Long latency muscle responses in cerebellar diseases

Clinical neurophysiology of Parkinson's disease and parkinsonism

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This review is part of the series on the clinical neurophysiology of movement disorders and focuses on Parkinson’s disease and parkinsonism.

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The pathophysiology of cardinal parkinsonian motor symptoms and myoclonus are reviewed.

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The recordings from microelectrode and deep brain stimulation electrodes are reported in detail.

This review is part of the series on the clinical neurophysiology of movement disorders. It focuses on Parkinson’s disease and parkinsonism. The topics covered include the pathophysiology of tremor, rigidity and bradykinesia, balance and gait disturbance and myoclonus in Parkinson’s disease. The use of electroencephalography, electromyography, long latency reflexes, cutaneous silent period, studies of cortical excitability with single and paired transcranial magnetic stimulation, studies of plasticity, intraoperative microelectrode recordings and recording of local field potentials from deep brain stimulation, and electrocorticography are also reviewed. In addition to advancing knowledge of pathophysiology, neurophysiological studies can be useful in refining the diagnosis, localization of surgical targets, and help to develop novel therapies for Parkinson’s disease.

Tremors: Essential Tremor and Beyond

Tremor is a fairly common movement disorder presenting to an outpatient pediatric neurology practice. Tremors can be primary or secondary to underlying neurologic or systemic diseases. When assessing a child with tremor, it is paramount to evaluate the phenomenology of the tremor, determine the presence or absence of other neurologic signs and symptoms, and the possible modifying influence of medications. Proper classification is essential for specific diagnosis and prompt adequate management. Treatment considerations should take into account objective assessment of tremor severity and the degree of disability or impairment experienced by the child. Overall effectiveness of pharmacologic treatments of tremor is unfortunately disappointing. In this article we review the clinical examination, classification, and diagnosis of tremor. The pathophysiology of the different forms of tremor is outlined, and treatment options are discussed.

Long Loop Reflex 2 in Patients with Cortical Dementias: A Pilot Study

INTRODUCTION

Dementia is a major public health problem and it appears to be a global epidemic. The prevalence is doubling every 5 years and it is expected that 70% of persons above 60 years will live in developing countries by 2020 and 15% of them are likely to suffer from dementia. Disease modifying treatments work only if initiated very early; however, diagnostic tools are not always able to clearly differentiate the different types in very early stage. Therefore, inexpensive and easily available biomarkers are needed to know if collectively they will improve the sensitivity of specific diagnosis. Therefore, in this pilot study, we have tried to analyze if long loop reflex (LLR2) is differentially affected in these two conditions early in the course of Alzheimer's dementia (AD) and frontotemporal dementia (FTD) based on hypothesis taking into account the anatomical substrates involved.

PATIENTS AND METHODS

Mild cases of clinically probable AD and FTD after appropriate inclusion criteria were subjected for LLR testing in the upper limb at median nerve. The presence or absence of LLR was assessed and also the latency, amplitude, and duration assessed.

RESULTS AND CONCLUSION

LLR 2 is differentially affected in both these conditions. Absence of LLR2 was consistently seen in FTD which can be explained by early break down of frontal subcortical circuits in this condition as against AD. This is likely to serve as a very cheap and very early biomarker to differentiate the two common types of cortical dementias.

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.

Location-specific and task-dependent modulation of cutaneous reflexes in intrinsic human hand muscles

OBJECTIVE

The current study was designed to determine location-specificity in long latency cutaneous reflexes in intrinsic human hand muscles while performing a simple abduction and a manual task.

METHODS

Subjects comprised of 13 neurologically intact healthy volunteers. Cutaneous reflexes following non-noxious electrical stimulation to the digits of the hand (digit 1, D1; digit 2, D2; and digit 5, D5) were elicited while the subjects performed isolated isometric contraction of the abductor pollicis brevis (APB), first dorsal interosseous (FDI) and abductor digiti minimi muscles (ADM). The cutaneous reflexes were also elicited while the subjects performed a pincer grip with D1 and D2 while slightly lifting the hand from the supporting surface by abduction of D5 (manual task).

RESULTS

While performing isolated tonic voluntary contraction of the APB, FDI and ADM, the magnitude of E2 (peak latency approximately 60-90 ms) was larger when stimulation was delivered to the homotopic digit (e.g. APB response following D1 stimulation) than to the heterotopic nearby (e.g. APB response following D2 stimulation) or heterotopic distant digit (e.g. APB response following D5 stimulation). I2 ( approximately 90-120 ms) and E3 ( approximately 120-180 ms) were significantly larger following D5 stimulation than D1 or D2 stimulation in all muscles tested. The size of each component in the ADM following D1 and D2 stimulation did not increase even when the contraction level of the ADM increased. However, while performing the manual task, the E2 response in the ADM following both D1 and D2 stimulation was significantly increased as compared to that recorded during isolated D5 abduction.

