Prolonged cortical relay time of long latency reflex and central motor conduction in patients with spinocerebellar ataxia type 6

Early Electrophysiology in Suspected Acute Guillain-Barré Syndrome: A Prospective Study of Comprehensive Testing

PURPOSE

Electrophysiologic changes in early Guillain-Barré Syndrome (GBS) can be nondiagnostic. Improved testing paradigms may improve hyperacute treatment.

METHODS

This work prospectively evaluated consecutive patients admitted to a metropolitan teaching hospital in Melbourne, Australia, with suspected acute GBS. We performed extensive neurophysiology at three different time points. Novel tests, including cutaneous silent periods, long latency reflexes, and contraction-induced H reflexes, were assessed.

RESULTS

Twenty-three participants were studied, including 13 cases of acute GBS. In total, 69% of acute cases of GBS were accurately diagnosed on the first nerve conduction study using published neurophysiologic criteria, with serial studies rarely altering the GBS subtype classification. Antidromic and orthodromic upper limb sensory studies were diagnostically equivalent. A sural sparing pattern was seen in 77% of cases of GBS at the first test. Long latency reflexes and contraction-induced H reflexes testing were abnormal in most participants but were limited by muscle weakness in some. Cutaneous silent periods testing was unobtainable in approximately 50% of cases because of weakness and did not discriminate from mimic disorders.

CONCLUSIONS

Abnormalities of long latency reflexes and contraction-induced H reflexes may be helpful where initial electrophysiology is nondiagnostic but are nonspecific. Cutaneous silent periods testing seems of limited value. Comprehensive testing provides diagnostic certainty in most cases of GBS from the very first study.

Long Latency Reflexes in Clinical Neurology: A Systematic Review

BACKGROUND

Long latency reflexes (LLRs) are impaired in a wide array of clinical conditions. We aimed to illustrate the clinical applications and recent advances of LLR in various neurological disorders from a systematic review of published literature.

METHODS

We reviewed the literature using appropriately chosen MeSH terms on the database platforms of MEDLINE, Web of Sciences, and Google Scholar for all the articles from 1st January 1975 to 2nd February 2021 using the search terms "long loop reflex", "long latency reflex" and "C-reflex". The included articles were analyzed and reported using synthesis without meta-analysis (SWiM) guidelines.

RESULTS

Based on our selection criteria, 40 articles were selected for the systematic review. The various diseases included parkinsonian syndromes (11 studies, 217 patients), Huntington's disease (10 studies, 209 patients), myoclonus of varied etiologies (13 studies, 127 patients) including progressive myoclonic epilepsy (5 studies, 63 patients) and multiple sclerosis (6 studies, 200 patients). Patients with parkinsonian syndromes showed large amplitude LLR II response. Enlarged LLR II was also found in myoclonus of various etiologies. LLR II response was delayed or absent in Huntington's disease. Delayed LLR II response was present in multiple sclerosis. Among the other diseases, LLR response varied according to the location of cerebellar lesions while the results were equivocal in patients with essential tremor.

CONCLUSIONS

Abnormal LLR is observed in many neurological disorders. However, larger systematic studies are required in many neurological disorders in order to establish its role in diagnosis and management.

The Electrophysiological Findings in Spinocerebellar Ataxia Type 6: Evidence From 24 Patients

PURPOSE

Peripheral neuropathy has been reported commonly in several spinocerebellar ataxia (SCA) types. To date, there is a lack of robust evidence for neuropathy or neuronopathy in SCA type 6 (SCA6). Here, we aim to evaluate the presence of neuropathy or neuronopathy in a cohort of SCA6 patients.

METHODS

Twenty-four individuals with genetically confirmed SCA6 underwent detailed neurophysiological assessment. This included nerve conduction studies, and in some, cutaneous silent periods, blink reflexes, tilt table tests, quantitative sudomotor axon reflex tests, and somatosensory (median and tibial) evoked potentials.

RESULTS

Mean age was 56.1 years (range, 22-94 years) at the time of testing. Four patients were presymptomatic of SCA6 at recruitment. The mean disease duration of symptomatic patients was 11.9 years (range, 1-40 years). Most patients (79.2%, 19/24) had no neurophysiological evidence of a peripheral neuropathy. One with impaired glucose tolerance had mild, large, and small fiber sensorimotor polyneuropathy. One elderly patient had length-dependent axonal sensorimotor polyneuropathy. Two had minor sensory abnormalities (one had type II diabetes and previous chemotherapy). One other had minor small fiber abnormalities. Ten patients (41.7%) had median neuropathies at the wrist. All somatosensory evoked potential (15/15), and most autonomic function tests (13/14) were normal.

