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Vertigo Caused by Semicircular Canal and Otolith Lesions

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Vertigo Caused by Semicircular Canal and Otolith Lesions ⅥSensory Organ Disorders Vertigo Caused by Semicircular Canal and Otolith Lesions JMAJ 46(7): 291–295, 2003 Toshiaki YAGI Professor and Chairman, Department of Head and Neck Surgery and Sensory Organ Science, Graduate School of Medicine, Nippon Medical School Abstract: The anatomical location of vestibular disorders that cause vertigo is commonly diagnosed simply as “peripheral” or “central”. As one can easily imagine, there are many anatomical regions in the central nervous system that can give rise to vertigo. At least two anatomical locations where peripheral vestibular lesions cause vertigo are possible: the semicircular canals and the otolith. Moreover, the semicircular canals on each side consist of three canals, anterior, lateral, and pos- terior, and the otolith consists of two organs, the saccule and utricle. The existence of localized lesions in the labyrinth has been noticed very recently. Lateral canal benign paroxysmal positional vertigo is a good sample of a localized labyrinthine lesion. However, very little is known about methods of diagnosis of these lesions. A very powerful tool for diagnosing localized lesions is three-dimensional analysis of spontaneous or induced nystagmus. The velocity vector of the nystagmus allows identification of the anatomical source of the lesion, that generates the nystagmus. Key words: Inner ear; Vertigo; Semicircular canals; Otoliths Introduction ing the symptom. The inner ear in one side contains the sen- Vertigo may result from various causes. The sors of balance and hearing. The sensors of symptom of vertigo may signify a problem in balance comprise the three semi-circular canals the inner ear and vestibular nerve, where (lateral, anterior, and posterior) which detect sensory signals are received and then trans- angular acceleration, and the otolith organs, mitted to the vestibular nuclei in the central namely the utricle and saccule, which moni- nervous system. In addition, the cerebellum tor linear acceleration and the orientation of and the brainstem, where sensory information the head relative to gravity (Fig. 1). The inner is integrated, play the important role for caus- ear lesions include entire or partial inner ear This article is a revised English version of a paper originally published in the Journal of the Japan Medical Association (Vol. 127, No. 9, 2002, pages 1479–1482). JMAJ, July 2003—Vol. 46, No. 7 291 T. YAGI Anterior semicircular canal Hair cells Cupula Lateral semicircular canal Utricle Saccule Ampullary crest Vestibular nerve Posterior semicircular Ampulla Cochlea canal Fig. 2 Ampulla of the semicircular canal Fig. 1 Membranous labyrinth dysfunction, which may occur unilaterally or Otolith bilaterally. Very recently, some clinicians became aware of the precise relationships between dysfunc- tion of individual labyrinthine organs, and the clinical manifestations. Morphology and Function of Hair cells the Semicircular Canals and Otoliths The bony labyrinth of the inner ear is a very Vestibular nerve dense shell that is filled with perilymph. Within the bony labyrinth, the membranous labyrinth filled with endolymph is located as the shape of Macula bony labyrinth. The vestibular labyrinth comprises the two Fig. 3 Macula of the otolith organ otolith organs, and the three semicircular canals. The three semicircular canals, which detect angular acceleration, are so arranged almost in planes orthogonal to one another, The sensor of the otolith organ, which com- as to detect angular acceleration. Each of the prises utricle and saccule, are called maculae. three semicircular canals has small swellings The utricle and saccule are set approximately (ampullae) at one end. Each ampulla has a at right angles. Both the utricular macula and crista having sensory cells (hair cells) on it’s saccular macula are covered with a gelatinous surface. In the crista, cilia arising from the hair mass containing an otoconia (otolith), to which cells are embedded in gelatinous material cilia from the hair cells are attached. When (cupula), which extends across the ampulla the head moves with linear acceleration, the (Fig. 2). The movement of the endolymph dur- otoconia lag behind and deflect the cilia, which ing angular acceleration results in displace- produces a change in the sensory signals emit- ment of the cupula, stimulating the hair cells. ted by the hair cells (Fig. 3). 