A novel mutation in the potassium channel gene KVLQT1 causes the Jervell and Lange-Nielsen cardioauditory syndrome

The Jervell and Lange-Nielsen (JLN) syndrome (MIM 220400) is an inherited autosomal recessive disease characterized by a congenital bilateral deafness associated with a QT prolongation on the electrocardiogram, syncopal attacks due to ventricular arrhythmias and a high risk of sudden death. JLN syndrome is a rare disease, which seems to affect less than one percent of all deaf children. Linkage to chromosome 11p15.5 markers was found by analysing four consanguinous families. Recombinants allowed us to map the JLN gene between D11S922 and D11S4146, to a 6-cM interval where KVLQT1, a potassium channel gene causing Romano-Ward (RW) syndrome, the dominant form of long QT syndrome, has been previously localized. An homozygous deletion-insertion event (1244, -7 +8) in the C-terminal domain of this gene was detected in three affected children of two families. We found that KVLQT1 is expressed in the stria vascularis of mouse inner ear by in situ hybridization. Taken together, our data indicate that KVLQT1 is responsible for both JLN and RW syndromes and has a key role not only in the ventricular repolarization but also in normal hearing, probably via the control of endolymph homeostasis.

Pathophysiology of Meniere's syndrome: are symptoms caused by endolymphatic hydrops?

BACKGROUND

The association of Meniere's syndrome with endolymphatic hydrops has led to the formation of a central hypothesis: many possible etiologic factors lead to hydrops, and hydrops in turn generates the symptoms. However, this hypothesis of hydrops as being the final common pathway has not been proven conclusively.

SPECIFIC AIM

To examine human temporal bones with respect to the role of hydrops in causing symptoms in Meniere's syndrome. If the central hypothesis were true, every case of Meniere's syndrome should have hydrops and every case of hydrops should show the typical symptoms.

METHODS

Review of archival temporal bone cases with a clinical diagnosis of Meniere's syndrome (28 cases) or a histopathologic diagnosis of hydrops (79 cases).

RESULTS

All 28 cases with classical symptoms of Meniere's syndrome showed hydrops in at least one ear. However, the reverse was not true. There were 9 cases with idiopathic hydrops and 10 cases with secondary hydrops, but the patients did not exhibit the classic symptoms of Meniere's syndrome. A review of the literature revealed cases with asymptomatic hydrops (similar to the current study), as well as cases where symptoms of Meniere's syndrome existed during life but no hydrops was observed on histology. We also review recent experimental data where obstruction of the endolymphatic duct in guinea pigs resulted in cytochemical abnormalities within fibrocytes of the spiral ligament before development of hydrops. This result is consistent with the hypothesis that hydrops resulted from disordered fluid homeostasis caused by disruption of regulatory elements within the spiral ligament.

CONCLUSION

Endolymphatic hydrops should be considered as a histologic marker for Meniere's syndrome rather than being directly responsible for its symptoms.

Changes in endolymphatic potential and crossed olivocochlear bundle stimulation alter cochlear mechanics

Mechanical nonlinearity in the cochlea produces acuoustic distortion products that can be measured in the ear canal. These distortion products can be altered by changes in the endolymphatic potential as well as by stimulation of the crossed olivocochlear bundle, which provides efferent innervation to cochlear hair cells.

High-frequency motility of outer hair cells and the cochlear amplifier

Outer hair cells undergo somatic elongation-contraction cycles in vitro when electrically stimulated. This "electromotile" response is assumed to underlie the high sensitivity and frequency selectivity of amplification in the mammalian cochlea. This process, presumably operating on a cycle-by-cycle basis at the frequency of the stimulus, is believed to provide mechanical feedback in vivo. However, if driven by the receptor potential of the cell, the mechanical feedback is expected to be severely attenuated at high frequencies because of electrical low-pass filtering by the outer hair cell basolateral membrane. It is proposed that electromotility at high frequencies is driven instead by extracellular potential gradients across the hair cell, and it is shown that this driving voltage is not subject to low-pass filtering and is sufficiently large. It is further shown that if the filtering properties of the cell membrane are canceled, taking advantage of the electrical characteristics of isolated outer hair cells in a partitioning glass microchamber, then the lower bound of the motor's bandwidth is approximately 22 kilohertz, a number determined only by the limitations of our instrumentation.

