Cerebellum: From the identification of the cerebellar motor syndrome to the internal models

Cerebellar circuitry is topographically arranged in closed loops with the cerebral cortex. The three cornerstones of clinical ataxia have emerged from studies on connectional anatomy and from clinical/neuropsychological observations, leading to the definition of clinical syndromes encountered in daily practice: (a) the cerebellar motor syndrome (CMS), (b) the vestibulocerebellar syndrome (VCS), and (c) the cerebellar cognitive affective syndrome/Schmahmann syndrome (CCAS/SS). These syndromes are either isolated or coexist, depending on the underlying pathological process and its degree of extension within the cerebellum. Dysmetria is the core feature of cerebellar deficits, encompassing motor dysmetria (hypermetria, hypometria) in CMS, oculomotor dysmetria in VCS, and dysmetria of thought in CCAS/SS. The leading hypothesis is that dysmetria results from errors in building or maintaining internal models, which are inherent to predictive behavior. Errors in prediction would lead to clumsiness and incoordination of limbs, oculomotor impairments, and aberrant cognitive/affective behavior. The cerebellum is currently viewed as a learning machine enriched with multiple plasticity mechanisms, allowing the permanent adaptation to the external world by generating and maintaining predictive operations, from motor to cognitive, affective, emotional, and social operations essential for daily human life.

The underpinnings of cerebellar ataxias

The human cerebellum contains more than 60% of all neurons of the brain. Anatomically, the cerebellum is divided into 10 lobules (I-X). The cerebellar cortex is arranged into three layers: the molecular layer (external), the Purkinje cell layer and the granular layer (internal). Purkinje neurons and interneurons are inhibitory, except for granule cells. The layer of Purkinje neurons inhibit cerebellar nuclei, the sole output of the cerebellar circuitry, as well as vestibular nuclei. The cerebellum is arranged into a series of olivo-cortico-nuclear modules arranged longitudinally in the rostro-caudal plane. The cerebro-cerebellar connectivity is organized into multiple loops running in parallel. From the clinical standpoint, it is now considered that cerebellar symptoms can be gathered into 3 cerebellar syndromes: a cerebellar motor syndrome (CMS), a vestibulocerebellar syndrome (VCS) and a cerebellar cognitive affective syndrome/Schmahmann syndrome (CCAS/SS). CMS remains a cornerstone of modern clinical ataxiology, and relevant lesions involve lobules I-V, VI and VIII. The core feature of cerebellar symptoms is dysmetria, covering motor dysmetria (errors in the metrics of motion) and dysmetria of thought. The cerebellar circuitry plays a key-role in the generation and maintenance of internal models which correspond to neural representations reproducing the dynamic properties of the body. These models allow predictive computations for motor, cognitive, social, and affective operations. Cerebellar circuitry is endowed with noticeable plasticity properties.

Limb lengthening with PRECICE magnetic nail in pediatric patients: A systematic review

BACKGROUND

Limb lengthening techniques play an increasingly important role in the pediatric orthopedic field. The principles of the osteogenesis distraction bonded traditionally with external fixators; however, the recent deployment of fully implantable systems has been able to overcome severities related to external fixators. The PRECICE^® is an implantable limb lengthening intramedullary nail system that is remotely controlled and magnetically driven.

AIM

To review the current literature available on this matter in order to assess the PRECICE clinical and radiological outcomes and its possible complications in a population of pediatric patients undergoing limb lengthening.

METHODS

Only five studies met the inclusion criteria and were consequently included in the review for a total of 131 patients and 135 femurs. The clinical and radiological outcomes of interest were: the main lengthening obtained, the distraction rate, the period of time to full weight bearing, the consolidation index, and the Association for the Study and Application of Methods of Ilizarov score.

RESULTS

In conclusion, data collected from the articles under investigation were comparable with the exception of the consolidation index. Unfortunately, the study population was too small and the patients’ follow-up was too short to make definitive conclusions.

CONCLUSION

This review shows that the PRECICE Nail System is still a therapeutic challenge in limb lengthening for pediatric orthopedic surgeons; however, careful pre-operative planning and an accurate surgical technique could allow the correction of more complex deformities with a low rate of complications.

