Some instructive cases concerning the circuitry of ocular smooth pursuit in the brainstem

Eye movement control and its disorders

We review the supranuclear control centers and pathways leading to individual cranial nerve nuclei in the brainstem. We discuss horizontal and vertical gaze and their abnormalities, and review the cranial nerves which subserve eye movements, III, IV, VI, including their clinical testing. We highlight the sites at which these nerves are clinically affected, which often result in characteristic associated features with neurological localizing value. Differential diagnoses of cranial nerve palsies including Duane and Möbius syndromes are also described. Lastly, we discuss the nature of childhood neuromuscular junction disorders such as myasthenia gravis, as well as disorders of the muscle itself (chronic progressive external ophthalmoplegia (CPEO), thyroid orbitopathy).

Movimentos oculares no bebê: o que eles nos indicam sobre o status oftalmológico e neurológico

Movimentos oculares anormais podem ser o resultado de um desenvolvimento visual anormal ou estar sinalizando a existência de uma doença neurológica ou neuromuscular. Assim, é importante que o clínico conheça essas anormalidades e possa distingui-las dos movimentos oculares normais, porém imaturos. Este trabalho oferece esclarecimentos sobre os diferentes tipos de movimentos oculares, como interpretá-los nos bebês saudáveis e quais os tipos de alterações encontradas nas diferentes doenças visuais e neurológicas.

Vertical nystagmus: clinical facts and hypotheses

The pathophysiology of spontaneous upbeat (UBN) and downbeat (DBN) nystagmus is reviewed in the light of several instructive clinical findings and experimental data. UBN due to pontine lesions could result from damage to the ventral tegmental tract (VTT), originating in the superior vestibular nucleus (SVN), coursing through the ventral pons and transmitting excitatory upward vestibular signals to the third nerve nucleus. A VTT lesion probably leads to relative hypoactivity of the drive to the motoneurons of the elevator muscles with, consequently, an imbalance between the downward and upward systems, resulting in a downward slow phase. The results observed in internuclear ophthalmoplegia suggest that the medial longitudinal fasciculus (MLF) is involved in the transmission of both upward and downward vestibular signals. Since no clinical cases of DBN due to focal brainstem damage have been reported, it may be assumed that the transmission of downward vestibular signals depends only upon the MLF, whereas that of upward vestibular signals involves both the MLF and the VTT. The main focal lesions resulting in DBN affect the cerebellar flocculus and/or paraflocculus. Apparently, this structure tonically inhibits the SVN and its excitatory efferent tract (i.e. the VTT) but not the downward vestibular system. Therefore, a floccular lesion could result in a disinhibition of the SVN-VTT pathway with, consequently, relative hyperactivity of the drive to the motoneurons of the elevator muscles, resulting in an upward slow phase. UBN also results from lesions affecting the caudal medulla. An area in this region could form part of a feedback loop involved in upward gaze-holding, originating in a collateral branch of the VTT and comprising the caudal medulla, the flocculus and the SVN, successively. Therefore, it is suggested that the main types of spontaneous vertical nystagmus due to focal central lesions result from a primary dysfunction of the SVN-VTT pathway, which becomes hypoactive after pontine or caudal medullary lesions, thereby eliciting UBN, and hyperactive after floccular lesions, thereby eliciting DBN. Lastly, since gravity influences UBN and DBN and may facilitate the downward vestibular system and restrain the upward vestibular system, it is hypothesized that the excitatory SVN-VTT pathway, along with its specific floccular inhibition, has developed to counteract the gravity pull. This anatomical hyperdevelopment is apparently associated with a physiological upward velocity bias, since the gain of all upward slow eye movements is greater than that of downward slow eye movements in normal human subjects and in monkeys.

Du noyau abducens à la mémoire spatiale : un parcours oculomoteur

Advances in our knowledge on eye movements over the last 25 years are reviewed, focusing on the author's experience. First, the advantages of binocular frontal vision, which is a common characteristic of all predator mammals, are compared to those of lateral vision, characterizing their preys. Binocular frontal vision implies a perfect parallelism of both eyes, which is ensured in the pons by means of the abducens nucleus, controlling abduction as well as adduction. The pathological example of the "one-and-a-half" syndrome, in which the abducens nucleus and the adjacent medial longitudinal fasciculus are simultaneously impaired, is described. The main brainstem syndromes involving vertical eye movements are also reviewed: in particular, the third nerve nucleus syndrome, in which both ipsilateral third nerve paralysis and contralateral superior rectus paralysis (with hypotropia) result from a unilateral third nerve nucleus lesion. A case of upbeat nystagmus (in the primary position of gaze) due to a small upper pontine lesion, probably affecting the ventral tegmental tract (VTT) is also reported. This is an opportunity to emphasize that, although a number of cases of upbeat nystagmus due to focal brainstem lesions affecting the upward vestibular pathway (UVP)--either at the upper pontine (VTT) or caudal medullary level--exist in the literature, no convincing cases with downbeat nystagmus (in the primary position of gaze) due to a focal brainstem lesion have been reported. Downbeat nystagmus could result from a UVP hyperactivity (secondary to a floccular lesion) and the notion that this pathway is physiologically predominant compared to the downward pathway, maybe due to gravity, is developed. A new hypothesis about the role of the caudal medulla in UVP is also proposed. Next, the cortical control of saccadic eye movements is reviewed, with a reminder that reflexive saccades are mainly triggered by the parietal eye field whereas intentional saccades depend upon the frontal eye field. The inhibition of reflexive saccades is mainly controlled by the dorsolateral frontal cortex (DLPFC), i.e. area 46 of Brodmann. A few examples of the use of saccades as a research tool and model in cognitive neurosciences are given. The use of memory-guided saccades allowed us to study spatial memory and led us to propose a relatively original conception of the cortical control of spatial memory in which the DLPFC, the parahippocampal cortex and the hippocampal formation could be involved successively according to specific periods of time. An experiment using functional magnetic resonance imaging in a paradigm studying decision is described, again with the involvement of the DLPFC in the decisional process. Lastly, the usefulness of eye movements in clinical neuro-ophthalmology at the bedside as well as in neurophysiological or even neuropsychological research in the laboratory is emphasized.

