Rotatory head response evoked by stimulating and destroying the interstitial nucleus and surrounding region

Clinical and topographic magnetic resonance imaging characteristics of suspected thalamic infarcts in 16 dogs

Sixteen dogs with acute-onset, non-progressive signs of brain dysfunction and magnetic resonance imaging (MRI) characteristics compatible with thalamic infarction are described. Topographically the MRI lesions could be grouped in three thalamic regions, namely, paramedian (8/16), extensive dorsal (5/16) and ventrolateral (3/16). Paramedian lesions resulted in signs typical of vestibular dysfunction. Extensive dorsal lesions were associated with vestibular ataxia, circling and contralateral menace response deficits. Ventrolateral lesions resulted in circling and contralateral proprioceptive deficits. In several dogs, regions other than the thalamus were also affected: four extended into the midbrain; six extended to the internal capsule, and two dogs had a second lesion in the cerebellum. Three clinical syndromes were identified in association with thalamic infarction. These signs varied somewhat, most likely because lesions were not confined to specific nuclear boundaries and involved different combinations of thalamic nuclei.

A-pattern strabismus with overdepression in adduction: a special type of bilateral skew deviation?

BACKGROUND

Skew deviation is an acquired vertical ocular misalignment caused by damage to the prenuclear vestibular inputs to the ocular motor nuclei. A-pattern strabismus often has bilaterally symmetric vertical incomitance and overdepression in adduction (superior oblique overaction) and can be associated with developmental delay, cerebral palsy, hydrocephalus, spina bifida, or posterior fossa or other brainstem disease. The purpose of this study is to describe the ocular motility and torsion findings in patients with A-pattern strabismus and bilateral overdepression in adduction (superior oblique muscle overaction) and to propose a possible brainstem mechanism underlying these observations.

RESULTS

Most of the 13 patients identified had other neurologic abnormalities, including spina bifida, hydrocephalus, perinatal stroke, or global delay. Only 2 patients had vertical ocular misalignment in primary gaze. Of the 13, 7 had incomitant vertical tropias during lateral gaze, and 12 had bilateral incyclotorsion documented on fundus examination. Despite having bilateral overdepression in adduction (superior oblique overaction), 11 of the 13 had no difference in vertical ocular misalignment with alternating head tilt rather than reversing hypotropias as would be expected from primary oblique dysfunction. The findings are consistent with damage to the utricular pathways corresponding to the anterior semicircular canal and a resulting posterior canal predominance to the extraocular muscle subnuclei that creates increased tonus to the depressors, bilaterally.

CONCLUSIONS

A-pattern strabismus may, in some cases, represent a special form of skew deviation. The ocular motility and clinical findings are consistent with bilateral damage to the utricular pathways corresponding to the anterior semicircular canals rather than bilateral primary superior oblique muscle overaction.

Skew Deviation Revisited

Skew deviation is a vertical misalignment of the eyes caused by damage to prenuclear vestibular input to ocular motor nuclei. The resultant vertical ocular deviation is relatively comitant in nature, and is usually seen in the context of brainstem or cerebellar injury from stroke, multiple sclerosis, or trauma. Skew deviation is usually accompanied by binocular torsion, torticollis, and a tilt in the subjective visual vertical. This constellation of findings has been termed the ocular tilt reaction. In the past two decades, a clinical localizing value for skew deviation has been assigned, and a cogent vestibular mechanism for comitant and incomitant variants of skew deviation has been proposed. Our understanding of skew deviation as a manifestation of central otolithic dysfunction in different planes of three-dimensional space is evolving. The similar spectrum of vertical ocular deviations arising in patients with congenital strabismus may further expand the nosology of skew deviation to include vergence abnormalities caused by the effects of early binocular visual imbalance on the developing visual system.