Mechanism generating endocochlear potential: role played by intermediate cells in stria vascularis

The endocochlear DC potential (EP) is generated by the stria vascularis, and essential for the normal function of hair cells. Intermediate cells are melanocytes in the stria vascularis. To examine the contribution of the membrane potential of intermediate cells (E(m)) to the EP, a comparison was made between the effects of K(+) channel blockers on the E(m) and those on the EP. The E(m) of dissociated guinea pig intermediate cells was measured in the zero-current clamp mode of the whole-cell patch clamp configuration. The E(m) changed by 55.1 mV per 10-fold changes in extracellular K(+) concentration. Ba(2+), Cs(+), and quinine depressed the E(m) in a dose-dependent manner, whereas tetraethylammonium at 30 mM and 4-aminopyridine at 10 mM had no effect. The reduction of the E(m) by Ba(2+) and Cs(+) was enhanced by lowering the extracellular K(+) concentration from 3.6 mM to 1.2 mM. To examine the effect of the K(+) channel blockers on the EP, the EP of guinea pigs was maintained by vascular perfusion, and K(+) channel blockers were administered to the artificial blood. Ba(2+), Cs(+) and quinine depressed the EP in a dose-dependent manner, whereas tetraethylammonium at 30 mM and 4-aminopyridine at 10 mM did not change the EP. A 10-fold increase in the K(+) concentration in the artificial blood caused a minor decrease in the EP of only 10.6 mV. The changes in the EP were similar to those seen in the E(m) obtained at the lower extracellular K(+) concentration of 1.2 mM. On the basis of these results, we propose that the EP is critically dependent on the voltage jump across the plasma membrane of intermediate cells, and that K(+) concentration in the intercellular space in the stria vascularis may be actively controlled at a concentration lower than the plasma level.

Localization and functional studies of pendrin in the mouse inner ear provide insight about the etiology of deafness in pendred syndrome

Immunolocalization studies of mouse cochlea and vestibular end-organ were performed to study the expression pattern of pendrin, the protein encoded by the Pendred syndrome gene (PDS), in the inner ear. The protein was restricted to the areas composed of specialized epithelial cells thought to play a key role in regulating the composition and resorption of endolymph. In the cochlea, pendrin was abundant in the apical membrane of cells in the spiral prominence and outer sulcus cells (along with their root processes). In the vestibular end-organ, pendrin was found in the transitional cells of the cristae ampullaris, utriculi, and sacculi as well as in the apical membrane of cells in the endolymphatic sac. Pds-knockout (Pds-/-) mice were found to lack pendrin immunoreactivity in all of these locations. Histological studies revealed that the stria vascularis in Pds-/- mice was only two-thirds the thickness seen in wild-type mice, with the strial marginal cells showing irregular shapes and sizes. Functional studies were also performed to examine the role of pendrin in endolymph homeostasis. Using double-barreled electrodes placed in both the cochlea and the utricle, the endocochlear potential and endolymph potassium concentration were measured in wild-type and Pds-/- mice. Consistent with the altered strial morphology, the endocochlear potential in Pds-/- mice was near zero and did not change during anoxia. On the other hand, the endolymphatic potassium concentration in Pds-/- mice was near normal in the cochlea and utricle. Together, these results suggest that pendrin serves a key role in the functioning of the basal and/or intermediate cells of the stria vascularis to maintain the endocochlear potential, but not in the potassium secretory function of the marginal cells.

Cellular localization of Na+,K+-ATPase in the mammalian cochlear duct: significance for cochlear fluid balance

Cytochemical and autoradiographic procedures were employed to determine the cellular distribution of Na+,K+-ATPase in the guinea pig cochlea. The highest activity was associated with the stria vascularis and was restricted almost entirely to the contraluminal extensions of the marginal cells. Elevated levels of activity were also observed in stromal cells of the spiral prominence, external sulcus, and spiral limbus. The pattern of activity in the latter tissues was unusual in being symmetrically distributed along the plasma membranes of the reactive cells. In the organ of Corti, only neural elements showed appreciable activity. Possible functions of the enzyme are discussed in relation to cochlear fluid balance.