CONCLUSIONS

A large proportion of subjects (79.2%) in our cohort had no evidence of large or small fiber neuropathy. This study does not support the presence of neuropathy or neuronopathy as a common finding in SCA6 and confirms the importance of considering comorbidities as the cause of neurophysiological abnormalities.

Rating scales and biomarkers for CAG-repeat spinocerebellar ataxias: Implications for therapy development

Spinocerebellar ataxias (SCAs) are a group of dominantly-inherited cerebellar ataxias, among which CAG expansion-related SCAs are most common. These diseases have very high penetrance with defined disease progression, and emerging therapies are being developed to provide either symptomatic or disease-modifying benefits. In clinical trial design, it is crucial to incorporate biomarkers to test target engagement or track disease progression in response to therapies, especially in rare diseases such as SCAs. In this article, we review the available rating scales and recent advances of biomarkers in CAG-repeat SCAs. We divided biomarkers into neuroimaging, body fluid, and physiological studies. Understanding the utility of each biomarker will facilitate the design of robust clinical trials to advance therapies for SCAs.

Transcranial magnetic stimulation in hereditary ataxias: Diagnostic utility, pathophysiological insight and treatment

Transcranial magnetic stimulation (TMS) is a valuable technique to assess and modulate human brain function in normal and pathological conditions. This critical review surveys the contributions of TMS to the diagnosis, insight into pathophysiology and treatment of genetically confirmed hereditary ataxias, a heterogeneous group of neurodegenerative disorders that can affect motor cortex and the corticospinal tract. Most studies were conducted on small sample sizes and focused on diagnostic approaches. The available data demonstrate early involvement of the corticospinal tract and motor cortex circuitry, and support the possible efficacy of cerebellar repetitive TMS (rTMS) as therapeutic approach. Further TMS-based studies are warranted, to establish biomarkers for early diagnosis and disease monitoring, explore the involvement of the cerebello-dentato-thalamo-cortical projection, study the effects of rTMS-induced plasticity, and utilize rTMS for treatment.

Predictive modeling by the cerebellum improves proprioception

Because sensation is delayed, real-time movement control requires not just sensing, but also predicting limb position, a function hypothesized for the cerebellum. Such cerebellar predictions could contribute to perception of limb position (i.e., proprioception), particularly when a person actively moves the limb. Here we show that human cerebellar patients have proprioceptive deficits compared with controls during active movement, but not when the arm is moved passively. Furthermore, when healthy subjects move in a force field with unpredictable dynamics, they have active proprioceptive deficits similar to cerebellar patients. Therefore, muscle activity alone is likely insufficient to enhance proprioception and predictability (i.e., an internal model of the body and environment) is important for active movement to benefit proprioception. We conclude that cerebellar patients have an active proprioceptive deficit consistent with disrupted movement prediction rather than an inability to generally enhance peripheral proprioceptive signals during action and suggest that active proprioceptive deficits should be considered a fundamental cerebellar impairment of clinical importance.

Cerebellar damage diminishes long-latency responses to multijoint perturbations

Damage to the cerebellum can cause significant problems in the coordination of voluntary arm movements. One prominent idea is that incoordination stems from an inability to predictively account for the complex mechanical interactions between the arm's several joints. Motivated by growing evidence that corrective feedback control shares important capabilities and neural substrates with feedforward control, we asked whether cerebellar damage impacts feedback stabilization of the multijoint arm appropriate for the arm's intersegmental dynamics. Specifically, we tested whether cerebellar dysfunction impacts the ability of posterior deltoid to incorporate elbow motion in its long-latency response (R2 = 45-75 ms and R3 = 75-100 ms after perturbation) to an unexpected torque perturbation. Healthy and cerebellar-damaged subjects were exposed to a selected pattern of shoulder-elbow displacements to probe the response pattern from this shoulder extensor muscle. The healthy elderly subjects expressed a long-latency response linked to both shoulder and elbow motion, including an increase/decrease in shoulder extensor activity with elbow flexion/extension. Critically, cerebellar-damaged subjects displayed the normal pattern of activity in the R3 period indicating an intact ability to rapidly integrate multijoint motion appropriate to the arm's intersegmental dynamics. However, cerebellar-damaged subjects had a lower magnitude of activity that was specific to the long-latency period (both R2 and R3) and a slightly delayed onset of multijoint sensitivity. Taken together, our results suggest that the basic motor pattern of the long-latency response is housed outside the cerebellum and is scaled by processes within the cerebellum.