292 JMAJ, July 2003—Vol. 46, No. 7 VERTIGO CAUSED BY THE SEMICIRCULAR CANAL AND OTOLITH LESIONS Vertigo Originating in the Semicircular Canals and/or Otoliths Vertigo can be divided into two major cat- egories: peripheral and central types. Meniere’s disease is characterized by a combination of symptoms, including severe episodes of vertigo, that are attributable to dysfunction of both the vestibular sensory organs and the auditory organ (cochlea). On the other hand, there are diseases that may damage only a portion of the vestibular Fig. 4 OVAR system (you can see the inside of sensory organs to cause vertigo. Actually, it has the chamber with the door left open.) been speculated that the pathology of benign paroxysmal positional vertigo (BPPV) comes from the impairment of the posterior semicir- cular canal. In this context, some investigators right or to the left, the eyes tend to rotate in have proposed that BPPV of lateral semicircu- the opposite direction (counterclockwise and lar canal type should be included as another clockwise to the patient, respectively). How- category of vertigo. ever, the normal eye rotation angles are only 6° when the head is tilted 45° and measurements How to Differentiate Vertigo of vary among patients. These features are not Semicircular Canal Origin from regarded as suitable for a daily clinical test. that of Otolith Origin Newer approaches for evaluation of otolithic functions include the off vertical axis rotation The functions of the semicircular canals (OVAR) test (Fig. 4), subjective visual horizon- can be tested clinically. They may be assessed tal determination, and elicitation of vestibular by rotating a patient in a computer-controlled evoked myogenic potentials (VEMPs) for esti- chair for creating angular acceleration to stim- mating otolith functions from cervical muscle ulate the endolymphatic flow (rotational test), contractions evoked by intense acoustic stimuli. or by irrigating the ear with cold or warm water In the OVAR test, the subject is seated on a (caloric test). The caloric test is especially use- chair in an enclosed large chamber. Nystagmus ful for detecting semicircular canal dysfunction is observed while the chair is rotated around of one or the other side. However, it is not the vertical axis. After the nystagmus disap- possible to test the functions of the individual pears at a constant velocity rotation, the rota- semicircular canals by the caloric test. A recent tional axis is tilted to various degrees to induce study has shown that an analysis of eye move- a new eye movement while continuing the ments induced by rapid rotation of the head on rotary stimulation. The newly evoked nystag- the target plane may be used to detect indi- mus in this condition is recorded for analysis vidual semicircular canal deficits (head impulse of the otolith functions, since the semicircular test).1) canals in this condition are no longer under In contrast, no reliable assessment methods stimulation.2) for otolith functions are available. One of the The subjective visual horizontal determina- tests that has been used is ocular counter- tion may be a relatively simple test to assess rolling, which is a test based on the observation otolith functions. The test is performed by ask- that when the head of the patient is tilted to the ing subjects to fix the position of the head and JMAJ, July 2003—Vol. 46, No. 7 293 T. YAGI to move the target luminous line set about 10° of spontaneous nystagmus may be used to to the right or the left until the target line determine the anatomical localization of the in a darkened room is viewed as horizontal. affected site. In this context, electronystag- Deviation of the horizontal line is small in mography (ENG), which has been performed subjects with normal otolith functions, while in for clinical diagnosis, records only horizontal with a unilateral inner ear dysfunction, espe- and vertical eye movements, and is not appro- cially of the otoliths, the target line tilts toward priate for quantitative analysis of vestibular the affected side. Similar results are obtained in nystagmus commonly associated with rotatory the subjective visual vertical determination. As eye movements. described above, subjective visual horizontal Simultaneous three-dimensional analysis of determination is easy to perform, but further eye movements (horizontal, vertical, and tor- studies are required to clarify the exact rela- sional) has a long history of research, but its tionships between the test results and the clinical application is still new. At our insti- affected site or severity of the disease. tution, we developed a unique video image VEMPs occurring in cervical muscles in analysis system (VIAS)4) for analyzing otolith- response to intense acoustic stimuli of short ocular movements induced by OVAR and duration (clicks) are considered to originate in spontaneous nystagmus.5) the saccule. These responses can be obtained BPPV was previously attributed to otolithic even in patients who are completely deaf, if dysfunction, but recent studies have empha- they have normal saccular functions. Elicita- sized the relationship between BPPV and tion of VEMPs
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