The nature of the negative endocochlear potentials produced by anoxia and ethacrynic acid in the rat and guinea-pig

1. The alterations in the Na+ and K+ concentrations of the cochlear endolymph and in the endocochlear potential were followed simultaneously by means of ion-sensitive and conventional micro-electrodes during simple anoxia, during anoxia after i.v. ethacrynic acid and after i.v. ethacrynic acid alone. The endolymphatic pH changes were measured separately and the effect of perilymphatic ethacrynic acid upon the endocochlear potential was investigated. 2. The over-all Na+:K+ permeability ratio for the endolymph system was determined in individual animals for the first time using an indirect method. The normal mean values of 0.27 (rat) and 0.38 (guinea-pig) were increased after ethacrynic acid. Permeability changes occurred during anoxia but were delayed in onset. 3. The negative endocochlear potentials in each situation behaved quantitatively like modified K+ diffusion potentials largely dependent upon the K+ and Na+ gradients between endolymph and perilymph.

Delayed endolymphatic hydrops

Delayed endolymphatic hydrops is a disease entity that can be differentiated from Ménière's disease. Typically it occurs in patients who have sustained a profound hearing loss in one ear, usually from infection or trauma, and then after a prolonged period of time develop either episodic vertigo from the same ear (ipsilateral delayed endolymphatic hydrops) or fluctuating hearing loss, also sometimes with episodic vertigo, in the opposite ear (contralateral delayed endolymphatic hydrops). The ipsilateral form of the disease may be treated by labyrinthectomy but no satisfactory therapy is available for the contralateral form of the disease.

Localizing signs in positional vertigo due to lateral canal cupulolithiasis

BACKGROUND

Different types of benign positional vertigo (BPV) have been recognized. The variant with permanent apogeotropic direction-changing lateral nystagmus in the supine position is particularly difficult to distinguish from central etiologies.

OBJECTIVE

To identify clinical features of this variant of BPV, helping to establish its peripheral etiology.

METHODS

In five patients without any evidence of neurologic disease and with this variant of positional vertigo, the behavior of nystagmus as a function of head position in space was studied.

RESULTS

In the supine position, a null point for lateral nystagmus was identified, beyond which the nystagmus changed direction. This null point was evident when the head was turned 10 to 20 degrees to the side. In this position, the ipsilateral cupula of the lateral semicircular canal is aligned with the gravity vector. In two of the five patients, a null point was identified in pitch, beyond which the nystagmus reversed direction. This null point corresponds to the head position where the lateral canals are in an earth horizontal plane.

CONCLUSION

From the behavior of lateral nystagmus in different head positions, the lateral canal system of the inner ear is shown to be gravity-sensitive and the side on which the cupula is affected can be determined.

Etiology, pathophysiology of symptoms, and pathogenesis of Meniere's disease

Endolymphatic hydrops is the pathologic feature associated with Meniere's disease. The development of endolymphatic hydrops appears to arise from multifactorial inheritance with alteration of endolymphatic homeostasis. Various factors associated with the phenomenon of hydrops include functional or anatomic obstruction of endolymphatic flow, malabsorption of endolymph, genetic anomalies, vasodilation, allergy, viral infection, and autoimmunity.