The Cerebellar Nuclei and Dexterous Limb Movements

Dexterous forelimb movements like reaching, grasping, and manipulating objects are fundamental building blocks of the mammalian motor repertoire. These behaviors are essential to everyday activities, and their elaboration underlies incredible accomplishments by human beings in art and sport. Moreover, the susceptibility of these behaviors to damage and disease of the nervous system can lead to debilitating deficits, highlighting a need for a better understanding of function and dysfunction in sensorimotor control. The cerebellum is central to coordinating limb movements, as defined in large part by Joseph Babinski and Gordon Holmes describing motor impairment in patients with cerebellar lesions over 100 years ago (Babinski, 1902; Holmes, 1917), and supported by many important human and animal studies that have been conducted since. Here, with a focus on output pathways of the cerebellar nuclei across mammalian species, we describe forelimb movement deficits observed when cerebellar circuits are perturbed, the mechanisms through which these circuits influence motor output, and key challenges in defining how the cerebellum refines limb movement.

Transient Dorsal Column Dysfunction After Extreme Cervical Deformity Correction: Report of 3 Cases

BACKGROUND

Chin-on-chest kyphotic cervical deformity can be debilitating. Surgical deformity correction serves to decompress neural elements and restore lordosis. This can be achieved through multiple osteotomies with instrumentation and fusion, sometimes requiring a staged approach. Such procedures carry a high risk of neurologic injury. Here we present examples of transient neurologic dysfunction not previously reported in the common literature.

CASE DESCRIPTION

The authors present 3 patients who underwent extreme cervical deformity correction for chin-on-chest deformity. Deformity correction in all cases was obtained through multiple osteotomies with multilevel cervicothoracic posterior instrumentation and arthrodesis. On postoperative examination, all 3 patients developed transient ataxia, dysmetria, and decreased proprioception in all 4 extremities-examination findings consistent with dorsal column dysfunction. All symptoms resolved within 2-3 weeks postoperatively.

CONCLUSIONS

Incomplete spinal cord syndromes such as posterior cord syndrome can be caused by compression or stretching of the ascending dorsal spinal tracts. Considering the large degree of correction obtained, we hypothesize the resulting shortening of the dorsal columns as the pathomechanism. Providers should be aware, and patients should be counseled preoperatively that these symptoms may occur. If these symptoms are present postoperatively, appropriate diligence is warranted with the understanding that these deficits may be transient.

Slow saccades in cerebellar disease

Eye movements are frequently considered diagnostic markers indicating involvement of the cerebellum. Impaired amplitude of saccades (saccade dysmetria), impaired gaze holding function (horizontal or downbeat nystagmus), and interrupted (choppy) pursuit are typically considered hallmarks of cerebellar disorders. While saccade dysmetria is a frequently considered abnormality, the velocity of saccades are rarely considered part of the constellation of cerebellar involvement. Reduced saccade velocity, frequently called “slow saccades” are typically seen in a classic disorder of the midbrain called progressive supranuclear palsy. It is also traditionally diagnostic of spinocerebellar ataxia type 2. In addition to its common causes, the slowness of vertical saccades is not rare in cerebellar disorders. Frequently this phenomenology is seen in multisystem involvement that substantially involves the cerebellum. In this review we will first discuss the physiological basis and the biological need for high saccade velocities. In subsequent sections we will discuss disorders of cerebellum that are known to cause slowing of saccades. We will then discuss possible pathology and novel therapeutic strategies.

Impaired Motor Learning in a Disorder of the Inferior Olive: Is the Cerebellum Confused?

An attractive hypothesis about how the brain learns to keep its motor commands accurate is centered on the idea that the cerebellar cortex associates error signals carried by climbing fibers with simultaneous activity in parallel fibers. Motor learning can be impaired if the error signals are not transmitted, are incorrect, or are misinterpreted by the cerebellar cortex. Learning might also be impaired if the brain is overwhelmed with a sustained barrage of meaningless information unrelated to simultaneously appearing error signals about incorrect performance. We test this concept in subjects with syndrome of oculopalatal tremor (OPT), a rare disease with spontaneous, irregular, roughly pendular oscillations of the eyes thought to reflect an abnormal, synchronous, spontaneous discharge to the cerebellum from the degenerating neurons in the inferior olive. We examined motor learning during a short-term, saccade adaptation paradigm in patients with OPT and found a unique pattern of disturbed adaptation, quite different from the abnormal adaption when the cerebellum is involved directly. Both fast (seconds) and slow (minutes) timescales of learning were impaired. We suggest that the spontaneous, continuous, synchronous output from the inferior olive prevents the cerebellum from receiving the error signals it needs for appropriate motor learning. The important message from this study is that impaired motor adaptation and resultant dysmetria is not the exclusive feature of cerebellar disorders, but it also highlights disorders of the inferior olive and its connections to the cerebellum.

