Effect of off-vertical tilt and macular ablation on postrotatory nystagmus in the squirrel monkey

The slow phase eye velocity (SPEV) and duration of post-rotatory nystagmus (PRN) were studied in squirrel monkeys (Saimiri sciureus) after a ramp speed rotation (0-200 degrees/sec, with 1 degree/sec2 angular acceleration). When the results were compared between straight upright vertical rotation, 9 degrees tilt rotation, and 18 degrees tilt rotation, faster decay both in SPEV and in duration was found in the tilt rotation situations. Difference in nystagmic decay curves by tilting rotation axis could be from the convergence of macula-semicircular canal inputs. Subsequently bilateral macular ablation (two-stage) was performed. The difference in nystagmus decay curves between three different rotations was reduced; therefore, the change of gravity direction perceived through gravity receptors other than macular endorgans was minimal and did not produce a difference in three different rotations.     

Development of postrotatory nystagmus time constants

Models of the slow phase portion of postrotatory nystagmus in the adult include time constants which describe primary and secondary nystagmus. The cupular time constant (T1) is believed to reflect the activity of the cupula of the semicircular canal, while the adaptation time constant (Ta) defines the rate of change of the baseline firing rate. Values for T1 and Ta have been published for the adult. In this study 37 normal infants under one year of age and 12 children, 3 to 13 years of age were examined for postrotatory nystagmus, and values for the two time constants were determined. The cupular time constant does not change with age and is concluded to be at an adult level at birth. This is correlated with the relative maturity of ear morphology at birth. The adaptation time constant increases from 79.5 sec at one month of age to 260.7 sec at 12-13 years of age, implying that maturation of adaption is a relatively slow process.     

Intrabeat relationship of postrotatory nystagmus in normal subjects

Intrabeat relationships between duration, amplitude and velocity of the slow phase of nystagmus and between amplitudes of the fast and slow phase of nystagmus were analysed from postrotatory responses in 10 normal subjects, using linear regression analysis. For 5 subjects, the tests were repeated five times. Highly significant correlation was found between velocity and amplitude of the slow phase in 9 of 10 subjects, whereas in one subject it was less significant. All subjects exhibited a significant correlation between velocity and duration of the slow phases. These intrabeat relationships were repeatable and representative for each individual. Highly significant correlation was found between amplitudes of the slow and fast phases of nystagmus in all subjects. No interindividual differences in these variables could be observed. The results indicate that the end-point of the slow phase of vestibular nystagmus in darkness is controlled by positional and durational corollary circuitries. The positional signal in displacement circuitry is probably derived by integration of the velocity signals from the labyrinths. The durational circuitry is presumably more dominant at high nystagmus velocities in order to permit sufficient time for fixation.     

Clinical evaluation of dysrhythmia of postrotatory nystagmus

Postrotatory responses of nystagmus were analysed in an exponential model by utilizing linear regression analysis. Four nystagmus qualities (velocity and duration of slow and fast phases) were studied in 10 patients with vestibular peripheral lesions, 10 patients with frontal lobe lesions and 10 patients with brain-stem lesions, together with 10 control subjects. In addition, pauses during the responses were quantified. Patients with frontal lobe lesions differed from other groups by scoring higher values of slow phase velocity and by exhibiting more pauses. The time constant was significantly shorter in patients with brain-stem lesion than in any other group. As regards other qualities, e.g. slow phase duration and fast phase velocity, or duration, no differences were observed. The pathological dysrhythmia may therefore be presented as changes in the gain and time constant of slow phase velocity as well as in pauses during nystagmus. Since all these changes may be encountered in normal subjects, one should be cautious in interpreting these changes as being pathological in each individual case.     