Calcium balance and mechanotransduction in rat cochlear hair cells

Auditory transduction occurs by opening of Ca(2+)-permeable mechanotransducer (MT) channels in hair cell stereociliary bundles. Ca(2+) clearance from bundles was followed in rat outer hair cells (OHCs) using fast imaging of fluorescent indicators. Bundle deflection caused a rapid rise in Ca(2+) that decayed after the stimulus, with a time constant of about 50 ms. The time constant was increased by blocking Ca(2+) uptake into the subcuticular plate mitochondria or by inhibiting the hair bundle plasma membrane Ca(2+) ATPase (PMCA) pump. Such manipulations raised intracellular Ca(2+) and desensitized the MT channels. Measurement of the electrogenic PMCA pump current, which saturated at 18 pA with increasing Ca(2+) loads, indicated a maximum Ca(2+) extrusion rate of 3.7 fmol x s(-1). The amplitude of the Ca(2+) transient decreased in proportion to the Ca(2+) concentration bathing the bundle and in artificial endolymph (160 mM K(+), 20 microM Ca(2+)), Ca(2+) carried 0.2% of the MT current. Nevertheless, MT currents in endolymph displayed fast adaptation with a submillisecond time constant. In endolymph, roughly 40% of the MT current was activated at rest when using 1 mM intracellular BAPTA compared with 12% with 1 mM EGTA, which enabled estimation of the in vivo Ca(2+) load as 3 pA at rest. The results were reproduced by a model of hair bundle Ca(2+) diffusion, showing that the measured PMCA pump density could handle Ca(2+) loads incurred from resting and maximal MT currents in endolymph. The model also indicated the endogenous mobile buffer was equivalent to 1 mM BAPTA.

The influence of semicircular canal morphology on endolymph flow dynamics. An anatomically descriptive mathematical model

The classic Steinhausen/Groen mathematical description of endolymph flow in a toroidal semicircular canal is extended to the case where the size, shape, and curvature of the canal lumen change continuously through the duct, utricle, and ampulla. The resulting second-order differential equation has three coefficients, unlike the equation of a torsion pendulum, which has only two. The salient anatomical parameters which determine endolymph motion are: the length of the central streamline occupying the center of the canal lumen; the area enclosed by this streamline as projected into the plane of rotation; the average inverse cross-sectional area of the lumen (taken around the central streamline); and the average inverse squared cross-sectional area, weighted by a local wall shape factor. These parameters are evaluated and the average displacement of the face of the cupula is estimated for the human, guinea pig, and rat, based on new anatomical data presented in companion papers. The model predicts that the dynamic range of human average cupula motion lies between 520 A and 10 microns.

Mixing model systems: using zebrafish and mouse inner ear mutants and other organ systems to unravel the mystery of otoconial development

Human vestibular dysfunction is an increasing clinical problem. Degeneration or displacement of otoconia is a significant etiology of age-related balance disorders and Benign Positional Vertigo (BPV). In addition, commonly used antibiotics, such as aminoglycoside antibiotics, can lead to disruption of otoconial structure and function. Despite such clinical significance, relatively little information has been compiled about the development and maintenance of otoconia in humans. Recent studies in model organisms and other mammalian organ systems have revealed some of the proteins and processes required for the normal biomineralization of otoconia and otoliths in the inner ear of vertebrates. Orchestration of extracellular biomineralization requires bringing together ionic and proteinaceous components in time and space. Coordination of these events requires the normal formation of the otocyst and sensory maculae, specific secretion and localization of extracellular matrix proteins, as well as tight regulation of the endolymph ionic environment. Disruption of any of these processes can lead to the formation of abnormally shaped, or ectopic, otoconia, or otoconial agenesis. We propose that normal generation of otoconia requires a complex temporal and spatial control of developmental and biochemical events. In this review, we suggest a new hypothetical model for normal otoconial and otolith formation based on matrix vesicle mineralization in bone which we believe to be supported by information from existing mutants, morphants, and biochemical studies.

Obliteration of the ductus reuniens

The ductus reuniens was successfully obliterated in 52 guinea pig ears. Histopathological study showed that a majority of these specimens demonstrated cochlear hydrops, saccular collapse and normal utricle. These results support the theory of longitudinal flow of endolymph from the cochlea toward the endolymphatic sac via the ductus reuniens and saccule. A major source of endolymph in the saccule appears to be the scala media. In another set of 11 animals in which the ductus reuniens was first obstructed and two months later the endolymphatic duct was blocked, endolymphatic hydrops was shown in the cochleae, saccules, and utricles of all but one. The evidence suggests that cochlear hydrops was caused by obliteration of the ductus reuniens, and the saccular and utricular hydrops occurred subsequently as the result of blockage of the endolymphatic duct. Remnants of otolithic membrane which were attached to the distended saccular wall indicate that the membrane which had collapsed onto the macula after obliteration of the ductus reuniens is capable of subsequent distension. This experiment supports the concept of endolymph flow from the utricle and canals toward the endolymphatic sac. A blocked cutus reuniens might also explain the pathophysiological basis for the auditory form of Meniere's disease.

