The vestibulo-collic reflex is abnormal in migraine.

Interictal evoked central nervous system responses are characterized in migraineurs by a deficit of habituation, at both cortical and subcortical levels. The click-evoked vestibulo-collic reflex (VCR) allows the assessment of otolith function and an oligosynaptic pathway linking receptors in the saccular macula to motoneurons of neck muscles. Three blocks of 75 averaged responses to monaural 95-dB normal hearing level 3-Hz clicks were recorded over the contracted ipsilateral sternocleidomastoid muscle in 25 migraineurs between attacks and 20 healthy subjects, without vestibular symptoms. Amplitudes, raw and corrected for baseline electromyography, were significantly smaller in migraine patients. Whereas in healthy volunteers the VCR habituated during stimulus repetition (-4.96% +/- 14.3), potentiation was found in migraineurs (4.34% +/- 15.3; P = 0.04). The combination with a reduced mean amplitude does not favour vestibular hyperexcitability as an explanation for the habituation deficit in migraine, but rather an abnormal processing of repeated stimuli in the reflex circuit.

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.

EMG responses to free fall in elderly subjects and akinetic rigid patients.

OBJECTIVES: The EMG startle response to free fall was studied in young and old normal subjects, patients with absent vestibular function, and patients with akinetic-rigid syndromes. The aim was to detect any derangement in this early phase of the "landing response" in patient groups with a tendency to fall. In normal subjects the characteristics of a voluntary muscle contraction (tibialis anterior) was also compared when evoked by a non-startling sound and by the free fall startle. METHODS: Subjects lay supine on a couch which was unexpectedly released into free fall. Latencies of multiple surface EMG recordings to the onset of free fall, detected by a head mounted linear accelerometer, were measured. RESULTS AND CONCLUSIONS: (1) EMG responses in younger normal subjects occurred at: sternomastoid 54 ms, abdominals 69 ms, quadriceps 78 ms, deltoid 80 ms, and tibialis anterior 85 ms. This pattern of muscle activation, which is not a simple rostrocaudal progression, may be temporally/spatially organised in the startle brainstem centres. (2) Voluntary tibialis EMG activation was earlier and stronger in response to a startling stimulus (fall) than in response to a non-startling stimulus (sound). This suggests that the startle response can be regarded as a reticular mechanism enhancing motor responsiveness. (3) Elderly subjects showed similar activation sequences but delayed by about 20 ms. This delay is more than can be accounted for by slowing of central and peripheral motor conduction, therefore suggesting age dependent delay in central processing. (4) Avestibular patients had normal latencies indicating that the free fall startle can be elicited by non-vestibular inputs. (5) Latencies in patients with idiopathic Parkinson's disease were normal whereas responses were earlier in patients with multiple system atrophy (MSA) and delayed or absent in patients with Steele-Richardson-Olszewski (SRO) syndrome. The findings in this patient group suggest: (1) lack of dopaminergic influence on the timing of the startle response, (2) concurrent cerebellar involvement in MSA may cause startle disinhibition, and (3) extensive reticular damage in SRO severely interferes with the response to free fall.

Torticollis due to disinhibition of the vestibulo-collic reflex in a patient with Steele-Richardson-Olszewski syndrome.

A patient with the clinical picture of Steele-Richardson-Olszewski syndrome and an unusual intermittent neck twisting is reported. He had virtually no voluntary ocular movements and only very slow, low-amplitude voluntary head movements. However, in response to optokinetic or vestibular stimulation, he developed full eye deviations in the direction of the slow phase of the expected nystagmus. No quick phases were observed, and the deviation outlasted the duration of the vestibular stimuli because of defective saccades. The head also turned fully during these stimuli, quicker than on attempted voluntary movements, and remained deviated similarly to the eyes. This suggests that the neck deviations in this patient were due to a disinhibited vestibulo-collic reflex and a disturbed head position resetting mechanism. Neck electromyographic responses in response to whole-body rotation indicated that the vestibulocollic reflex responsible for the torticollis in this patient had a short latency of approximately 30 ms.

The perception of body verticality (subjective postural vertical) in peripheral and central vestibular disorders.

