New insights into the upward vestibulo-oculomotor pathways in the human brainstem

Brainstem white matter microstructure is associated with hyporesponsiveness and overall sensory features in autistic children

BACKGROUND

Elevated or reduced responses to sensory stimuli, known as sensory features, are common in autistic individuals and often impact quality of life. Little is known about the neurobiological basis of sensory features in autistic children. However, the brainstem may offer critical insights as it has been associated with both basic sensory processing and core features of autism.

METHODS

Diffusion-weighted imaging (DWI) and parent-report of sensory features were acquired from 133 children (61 autistic children with and 72 non-autistic children, 6-11 years-old). Leveraging novel DWI processing techniques, we investigated the relationship between sensory features and white matter microstructure properties (free-water-elimination-corrected fractional anisotropy [FA] and mean diffusivity [MD]) in precisely delineated brainstem white matter tracts. Follow-up analyses assessed relationships between microstructure and sensory response patterns/modalities and analyzed whole brain white matter using voxel-based analysis.

RESULTS

Results revealed distinct relationships between brainstem microstructure and sensory features in autistic children compared to non-autistic children. In autistic children, more prominent sensory features were generally associated with lower MD. Further, in autistic children, sensory hyporesponsiveness and tactile responsivity were strongly associated with white matter microstructure in nearly all brainstem tracts. Follow-up voxel-based analyses confirmed that these relationships were more prominent in the brainstem/cerebellum, with additional sensory-brain findings in the autistic group in the white matter of the primary motor and somatosensory cortices, the occipital lobe, the inferior parietal lobe, and the thalamic projections.

LIMITATIONS

All participants communicated via spoken language and acclimated to the sensory environment of an MRI session, which should be considered when assessing the generalizability of this work to the whole of the autism spectrum.

CONCLUSIONS

These findings suggest unique brainstem white matter contributions to sensory features in autistic children compared to non-autistic children. The brainstem correlates of sensory features underscore the potential reflex-like nature of behavioral responses to sensory stimuli in autism and have implications for how we conceptualize and address sensory features in autistic populations.

Weak Vestibular Response in Persistent Developmental Stuttering

Vibrational energy created at the larynx during speech will deflect vestibular mechanoreceptors in humans (Todd et al., 2008; Curthoys, 2017; Curthoys et al., 2019). Vestibular-evoked myogenic potential (VEMP), an indirect measure of vestibular function, was assessed in 15 participants who stutter, with a non-stutter control group of 15 participants paired on age and sex. VEMP amplitude was 8.5 dB smaller in the stutter group than the non-stutter group (p = 0.035, 95% CI [−0.9, −16.1], t = −2.1, d = −0.8, conditional R² = 0.88). The finding is subclinical as regards gravitoinertial function, and is interpreted with regard to speech-motor function in stuttering. There is overlap between brain areas receiving vestibular innervation, and brain areas identified as important in studies of persistent developmental stuttering. These include the auditory brainstem, cerebellar vermis, and the temporo-parietal junction. The finding supports the disruptive rhythm hypothesis (Howell et al., 1983; Howell, 2004) in which sensory inputs additional to own speech audition are fluency-enhancing when they coordinate with ongoing speech.

Three-Dimensional Identification of the Medial Longitudinal Fasciculus in the Human Brain: A Diffusion Tensor Imaging Study

Background: The medial longitudinal fasciculus (MLF) interacts with eye movement control circuits involved in the adjustment of horizontal, vertical, and torsional eye movements. In this study, we attempted to identify and investigate the anatomical characteristics of the MLF in human brain, using probabilistic diffusion tensor imaging (DTI) tractography. Methods: We recruited 31 normal healthy adults and used a 1.5-T scanner for DTI. To reconstruct MLFs, a seed region of interest (ROI) was placed on the interstitial nucleus of Cajal at the midbrain level. A target ROI was located on the MLF of the medulla in the reticular formation of the medulla. Mean values of fractional anisotropy, mean diffusivity, and tract volumes of MLFs were measured. Results: The component of the MLF originated from the midbrain MLF, descended through the posterior side of the medial lemniscus (ML) and terminated on the MLF of medulla on the posterior side of the ML in the medulla midline. DTI parameters of right and left MLFs were not significantly different. Conclusion: The tract of the MLF in healthy brain was identified by probabilistic DTI tractography. We believe this study will provide basic data and aid future comparative research on lesion or age-induced MLF changes.

