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Biomedical subjects

A M Bronstein

Publications and source records attributed to A M Bronstein.

At least 19 recordsLinked to original sources
PubMed1992

Visually controlled spatial stabilisation of the human head: compensation for the eye's limited ability to roll.

During movements of the head in pitch (yes-yes) or in yaw (no-no) the visual scene appears stable whereas rolling the head (ear down to shoulder) induces an apparent swinging of the world in the opposite direction. This visual instability is due to the inadequacy, in the roll plane, of the reflex eye movements which are effective in stabilising the eyes in space during pitch and yaw. We investigated whether head is stabilised in roll to protect against visual instability. Human subjects were fixed in a gimbal with their heads free and were exposed to unpredictable oscillatory movement in pitch and, for comparison roll, about axes aligned with the head. With vision, during roll motion, the head was displaced from upright by approximately half the amplitude of the gimbal motion. In comparison, with eyes closed relying on vestibular and proprioceptive cues and during pitch stimuli with or without vision, the magnitude of head displacement from upright was approximately equal to that to the gimbal. The superior head stability in roll, dependent on a visual frame of reference, compensates for poverty of eye movement in this plane.

Eye Movements↗
PubMed1992

Neurology of otolith function. Peripheral and central disorders.

The otolith organs detect gravitoinertial forces acting on the head providing signals to the brain which are essential for spatial orientation, sensing motion and organizing motor behaviour. Although the pathophysiology of otolithic dysfunction is poorly understood, a disorder of otolith function, at a peripheral or central level, may be suspected when a patient describes symptoms of false sensations of linear motion or tilt or shows signs of specific derangements of ocular motor and postural, orienting and balancing responses. When disorientation is severe the patient may describe symptoms which sound bizarre, raising doubts over the organic basis of the disease. Our recognition of an otolithic disorder and understanding otolithic involvement in a wider neurological context may be guided by knowledge of otolith physiology and of the characteristics of the few proven otolithic syndromes.

Brain↗
PubMed1992

Torsional nystagmus. A neuro-otological and MRI study of thirty-five cases.

Thirty-five patients with torsional nystagmus (TN) underwent vestibular and ocular motor assessment and magnetic resonance image (MRI) scanning of the head. Patients were divided into two groups according to whether TN was predominant and present in primary gaze (Group I, 23 patients) or elicited by head positioning or gaze deviation and less prominent than other concurrent nystagmus (Group II, 12 patients). The main aetiologies in both groups were demyelination, vascular disease and posterior fossa tumours. In Group I, a frequent pattern of findings, occurring in 30-50% of cases, was a caloric canal paresis contralateral to the direction of the fast phases ('beat') of the TN, whereas the duration of horizontal caloric/rotational nystagmus and the slow-phase eye velocity of pursuit and of optokinetic nystagmus were all reduced in the direction of beating. The TN was more frequently and consistently modulated by vertical canal stimuli (head oscillation in roll) than by otolith stimuli (static tilt). Statistical analysis of the MRI showed significant overlap of abnormal MRI signals in the area of the vestibular nuclei, on the side opposite to the beat direction of TN. These results suggest that TN originates in a central imbalance of vertical semicircular canal function, resulting from lesions involving the vestibular nuclei on the opposite side of the TN. Group II was heterogeneous with no consistent pattern of neuro-otological findings, although lesions ipsilateral to the TN were frequent occurrence; in these cases cerebellar system lesions may have produced ipsilateral vestibular nuclei disinhibition.

Adolescent↗
PubMed1992

The site of brainstem lesions causing semicircular canal paresis: an MRI study.

Ten patients with canal paresis of central origin and ten patients with peripheral canal paresis were studied using MRI of the brainstem to identify lesions within the central vestibular pathways. In the central group, the magnitude of the canal paresis was generally lower than in the peripheral group and removal of fixation had little effect on the nystagmic response. In the peripheral group, removal of fixation enhanced the nystagmus and lessened the discrepancy between the two ears. Statistical processing of the MRI showed that in the central group significant spatially coincident lesions occurred within the medial vestibular nucleus, lateral vestibular nucleus and proximal portion of the vestibular fascicle.

Adult↗
PubMed1992

Pendular pseudonystagmus arising as a combination of head tremor and vestibular failure.

