Short latency afferent inhibition: a biomarker for mild cognitive impairment in Parkinson's disease?

BACKGROUND

Mild cognitive impairment in Parkinson's disease (PD) is common and predicts those at risk of dementia. Cholinergic dysfunction may contribute to its pathophysiology and can be assessed using short latency afferent inhibition.

METHODS

Twenty-two patients with PD (11 cognitively normal; 11 with mild cognitive impairment) and 22 controls participated. Short latency afferent inhibition was measured by conditioning motor evoked potentials, which were elicited by transcranial magnetic stimulation of the motor cortex with electrical stimuli delivered to the contralateral median nerve at varying interstimulus intervals.

RESULTS

There was no significant difference between cognitively normal PD and controls for short latency afferent inhibition (62.8±30.3% vs. 55.7±21.7%; P=0.447). The PD-mild cognitive impairment group had significantly less inhibition (88.4±25.8%) than both cognitively normal PD (P=0.021) and controls (P=0.01).

CONCLUSIONS

Cholinergic dysfunction occurs early in those with PD-mild cognitive impairment. Short latency afferent inhibition may be a useful biomarker of increased risk of dementia in PD patients. © 2013 Movement Disorder Society.

Cholinergic dysfunction contributes to gait disturbance in early Parkinson's disease

Gait disturbance is an early feature in Parkinson's disease. Its pathophysiology is poorly understood; however, cholinergic dysfunction may be a non-dopaminergic contributor to gait. Short-latency afferent inhibition is a surrogate measure of cholinergic activity, allowing the contribution of cholinergic dysfunction to gait to be evaluated. We hypothesized that short-latency afferent inhibition would be an independent predictor of gait dysfunction in early Parkinson's disease. Twenty-two participants with Parkinson's disease and 22 age-matched control subjects took part in the study. Gait was measured objectively using an instrumented walkway (GAITRite), and subjects were asked to walk at their preferred speed for 2 min around a 25-m circuit. Spatiotemporal characteristics (speed, stride length, stride time and step width) and gait dynamics (variability described as the within subject standard deviation of: speed, stride time, stride length and step width) were determined. Short-latency afferent inhibition was measured by conditioning motor evoked potentials, elicited by transcranial magnetic stimulation of the motor cortex, with electrical stimuli delivered to the contralateral median nerve at intervals ranging from N20 (predetermined) to N20 + 4 ms. Short-latency afferent inhibition was determined as the percentage difference between test and conditioned response for all intervals and was described as the group mean. Participants were optimally medicated at the time of testing. Participants with Parkinson's disease had significantly reduced gait speed (P = 0.002), stride length (P = 0.008) and stride time standard deviation (P = 0.001). Short-latency afferent inhibition was also significantly reduced in participants with Parkinson's disease (P = 0.004). In participants with Parkinson's disease, but not control subjects, significant associations were found between gait speed, short-latency afferent inhibition, age and postural instability and gait disorder score (Movement Disorders Society Unified Parkinson's Disease Rating Scale) and attention, whereas global cognition and depression were marginally significant. No other gait variables were associated with short-latency afferent inhibition. A multiple hierarchical regression model explored the contribution of short-latency afferent inhibition to gait speed, controlling for age, posture and gait symptoms (Postural Instability and Gait Disorder score-Movement Disorders Society Unified Parkinson's Disease Rating Scale), attention and depression. Regression analysis in participants with Parkinson's disease showed that reduced short-latency afferent inhibition was an independent predictor of slower gait speed, explaining 37% of variability. The final model explained 72% of variability in gait speed with only short-latency afferent inhibition and attention emerging as independent determinants. The results suggest that cholinergic dysfunction may be an important and early contributor to gait dysfunction in Parkinson's disease. The findings also point to the contribution of non-motor mechanisms to gait dysfunction. Our study provides new insights into underlying mechanisms of non-dopaminergic gait dysfunction, and may help to direct future therapeutic approaches.