The caloric vestibular reaction in space. Physiological considerations

Caloric stimulus testing was performed as part of the vestibular research program during the European Spacelab 1 mission in Nov/Dec 1983. Contrary to prediction according to the classical endolymph flow theory originally forwarded by Bárány, caloric nystagmus was elicited in both tested astronauts. The intensity of the response was found comparable to that measured on earth. The theoretical consequences of these findings are discussed and possible mechanisms are considered. The direct volume displacement hypothesis is favoured as the primary effect responsible for the observed vestibulo-ocular nystagmus. Estimated differential pressure conditions resulting in the endolymph canal support this hypothesis and are in agreement with the observed response intensity. It is further speculated that interaction in the central vestibular system between canal and otolith signals be responsible for the well-known body position modulation of the observed nystagmus.

Cyclic AMP and adenylate cyclase in the inner ear

The activity of adenylate cyclase and the steady state levels of cyclic AMP (cAMP) were determined in stria vascularis (SV) and organ of Corti (OC) of the guinea pig cochlea. The activities are 12 and 19 pmoles/mg dry weight/minute for OC and SV, respectively. The activity was increased two to four-fold by NaF. The base level of cAMP is 4.2 and 4.4 nmoles/g dry weight in OC and SV, respectively. In contrast to brain, neither ischemia nor barbiturates produced major changes of the steady state levels of cAMP. No in vitro effect of cAMP upon the state of activation of glycogen phosphorylase was noticeable in either tissue. cAMP did not exert a significant in vitro inhibition of strial Na+K+-ATPase. Perilymphatic perfusion of cAMP (10-3 M) and of theophylline (5 times 10-3 M) did not produce changes in the endolymphatic potential (EP), but dibutyryl cAMP (10-3 M) led to a significant increase of EP. The alpha adrenergic blocking agent, phentolamine, produced very complex changes of the cochlear potentials. A possible role of catecholamines and cAMP in the secretory phenomena of the SV and in the transduction and/or transmission processes of the auditory sense organ are discussed.

The nature of the ototoxic actions of ethacrynic acid upon the mammalian endolymph system. I. Functional aspects

The endolymphatic changes produced by an intravenous injection of 60 mg kg-1 ethacrynic acid were followed for up to 120 min using conventional and ion-sensitive (Na+, K+ and pH) microelectrodes in the rat. They were found to be caused by three distinct effects upon the endolymph system. Initially, the drug completely inhibited the strial potential-producing and cation-transporting processes. Recovery began quickly and was rapid at first. Then its rate declined considerably, probably due to diminution in strial energy production of delayed onset and prolonged duration. Coincident with these actions upon active transport, there was a decrease in the overall cation permeability of the endolymph system. This followed a different time course and affected K+ much more than Na+. The findings also provided further information about the mechanisms responsible for the normal endolymphatic composition. Qualitatively similar results were obtained in a subsidiary study on guinea pigs.

Effects of aging on potassium homeostasis and the endocochlear potential in the gerbil cochlea