Motor cortex rTMS improves dexterity in relapsing-remitting and secondary progressive multiple sclerosis

The motor cortex (MC) receives an excitatory input from the cerebellum which is reduced in patients with cerebellar lesions. High-frequency repetitive transcranial magnetic stimulation (rTMS) induces cortical facilitation which can counteract the reduced cerebellar drive to the MC. Our study included 24 relapsing-remitting multiple sclerosis (RRMS) and secondary progressive multiple sclerosis (SPMS) patients with dysmetria. The patients were divided into two groups: Group A received two sessions of real MC rTMS and Group B received one session of real rTMS and one session of sham rTMS. Ten healthy volunteers formed group C. Evaluation was carried out using the nine-hole pegboard task and the cerebellar functional system score (FSS) of the expanded disability status scale (EDSS). Group A patients showed a significant improvement in the time required to finish the pegboard task (P = 0.002) and in their cerebellar FSS (P = 0.000) directly after the second session and 1 month later. The RRMS patients showed more improvement than the SPMS patients. Group B patients did not show any improvement in the pegboard task or the cerebellar FSS. These results indicate that MC rTMS can be a promising option in treating both RRMS or SPMS patients with cerebellar impairment and that its effect can be long-lasting.

Consensus Paper: Revisiting the Symptoms and Signs of Cerebellar Syndrome

The cerebellum is involved in sensorimotor operations, cognitive tasks and affective processes. Here, we revisit the concept of the cerebellar syndrome in the light of recent advances in our understanding of cerebellar operations. The key symptoms and signs of cerebellar dysfunction, often grouped under the generic term of ataxia, are discussed. Vertigo, dizziness, and imbalance are associated with lesions of the vestibulo-cerebellar, vestibulo-spinal, or cerebellar ocular motor systems. The cerebellum plays a major role in the online to long-term control of eye movements (control of calibration, reduction of eye instability, maintenance of ocular alignment). Ocular instability, nystagmus, saccadic intrusions, impaired smooth pursuit, impaired vestibulo-ocular reflex (VOR), and ocular misalignment are at the core of oculomotor cerebellar deficits. As a motor speech disorder, ataxic dysarthria is highly suggestive of cerebellar pathology. Regarding motor control of limbs, hypotonia, a- or dysdiadochokinesia, dysmetria, grasping deficits and various tremor phenomenologies are observed in cerebellar disorders to varying degrees. There is clear evidence that the cerebellum participates in force perception and proprioceptive sense during active movements. Gait is staggering with a wide base, and tandem gait is very often impaired in cerebellar disorders. In terms of cognitive and affective operations, impairments are found in executive functions, visual-spatial processing, linguistic function, and affective regulation (Schmahmann's syndrome). Nonmotor linguistic deficits including disruption of articulatory and graphomotor planning, language dynamics, verbal fluency, phonological, and semantic word retrieval, expressive and receptive syntax, and various aspects of reading and writing may be impaired after cerebellar damage. The cerebellum is organized into (a) a primary sensorimotor region in the anterior lobe and adjacent part of lobule VI, (b) a second sensorimotor region in lobule VIII, and (c) cognitive and limbic regions located in the posterior lobe (lobule VI, lobule VIIA which includes crus I and crus II, and lobule VIIB). The limbic cerebellum is mainly represented in the posterior vermis. The cortico-ponto-cerebellar and cerebello-thalamo-cortical loops establish close functional connections between the cerebellum and the supratentorial motor, paralimbic and association cortices, and cerebellar symptoms are associated with a disruption of these loops.