Beta-band intermuscular coherence: a novel biomarker of upper motor neuron dysfunction in motor neuron disease

In motor neuron disease, the focus of therapy is to prevent or slow neuronal degeneration with neuroprotective pharmacological agents; early diagnosis and treatment are thus essential. Incorporation of needle electromyographic evidence of lower motor neuron degeneration into diagnostic criteria has undoubtedly advanced diagnosis, but even earlier diagnosis might be possible by including tests of subclinical upper motor neuron disease. We hypothesized that beta-band (15–30 Hz) intermuscular coherence could be used as an electrophysiological marker of upper motor neuron integrity in such patients. We measured intermuscular coherence in eight patients who conformed to established diagnostic criteria for primary lateral sclerosis and six patients with progressive muscular atrophy, together with 16 age-matched controls. In the primary lateral sclerosis variant of motor neuron disease, there is selective destruction of motor cortical layer V pyramidal neurons and degeneration of the corticospinal tract, without involvement of anterior horn cells. In progressive muscular atrophy, there is selective degeneration of anterior horn cells but a normal corticospinal tract. All patients with primary lateral sclerosis had abnormal motor-evoked potentials as assessed using transcranial magnetic stimulation, whereas these were similar to controls in progressive muscular atrophy. Upper and lower limb intermuscular coherence was measured during a precision grip and an ankle dorsiflexion task, respectively. Significant beta-band coherence was observed in all control subjects and all patients with progressive muscular atrophy tested, but not in the patients with primary lateral sclerosis. We conclude that intermuscular coherence in the 15–30 Hz range is dependent on an intact corticospinal tract but persists in the face of selective anterior horn cell destruction. Based on the distributions of coherence values measured from patients with primary lateral sclerosis and control subjects, we estimated the likelihood that a given measurement reflects corticospinal tract degeneration. Therefore, intermuscular coherence has potential as a quantitative test of subclinical upper motor neuron involvement in motor neuron disease.

Contributions of descending and ascending pathways to corticomuscular coherence in humans

Non-technical summary

Neural activity in parts of the cerebral cortex related to movement oscillates at frequencies around 20 Hz. These oscillations are correlated with similar rhythms in contracting muscles on the opposite side of the body. In this work, we used an analysis method called directed coherence to investigate the direction of oscillatory coupling. We find that oscillations travel not only from cortex to muscle (as expected for a motor command), but also back from muscle to cortex (reflecting sensory input). This oscillatory loop may allow the cortex to measure features of the limb state, integrating sensory inflow with the motor command.

Abstract

Corticomuscular coherence in the beta frequency band (15–30 Hz) has been demonstrated in both humans and monkeys, but its origin and functional role are still unclear. Phase–frequency plots produced by traditional coherence analysis are often complex. Some subjects show a clear linear phase–frequency relationship (indicative of a fixed delay) but give shorter delays than expected; others show a constant phase across frequencies. Recent evidence suggests that oscillations may be travelling around a peripheral sensorimotor loop. We recorded sensorimotor EEGs and EMGs from three intrinsic hand muscles in human subjects performing a precision grip task, and applied directed coherence (Granger causality) analysis to explore this system. Directed coherence was significant in both descending (EEG→EMG) and ascending (EMG→EEG) directions at beta frequencies. Average phase delays of 26.4 ms for the EEG→EMG direction and 29.5 ms for the EMG→EEG direction were closer to the expected conduction times for these pathways than the average delays estimated from coherence phase (7.9 ms). Subjects were sub-divided into different groups, based on the sign of the slope of the linear relation between corticomuscular coherence phase and frequency (positive, negative or zero). Analysis separated by these groups suggested that different relative magnitudes of EEG→EMG and EMG→EEG directed coherence might underlie the observed inter-individual differences in coherence phase. These results confirm the complex nature of corticomuscular coherence with contributions from both descending and ascending pathways.