The effect of lateral head tilt on horizontal postrotatory nystagmus I and II and the Purkinje effect

The influence of an active lateral head tilt on postrotatory nystagmus I and II (PI, PII) was explored in normal humans. During postrotatory nystagmus the head was tilted laterally either towards the direction of the previous rotation (ipsilateral tilt) or towards the opposite direction (contralateral tilt) or the head was kept erect. Both ipsi- and contralateral head tilts led to a substantially weaker and shorter PI and PII as compared with the trial without head tilt. The time constants of the decline of PI were shortened to the ones observed in the peripheral nerve of animals. The reduction of PI and PII suggests a position-dependent tonic inhibition of the vestibular storage mechanism in the vestibular nuclei probably by the otoliths. The stronger reduction of time constant and cumulative amplitude of PI after contralateral as compared with ipsilateral head tilt may be explained by different combinations of asymmetric responses in canal and otolith afferents.     

Suppression of optokinetic velocity storage in humans by static tilt in roll.

The effects of static tilts about the roll (anterior-posterior) axis on human horizontal optokinetic afternystagmus (HOKAN) were examined. Static tilts in roll, with subjects lying on their left side, produced significant tilt-dependent HOKAN suppression. Only the slow (indirect pathway) component time constant (1/D) of the double exponential model for human HOKAN decreased with angle of roll tilt. The effect was direction specific in that suppression occurred only following a leftward-going stimulus. These findings provide further support for the postulate that otolith-organ-mediated activity can couple to the horizontal velocity storage mechanism in humans. A slight trend towards a tilt-dependent reduction of coefficient A (initial slow phase velocity of fast component decay) was revealed, suggesting the possibility that otolith-organ-mediated activity could couple to direct (pursuit-mediated?) pathways as well. No horizontal-to-vertical cross-coupling occurred, indicating that this aspect of the 3-dimensional model for velocity storage proposed by Raphan & Cohen (1988) may not completely apply to humans.     

Intrabeat relationship of postrotatory nystagmus in patients with neurological disorders

The intrabeat relationships between velocity, amplitude and duration of the slow phase of nystagmus as well as between amplitudes of slow vs. fast phase of nystagmus were analysed during a postrotatory nystagmus response by using linear regression analysis. Three groups of patients were studied, each consisting of 10 subjects with lesions either in the peripheral vestibular system, in the frontal lobe, or in the brain stem. Irrespective of the site of the lesion, all groups exhibited the same response pattern: a reduction in amplitude control of the end-point of nystagmus and an increase in durational control. The most prominent changes were observed in patients with brain-stem lesion in whom the durational control of the slow phase of nystagmus was constant, with a mean slow phase duration of 150 to 250 ms in the case of 3 to 4 fast phases per second. No alterations were observed in the relationship between amplitude of slow vis-à-vis fast phase. The results indicate that analysis of intrabeat relationship may provide additional data on the side of the disorder affecting the vestibular system.     

Human ocular counter-rolling and roll tilt perception during off-vertical axis rotation after spaceflight

Ocular counter-rolling (OCR) induced by whole body tilt in roll has been explored after spaceflight as an indicator of the adaptation of the otolith function to microgravity. It has been claimed that the overall pattern of OCR responses during static body tilt after spaceflight is indicative of a decreased role of the otolith function, but the results of these studies have not been consistent, mostly due to large variations in the OCR within and across individuals. By contrast with static head tilt, off-vertical axis rotation (OVAR) presents the advantage of generating a sinusoidal modulation of OCR, allowing averaged measurements over several cycles, thus improving measurement accuracy. Accordingly, OCR and the sense of roll tilt were evaluated in seven astronauts before and after spaceflight during OVAR at 45 degrees/s in darkness at two angles of tilt (10 degrees and 20 degrees). There was no significant difference in OCR during OVAR immediately after landing compared to preflight. However, the amplitude of the perceived roll tilt during OVAR was significantly larger immediately postflight, and then returned to control values in the following days. Since the OCR response is predominantly attributed to the shearing force exerted on the utricular macula, the absence of change in OCR postflight suggests that the peripheral otolith organs function normally after short-term spaceflight. However, the increased sense of roll tilt indicates an adaptation in the central processing of gravitational input, presumably related to a re-weighting of the internal representation of gravitational vertical as a result of adaptation to microgravity.     

