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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
Vertigo may result from various causes. The symptom of vertigo may signify a problem in the inner ear and vestibular nerve, where sensory signals are received and then trans- mitted to the vestibular nuclei in the central nervous system. In addition, the cerebellum and the brainstem, where sensory information is integrated, play the important role for caus-
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).
ing the symptom. The inner ear in one side contains the sen- sors of balance and hearing. The sensors of balance comprise the three semi-circular canals (lateral, anterior, and posterior) which detect angular acceleration, and the otolith organs, namely the utricle and saccule, which moni- tor linear acceleration and the orientation of the head relative to gravity (Fig. 1). The inner ear lesions include entire or partial inner ear
Sensory Organ Disorders
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dysfunction, which may occur unilaterally or 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
the Semicircular Canals and Otoliths
The bony labyrinth of the inner ear is a very dense shell that is filled with perilymph. Within the bony labyrinth, the membranous labyrinth filled with endolymph is located as the shape of bony labyrinth. The vestibular labyrinth comprises the two 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, as to detect angular acceleration. Each of the three semicircular canals has small swellings (ampullae) at one end. Each ampulla has a crista having sensory cells (hair cells) on it’s surface. In the crista, cilia arising from the hair cells are embedded in gelatinous material (cupula), which extends across the ampulla (Fig. 2). The movement of the endolymph dur- ing angular acceleration results in displace- ment of the cupula, stimulating the hair cells.
The sensor of the otolith organ, which com- prises utricle and saccule, are called maculae. The utricle and saccule are set approximately at right angles. Both the utricular macula and saccular macula are covered with a gelatinous mass containing an otoconia (otolith), to which cilia from the hair cells are attached. When the head moves with linear acceleration, the otoconia lag behind and deflect the cilia, which produces a change in the sensory signals emit- ted by the hair cells (Fig. 3).
Lateral semicircular canal
Anterior semicircular canal
Posterior semicircular canal
Utricle Saccule
Ampulla Cochlea
Fig. 1 Membranous labyrinth
Otolith
Hair cells
Vestibular nerve
Macula Fig. 3 Macula of the otolith organ
Ampullary crest Vestibular nerve
Hair cells Cupula
Fig. 2 Ampulla of the semicircular canal
to move the target luminous line set about 10° to the right or the left until the target line in a darkened room is viewed as horizontal. Deviation of the horizontal line is small in subjects with normal otolith functions, while in with a unilateral inner ear dysfunction, espe- cially of the otoliths, the target line tilts toward the affected side. Similar results are obtained in the subjective visual vertical determination. As described above, subjective visual horizontal determination is easy to perform, but further studies are required to clarify the exact rela- tionships between the test results and the affected site or severity of the disease. VEMPs occurring in cervical muscles in response to intense acoustic stimuli of short duration (clicks) are considered to originate in the saccule. These responses can be obtained even in patients who are completely deaf, if they have normal saccular functions. Elicita- tion of VEMPs has, over a period of time, be- come popular as an otolith function test. Incor- poration of some ingenuity in the procedure may enhance its usefulness as a routine clinical vestibular test. Although these above mentioned tests can reveal impaired otolith functions, further improvements in the tests may not be easily accomplished, and confirming whether vestib- ular disorders arise from impairment in the semicircular canals or otoliths, remains a chal- lenge in patients with vestibular organ dysfunc- tion in the clinical setting. Data from patients with partial ablation of the inner ear, or those with congenital deficiency of the semicircular canals or otoliths could prove very helpful for improving the algorithms for these test procedures.
Estimation of Partial Labyrinthine
Dysfunction Based on Analysis of
Eye Movements
Acute lesion of the semicircular canals or otoliths causes disequilibrium and spontane- ous nystagmus. Accordingly, accurate analysis
of spontaneous nystagmus may be used to determine the anatomical localization of the affected site. In this context, electronystag- mography (ENG), which has been performed for clinical diagnosis, records only horizontal and vertical eye movements, and is not appro- priate for quantitative analysis of vestibular nystagmus commonly associated with rotatory eye movements. Simultaneous three-dimensional analysis of eye movements (horizontal, vertical, and tor- sional) has a long history of research, but its clinical application is still new. At our insti- tution, we developed a unique video image analysis system (VIAS)4)^ for analyzing otolith- ocular movements induced by OVAR and spontaneous nystagmus.5) BPPV was previously attributed to otolithic dysfunction, but recent studies have empha- sized the relationship between BPPV and impairment of function of the posterior semi- circular canal. The vector of the slow phase velocity of the nystagmus obtained from three- dimensional analysis of positional nystagmus using our VIAS has been shown to be consis- tent with that of the semicircular canal in some patients with BPPV, and inconsistent with that of any semicircular canal in other patients with BPPV. Thus, BPPV may also be caused by impaired otolith function.6) In addition, we carried out three-dimensional analysis of positional nystagmus in patients with BPPV of lateral semicircular canal type, which has recently been proposed as a category of vertigo. The results showed that BPPV in this category may also be classified into two types: of lateral semicircular canal origin and of otolithic origin.7)^ Therefore, detailed analysis of spontaneous or induced nystagmus would be useful for differentiating between semicircular canal and otolithic impairment in patients with vertigo.
Conclusion
We believe that vertigo is caused by not only
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entire inner ear dysfunction, but also partial lesion. Further studies, however, are required to establish reliable test procedures to differen- tiate among impairments of individual sensory organs.
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VERTIGO CAUSED BY THE SEMICIRCULAR CANAL AND OTOLITH LESIONS