Multi-system neurological disease is common in patients with OPA1 mutations

Additional neurological features have recently been described in seven families transmitting pathogenic mutations in OPA1, the most common cause of autosomal dominant optic atrophy. However, the frequency of these syndromal 'dominant optic atrophy plus' variants and the extent of neurological involvement have not been established. In this large multi-centre study of 104 patients from 45 independent families, including 60 new cases, we show that extra-ocular neurological complications are common in OPA1 disease, and affect up to 20% of all mutational carriers. Bilateral sensorineural deafness beginning in late childhood and early adulthood was a prominent manifestation, followed by a combination of ataxia, myopathy, peripheral neuropathy and progressive external ophthalmoplegia from the third decade of life onwards. We also identified novel clinical presentations with spastic paraparesis mimicking hereditary spastic paraplegia, and a multiple sclerosis-like illness. In contrast to initial reports, multi-system neurological disease was associated with all mutational subtypes, although there was an increased risk with missense mutations [odds ratio = 3.06, 95% confidence interval = 1.44-6.49; P = 0.0027], and mutations located within the guanosine triphosphate-ase region (odds ratio = 2.29, 95% confidence interval = 1.08-4.82; P = 0.0271). Histochemical and molecular characterization of skeletal muscle biopsies revealed the presence of cytochrome c oxidase-deficient fibres and multiple mitochondrial DNA deletions in the majority of patients harbouring OPA1 mutations, even in those with isolated optic nerve involvement. However, the cytochrome c oxidase-deficient load was over four times higher in the dominant optic atrophy + group compared to the pure optic neuropathy group, implicating a causal role for these secondary mitochondrial DNA defects in disease pathophysiology. Individuals with dominant optic atrophy plus phenotypes also had significantly worse visual outcomes, and careful surveillance is therefore mandatory to optimize the detection and management of neurological disability in a group of patients who already have significant visual impairment.

Corticospinal activation confounds cerebellar effects of posterior fossa stimuli

Objective

To investigate the efficacy of magnetic stimulation over the posterior fossa (PF) as a non-invasive assessment of cerebellar function in man.

Methods

We replicated a previously reported conditioning-test paradigm in 11 healthy subjects. Transcranial magnetic stimulation (TMS) at varying intensities was applied to the PF and motor cortex with a 3, 5 or 7 ms interstimulus interval (ISI), chosen randomly for each trial. Surface electromyogram (EMG) activity was recorded from two intrinsic hand muscles and two forearm muscles. Responses were averaged and rectified, and MEP amplitudes were compared to assess whether suppression of the motor output occurred as a result of the PF conditioning pulse.

Results

Cortical MEPs were suppressed following conditioning-test ISIs of 5 or 7 ms. No suppression occurred with an ISI of 3 ms. PF stimuli alone also produced EMG responses, suggesting direct activation of the corticospinal tract (CST).

Conclusions

CST collaterals are known to contact cortical inhibitory interneurones; antidromic CST activation could therefore contribute to the observed suppression of cortical MEPs.

Significance

PF stimulation probably activates multiple pathways; even at low intensities it should not be regarded as a selective assessment of cerebellar function unless stringent controls can confirm the absence of confounding activity in other pathways.

Stiff person syndrome

Stiff person syndrome (SPS), stiff limb syndrome, jerking SPS and progressive encephalomyelitis with rigidity and myoclonus (PERM) are a family of rare, insidiously progressive diseases of the central nervous system. They all share the core clinical features of appendicular and axial rigidity caused by continuous involuntary motor unit activity, and superimposed stimulus-sensitive spasms. There is good evidence for a primary auto-immune aetiology. Anti-glutamic acid decarboxylase (anti-GAD) antibodies, specifically to the GAD65 isoform, are present in serum or cerebrospinal fluid of 60-80% of patients with SPS and its variants. A paraneoplastic form of SPS is recognized in about 5%, associated with a different profile of auto-antibodies. Repeated intravenous immunoglobulin is the mainstay of disease-modifying therapy in SPS. Rigidity and spasms may be treated symptomatically with benzodiazepines, baclofen, tiagabine and levetiracetam. After an initial progressive phase, patients with SPS generally stabilize over a period of months to years. However, 10% will require prolongedadmission to intensive care at some stage during the disease. Sudden death has been reported in asmany as 10% of patients because of unexplained metabolic acidosis or autonomic crises. The prognosis in paraneoplastic SPS, jerking SPS and PERM, in terms of mortality, is generally worse than in primary SPS.