Abnormal supranuclear eye movements in the child: a practical guide to examination and interpretation

Abnormal eye movements in the infant or voting child can be congenital or acquired. They may be a result of abnormal early visual development or a sign of underlying neurologic or neuromuscular disease. It is important to be able to detect these abnormalities and to distinguish them from normal but immature eye movements. The spectrum of disease in children differs from that in adults. Serious, potentially fatal but treatable disorders can be acquired in infancy, and abnormal eye movements in a sometimes apparently well child should never be labeled as congenital or benign without careful investigation. Eye movement analysis can indicate the presence of an underlying condition and help the clinician to classify different neurologic diseases. It is important to carefully examine the ocular motor system in any children at risk of neurologic disease. This review provides a practical guide to the examination and interpretation of eye movements in the child and includes recent literature on eye movement disorders of childhood. We describe supranuclear abnormalities of the ocular motor system in the order in which we would normally examine it: saccades, pursuit, convergence, vestibulo-ocular reflex, and optokinetic nystagmus. Nystagmus, internuclear ophthalmoplegia, cranial nerve abnormalities, and "miswiring" phenomena (such as Duane's syndrome and synergistic divergence) are not discussed.

Unilateral saccadic pursuit in patients with sensory stroke: sign of a pontine tegmentum lesion

BACKGROUND AND PURPOSE

Pure hemisensory syndrome can be caused by small strokes occurring in a number of regions, including the thalamus and pons. Differentiation of the pontine sensory syndrome from the thalamic sensory syndrome has generally been made on the basis of distribution of sensory loss and involvement of specific sensory modalities but not without uncertainties and difficulties. Because the pontine tegmentum plays a pivotal role in generating horizontal eye movement, we attempted to discriminate these 2 syndromes by analyzing horizontal eye movements in stroke patients with pure hemisensory syndrome.

METHODS

Horizontal saccade, pursuit, vestibulo-ocular reflex (VOR), and VOR cancellation (VORC) were evaluated using electro-oculography in 6 patients with hemisensory syndromes, 3 due to pontine stroke and 3 due to thalamic stroke, and all were verified by MRI or CT. In addition, somatosensory evoked potentials (SEPs) were recorded.

RESULTS

Smooth pursuit and VORC directed toward the side of the lesion were impaired unilaterally in patients with pontine sensory stroke, whereas those 2 movements were intact bilaterally in patients with thalamic sensory stroke. Saccade and VOR were preserved in all patients. SEPs were normal in all patients with pontine and thalamic sensory strokes. No difference was found in the pattern of sensory disturbance between the 2 types of stroke patients.

CONCLUSIONS

Ipsilateral impairment of the smooth pursuit system may be a sign of a pontine lesion in patients with hemisensory stroke.

Comparison of the smooth eye tracking disorder of schizophrenics with that of nonhuman primates with specific brain lesions

The smooth pursuit eye tracking deficit (ETD) often associated with schizophrenia has generated enormous interest over the last 20 years. The deficit is observed in about 80% of schizophrenics and in half of their first degree relatives. It is not affected by neuroleptic medication and is not due to inattention. A review of 52 studies (and actual records when available) on ETD in schizophrenia reveals that the deficit can consistently be described as low gain pursuit augmented with catch-up saccades and often peppered with intrusive saccades. A review of the brain areas that have been shown to be involved in pursuit provides the necessary background for the subsequent section which details the nature of the smooth tracking deficits following experimental lesions. This section reveals that the ETD following lesions of the frontal lobe is unique in that it closely resembles the ETD of schizophrenics. This finding lends further support for frontal lobe theories of schizophrenia.

Paresis of contralateral smooth pursuit and normal vestibular smooth eye movements after unilateral brainstem lesions

Pursuit and vestibular smooth eye movements were measured in patients with lesions of the caudal brainstem tegmentum identified by magnetic resonance imaging (MRI) and computed tomography (CT), with neuropathological correlation in 1 patient. Contralateral smooth pursuit gain was significantly lower than ipsilateral gain in each patient. Ipsilateral smooth pursuit gain was also subnormal in patients with unilateral pontine damage that caused slowing of ipsilateral saccades. Horizontal vestibulo-ocular reflex gain and phase were normal. These quantitative correlations indicate that lesions of the pontine tegmentum that paralyze ipsilateral saccades can spare the vestibulo-ocular reflex, and that smooth pursuit movement and the vestibulo-ocular reflex can be impaired independently by pontine or medullary lesions. In contrast to lesions at other sites, unilateral lesions of the pontine or medullary tegmentum impair contralateral smooth pursuit more than ipsilateral pursuit movements. These findings provide evidence that a double decussating pathway mediates smooth pursuit; the first decussation is from the pons to the cerebellum, and the second decussation is from the vestibular nucleus to the contralateral abducens nucleus.