Vestibulo-ocular responses during static head roll and three-dimensional head impulses after vestibular neuritis

This study aimed to investigate whether unilateral vestibular neuritis (VN) causes the same deficits of ocular counter-roll during static head roll (OCR(S)) and dynamic vestibulo-ocular reflex gains during head impulses (VOR(HI)) as unilateral vestibular deafferentation (VD). Ten patients with acute and 14 patients with chronic vestibular paralysis after VN were examined. The testing battery included fundus photography of both eyes with the head upright (binocular cyclorotation) and dual search coil recordings in a three-field magnetic frame. With one dual search coil on the right eye and the other on the forehead, the following stimuli were given: i) Halmagyi-Curthoys head impulses about the vertical, horizontal and torsional axes. ii) Static roll positions of the head up to 20 degrees right- and left-ear-down by movement of the neck. The comparison group consisted of 19 healthy subjects. Compared with the VD-patients, as reported in the literature, acute VN-patients showed the same pattern of OCR(S) gain reduction and binocular cyclorotation (CRb). The main feature that distinguished chronic VN-patients from chronic VD-patients was the normalization of the torsional VOR(HI) gain to the affected side, whereas the VOR(HI) gains in the horizontal and vertical directions did not show recovery (as in the patients with chronic VD). Chronic VN-patients differed from acute VN-patients by: i) symmetrical OCR(S) gains, ii) a less pronounced CRb toward the affected side, and iii) a normal torsional VOR(HI) gain toward the affected side. Since the ipsilesional torsional VOR(HI) gain did not recover in VD-patients, the normalization of this gain in our VN-patients can only be explained by a (partial) recovery of otolith function on the side of the lesion after the neuritis.     

The interpretation of static positional nystagmus in a balance clinic

Objective: To review the current criteria for the interpretation of positional nystagmus (PN). Design: Videonystagmography data on PN was collected for 90 patients presenting for balance testing with a history of vertigo or imbalance (excluding those diagnosed with benign paroxysmal positional vertigo). These were compared to normative data from 90 asymptomatic subjects. A literature review was carried out regarding threshold criteria for clinically significant PN. These were evaluated using the data collected. Study sample: Clinic group: 90 patients; 37 male, 53 female. Age range 15–82 years. Normative group: Ninety subjects; 31 male, 59 female. Age range 21–79 years.

Results: PN was seen in 75.6% of the clinic group and in 50% of the normative group. The prevalence of nystagmus of slow phase velocity (SPV)?>?3°/s was significantly higher in the clinic group than the normative group. The maximum SPV recorded in each position was greater from the clinic group than the normative group. Conclusions: This study supports the use of normative thresholds for SPV for both horizontal and vertical PN. A normative threshold of >3°/s for horizontal and >7°/s for vertical nystagmus is proposed.     

Three-dimensional analysis of eye movements induced by roll and pitch tilt

The torsional eye movements induced by the right and left ear-down position are known as "ocular counter-rolling" but the mechanism of the horizontal and vertical eye movements has not been clarified. The vertical eye movements induced by backward tilt are known as the "doll reflex", but clear eye movements are induced by forward tilt. The mechanism of horizontal and torsional eye movements are unclear. To detect 3D eye movements in the roll and pitch planes, we performed 3D analysis of eye movements by our video-oculographic method. During roll tilt, counter-torsional eye movements were clearly induced, but no definite, eye movements were observed in the horizontal and vertical components of eye movements. During pitch tilt, downward eye movements were observed following backward tilt, but no clear eye movements were observed following forward tilt. Moreover, no clear horizontal or torsional eye movements were observed during pitch tilt. These results suggest that the response to pitch tilt depends on the saccular input.     

Direction and angle of active head tilts influencing the Purkinje effect and the inhibition of postrotatory nystagmus I and II

Postrotatory nystagmus I and II (P I, P II) were evoked in four normal humans by velocity steps (prior velocity of rotating chair 90 degrees/s). 4 s after the stop the head was actively tilted by the subject 90 degrees forwards, backwards, to the shoulder of the previous direction of rotation -'ipsilaterally', or to the other shoulder-'contralaterally'. In control trials, the head was kept in the previous erect position. Compared with the control experiments, P I was significantly reduced by all head tilts. Inhibition of P I was strongest with forward and weakest with backward tilts. This difference is explained by the inclination of the utricular base by 30 degrees backward with respect to the horizontal of the skull and by the elastic properties of the sensory matrix. A smaller amplitude (45 degrees) of head tilt about the roll axis leads to a weaker inhibition (28.5%) than a 90 degree tilt, which corresponds to the difference of the sine of the tilt angle and thereby reflects the mechanical force acting on the receptor layer.     

