The Role of Vestibular Evoked Myogenic Potentials in the Diagnosis of Dizziness

MethodologySubjects

Thirty-five subjects were studied. Subjects were divided into two groups: (1) normal subjects (controls, n=10 with 2 males and 8 females, with a mean age of 34.9 years) (2) subjects with balance disorders ( n=25 with13 males and 12 females, with a mean age of 50.9 years)

Testing was carried out in a sound proof booth with the subjects in a seated position in a comfortable chair.

Methods

The surface electromyographic activity was recorded from symmetrical sites with a two channel setup and five electrodes. The two active electrodes were placed, one on each mid-point of the sternocleidomastoid mucscle (SCM), the two reference electrodes were placed, one on each mastoid and the common electrode was placed on the forehead. During the recoding the patients were instructed to lift there heads from the head rest and turn it as far as possible opposite to the side of the stimulated ear. This would create the appropriate SCM tension to elicit a vestibular myogenic potential response. Electromyographic activity was monitored during the recording to maintain appropriate muscle activation at a constant levels in each patient.

The sensitivity measure was set at 150 µV and the noise rejection level was at 25 + µV. There was a high band pass filter of 5Hz with gradient of -6 dB to 12dB per octave and low pass filter of 750Hz with a gradient of 40dB per octave. Alternating 500Hz Blackman Pips (2-1-2 cycles) at 90 dB normal hearing level were presented via headphones. The stimulation rate was 9.09 Hz, and the analysis time window was 100 milliseconds. The responses were averaged for 500 sweeps and were repeated to ensure that the response was consistent and repeatable.

IntroductionThe electro-myogenic activity recorded from a tonically activated sternocleidomastoid (SCM) muscle with acoustic stimulation is considered to be vestibular in origin and has been termed vestibular evoked myogenic potentials (VEMPs)

Although sound activated, the VEMP has been shown to be independent of sensori-neural hearing loss but dependent on the integrity of the vestibular nerve (Colebatch and Halmagyi 1992; Colebatch et al 1994).

The largest P1-N1 amplitude has been obtained with 500Hz stimuli (Akin et al 2003) which is considered to be the most responsive region of the vestibular nerve afferent fibres (McCue and Guinan 1995) . The latency of P1 has been reported in normal control subjects from 11-13ms with click stimuli at 100dBnHL and within a range of 13-16ms for 500Hz stimuli.

Previous studies indicate that VEMPs may be absent in patients with Acoustic Neuroma and vestibular nerve section and may have an increased sensitivity in patients with the Tullio phenomenon. It has also been suggested that the caloric test findings do not correlate with the results of VEMPs. This study examined the VEMPs in patients with dizziness or imbalance and compared these with videonystagmographic results and caloric testing as well as a control group of subjects.

Results and DiscussionThe results from the first 25 patients tested to date show significant abnormalities of the response in this patient group with vestibular symptoms. Ten of the 25 patients had absent VEMPs. It is clear that a significant proportion (40%) of patients with symptoms of disequilibrium have saccule dysfunction as indicated by abnormalities of the VEMPs.

Of the seven patients who had a unilateral canal paresis on caloric testing, 4 had abnormal VEMPs. A reduced semi-circular canal function as indicated by the slow component velocity on bithermal caloric irrigation induced nystagmus provides a measure of the asymmetry in the dynamic vestibular system which generally results in vertigo but those with additional abnormalities of VEMPs may suffer from a combination of static and dynamic balance system dysfunction resulting in a more broader set of balance symptoms.

Of the 11 patients who had abnormalities on Videonystagmography (VNG), 5 had abnormal VEMPs.

Of the 15 patients that had normal VEMPs, 3 had a canal paresis and 5 had abnormal VNG findings.

These results provide further evidence of complementary information provided by the two sets of tests.

The mean latencies for P1 and N1 were 15.45ms and 23.6ms for the controls and 17.4 ms and 25.45 ms for the patients respectively. There was less variability in the latency of N1 across subjects than that for the P1 for both groups.

Deepak Prasher, Shasa Nur Abdul Aziz, Sriram Vangapalli, Nicola Topass, Kerry Nee and Nicola Mudie

Audiology Department, Royal Surrey County Hospital, Guildford, Surrey GU2 7XX

Groups M/F Mean Age Left P1 Left N1Left P1-N1 amp

Right P1 Right N1Right P1-N1amp

Controls 2/8 34.9+13.8 15.3+1.3 23.9+1.3 54.5+35.1 15.6+1.8 23.3+0.7 49.8+30.5

Patients 13/12 50.9+15.0 17.0+1.5 25.4+2.5 26.9+15.0 17.8+2.2 25.5+1.6 24.5+9.8

Patients with absent N1-P1

4 6

Mean N1-P1 Latencies and Amplitudes for Controls and Patients

The analysis shows that there was a difference in the mean latency of P1 and N1 between patients and controls with patients having a prolonged latency for both P1 and N1 on average of 2ms in comparison with the controls. The mean amplitude of the P1-N1 complex was significantly reduced in patients with a mean of 25.7uV compared to an average of 52.15uV in controls. The difference in the mean age of the controls and patients may be a contributory factor in the mean latency and amplitude differences observed but individual differences in the different age groups in the controls appeared not to bear this out.

ConclusionsThe results indicate a significant role for VEMPs in the diagnosis of vestibular pathology in patients with balance dysfunction.

The static and dynamic system testing provides complementary information which needs to be related to the symptoms for each patient.

The symptom complex may provide an indication of whether both saccule and semi-circular canal functions are the cause of the observed effects in the individual patient. This is currently being examined.

Vestibular Evoked Myogenic Potentials

Right Side Left Side

Patients

CH

Calorics:

Right Canal Paresis

VNG:Normal

RH

Calorics:

Bilateral Hypofunction

VNG: Abnormal

Control

KN

Correspondence - e-mail deepak.prasher@royalsurrey.nhs.uk

P&

G 0

8112

031

The Royal Surrey County HospitalNHS Trust