August 2010 Articles

Assessment of semicircular canal function

Since the invention of the caloric test by Bárány about 100 years ago [4], caloric irrigation and rotatory chair tests were the primary methods used to investigate semicircular canal (SCC) function up until the end of the last century. These tests, however, have poor reliability, mainly due to the following disadvantages: caloric irrigation (i.e., the thermal gradient across the inner ear) depends on anatomical factors of the middle ear, temporal bone thickness, and on the technician’s skills, and conventional rotatory chairs deliver rather low- testing stimuli (usually <100 deg/s peak velocity and between 0.1 and 2 Hz frequency) [5,6]. Both methods, moreover, are almost exclusively used for investigating the horizontal SCC. In 1988, Halmagyi and Curthoys [7] introduced a new diagnostic method known as the head impulse test (HIT), which has the significant advantage of assessing the function of an individual SCC. It can be performed at the bedside and has been shown to have a high sensitivity (~70%) and specificity (~70%) for detecting horizontal peripheral vestibular hypofunction even when conducted by physicians that do not specialize in neuro-otology [8]. In contrast to the caloric irrigation and rotatory chair tests, the quantitative measurement of HIT performed by the dual search-coil method is technically complex and expensive and thus only available in tertiary referral centers, despite allowing functional testing of all six individual canals [9].

Direct comparison of the diagnostic utility of caloric irrigation, rotatory chair testing, and HIT is problematic, as these three methods probe different frequency ranges of vestibular function. In Ménière’s disease, for example, it has been shown that low-frequency vestibular function as assessed by caloric irrigation is affected earlier than high-frequency vestibular function as investigated by HIT [10]. The opposite is observed in Fabry disease, a congenital lysosomal storage disease [11]. These differences possibly result from the affection of different types of hair cells on the cupula. Moreover, in vestibular neuritis (a sudden unilateral deficit of the peripheral vestibular organ), recovery of vestibular imbalance is frequency dependent: while the low-frequency vestibular function determined by caloric irrigation becomes symmetrical, the high-frequency function assessed by HIT often remains impaired [12,13].

Assessment of otolithic function

Estimation of the subjective visual vertical (SVV) is a simple method to assess utricular otolith function. In general, it is performed with the subject sitting upright in a stationary position in darkness and adjusting a luminous line to match the perceived earth-vertical. SVV estimation might be normal in a patient with chronic unilateral peripheral utricular hypofunction tested in a stationary position, however, deficits may be unmasked during eccentric rotation or by off-vertical axis rotation (OVAR) because any utricular otolith function asymmetry will be enhanced, thereby increasing the likelihood of detection. Rotational testing, however, is technically demanding and OVAR can induce motion sickness. This explains why both tests have been applied relatively seldomly to patients.

Diagnosis

In recent years there have been major advances in the methods used to diagnose vestibular organ function, allowing much better and easier diagnosis in dizzy patients. Some of these advances have been outlined below.

Treatment

Vestibular function assessment

The implementation of the Video-HIT and DVA makes quantitative assessment of peripheral vestibular semicircular canal function now possible in any clinical setting. Not only do Video-HIT and DVA help clinicians to diagnose a peripheral vestibular deficit, but they also assist in monitoring vestibular recovery, such as after vestibular neuritis.

VEMP should be performed whenever SCDS, a perilymphatic fistula, or a large vestibular aqueduct syndrome is suspected [55,56]. Typical VEMP responses in patients with these conditions show increased amplitudes and lowered thresholds compared with normal control subjects [57]. VEMPs can also be used to differentiate superior vestibular neuritis (i.e., the affection of the superior division of the vestibular nerve) from a total or inferior vestibular neuritis (i.e., the affection of the inferior division of the vestibular nerve), since in the former the saccular function is mostly spared and, thus, VEMP responses are normal. It has not yet been shown whether the little part of the sacculus being innervated by the superior vestibular nerve has a significant effect on the VEMP in superior vestibular neuritis.

Treatments of peripheral vestibular disorders

Clinicians are advised to initiate drug therapy in patients with vestibular neuritis with methylprednisolone (e.g., 100 mg/day for 7 days) within 1 week (or may be even up to 4 weeks) after the onset of symptoms. The time of treatment onset for corticoid administration has not yet been studied and whether an increased dose (e.g., 250 mg/day instead of 100 mg/day methylpredisone) has a better effect is a matter of debate. In patients severely affected by vestibular neuritis, clinicians might consider the administration of valacyclovir (1000 mg three times a day for 7 days) if diagnosis is made within 3 days of symptom onset.

Since intratympanic steroid injections reach a higher level in the inner ear fluids than oral or intravenous administrations, this application technique may have a better effect in acute vestibulopathies. However, vestibular disorders are often thought of as being the result of nerve damage (neuritis), and it would be unlikely that the nerves would be reached by intracochlear steroids. No studies about intratympanic treatment exist, so this point has not yet been validated.

When treating Ménière’s disease, intratympanic steroid injections are appropriate if the disease does not respond to conventional medical treatment with betahistine and cinnarizine, and should be considered before more invasive and vestibulotoxic interventions like intratympanic gentamicin injections or surgery.

Treatments of central vestibular diseases

When treating vestibular migraine, clinicians are advised to closely follow the treatments proposed by Neuhauser and Lempert [58] and are recommended to read a recent review by Lempert et al. on vertigo as a symptom of migraine [59]. Those making decisions about drug selection for the treatment of vestibular migraine should also consider potential side-effects, such as increased depression with flunarazine treatment or orthostatic hypotension with beta-blockers. A 50% reduction of attacks is a reasonable goal to aspire to [60]. It is advised that patients monitor the severity and frequency of attacks on a calendar and that the efficacy of drugs be evaluated after 3 months. Moreover, lifestyle modifications approved for migraine prophylaxis (e.g., regular exercise, sleep, and diet) should be discussed.

Finally, it has to be emphasized that from what is known so far, aminopyridine seems to be effective predominantly in patients with cerebellar atrophy and with DBN of an unknown etiology, and less effective in patients with structural cerebellar lesions [48,50,61].