Previous work has shown that the endocochlear potential (EP) decreases with age in the gerbil. Concomitant with the EP decrease is an age-related loss of activity of Na,K-ATPase in the lateral wall and stria vascularis. We hypothesized that the EP decrease is associated with a similar decrease in the endolymphatic potassium concentration [Ke+]. This hypothesis was tested using double-barrelled, K(+)-selective electrodes introduced into scala media through the round window in young and quiet-aged gerbils. Results show that the means (+/- S.D.) of the [Ke+] in young and aged gerbils were not significantly different (178.2 +/- 14.2 mM and 171.2 +/- 34.4 mM, respectively), although the intersubject variability was much greater in the aged animals than in the young. These values of [Ke+] are slightly higher than those found for other mammals and may reflect the higher plasma osmolarity found in the gerbil. The concentration of perilymphatic potassium [Kp+] in scala tympani at the round window was also similar for the young and aged groups (3.57 +/- 1.17 mM and 4.18 +/- 2.03 mM, respectively). On the other hand, mean EP values in the young and aged gerbils were 92.0 +/- 5.7 mV and 64.8 +/- 15.8 mV, respectively and were statistically different (P < 0.001). Overall, EP and [Ke+] showed little correlation (R2 = 0.23), except that when [Ke+] fell below 150 mM, the EP was always less than 60 mV. An analysis of the chemical potential for Ke+ with respect to Kp+ shows that it was similar for young and aged gerbils (overall mean of 103.1 +/- 13.7 mV) and remained constant with respect to the EP, in spite of an overall electrochemical potential of Ke+ that varied from 120 to 210 mV. Thus, the system maintains Ke+ homeostasis at the expense of the EP, even when the EP is on the verge of collapse.

Direct measurement of longitudinal endolymph flow rate in the guinea pig cochlea

The rate of longitudinal endolymph flow in the guinea pig cochlea has been measured with a novel tracer technique. The tracer we utilized was the tetramethylammonium (TMA) ion, the movement of which was monitored by ion-sensitive microelectrodes. Extremely small amounts of tracer were required as the electrodes could readily detect TMA concentrations in endolymph as low as 10 microM. TMA was introduced into scala media in the form of a small bolus, varying from 2-20 nl in volume. To examine whether longitudinal flow affects the dispersion of TMA in endolymph, we compared the characteristics of TMA spread to turn I following injection into turn II, with those of TMA spread to turn II following injection into turn I. The comparison of these data with an analytical model combining the processes of diffusion and volume flow demonstrates that the spread of tracer is dominated by passive diffusion processes with very little contribution from longitudinal endolymph flow. The rate of longitudinal endolymph flow between turn I and turn II was estimated to be less than 0.01 mm/min directed towards the basal turn. This value is considerably lower than recently published estimates using other techniques.

Ionic environment of cochlear hair cells

The scala media of the adult cochlea in mammals comprises a morphologically closed compartment sealed with tight junctions of the intermediate to tight types. The unique ionic composition of endolymph is maintained by the stria vascularis through active reabsorption of sodium and active secretion of potassium against ionic gradients. The subtectorial space is only a partially closed compartment which communicates with the endolymph via holes in the tectorial membrane at its outer insertion to the organ of Corti. Hardesty's membrane divides the subtectorial space into two compartments: one facing the surfaces of inner hair cells and one facing the surfaces of outer hair cells. In the study of comparative anatomy, hair cells, e.g. in the lizard, basilar papilla are of two types: those covered with a tectorial membrane and those being free-standing lacking the tectorial membrane. The ionic environment of the hair cell surface seems to be the same, independent of whether covered with a tectorial membrane or not. The tectorial membrane itself is semipermeable to ions in the endolymphatic space. Only the surface structures of the hair cell with the sensory hairs facing the subtectorial space are exposed to the high concentration of potassium, whereas the remaining parts of the hair cell are surrounded by a fluid having a more normal extracellular type of ionic composition (cortilymph/perilymph). During embryonic development the ionic composition of endolymph develops in parallel with the morphologic maturation of the stria vascularis. A completely mature composition of endolymph is reached before any electrophysiological potentials in the cochlea can be elicited. The sensory hair surface of hair cells has reached a mature morphology prior to the maturation of endolymph. In several species the tectorial membrane is morphologically only partially mature when the increase of the potassium concentration of endolymph starts. Drugs primarily affecting the stria vascularis causing a transient change of the ionic composition of endolymph result in a transient dysfunction of inner ear potentials. If the ionic changes persist for longer time, morphological changes can occur in both the stria vascularis and the hair cells of the organ of Corti. Whether such changes are primarily caused by the ototoxic drug itself or by changes in the ionic composition of endolymph has to be explored further.