We describe three patients with spontaneous pendular oscillation of the eye during funduscopy. All patients had blurred, shimmering vision or oscillopsia, exacerbated by concentration, reading, or trivial head movements, and had a history of unsteadiness. Examination revealed a fine head tremor, mild unsteadiness, absent vestibulo-ocular reflex (VOR), and otherwise normal neurologic and ocular motor findings. Rigid immobilization of the head abolished the retinal oscillations. Simultaneous precision recordings of head and eye movements showed that the eye movement was in the compensatory direction to the head tremor but that, in contrast to normal VOR, it was in phase error. We conclude that the essential head tremor was provoking oscillopsia and retinal oscillation because of the absence of VOR. Recognizing the association of head tremor with absent VOR is important since in all these patients the presence of this pendular pseudonystagmus on ophthalmoscopy raised the diagnostic possibility of brainstem disease.

Aged↗
PubMed1992

Testing otolith function.

Otolithic signals contribute to; (1) perception of orientation and linear motion, (2) generate compensatory eye movements in response to linear acceleration of the head and (3) participate in the co-ordination of movement and balance. Tests of these functions shown to be useful in identifying clinical disorders have been reviewed: (1) Evaluation of orientation to gravity, as estimated by adjustment of the visual vertical, indicates deranged otolith function at a peripheral or central level and the sensitivity of this test can be enhanced by performing estimates during centrifugation on a motorised turntable. Estimation of thresholds of self motion on a parallel swing identifies global reduction or unilateral loss of peripheral function, with central disorders awaiting study. (2) Otolith ocular reflexes to linear head translation can be used to demonstrate overall integrity of peripheral function and reveal central abnormalities. Counter-rolling responses to head roll-tilt and measurements of cyclodeviation of the eyes demonstrate functional asymmetries, with some lateralising value, particularly in central lesions. Global function and asymmetries may also be evaluated by 'head eccentric' rotational testing, which adds a tangential linear acceleration to the angular stimulus. The linear acceleration enhances the canal response by adding an otolith component. (3) Latency and amplitude of surface electro-myography (EMG) responses in the limbs to sudden falls, which can be recorded with the subject suspended on a hinged bed, indicate gross peripheral abnormality of function and can lateralize disorders of CNS motor pathways. It is concluded that some tests of otolith function can be of use in indicating global loss of peripheral otolith function, others are capable of lateralizing a marked loss of function and all have the potential to give information about central disorders. They all have to be interpreted within the clinical context and, unfortunately, none have yet been shown to be sensitive to partial, particularly unilateral, dysfunction.

Adult↗
PubMed1991

Compensatory eye movements in the presence of conflicting canal and otolith signals.

Orbital motion of the head with the face directed towards the axis of rotation is a stimulus to the otolith organs which is in the opposite rightwards-left-wards sense to the rotational stimulus to the semicircular canals. This can be experienced, for example, by a child held at arms length "en face' and swung from side to side. As one swings, say to the right, the child's head rotates to its right yet moves linearly to its left. Eye movement responses to a transient orbital movement were observed whilst subjects fixated earth-fixed targets. i) a "near target" placed between the head and the axis whose relative displacement is in the same direction as head rotation, and ii) a "far target" placed beyond the axis whose relative motion is in the opposite direction to head rotation. The motion stimuli evoked slow phase eye movements at 45 ms latency, always in the opposite direction to head rotation, thus compensating for the motion of the far target but in the wrong direction for fixating the near target. Theoretically, fixating the near target demands a predominance of the otolith ocular-reflex, which would give an eye movement in the correct direction. However, despite visual cues, it seems that if the canal and otolith-ocular reflexes are evoked in opposing directions, the otolith reflex fails to operate at a sensitivity sufficiently high to reverse the direction of the canal-reflex.

Adult↗
PubMed1991

Cervico-ocular function in patients with spasmodic torticollis.

The cervico-ocular (COR) and active and passive vestibulo-ocular reflexes (VOR) were measured in seven patients with spasmodic torticollis (ST) and six normal controls. The COR was found to be weak or absent in both groups. The VOR gain was similar in the two groups but five patients had a significant asymmetry of the response. There was no evidence of abnormal cervico-vestibular interaction during active head rotation. The study suggests that the VOR asymmetry frequently found in ST cannot be explained on the basis of an abnormal cervical input.

Adult↗
PubMed1991

Congenital-type nystagmus emerging in later life.

We describe six patients who, as teenagers or adults, developed florid nystagmus with consequent visual symptoms without any other manifestation of disease. In three patients, previous ophthalmologic examination had excluded nystagmus, and there was medically informed witness to its onset. The remainder may or may not have had a milder, unsuspected nystagmus before the development of symptoms. Ophthalmologic and neurologic investigations were negative, and follow-up of 2 to 15 years has been uneventful. Eye movement recordings showed the characteristics of the nystagmus to be indistinguishable from congenital nystagmus, which normally becomes manifest in early infancy. We conclude that a congenital-type nystagmus can emerge or enhance in later life without apparent provocation and is probably associated with a benign pathophysiology.

