1. To examine head-shaking nystagmus (HSN), horizontal head-shaking test should be performed routinely, supplemented by vertical head-shaking test when a central lesion is suspected. 2. HSN with more than four distinct consecutive beats is generally pathological, especially when observed using Frenzel's glasses in a dark room. However, HSN may be physiological if the patient's vision is completely blocked. 3. Horizontal HSN suggests the existence of an imbalance between right and left in the vestibular system, and it may have either a peripheral or a central orgin. A distinct vertical HSN strongly suggests the existence of a central lesion. 4. HSN usually appears in a monophasic or biphasic pattern. The pattern which appears in cases of peripheral vestibular disorder (PVD) depends mainly on the stage of recovery. 5. In PVD, HSN beats toward the normal ear in about 75% of monophasic cases and in more than 85% of biphasic cases during the first phase. 6. In many patients with long-lasting PVD, both spontaneous nystagmus (SPN) and HSN reverse their nystagmus directions during the course of recovery in the following order: SPN toward the impaired ear (irritative nystagmus), SPN toward the normal ear (paretic nystagmus), monophasic HSN toward the normal ear, biphasic HSN in the first phase toward the normal ear, monophasic HSN toward the impaired ear, and finally SPN toward the impaired ear (recovery nystagmus). 7. In patients with long-lasting PVD, who show monophasic nystagmus toward the impaired side, the function of the impaired labyrinth is usually recovering (reversed monophasic HSN). This type of HSN can also be observed in cases of Meniere's disease. 8. Biphasic HSN in which the nystagmus in the first phase beats toward the impaired ear (reversed biphasic HSN) is rarely encountered in cases of long-lasting PVD. In cases of Meniere's disease, this type of biphasic HSN is relatively common. 9. It is highly probable (95%) that CP does not exist when HSN is not present. The presence of CP cannot be determined by positive HSN alone.
We investigated the differences and similarities between humans and experimental animals with regard to the vertebro-basilar arterial systems (VBS) and the microvasculature in the brainstem. For that purpose, in the animals, we observed the vascular architecture in the brainstem, the vasculature and local blood distribution in the vestibular nucleus (VN), and cochlear nucleus (CN), the glucose utilization in the VN, blood distribution in the cerebellum, the brainstem and the inner ear, and blood flow regulation in the brainstem and the inner ear. The results are as follows. 1) In the brainstem of the Mongolian gerbil, the VN was supplied by the terminal arteries, one was the median pontine branch and the others were the lateral pontine branches. Those vessels were straight and showed a parallel arrangement. The CN was only supplied by the straight lateral pontine branches. These findings appeared similar to findings in the human posterior circulation. 2) Three types of perforating vessels were identified in the brainstem of the Mongolian gerbil. The VN was supplied by type III perforators (the diameter of those vessels ranged from 35-55pm). The CN was supplied by type II perforators (the diameter of those vessels ranged from 20-25pm). There were differences in the diameters and densities between the capillary blood vessels in those areas. The diameters of the capillary blood vessels ranged from 4-5μm and those blood vessels showed the same reticular arrangement. 3) Using the surface count of microspheres, blood flow in several areas was measured in rats. Blood flow in the cerebellum was richest next to that in the brainstem and that in the inner ear was the poorest of those areas. Also the results of experiments using autoradiographic and microsphere surface techniques showed that blood flow in the VN was almost equal to that in the CN. 4) When the unilateral inner ear was stimulated by the injection of ice water, glucose utilization in the ipsilateral VN was higher than that in the contralateral VN in rats. 5) In rats, the brainstem, which includes the VN and CN, as well as the inner ear have autoregulation of blood flow. These areas are also chemically regulated to increase blood flow in response to hypercapnia. The brainstem has a stronger vascular response than the inner ear. Based on these findings and the results of the simulation model experiments we reported previously, we discuss the characteristics of the VBS that are similar in humans and animals.
To determine age-related changes in vestibular compensation we measured the frequency of spontaneous nystagmus and yaw head tilt after unilateral labyrinthectomy. Albino guinea pigs were divided into two groups: the control group (3 to 12 months of age; n=8) and the aged group (36 to 39 months of age; n=3). We found that the frequency of spontaneous nystagmus in the aged group was significantly lower than in the control group (p<.05). The time constant for spontaneous nystagmus in the aged group was shorter than that in the control group. No significant difference was found in yaw head tilt between the two groups. We conclude that the function of vestibular compensation is not affected by aging. We suggest that the decline of spontaneous nystagmus in the aged group may be due to the diminution of resting activity in two vestibular nuclei.