Acute thrombosis of the carotid artery associated with positional nystagmus at multiple planes

Objective

An acute onset central pathology without any clear neurological symptoms may mimic peripheral vestibular problem in an emergency setting. A 54-year-old man suddenly developed dizziness without any cranial nerve symptoms, paresis, cerebellar signs or sensory disturbances except upbeat positional nystagmus at multiple provoked positions which alerted for a possible acute central pathology.

Findings

An instantaneous magnetic resonance imaging and angiography studies further showed obstruction of the left internal carotid artery above the bifurcation. The patient’s subsequent prognosis was consistent with good recovery following anti-coagulant therapy. A follow-up MRI and angiography showed resolution of thrombosis.

Conclusion

It should be kept in mind that positional nystagmus is likely to occur in central pathologies. Differentiation between benign paroxysmal positional vertigo and central positioning nystagmus is critical.

[Etiological assessment of a nystagmus in childhood]

Apart from the latent nystagmus, which arises as a consequence of failure to develop binocular vision, every case of childhood nystagmus needs an etiological assessment. Knowledge of the pathogenesis of the various types of nystagmus guides this assessment, particularly considering the morphological characteristics of the nystagmus. The clinical ophthalmologic examination is complemented by OCT and electrophysiologic testing (ERG, VEP). If this testing is normal, an MRI and genetic assessment are required.

Calretinin inputs are confined to motoneurons for upward eye movements in monkey

Motoneurons of extraocular muscles are controlled by different premotor pathways, whose selective damage may cause directionally selective eye movement disorders. The fact that clinical disorders can affect only one direction, e.g., isolated up-/downgaze palsy or up-/downbeat nystagmus, indicates that up- and downgaze pathways are organized separately. Recent work in monkey revealed that a subpopulation of premotor neurons of the vertical eye movement system contains the calcium-binding protein calretinin (CR). With combined tract-tracing and immunofluorescence, the motoneurons of vertically pulling eye muscles in monkey were investigated for the presence of CR-positive afferent terminals. In the oculomotor nucleus, CR was specifically found in punctate profiles contacting superior rectus and inferior oblique motoneurons, as well as levator palpebrae motoneurons, all of which participate in upward eye movements. Double-immunofluorescence labeling revealed that CR-positive terminals lacked the γ-aminobutyric acid (GABA)-synthesizing enzyme glutamate decarboxylase, which is present in inhibitory afferents to all motoneurons mediating vertical eye movements. Therefore, CR-containing afferents are considered to be excitatory. In conclusion, a strong CR input is confined to motoneurons mediating upgaze, which derive from premotor pathways mediating saccades and smooth pursuit, but not from secondary vestibulo-ocular neurons in the magnocellular part of the medial vestibular nucleus. The functional significance of CR in these connections is unclear, but it may serve as a useful marker to locate upgaze pathways in the human brain.

Vertical supranuclear gaze palsy in Niemann-Pick type C disease

Vertical supranuclear gaze palsy (VSGP) is a key clinical feature in patients with Niemann-Pick type C disease (NP-C), a rare, autosomal recessive, neuro-visceral disorder caused by mutations in either the NPC1 or NPC2 gene. VSGP is present in approximately 65 % of the cases and is, with gelastic cataplexy, an important risk indicator for NP-C. VSGP in NP-C is characterized by a paralysis of vertical saccades, especially downward, with the slow vertical eye movement systems (smooth pursuit and the vestibulo-ocular reflex) spared in the early phase of the disease. This dissociation is caused by a selective vulnerability of the neurons in the rostral interstitial nuclei of the medial longitudinal fasciculus (riMLF) in NP-C. Here we discuss VSGP in NP-C and how clinicians can best elicit this sign.

Sources of calretinin inputs to motoneurons of extraocular muscles involved in upgaze

Recent monkey studies showed that motoneurons of the oculomotor nucleus involved in upward eye movements receive a selective input from afferents containing calretinin (CR). Here, we investigated the sources of these CR-positive afferents. After injections of tract-tracers into the oculomotor nucleus (nIII) of two monkeys, the retrograde labeling was combined with CR-immunofluorescence in frozen brainstem sections. Three sources of CR inputs to nIII were found: the rostral interstitial nucleus of the medial longitudinal fascicle (RIMLF), the interstitial nucleus of Cajal, and the y-group. CR is not present in all premotor upward-moving pathways. The excitatory secondary vestibulo-ocular neurons in the magnocellular part of the medial vestibular nuclei contained nonphosphorylated neurofilaments, but no CR, and they received a strong supply of large CR-positive boutons. In conclusion, the present study presents evidence that only specific premotor pathways for upward eye movements--excitatory upgaze pathways--contain CR, but not the up vestibulo-ocular reflex pathways. This property may help to differentiate between premotor up- and downgaze pathways in correlative clinico-anatomical studies in humans.

