanometry, were key pieces of evidence that helped lead to the proper diagnosis and treatment. This case also serves as a good example of the holistic approach required to interpret comprehensive vestibular diagnostics. The abnormal results obtained from the velocity-step test did not correlate with any other positive signs for central or peripheral abnormalities, and so that data was impossible to interpret. The vast majority of the testing showed normal central and peripheral responses. In hindsight, it may be obvious that VEMP analysis would be the only test required for this patient. However, SCD is a relatively rare disease, and prudence dictates the most common causes of his symptoms should be ruled out first. Mark Parker, PhD, is an assistant professor of otolaryngology at Tufts University School of Medicine and director of audiology at Steward St. Elizabeth s Medical Center. References Li CW, Hooper RE, Cousins VC. (1991) Sinusoidal harmonic acceleration testing in normal humans. Laryngoscope 101(2): Stockwell C, Bojrab DI. (1997) Interpretation and usefulness of rotational testing. In: Jacobson GP, Newman CW, Kartush J, eds. Handbook of Balance Function Testing. San Diego: Singular, Thomson Learning, FIGURE 9. CT scan of the right temporal bone (light grey) shows the thinned bone (arrow) of the superior canal that produced these symptoms. 54