Adolescent↗
PubMed1991

The neck-eye reflex in patients with reduced vestibular and optokinetic function.

It is accepted that the neck-eye loop (cervico-ocular reflex, COR) is enhanced following loss of vestibular function and that this helps to restore gaze stability during head movements. In this paper we address the question of which structures and/or mechanisms may participate in such plastic enhancement by investigating the COR in 2 patients with absent vestibular function and reduced smooth pursuit-optokinetic eye movements (SP-OKN). The patients had multisystem atrophy involving the vestibular system and the cerebellum. The COR (elicited by angular motion of the trunk relative to the fixed head and angular motion of the head relative to the fixed trunk) was not enhanced in these 2 patients when compared with normal subjects, in contrast to previous findings in a group of patients with absent vestibular function alone. Measurements of slow phase eye movement velocity during SP-OKN stimuli and during combined COR-OKN stimulation (head oscillation relative to the stationary trunk in the light) showed identical values in these two conditions, which indicates that the neck-eye loop did not contribute to gaze stability during head movements. The absence of plastic enhancement of the COR in these patients may be secondary to interruption of SP-OKN pathways at various possible sites and/or to involvement of the vestibulocerebellum, which is known to mediate adaptive plasticity in the vestibulo-ocular reflex.

Darkness↗
PubMed1990

Convergence nystagmus associated with Arnold-Chiari malformation.

A case of convergence nystagmus associated with an Arnold-Chiari type I malformation is presented. The nystagmus appeared in the absence of fixation, was provoked during Valsalva's maneuver and neck flexion and extension, and attenuated on deep inspiration. Sagittal magnetic resonance images showed that the diameter of the cerebral aqueduct increased with the neck in full flexion and in full extension. Surgical foramen magnum decompression considerably reduced the nystagmus and abolished the postural variation of aqueduct diameter. It is postulated that this nystagmus was due to a combination of mechanical distortion and abnormal transmission of cerebrospinal fluid pressure to the aqueductal region.

Adolescent↗
PubMed1990

Visual control of balance in cerebellar and parkinsonian syndromes.

The role of vision in the control of balance in patients with Parkinson's disease (PD) and cerebellar disease (CD) was studied by measuring body sway with eyes open, closed, and in response to visual stimuli generated by discrete lateral displacements of a moveable room which enclosed the subjects. In response to room movement, normal subjects swayed by an amount intermediate between sway with eyes open and eyes closed and their response attenuated on repetition of the movement, a process depending on shifting from predominantly visual to proprioceptive control. CD patients swayed more than controls with eyes open or closed and as shown by high 'Romberg quotients' (eyes closed/eyes open sway ratio) were able to use visual information to control much of their unsteadiness. CD patients had a normal attenuation of response to repetition of the room movement. PD patients had normal sway with eyes open or closed but their responses to room movement were abnormal, being proportionately larger and failing to attenuate during successive stimuli. The results indicate that cerebellar lesions seem largely to spare the visuopostural loop and also spare the ability to shift from a visual to a proprioceptive control of postural sway. In contrast, the findings in PD suggest that the visuopostural loop is hyperactive and that its influence cannot easily be de-emphasized when visual information is misleading. The latter finding suggests that basal ganglia participation in posture is concerned with the reweighting of the various sensorimotor loops controlling posture in the process of adapting to novel situations.

Adaptation, Physiological↗
PubMed1990

Abnormalities of horizontal gaze. Clinical, oculographic and magnetic resonance imaging findings. I. Abducens palsy.

Fifty one patients with abnormalities of horizontal gaze were studied with magnetic imaging of the brain (MRI) and eye movement recordings to identify the loci of lesions responsible for isolated abducens palsy, conjugate gaze palsy and different types of internuclear ophthalmoplegias. The lesions responsible for a particular disorder were identified by overlapping enlarged drawings of the individual scans at comparable brain-stem levels and identifying the areas where the abnormal MRI signals intersected. A statistical procedure was devised to exclude the possibility that the areas of overlap occurred by chance. In this paper, the findings in the group of patients with VI nerve palsy are reported since the location of their lesions could be predicted from known anatomy, so validating the procedure. The results were independently obtained with the overlapping technique and the statistical procedure and showed that the lesions were located in a region corresponding to the posterior part of the abducens fasciculus. This confirms that central lesions producing isolated lateral rectus weakness spare the abducens nuclei. The agreement between the procedures used and earlier clinical and experimental results suggest that the method we describe can be applied to locate the site of lesions on MRI scans in other groups of patients with more complex gaze disorders.

Abducens Nerve↗