Читать книгу Vestibular Disorders - Группа авторов - Страница 16

The question here is “does the patient have BPV or not?” rather than “is there positional nystagmus?” BPV should be diagnosed only when paroxysmal positional nystagmus that is in the plane of a single semicircular canal is observed [9]. If these attributes are not met, it is best to leave the diagnosis open. To check for posterior and anterior canal BPV, rotate the patient’s head 45 degrees to one side and lower the head 30 degrees below horizontal (Fig. 4). Observe for 30 s since nystagmus onset can be delayed. If positional nystagmus is seen, maintain the patient in the Hallpike position for at least 1 min, to check that it ends. Consider typical left posterior canal BPV; when the patient is upright, no nystagmus is observed. In the left Hallpike position, movement of otoconia along the posterior canal activates the canal afferents, producing a brief paroxysm of upbeating and leftward torsional (geotropic) nystagmus (Fig. 4; online suppl. Video 4).

Fig. 3. Nystagmus patterns observed in a left peripheral vestibulopathy and in cerebellar pathology. The top panel shows typical peripheral right beating nystagmus that is unidirectional and enhances on rightward gaze (“Alexander’s Law”) in a left peripheral vestibulopathy. The bottom panel shows typical gaze-evoked nystagmus, which beats leftward on left gaze and rightward on right gaze.

To test for horizontal canal BPV, the subject begins by assuming the supine position, lying on a single pillow, with the head elevated 30 degrees from horizontal, to align the horizontal canals with earth vertical. Consider right horizontal canalithiasis where rotating to the affected right side provokes a paroxysm of horizontal geotropic nystagmus beating toward the right side (Fig. 5; online suppl. Video 5). Rotation to the left provokes a similar but less intense paroxysm to the left. Lying face down from an initial upright position or pitching the head down provokes horizontal right-beating nystagmus and lying supine provokes left-beating nystagmus. To correctly identify the affected ear, nystagmus intensity with either ear down can be compared. If an examiner has access to video Frenzel goggles, supine (contraversive) and prone (ipsiversive) nystagmus also help identify the affected side; in the prone position, nystagmus fast phases beat to the affected ear and when supine, towards the unaffected ear [10]. Persistent, direction-changing horizontal positional nystagmus is observed in horizontal canal BPV secondary to cupulolithiasis, where otoconia are adherent to the horizontal canal cupula. In contrast to horizontal canalithiasis, the positional nystagmus is apogeotropic, meaning that the nystagmus beats away from the lowermost ear during roll testing. The nystagmus is more pronounced with the unaffected ear down [10]. Persistent geotropic and apogeotropic horizontal positional nystagmus can also be seen in central vestibular disorders, inclusive of VM [11].

Fig. 4. Left posterior canal BPV. a In the upright subject, the otoconia lie in the lowermost part of the left posterior canal. No spontaneous nystagmus is seen. b As the head is turned 45 degrees to the left and lowered to the Hallpike position, otoconia drift downward (ampullofugally). Ampullofugal movement of the cupula excites vertical canal afferents. Left posterior canal afferents are activated, briefly resulting in a paroxysm of upbeat geotropic torsional nystagmus consistent with left posterior canal BPV. Hallpike testing with the unaffected ear down (c) yields no nystagmus. The panels on the right side illustrate eye position and slow-phase velocity as a function of time during the left Hallpike test, with a crescendo-decrescendo profile.

Anterior canal BPV is rare and is also elicited with a Hallpike test on the affected or unaffected side. Consider left anterior canal BPV that could result in a positive right or left Hallpike test or both. The nystagmus, regardless of which ear is down, is downbeating, with a torsional component that beats to the affected left side (online suppl. Video 6). So rare is anterior canal BPV that torsional downbeat nystagmus on positional testing should first raise the possibility of an underlying central cause, unless of course the nystagmus abolishes after a successful liberatory manoeuver.