Localization of static positional nystagmus with the ocular fixation test

OBJECTIVE: Characteristics of static positional nystagmus (SPN) (i.e., persistency, direction fixed, direction changing) are observed in both peripheral and central disturbances and possess no localizing value for vestibular lesions. Our objective was to investigate whether the ocular fixation test as applied to SPN could assist in localizing vestibular lesions. STUDY DESIGN: A 3-year prospective study that included 43 patients with SPN. METHODS: All patients underwent a standard vestibular test battery and cerebral imaging (7, computed tomography scan; 36, magnetic resonance imaging). The ocular fixation index (OFI) was calculated by the ratio of the mean slow component velocity of SPN (measured with red light-emitting diode fixation) to that measured in darkness, multiplied by 100. An OFI less than 50 was considered normal. RESULTS: In 33 of 35 patients whose OFI was less than 50, the cerebral imaging was normal and a peripheral vestibular lesion was diagnosed (two benign tumors of the fourth ventricle were missed). In all eight patients whose OFI was greater than 50, the cerebral imaging was abnormal and a central vestibular lesion was noted. CONCLUSIONS: These results indicate that the visual suppression of SPN does, indeed, permit the localization of vestibular lesions. The predictive value of the ocular fixation test on the origin of SPN is greater than 94% for peripheral lesions and 100% for central disorders.     

Head tilt is pronounced after an ipsilateral head roll in patients with vestibular schwannoma

The study aimed to measure utricular function by directly quantifying head tilt in vestibular schwannoma (VS) patients using regular video-oculography (VOG) equipment with integrated head-position sensor, and to correlate the results with patients’ symptoms and signs. We recorded head tilting after exclusion of visual cues (static head tilt), and after returning to the centre following lateral head rolls towards each side [subjective head vertical (SHV)]. Head tilt in 43 patients was measured preoperatively and approximately 4 months postoperatively, and compared to that of 20 healthy subjects. Symptoms were assessed with a structured questionnaire. Static head tilt in patients was significantly greater than in controls (1.0° ± 0.9°) preoperatively (1.6° ± 1.5°, p = 0.04) and postoperatively (1.7° ± 1.5°, p = 0.01). Mean SHV in patients was significantly greater than in controls (1.2° ± 1.0°) preoperatively (2.0° ± 1.9°, p = 0.03) and postoperatively (2.5° ± 1.8°, p = 0.001), increasing non-significantly after surgery (p = 0.3). Side-specific SHV after ipsilateral head rolls was significantly greater than after contralateral head rolls preoperatively (2.8° ± 3.3° vs. ?0.5° ± 3.0°, p = 0.001) and postoperatively (3.3° ± 3.0° vs. 0.6° ± 3.2°, p < 0.001). The intensity of dizziness increased postoperatively (p = 0.04), but its effect on quality of life remained unchanged. In conclusion, commercial VOG equipment including a head-position sensor allows direct evaluation of head tilt in VS patients. The slight head tilt towards the ipsilateral side becomes most evident after returning from an ipsilateral head roll.     

Suppression of optokinetic velocity storage in humans by static tilt in pitch.

The effects of static tilts about the pitch axis on human horizontal optokinetic after-nystagmus OKAN (HOKAN) were examined. Static tilts in pitch produced tilt-dependent HOKAN suppression. The slow decay (indirect pathway) component (coefficient C and long time constant 1/D) of the two-component model for OKAN was significantly reduced, while the short decay (direct pathway) component (coefficient A and short time constant 1/B) remained invariant as angle of tilt was increased. These results provide further evidence that otolith organ activity can couple to horizontal velocity storage in humans, in accordance with models proposed in the literature.