Central motor conduction time

Central motor conduction time (CMCT) is the time taken for neural impulses to travel through the central nervous system on their way to the target muscles. When the motor cortex is stimulated with transcranial magnetic stimulation (TMS), CMCT is calculated by subtracting the peripheral conduction time from the motor evoked potential latency elicited by motor cortical TMS. CMCT in infants and children reaches adult level at about age of 6 years for the lower limbs. The alterations of CMCT in various neurological conditions are reviewed in this chapter. Prolongation of CMCT occurs due to slowing of conduction through rapidly conducting corticospinal fibers, as seen in various disorders such as demyelinating diseases (multiple sclerosis, MS), amyotrophic lateral sclerosis, structural lesions in the corticospinal tract such as stroke and compressive myelopathy, and neurodegenerative disorders including multiple system atrophy and progressive supranuclear palsy. As CMCT is prolonged in certain clinical conditions, it is of diagnostic value in some neurological disorders such as myelopathy, amyotrophic lateral sclerosis, and MS when used together with other clinical and electrophysiological measures. It could also be used as a prognostic marker in some of neurological conditions, such as myelopathy and MS.

Prolonged cortical silent period but normal sensorimotor plasticity in spinocerebellar ataxia 6

Spinocerebellar ataxia 6 (SCA6) is a hereditary disease characterized by a trinucleotide repeat expansion in the CACNA1A gene and late-onset bilateral cerebellar atrophy. It is unclear if there is significant pathology outside of the cerebellum. We used transcranial magnetic stimulation to assess sensorimotor cortical circuits and cortical plasticity in 8 SCA6 patients and 8 age-matched controls. Behavioral performance was assessed using a rhythmic tapping task. Neurophysiological measures of SCA6 patients showed a prolonged cortical silent period (CSP) but normal MEP recruitment curve, short-latency afferent inhibition, long-latency afferent inhibition and ipsilateral silent period. Paired-associative stimulation induction also increased motor-evoked potentials normally. SCA6 patients had greater variability with cued rhythmic tapping than normals and deteriorated when the cue was removed; in comparison, normal subjects had similar variability between cued and uncued rhythmic tapping. Analysis using a Wing-Kristofferson timing model indicated that both clock variance and motor delay variance were abnormal. Conclusion. In SCA6, the circuits for sensorimotor integration and the mechanisms for LTP-like plasticity in the sensorimotor cortex are unimpaired. A prolonged CSP in SCA6 just like in other cerebellar atrophies would suggest that this neurophysiological change typifies cerebellar dysfunction.

Regional patterns of cerebral glucose metabolism in spinocerebellar ataxia type 2, 3 and 6 : a voxel-based FDG-positron emission tomography analysis

The purpose of this study was to investigate the regional patterns of cerebral metabolic deficits by voxel-based FDGPET analysis in patients with distinct spinocerebellar ataxia (SCA) genotypes, including SCA type 2 (SCA2), SCA3, and SCA6. Nine patients with SCA2, 12 with SCA3, seven with SCA6, and 23 healthy control subjects were recruited. The clinical severity of the patients' cerebellar ataxia was evaluated according to the International Cooperative Ataxia Rating Scale. The brain glucose metabolism was evaluated with 2- [fluorine 18]-fluoro-2-deoxy-D: -glucose (FDG) positron emission tomography. Group data were analyzed and compared by voxelbased analysis. In SCA2, FDG utilization was significantly reduced in the cerebellum, pons, parahippocampal gyrus and frontal cortex. In SCA3, FDG metabolism in the cerebellum, parahippocampal gyrus of the limbic system, and lentiform nucleus was decreased. In SCA6, FDG metabolism was diminished in the cerebellum and the frontal and prefrontal cortices. On group comparisons, while all SCAs have impaired cerebellar functions, the cerebellar FDG metabolism was most severely compromised in SCA2. Instead, the FDG metabolism in the lentiform nucleus and medulla was characteristically worst in SCA3. There was no brainstem involvement in SCA6.

Altered soleus responses to magnetic stimulation in pure cerebellar ataxia

OBJECTIVE

Transcranial magnetic stimulation (TMS) over the leg motor area elicits a soleus primary response (SPR) and a soleus late response (SLR). We evaluated the influence of the cerebellofugal pathway on the SPR and SLR in patients with 'pure' cerebellar ataxia.