The contribution of midbrain circuits in the control of gaze

The midbrain contains several structures important for the generation of torsional and vertical eye movements including the rostral interstitial nucleus of the MLF (riMLF) and the interstitial nucleus of Cajal (iC). While the riMLF is the immediate premotor structure for the generation of torsional and vertical saccades, the iC is considered a major part of the neural integrator for torsional and vertical eye movements. Experiments in monkeys show that a unilateral inactivation of the riMLF with muscimol leads to spontaneous contralesional torsional nystagmus, whereas an iC inactivation causes ipsilesional torsional nystagmus. In addition, inactivation of either structure leads to a tonic ocular torsion to the contralesional side. While the deficits after a riMLF lesion are thought to result from an imbalance of the saccade generator, a vestibular imbalance probably causes the deficits after an iC lesion. Contralesional and ipsilesional torsional nystagmus is also found in patients with unilateral mesencephalic lesions. A detailed analysis of the lesions from MRI scans shows a preferential involvement of the riMLF for patients with contralesional torsional nystagmus, and a major involvement of iC in cases with ipsilesional torsional nystagmus. Thus, the direction of torsional nystagmus appears to be a valuable topodiagnostic sign for patients with midbrain lesions.

Electron microscopic investigation of the vestibular projection to the cat trochlear nuclei

Ultrastructural degeneration studies were carried out on the cat trochlear nucleus following lesion of the vestibulo-trochlear pathway in order to characterize the location and type of presynaptic endings involved in this pathway. Four types of boutons are found in the normal trochlear nucleus. Types I and II are large and demonstrate typical en passant profiles with small diameter synaptic vesicles (35 and 40 nm). These terminals are characterized by the absence of neurofilaments in the Type II endings. Types III and IV are smaller boutons, located more axondendritically, and contain larger diameter synaptic vesicles (45 nm). Type V terminals contain large, granulated vesicles and occur only rarely. Following the interruption of the ascending projection from the ipsilateral superior and medial vestibular nuclei by parasagittal medullary lesions, degeneration of Type II boutons was commonly encountered in the ipsilateral trochlear nucleus. Predominantly Type III degeneration was found in the contralateral trochlear nucleus. Electrical stimulation of the vestibular nerve showed that these lesions resulted in (1) a complete loss of inhibition in the ipsilateral trochlear nucleus and (2) a significant (75-90%) reduction in the contralateral excitatory pathway to the trochlear nucleus. Midline sagittal lesions in the floor of the fourth ventricle interrupting the decussating fiber projection from the bilateral medial vestibular nuclei resulted in selective degeneration of only Type III boutons in both trochlear nuclei. We conclude that inhibitory vestibular neurons eminating from the superior vestibular nucleus terminate on trochlear motoneurons with Type II boutons and excitatory vestibular neurons from the contralateral medial vestibular nucleus end on trochlear motoneurons with Type III boutons.

Vertical diplopia

An accurate clinical evaluation of vertical diplopia is predicated upon meticulous history-taking, observations regarding the presence and pattern of an anomalous head position, and the analysis of several subjective and objective tests of extraocular muscle function. To reach a final diagnosis with minimum risk and expense to the patient the examiner must be familiar with the neuroanatomy of the supranuclear and infranuclear pathways which control the actions of the vertically-acting extraocular muscles, the clinical methods and pitfalls of a number of clinical techniques which are used to identify an underacting extraocular muscle, and the hallmark characteristics of a supranuclear, infranuclear and restrictive ophthalmopathy.

Ocular tilt reaction resulting from vestibuloacoustic nerve surgery

The ocular tilt reaction (OTR) is a triad of head-eye synkinesis composed of head tilt, conjugate ocular torsion in the direction of head tilt, and skew deviation. The OTR represents a normal compensatory response to lateral head tilts and is produced by activation of the utricle of the lowermost ear. A pathological OTR results when otolith activity is unopposed as the result of injury to the opposite utricle or its nerve. Vertical diplopia may be the only symptom of OTR in patients who have undergone surgery involving the vestibuloacoustic nerve. We report a series of patients with OTR after surgery for acoustic neuroma or Meniere's disease. In each patient, the manifesting symptom was vertical diplopia. Bedside neuro-ophthalmological testing readily excluded a brain stem cause for the double vision. We conclude that OTR after vestibuloacoustic surgery is a benign condition with spontaneous resolution of symptoms within several months.