Aspects on endolymphatic sac morphology and function

Morphological evidence indicate that the main function of the endolymphatic sac is to act as a reabsorptive and defensive mechanism for the inner ear. This activity is markedly enhanced in labyrinthine trauma, such as injection of foreign particles into the labyrinth, blocking of the endolymphatic duct, and cryosurgical destruction of vestibular sensory epithelia. Light and dark epithelial cells of the intermediate portion of the sac are capable of reabsorbing endolymph and digesting cellular debris respectively. The extensive capillary network surrounding the endolymphatic sac exhibits endothelial characteristics suggestive of active fluid transport. The "dynamic-flow theory" of endolymph circulation suggests that a radial-flow should be considered for energy metabolism and ion exchange around the sensory cell regions whereas a longitudinal-flow should be considered for reabsorption of endolymph and disposal of high molecular waist products and debris by the endolymphatic sac. The earlier concepts of endolymph circulation thus need not any longer be considered conflicting.

Repositioning maneuver for the treatment of the apogeotropic variant of horizontal canal benign paroxysmal positional vertigo

OBJECTIVE

The purpose of this study was to determine the effectiveness of a new physical maneuver in the treatment of the apogeotropic variant of horizontal canal benign paroxysmal positional vertigo.

STUDY DESIGN

Case review.

SETTING

Outpatient clinic.

PATIENTS

The diagnosis of apogeotropic horizontal canal benign paroxysmal positional vertigo was based on the history of recurrent sudden crisis of vertigo associated with bursts of horizontal apogeotropic paroxysmal nystagmus provoked by turning the head from the supine to either lateral position. The patients were three men and five women ranging in age from 31 to 73 years (average, 49.2 yr).

INTERVENTIONS

All patients were treated with a repositioning maneuver based on the hypothesis that the syndrome is caused by the presence of free-floating dense particles inside the endolymph of the anterior arm of the horizontal canal. The maneuver favors their shifting into the posterior arm of the canal. Patients were reexamined immediately after the treatment and underwent Gufoni's liberatory maneuver for the geotropic variant of horizontal canal benign paroxysmal positional vertigo.

MAIN OUTCOME MEASURE

The treatment outcome was considered as responsive when, after one repositioning maneuver, nystagmus shifted from apogeotropic to geotropic.

RESULTS

The repositioning maneuver resulted in a transformation from the apogeotropic variant into a geotropic variant of horizontal canal benign paroxysmal positional vertigo in all patients.

CONCLUSION

This maneuver represents a simple and effective approach to the treatment of the apogeotropic variant of horizontal canal benign paroxysmal positional vertigo. It favors the shifting of the canaliths from the anterior into the posterior arm of the horizontal canal from where they can migrate into the utricle with Gufoni's maneuver.

Purinergic modulation of cochlear partition resistance and its effect on the endocochlear potential in the Guinea pig

Introduction of adenosine 5'-triphosphate (ATP) into the endolymphatic compartment of the guinea-pig cochlea decreases the endocochlear potential (EP). To determine if this is due to an ATP-induced change in compartment resistance, the cochlear partition resistance (CoPR) was measured using constant current injections into scala media before, during, and after microinjection of ATP into the same compartment. The CoPR (mean = 3.13 +/- 0.13 kOmega) decreased with ATP in a dose-dependent manner (25.1 +/- 3.0% decrease in relation to baseline values) and this was linearly correlated ( R(2) = 0.91) to the magnitude of the ATP-induced decline in EP (41.6 +/- 7.0% decline in relation to the baseline). Pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS, a P2X receptor antagonist) injected prior to ATP application blocked this ATP-induced reduction in EP and CoPR. This indicates that ATP-gated ion channels (P2X receptors) provide a latent shunt capable of regulating the majority of the electrical potential across the luminal surface of the sensory hair cells, which is necessary for sound transduction. The results suggest a novel sound transduction regulatory mechanism, which, via extracellular ATP, has the capability of adjusting hearing sensitivity.