METHODS

SPRs and SLRs were recorded from 11 healthy subjects and 9 patients with 'pure' cerebellar cortical degeneration; 5 with spinocerebellar ataxia type 6 (SCA6), and 4 with late cortical cerebellar ataxia (LCCA). In addition, three patients with localized cerebellar lesions were tested.

RESULTS

The SPR latency was significantly longer in patients than in controls, but primary responses in the tibialis anterior muscle were normal. The frequency of abnormal SLR was 38.9% in the supine position and 83.3% in the standing position. Two out of three patients with localized cerebellar lesions also showed abnormal SLR.

CONCLUSIONS

Altered SPRs in patients may result from a dysfunction of the primary motor cortex caused by crossed cerebello-cerebral diaschisis. In addition, our results suggest that 'pure' cerebellar degeneration involves the mechanism responsible for evoking SLR which is related to the control of posture.

SIGNIFICANCE

SLR can be a useful neurophysiological parameter for evaluating cerebellofugal function.

Dissociation of grey and white matter reduction in spinocerebellar ataxia type 3 and 6: A voxel-based morphometry study

The aim of this study was to examine the different patterns of cerebellar and/or brainstem atrophy in spinocerebellar ataxia (SCA) type 3 and 6. Eighteen patients (SCA3 n=9, SCA6 n=9) and 15 healthy volunteers were studied. Voxel-based morphometry (VBM) was applied to segmented grey matter (GM) and white matter (WM) of high-resolution T1-weighted brain volumes of each group. We found reduction of grey matter in the pons as well as in the vermis in SCA3 as compared to control subjects. In SCA6 significant grey matter loss was found in hemispheric lobules bilaterally as well as in the vermis. White matter analysis revealed significant changes in SCA3, especially in the pons, in the white matter surrounding the dentate nucleus (DN) and in the cerebellar peduncles, whereas no significant white matter reduction was found in SCA6 patients. Our results demonstrate different patterns of grey and white matter affection detected by magnetic resonance imaging (MRI) in SCA3 and SCA6 patients, confirming the pathological concept of cortical cerebellar atrophy in SCA6. In contrast, SCA3 represents a form of ponto-cerebellar atrophy with predominant affection of pontine nuclei and fibre tracts.

Prolonged intracortical delay of long-latency reflexes: electrophysiological evidence for a cortical dysfunction in multiple sclerosis

Convincing evidence suggests that long-latency reflexes (LLRs) are capable of testing the transcortical sensorimotor reflex arch. By subtracting the sum of the latencies of N20 (afferent branch) and transcranially elicited motor evoked potentials (MEP; efferent branch) from the LLR II latency, the cortical relay time (CRT) can also be obtained, which is alleged to represent the time required for the cortical sensorimotor integration. The aim of the present study was to investigate if a cortical dysfunction occurs in multiple sclerosis (MS). Median nerve somatosensory evoked potentials (SEPs), MEPs and LLRs were recorded from the upper limbs of 23, not severely disabled MS patients in acute phases of the disease. Eighteen age and sex matched healthy volunteers served as controls. N20, MEP, LLR II latencies were measured, and the CRT was calculated for each limb. The statistical comparison between patients and controls was only weakly significant by taking into account conduction times along either the afferent (N20) or the efferent (MEP) pathways. On the contrary, it turned out to be considerably significant if both branches of the transcortical sensorimotor reflex arch, together with the intracortical pathway, were simultaneously tested by means of the LLRs. Moreover, the patients showed a significantly higher CRT compared with that found in the control subjects. These findings are consistent with a prolonged intracortical delay of LLRs in the MS group and suggest the occurrence of conduction velocity slowing and/or synaptic transmission impairment along the sensorimotor intracortical pathway in MS.

Triple stimulation technique in patients with spinocerebellar ataxia type 6

OBJECTIVE

To establish further evidence that SCA6 may not be a pure cerebellar syndrome.

METHODS

Seven patients with genetically confirmed SCA6 and 9 age-matched normal controls were studied. Recordings of the CMAP were obtained from the right first dorsal interosseus muscle. Transcranial magnetic stimulation of the left motor cortex was applied to the contralateral scalp with a plane figure-of-8 coil. Conventional transcranial magnetic stimulation (TMS), central motor conduction time (CMCT) by F-wave method and the triple stimulation technique (TST) amplitude ratio (TST test/TST control) were investigated.

RESULTS

The mean resting motor threshold and mean CMCT did not show significant differences between normal controls and patients, but the mean TST amplitude ratio was significantly smaller in patients than in controls.