The anatomy and physiology of the ocular motor system

Accurate diagnosis of abnormal eye movements depends upon knowledge of the purpose, properties, and neural substrate of distinct functional classes of eye movement. Here, we summarize current concepts of the anatomy of eye movement control. Our approach is bottom-up, starting with the extraocular muscles and their innervation by the cranial nerves. Second, we summarize the neural circuits in the pons underlying horizontal gaze control, and the midbrain connections that coordinate vertical and torsional movements. Third, the role of the cerebellum in governing and optimizing eye movements is presented. Fourth, each area of cerebral cortex contributing to eye movements is discussed. Last, descending projections from cerebral cortex, including basal ganglionic circuits that govern different components of gaze, and the superior colliculus, are summarized. At each stage of this review, the anatomical scheme is used to predict the effects of lesions on the control of eye movements, providing clinical-anatomical correlation.

Effect of gravity on vertical eye position

There is growing evidence that gravity markedly influences vertical eye position and movements. A new model for the organization of brainstem upgaze pathways is presented in this review. The crossing ventral tegmental tract (CVTT) could be the efferent tract of an "antigravitational" pathway terminating at the elevator muscle motoneurons in the third nerve nuclei and comprising, upstream, the superior vestibular nucleus and y-group, the flocculus, and the otoliths. This pathway functions in parallel to the medial longitudinal fasciculus pathways, which control vertical eye movements made to compensate for all vertical head movements and may also comprise the "gravitational" vestibular pathways, involved in the central reflection of the gravity effect. The CVTT could provide the upgaze system with the supplement of tonic activity required to counteract the gravity effect expressed in the gravitational pathway, being permanently modulated according to the static positions of the head (i.e., the instantaneous gravity vector) between a maximal activity in the upright position and a minimal activity in horizontal positions. Different types of arguments support this new model. The permanent influence of gravity on vertical eye position is strongly suggested by the vertical slow phases and nystagmus observed after rapid changes in hypo- or hypergravity. The chin-beating nystagmus, existing in normal subjects with their head in the upside-down position, suggests that gravity is not compensated for in the downgaze system. Upbeat nystagmus due to brainstem lesions, most likely affecting the CVTT circuitry, is improved when the head is in the horizontal position, suggesting that this circuitry is involved in the counteraction of gravity between the upright and horizontal positions of the head. In downbeat nystagmus due to floccular damage, in which a permanent hyperexcitation of the CVTT could exist, a marked influence of static positions of the head is also observed. Finally, the strongest argument supporting a marked role of gravity in vertical eye position is that the eye movement alterations observed in the main, typical physiological and pathological conditions are precisely those that would be expected from a direct effect of gravity on the eyeballs, with, moreover, no single alternative interpretation existing so far that could account for all these different types of findings.

Vertigo and vestibular abnormalities in spinocerebellar ataxia type 6

Spinocerebellar ataxia type 6 (SCA6) is a calcium channelopathy due to a pathological CAG repeat expansion in CACNL1A4. Patients frequently describe paroxysmal vertigo early in the disease course, but it is not clear whether this is central or labyrinthine in origin. To address this issue we studied 21 SCA6 patients. Symptoms of vertigo were defined using a structured questionnaire. Signs were recorded during a standardised bed-side vestibular examination that included systematic positional testing with Frenzel goggles.Brief, recurrent attacks of vertigo occurred in 13 patients, usually preceding the onset of ataxia. Nystagmus was observed behind Frenzel goggles in 14 patients, and was induced either during positional testing, or head shaking in 20 patients. Only one patient had findings typical of benign paroxysmal positional vertigo (BPPV). Combined downbeat and horizontal gaze-evoked nystagmus ("side-pocket") was the most common form, occurring most commonly in supine and head-hanging positions, and following horizontal head-shaking. Nystagmus beating away from the ground (apogeotropic) occurred in 9 patients as they lay on their side.In conclusion, vertigo and abnormalities on bedside vestibular examination are common in SCA6, with forms of nystagmus typical of cerebellar, rather than labyrinthine, disease. These findings demonstrate phenotypic overlap between SCA6 and episodic ataxia type 2, which are both due to mutations in CACNL1A4.