CONCLUSIONS

Long latency cutaneous reflexes following non-noxious electrical stimulation are organized in a highly location-specific as well as task-dependent manner.

SIGNIFICANCE

Our findings provide further insight into the nature and functional significance of long latency cutaneous reflexes in human intrinsic hand muscles.

Rhythmic cortical myoclonus in a case of HIV-related encephalopathy

We describe a 66-year-old, HIV-seropositive patient presenting with ataxia and upper limb rhythmic myoclonus activated by postural maintenance. Electromyograph (EMG) recordings of the forearm muscles showed 50-msec bursts, with a frequency of 10 Hz, concurring with frontocentral electroencephalograph (EEG) rhythmic activity. Autoregressive spectral analysis applied to the EEG-EMG traces made it possible to detect significant coherence between the rhythmic EEG discharges and EMG bursts. The amplitude of the middle-latency somatosensory evoked potentials was increased. Long-latency reflexes were enhanced. On the basis of the electrophysiological findings, the movement disorder should be considered a rhythmic variant of cortical myoclonus. In our patient, HIV infection may have caused a dysfunction in the central nervous system pathways involving the cerebellum and sensorimotor cortex, similar to that occurring in genetically determined conditions characterised by cortical myoclonus.

Pathophysiology of nonparkinsonian tremors

Patients with nonparkinsonian tremors are the second largest group treated with functional neurosurgery. We summarize the present pathophysiological knowledge of these conditions. Essential tremor (ET) may be due to oscillations within the olivocerebellar circuit. There is experimental evidence from animal models for such a mechanism, and clinical data indicate an abnormal function of the cerebellum in ET. Cerebellar tremor may be closely related to the tremor seen in advanced ET. The malfunction of the cerebellum causes a pathological feed-forward control. Additionally an oscillator within the cerebellum or its input/output pathways may cause cerebellar tremor. Almost nothing is known about the pathophysiology of dystonic tremor. Holmes tremor is based on a nigral and a cerebellar malfunction and presents clinically as the combination of tremor in Parkinson's disease and cerebellar tremor. Neuropathic tremor can be extremely disabling and is thought to be due to an abnormal interaction of the disturbances within the periphery and abnormal cerebellar feedback. Unlike the case of Parkinson's disease, functional neurosurgery of nonparkinsonian tremors is not yet based on a solid pathophysiological background.

The pathophysiology of tremor

Tremor is defined as rhythmic oscillatory activity of body parts. Four physiological basic mechanisms for such oscillatory activity have been described: mechanical oscillations; oscillations based on reflexes; oscillations due to central neuronal pacemakers; and oscillations because of disturbed feedforward or feedback loops. New methodological approaches with animal models, positron emission tomography, and mathematical analysis of electromyographic and electroencephalographic signals have provided new insights into the mechanisms underlying specific forms of tremor. Physiological tremor is due to mechanical and central components. Psychogenic tremor is considered to depend on a clonus mechanism and is thus believed to be mediated by reflex mechanisms. Symptomatic palatal tremor is most likely due to rhythmic activity of the inferior olive, and there is much evidence that essential tremor is also generated within the olivocerebellar circuits. Orthostatic tremor is likely to originate in hitherto unidentified brainstem nuclei. Rest tremor of Parkinson's disease is probably generated in the basal ganglia loop, and dystonic tremor may also originate within the basal ganglia. Cerebellar tremor is at least in part caused by a disturbance of the cerebellar feedforward control of voluntary movements, and Holmes' tremor is due to the combination of the mechanisms producing parkinsonian and cerebellar tremor. Neuropathic tremor is believed to be caused by abnormally functioning reflex pathways and a wide variety of causes underlies toxic and drug-induced tremors. The understanding of the pathophysiology of tremor has made significant progress but many hypotheses are not yet based on sufficient data. Modern neurology needs to develop and test such hypotheses, because this is the only way to develop rational medical and surgical therapies.

The clinical use of brainstem reflexes and hand-muscle reflexes

Brainstem reflexes and hand-muscle reflexes can be elicited and recorded with routine EMG equipment. Not all these reflexes are useful in clinical neurology. But those that are - the subject of this review - exhibit distinct patterns of abnormality that have clinical diagnostic and localizing value in various diseases, including cranial neuropathies, focal lesions within the cervical cord, brainstem, and brain, movement disorders, and pain.