CONCLUSIONS

An abnormal TST represents upper motor neuron loss, central axon lesions or conduction blocks, or inexcitability in response to TMS. The lack of pathological changes in the corticospinal tract of patients with SCA6 indicates that this abnormality may be caused by crossed cerebellar diaschisis, or a functional disorder in the brain resulting from CACNA1A mutations.

SIGNIFICANCE

TST is a useful method for quantifying corticospinal tract dysfunction.

Identification of quantitative trait loci controlling cortical motor evoked potentials in experimental autoimmune encephalomyelitis: correlation with incidence, onset and severity of disease

Experimental autoimmune encephalomyelitis (EAE) is a polygenic chronic inflammatory demyelinating disease of the nervous system, commonly used as an animal model of multiple sclerosis. Previous studies have identified multiple quantitative trait loci (QTLs) controlling different aspects of disease pathogenesis. However, direct genetic control of cortical motor evoked potentials (cMEPs) as a straightforward measure of extent of demyelination or synaptic block has not been investigated earlier. Here, we examined the genetic control of different traits of EAE in a F2 intercross population generated from the EAE susceptible SJL/J (SJL) and the EAE resistant C57BL/10.S (B10.S) mouse strains involving 400 animals. The genotypes of 150 microsatellite markers were determined in each animal and correlated to phenotypic data of onset and severity of disease, cell infiltration and cMEPs. Nine QTLs were identified. Three sex-linked QTLs mapped to chromosomes 2, 10 and 18 linked to disease severity in females, whereas QTLs on chromosomes 1, 8 and 15 linked to the latency of the cMEPs. QTLs affecting T-lymphocyte, B-lymphocyte and microglia infiltration mapped on chromosomes 8 and 15. The cMEP-associated QTLs correlated with incidence, onset or severity of disease, e.g. QTL on chromosome 8, 32-48 cM (EAE 31) (LOD 6.9, P<0.001), associated to cMEP latencies in non-immunized mice and correlated with disease onset and EAE 32 on chromosome 15 linked to cMEP latencies 15 days post-immunization and correlated with disease severity. Additionally, applying tissue microarray technology, we identified QTLs associated to microglia and lymphocytes infiltration on chromosomes 8 and 15, which are different from the QTLs controlling cMEP latencies. There were no alterations in the morphological appearance of the myelin sheaths. Our findings suggest a possible role of myelin composition and/or synaptic transmission in susceptibility to EAE.

Prolonged central motor conduction time of lower limb muscle in spinocerebellar ataxia 6

We investigated the function of corticospinal tract in spinocerebellar ataxia 6 (SCA6) by measuring the central motor conduction time (CMCT). Motor evoked potentials (MEP) of tibialis anterior (TA) muscle were elicited by magnetic stimulation to motor cortex and spinal cord in 9 SCA6 patients and 10 normal height- and age-matched subjects. CMCT in lower limb of SCA6 patients (18.1+/-1.9 ms) was significantly prolonged than that of the normal subjects (15.0+/-1.0 ms) ((p < 0.001). The prolonged CMCT was well correlated with the duration of disease (p = 0.005), but MEP amplitudes and stimulation intensities were not significantly different. These results indicate that the corticospinal tract function is also impaired and correlate with the disease duration in SCA6.

Long-latency reflexes in patients with Behçet's disease

OBJECTIVE

Recent studies demonstrate that the subclinical involvement of motor pathways is frequently observed in patients with Behçet's disease (BD). Long-latency reflexes (LLR) provide information about the continuity of both ascending and descending neural pathways. Our aim was to evaluate the utility of LLR and somatosensory-evoked potentials (SEP) in demonstrating subclinical neural involvement in patients with BD.

METHODS

Twenty-nine patients with BD were studied by means of SEP and LLR. Bilateral median nerve SEPs and LLRs evoked by electrical stimulation of both median nerves were recorded. The latency of second component of LLR (LLR2), the duration of LLR2-HR (Hoffmann reflex, spinal reflex component of LLR) interval, peak to peak amplitude of LLR2 and the amplitude ratio of LLR2/HR were analyzed. The data obtained from patients were compared with those of 20 control subjects.

RESULTS

LLR2 latencies and the durations of LLR2-HR interval were significantly prolonged in patients with BD (p=0.001 for both parameters). Increased duration of LLR2-HR interval was the most frequent abnormality observed in the study (37.9%).

CONCLUSION

Our findings suggest that LLR is a useful technique to demonstrate subclinical neural involvement in patients with BD.