Paramedian suboccipital transmeatal vestibular neurectomy was performed in 7 cases of intractable vertigo. Four patients had Meniere's disease, two had unilateral inner ear disease, and one had sudden deafness. The seven patients consisted of 6 males and 1 female with ages ranging from 21 to 61 years old (mean age 45.3). Vertiginous attacks ceased in all cases following surgery with a mean follow-up period of 1 year and 5 months. None of the patients showed a significant change in hearing after surgery except for one case, in which preoperative 86.3 dB in PTA became anacusis. Differing from the conventional retrosigmoid approach, the paramedian route enables us to apply the drill in a more anterior-posterior direction and to reach the fundus without destroying the labyrinth. Near the fundus, we can easily section the superior and inferior vestibular nerves, while keeping the facial and cochlear nerves intact. In these cases, with routine neuroanesthetic techniques controlling the intracranial pressure, wider retraction of the cerebellum did not cause any cerebellar symptoms. Hearing preservation after surgery was also acceptable. Among various approaches for vestibular neurectomy to relieve uncontrollable vertigo, the paramedian suboccipital transmeatal route might be the most reasonable from the perspective of surgical anatomy and safety.
Colebatch and Halmagyi (1992) reported that human average evoked myogenic potential in response to clicks shows a characteristic short-latency positivity-negativity, and that this is abolished by selective vestibular neurectomy. Therefore, this clickevoked myogenic potential is thought to be due to vestibulo-collic reflex, and expected to respond to a new vestibular test. We have examined this response in healthy normal subjects and patients with neurotological disease since 1994, and will discuss these cases to evaluate the significance of this test. In normal subjects, the myogenic potential to the click in the unilateral ear, whose peak latency of positive and negative wave were averaged at 13.6 and 23.0 msec, respectively, was found ipsilaterally. In addition, averaged threshold of the potential was 87 dB. Wave form and peak latency of myogenic potential were similar on each side. In patients with profound hearing loss and normal caloric response, the myogenic potential was present. However, in patients with moderate hearing loss and no caloric response, the myogenic potential was lost. These findings suggest that the click-evoked short-latency myogenic potential may originate not from the cochlear but vestibular end organ.
The fluid and ion transports in the inner ear are reportedly regulated by several circulating hormones including antidiuretic hormone (ADH). To address the involvement of an ADH-sensitive water channel (AQP2) in the formation of endolymphatic hydrops in Meniere's disease, molecular biological and immunohistochemical techniques were employed in this study. Total RNA was extracted from the endolymphatic sac of the rat inner ear using the CsTFA-GTC method. AQP2 cDNA was amplified by RT-PCR and the sequences of the PCR products were analyzed. The localization of AQP2 protein was determined by fluorescence- and ABC-immunocytochemistry using a specific antibody to the C-terminus peptides of AQP2. RT-PCR study identified the expression of AQP2 message in the endolymphatic sac. An immunocytochemical study showed an intense expression of AQP2 protein along the epithelium of the endolym-phatic sac. AQP2 was localized mainly at the apical surface of the absorptive epithelium. The results suggest that the AQP2 water channel plays a very important role in volume regulation of the endolymph and maintenance of the endolymphatic fluid/ion balance, and that the impairment of this channel may result in the formation of endolymphatic hydrops.
Both the distance between the posterior semicircular canal and the posterior petrous surface (P-P distance), and that between the vestibule and the posterior petrous surface (V-P distance) were measured on inner ear CT slice encompassing the lateral semicircular canal in 45 patients with unilateral Meniere's disease. Statistical analysis of these measurements revealed that the affected side was significantly shorter than the non-affected side in both the P-P distance (p=0.004) and the V-P distance (p=0.029). Based on these CT measurements, the side showing shorter distances was temporarily defined as the CT-based affected side. Cases in which the CT-based affected side coincided with the actually affected side were determined. Of 45 cases, 31 (68.9%) were affected on the side with a short P-P distance and 31 others (68.9%) were affected on the side with a short V-P distance. Although the affected side in unilateral Meniere's disease is generally diagnosed on the basis of clinical findings, CT findings such as P-P distance and V-P distance seemed to be useful tools for determing the affected side.
The caloric test (irrigation with 5 ml of 20°C water) was performed in 89 otoneurologically healthy subjects (164 ears), from 22 to 84 years of age, to examine the influences of age and gender. 1) Regarding the duration and latency of caloric nystagmus response, there was neither an age-dependent change nor a significant gender-related difference. 2) The maximal slow phase eye velocity of caloric nystagmus response (SPEV) remained unchanged till the quite advanced age, and then lowered from 70.6 years on average in men and from 78.5 years on average in women, indicating that SPEV was more significantly lowered in men than in women between the ages of 65 to 74 years without gender-related difference in the age group over 75 years of age. Therefore it can be assumed that the influence of age on the vestibular system begins earlier in men than in women. 3) Since caloric test does not show canal function alone but is influenced by the central nervous system, it seems possible that the test reflects different aspects especially in the advanced age group. As for SPEV, however, it was significantly lowered in both men and women over 75 years of age.