Changes in the motor pattern of learned and unlearned responses following cerebellar lesions: a kinematic analysis of the nictitating membrane reflex

Kinematic and dynamic analyses were employed to study the effects of cerebellar lesions on conditioned and unconditioned nictitating membrane responses in the rabbit. It was found that conditioned responses acquired to an auditory stimulus accelerated in two bursts as indicated by two distinct peaks of acceleration. The second peak of acceleration was very weak during the early portions of conditioning but became a prominent feature of the conditioned response over 16 sessions of conditioning. The second peak of acceleration in the conditioned response was more sensitive to cerebellar damage than was the first peak. When lesions of the cerebellum permanently reduced the amplitude of conditioned responses, but did not affect their frequency, the second peak of acceleration was nearly abolished while the first peak was unaffected. When cerebellar lesions profoundly impaired both the amplitude and frequency of conditioned responses, large and permanent impairments occurred in both peaks of acceleration. Lesions of the anterior interpositus nucleus most severely impaired both peaks of acceleration in the conditioned response and significantly reduced the acceleration of unconditioned responses across a wide range of intensities of corneal air puff. The deficit in the acceleration of unconditioned responses became manifest only after membrane extension exceeded 0.12 mm. The impairment in the amplitude of the unconditioned response after cerebellar lesions more closely approximated the impairment in the amplitude of the conditioned response when the force-generating properties of the conditioned and unconditioned stimuli were equated. It was hypothesized, therefore, that one reason why conditioned responses are so easily disrupted by cerebellar lesions is because they are of low force and not simply because they are learned. It was proposed that the two peaks of acceleration that characterize the conditioned response represent the function of two distinct anatomical systems. The first, a short-latency system, initiates the response and is most likely mediated by circuits that traverse the pontomedullary reticular formation. The second, a longer-latency system, amplifies response amplitude and its neural basis remains to be elucidated. The two components of the conditioned response may reflect two sequential bursts of activity in the accessory abducens nucleus, the principal site of the motoneurons for the retractor bulbi muscle, or may reflect the synergistic activity of the accessory abducens nucleus and the motor nuclei of the other extraocular muscles. It was concluded that the vulnerability of the second component of the conditioned response to cerebellar damage reflects an important role for the cerebellum in modulating the degree to which long-latency neural systems contribute to the ongoing performance of learned and unlearned behaviors.

Physiology and clinical applications of hand muscle reflexes

Hand muscle reflexes to stretch or electric stimulation of mixed nerves consist of two main components, the short latency reflex (SLR, M1) or Hoffmann reflex (HR) and the long latency reflex (LLR, M2). The SLR is most likely a spinal, monosynaptic reflex and all the evidence presently available supports a transcortical pathway of the LLR. Investigations in normal subjects demonstrate that the LLR is a reflex mediated by fast conducting muscle and cutaneous afferents. Group II muscle afferents do not significantly contribute to this reflex and it cannot be explained by repetitive excitation of spinal oligosynaptic pathways. These findings should not be uncritically generalized to other muscle groups, because the central and peripheral mechanisms apparently differ according to the body region and mode of stimulation. The LLR of hand muscles is most likely involved in skillful movements of the fingers. It is believed to assist rapid compensatory responses to unexpected disturbances. In addition to the main component of the LLR, which is called LLR II, the study of electrically elicited thenar reflexes following stimulation of the median nerve disclosed further LLR components, the LLR I and the LLR III. The latter reflexes are rarely seen in normal subjects but have a significance in several diseases. Several abnormalities could be demonstrated in different diseases. Enhanced HR and reduced LLR are found in spasticity of various origin. Enhanced LLR I are frequently seen in Parkinson's disease, essential tremor and reflex myoclonus. Absent or reduced LLR II is found in Huntington's disease and in different focal brain lesions but not in symptomatic choreatic syndromes of other origin. Delayed latencies of the LLR II or absent LLR II have been described in multiple sclerosis. Enhanced LLR III may occur in cerebellar diseases. The method to elicit LLR of thenar muscles by electric stimulation may prove to be useful for clinical neurophysiology.

Central motor conduction in degenerative ataxic disorders: a magnetic stimulation study

Central motor conduction to small hand muscles was measured using magnetic stimulation of the motor cortex and electrical stimulation of proximal motor roots in 11 patients with Friedreich's ataxia, 10 patients with early onset cerebellar ataxia with retained tendon reflexes (EOCA) and 13 patients with late onset degenerative cerebellar disease (LOCD). Central motor conduction was abnormal in 91% with Friedreich's ataxia, 70% with EOCA and 38% with LOCD. Central motor conduction abnormalities were not specific to individual disorders but were more severe and were related to disease duration in Friedreich's ataxia and EOCA.