Читать книгу Vestibular Disorders - Группа авторов - Страница 19

To perform the head impulse test, the patient fixates on a target approximately 1 m away (the examiner’s nose). The examiner firmly grasps the patient’s head with both hands and turns it briskly to the left or the right (Fig. 6). The impulses should be unpredictable, low-amplitude (10–20 degrees), high-acceleration movements. When the VOR is intact, each head movement is associated with an equal and opposite eye movement, thus maintaining visual fixation on the target (Fig. 6a). When the VOR is impaired unilaterally, a head impulse to the affected side causes the head and eyes to initially move together (Fig. 6c), followed by a rapid corrective eye movement or catch-up saccade, which rapidly refixates the eye (Fig. 6d). A positive catch-up saccade in the horizontal canal plane is usually evident in the subject, presenting with an acute vestibular syndrome secondary to vestibular neuritis [13]. To confidently separate vestibular neuritis from an acute vestibular syndrome of central origin, it is essential to prove the presence or absence of all the attributes of neuritis. A positive “HINTS plus” test battery (positive head impulse test, typical peripheral nystagmus, absence of skew deviation, and normal hearing) has been shown to separate central from peripheral causes of acute vestibular syndrome with greater accuracy than an MRI scan [13].

Fig. 5. BPV: right horizontal canalolithiasis. a In the upright subject, otoconia lie along the lowest part of the duct of the horizontal canal. No spontaneous nystagmus is seen. When lying on the affected right side (b), the otoconia drift ampullopetally. For the horizontal canals, ampullopetal movement of the cupula constitutes excitation and excitatory right-beating nystagmus follows. The panel on the right side illustrates eye position and slow-phase velocity as a function of time. The nystagmus slow-phase velocity shows the typical rise and fall. When lying on the unaffected left side (c), the otoconia drift ampullofugally and inhibit the right horizontal canal afferents, and inhibitory (left-beating) nystagmus follows. The nystagmus profiles with the affected and unaffected ears down are similar, but the peak velocity is higher with the affected ear down. The side with the more pronounced nystagmus is considered the affected side. BPV, benign positional vertigo.

Fig. 6. The head impulse test. When the head is turned right (a), the intact right horizontal VOR produces an equal and opposite eye movement that enables the eye to maintain fixation on the target. b The primary position at rest. When the head is turned leftward, the eyes initially move with the head (c). A refixation saccade, or catch-up saccade, returns the eye back to the target (d).