The perception of body verticality (subjective postural vertical, SPV) was assessed in normal subjects and in patients with peripheral and central vestibular lesions and the data were compared with conventional neuro-otological assessments. Subjects were seated with eyes closed in a motorized gimbal which executed cycles of tilt at low constant speed (1.5 degrees s-1), both in the frontal (roll) and sagittal (pitch) planes. Subjects indicated with a joystick when they entered and left verticality, thus defining a sector of subjective uprightness in each plane. The mean angle of tilt (identifying a bias of the SPV) and the width of the sector (defining sensitivity of the SPV) were then determined. In normal subjects, the angle of the "verticality' sector was 5.9 degrees for pitch and roll. Patients with bilateral absence of vestibular function, patients with vertigo, i.e. acute unilateral lesions, benign paroxysmal positional vertigo (BPPV) and Ménière's disease, and patients with positionally modulated up-/downbeat nystagmus all had enlarged sectors (i.e. loss in sensitivity). Mean sector angle in these groups ranged from 7.8 to 11 degrees and the abnormality was present both in pitch and roll, regardless of the direction of nystagmus or body sway. Patients with chronic unilateral peripheral vestibular lesions and those with position-independent vertical nystagmus had normal sensitivities. No significant bias of the SPV was found in any patient group, not even those with acute unilateral vestibular lesions who had marked tilts of the subjective visual vertical (SVV). Complementary experiments in normal subjects tested under galvanic vestibular or roll-plane optokinetic stimulation also failed to show biases of the SPV. In contrast, a significant bias in the SPV could be induced in normal subjects by asymmetric cycles of gimbals tilt, presumably by proprioceptive adaptation. The following conclusions can be drawn. (i) The perception of body verticality whilst seated is mainly dependent on proprioceptive/contact cues but these are susceptible to tilt-mediated adaptation. (ii) Vestibular input improves the sensitivity of the SPV, even in vestibular disorders, as long as the abnormality is stable. (iii) There can be marked dissociation between vestibulo-motor (ocular and postural) phenomena and the perception of body verticality, and between the SPV and SVV. (iv) The postural sway asymmetries in patients with peripheral and central vestibular lesions, like those induced by galvanic or optokinetic stimulation in normal subjects, are not consequences of changes of the SPV.

Subjective postural vertical in peripheral and central vestibular disorders.

The perception of subjective postural vertical was assessed in normals and patients with peripheral and central vestibular disorders and spasmodic torticollis. The subjects were seated in a motorized gimbal with the head and torso restrained and their eyes closed. The gimbal executed 7-10 cycles of tilt around the vertical at 1.5 degrees/s in either pitch or roll. Subjects indicated when they began to feel upright and again when they began to feel tilted by an analogous 3-position joystick. Normal subjects felt upright within a sector of 5-6 degrees around vertical in pitch and roll. Five patients with absent vestibular function, 25 torticollis patients and 3 patients with acute unilateral peripheral vestibular lesions showed a significant increase of the sector in pitch and roll, but only the latter had a mild directional bias. Two patients with long standing complete unilateral vestibular deficit and 8 patients with up or downbeat nystagmus in the vicinity of upright had abnormally large sectors within which they felt to be upright. The results suggest that vestibular function is important for the accurate perception of the postural vertical and that a directional asymmetry in vestibulo-ocular function or a head tilt does not necessarily correlate with a directional bias of subjective verticality.

EMG-responses to sudden onset free fall.

Recordings of axial and limb muscles in reaction to a free fall induced startle were performed in subjects while they were lying on a tilting couch with their eyes closed. Young normals (n = 24, aged 31.1 +/- 6.6 years) showed an activation sequence consisting of sternomastoid (N.XI: 57 ms), abdominal muscles (T10: 65 ms), quadriceps (L3: 75 ms) and deltoid (C5: 78 ms) and tibialis anterior (L4: 80 ms). The sequence of activation is not compatible with the current hypothesis of the startle being produced by a single volley spreading rostrally and caudally from the lower brainstem. Instead it is suggested that the startle is a patterned response organized by a putative reticular generator capable of spatio-temporal sequencing. Two avestibular patients had responses at mildly delayed latencies, showing that these can be elicited by non-vestibular inputs. Similar testing of 11 subjects aged 70-80 years showed a latency delay of ca. 26% in the EMG response but a similar activation sequence. The amount of delay in the elderly can only partially be attributed to age-dependent motor conduction slowing and suggests a prolongation of central processing time. In patients with advanced stages of akinetic-rigid syndromes abnormalities were seen in cases with an involvement of the brainstem reticular formation.

Responses in neck and facial muscles to sudden free fall and a startling auditory stimulus.

EMG responses elicited by sudden onset of free fall and a startling auditory stimulus were investigated in healthy subjects while lying on a couch with their eyes closed. Muscle responses were recorded from masseter (V cranial nerve), orbicularis oculi and mentalis (VII nerve) and sternomastoid and trapezoid (XI nerve). A similar sequence of muscle activation and absolute latencies occurred in response to both stimulus modalities, consisting of a blink (30 msec) followed simultaneously by mentalis, sternomastoid and trapezoid (55 msec). Masseter could either be simultaneously activated with the latter muscles or follow after a delay of 10-20 msec. A patient with bilateral cochleo-vestibular nerve section had responses at comparable latencies in the free fall experiment. The similarities between the reaction to free fall and a startling auditory stimulus indicate that the early response to free fall constitutes a startle and that various stimuli converge onto a common response generator. The latency pattern of neck and facial muscles does not follow a sequence of innervation with increasing segmental distance from a single centre. Therefore, our data do not support the concept that the startle response is produced by a caudally and rostrally spreading volley from a putative pontomedullary centre. It is suggested that the startle response is a polysynaptically generated patterned muscle activation.