Turning responses evoked by stimulation of visuomotor pathways

Lateral eye, head, and body movements are produced by electrical stimulation of many brain regions from frontal cortex to pons. A new collision method shows that at least 5 separate axon bundles mediate stimulation-elicited lateral head and body movements in rats. One bundle passes between the rostromedial tegmentum and medial pons, with conduction velocities of 0.8-18 m/s. A second bundle passes between the superior colliculus and contralateral medial pons, with conduction velocities of 1.7-13 m/s. A third bundle passes between the superior colliculus and ventrolateral pons, with conduction velocities of 1.3-20 m/s. A fourth bundle passes between the internal capsule and medial substantia nigra, with conduction velocities of 0.9-4.4 m/s. A fifth bundle passes between the anteromedial cortex and rostral striatum, with conduction velocities of 2.4-36 m/s. Collision effects have not been observed between the anteromedial cortex and the internal capsule, medial substantia nigra, superior colliculus, rostromedial tegmentum, or medial pons, which suggests that these sites are not connected by axons mediating turning. Possible synaptic linkages between the 5 bundles and possible transmitters are discussed.

Two converging brainstem pathways mediating circling behavior

Ipsiversive circling results from stimulation of the rostromedial tegmentum (RMT) or medial pons (PONS), and contraversive circling results from stimulation of the superior colliculus (SC). To determine whether these sites are functionally connected, the collision method of Shizgal, Bielajew, Corbett, Skelton and Yeomans (1980) was used in rats. Pairs of stimulation pulses were presented to two sites, and the degree of collision between stimulation-evoked action potentials was assessed by measuring the frequency required for circling at short and long intrapair conditioning-testing (C-T) intervals. Collision was evidenced when the required frequencies were higher at short C-T intervals than at long C-T intervals. Collision of 46-62% was observed between RMT and PONS, and collision of 15-29% was observed between SC and PONS. Sites from which collision was obtained were located along the trajectories of the medial tegmental tract and the crossed tectospinal pathway. Refractory periods in all sites were similar, ranging from 0.3 to 1.7 ms. Conduction velocities of axons connecting RMT and PONS and SC and PONS were comparable, ranging from 0.8 to 13.3 m/s and 1.7 to 13.8 m/s, respectively, with lower conduction velocities associated with more ventral pontine sites. Thus, RMT and PONS, and SC and PONS are connected by myelinated axons that mediate circling.

Descending projections to the inferior olive from the mesencephalon and superior colliculus in the cat. An autoradiographic study

Descending projections from the mesencephalon and superior colliculus to the inferior olive were analyzed by an autoradiographic tracing method. Injections of tritium-labelled leucine were placed in regions which had previously been identified as sources of afferents to the olive. These were located adjacent to the central gray and extended from the rostral red nucleus to the posterior thalamus. Additional injections were made in the superior colliculus. Other injections were placed in the basal ganglia and thalamus. Injections restricted to one side of the central mesencephalon resulted in predominantly ipsilateral labelling of the olive. After injections in the caudo-medial parafascicular and subparafascicular nuclei and rostral nucleus of Darkschewitsch, deposits of grains were observed in the rostral pole of the medial accessory olive and adjacent ventral lamella of the principal olive. The medial accessory olive contained grains into its middle third. More caudal injections which involved the interstitial nucleus of Cajal as well as the nucleus of Darkschewitsch and rostral red nucleus resulted in the dense labeling of the entire principal olive (except the dorsal cap), the entire medial accessory olive (except subnucleus beta and the caudo-medial pole) and the caudo-dorsal accessory olive. Injections centered in the caudal magnocellular red nucleus and extending into the rostral parvocellular division labelled the dorsal lamella of the principal olive almost exclusively. When only the caudal part of the red nucleus was involved in the injection, the olive was entirely clear of grains. Minor contralateral distributions were observed in the dorsomedial cell column, the medial tip of the dorsal lamella and in the caudal medial accessory olive. The deep layers of the superior colliculus were found to project strongly to the contralateral medial accessory olive immediately beside subnucleus beta and weakly to the same area ipsilaterally. Four pathways were identified as contributing fibers to the olivary projections. These were the medial longitudinal fasciculus, the medial tegmental tract, the central tegmental tract and tectospinal or tectobulbar fibers. The rubrospinal tract did not contribute projections to the olive. Injections in the caudate nucleus, entopeduncular nucleus and ventral anterior and ventral lateral thalamic nuclei, did not result in any labeling in the olive.