Muscle responses to transcranial stimulation in man depend on background oscillatory activity

Muscle responses to transcranial stimulation show high sweep-to-sweep variability, which may reflect an underlying noise process in the motor system. We examined whether response amplitude correlated with the level of prestimulus background EMG, and network oscillations. Transcranial magnetic or electrical stimulation was delivered to primary motor cortex whilst human subjects performed a precision grip task known to promote beta-band ( approximately 20 Hz) cortical oscillations. Responses were recorded from two intrinsic hand muscles. Response magnitude correlated significantly with the level of background EMG (mean r(2) = 0.20). Using a novel wavelet method, we quantified the amplitude and phase of oscillations in prestimulus sensorimotor EEG. Surprisingly, response magnitude showed no significant correlation with EEG oscillations at any frequency. However, oscillations in the prestimulus EMG were significantly correlated with response size; the correlation coefficient had peaks around 20 Hz. When oscillations in one muscle were used to predict response amplitude in a different muscle, correlations were substantially smaller. Finally, for each recording, we calculated the best possible prediction of response size obtainable from up to 20 measures of prestimulus EEG and EMG oscillations. Such optimal predictions had low correlation coefficients (mean r(2) = 0.2; 76% were below 0.3). We conclude that prestimulus oscillations, mainly in the beta-band, do explain some of the variability in responses to transcranial stimulation. Oscillations may likewise increase the noise of natural motor processing, explaining why this form of network activity is usually suppressed prior to dynamic movements. However, the majority of the variation is determined by other factors, which are not accessible by noninvasive recordings.

Non-uniform olivocerebellar conduction time in the vermis of the rat cerebellum

It has been proposed that the conduction velocities of cerebellar climbing fibre (olivocerebellar) axons are tuned according to length, in order to precisely fix the conduction time between the inferior olive and cerebellar cortex. Some data conflict with this view. We have re-evaluated this issue using the climbing fibre reflex. The white matter of the tip of one folium in lobule VI or VII was stimulated electrically 0.5-1 mm below the surface and recordings were made from Purkinje cells in lobules VIII and IX. Reflex evoked climbing fibre (CF) responses (33 units) were recorded at different depths from Purkinje cells found in a narrow sagittal zone of cortex as complex spikes. The responses had latencies ranging from 4.3 ms to 11.3 ms. A consistent trend was that Purkinje cell responses recorded at greater depth had shorter CF reflex latencies than those recorded more superficially, both in individual experiments and in grouped data. These data show that the CF reflex latency is not constant, but is directly proportional to the distance an action potential has to travel along a CF. These data are not consistent with tuning of CF conduction velocities to normalize olivocerebellar conduction time, but are consistent with a CF conduction velocity in the cortex of approximately 0.6 m s-1. This suggests that climbing fibres projecting to different parts of the cerebellar cortex may have differences in spike conduction time of a few milliseconds, and that submillisecond precision is not an important element of the climbing fibre signal.

Treatment of stiff person syndrome with rituximab

This case report is about the novel use of the anti-CD20 antibody, rituximab, in the treatment of a 41 year old woman with stiff person syndrome. She was admitted to hospital as an emergency with prolonged and painful extensor spasms affecting the neck and back, arms, and legs. The disease had progressed despite a favourable initial response to conventional treatment with intravenous immunoglobulin and cytotoxics. Treatment with rituximab induced a lasting clinical remission.

The effect of carbamazepine on human corticomuscular coherence

EEG recordings from motor cortex show oscillations at approximately 10 and 20 Hz. The 20-Hz oscillations are coherent with contralateral EMG; in most studies those at 10 Hz are not. However, significant 10-Hz coherence has recently been reported in a group of epileptic patients, all of whom were taking the anticonvulsant drug carbamazepine (CBZ). In a double blind study, we investigated the effects of CBZ on corticomuscular coherence in eight healthy human subjects (all male). Subjects performed a precision grip task against an auxotonic load, whilst left sensorimotor EEG and EMGs from five muscles in the right hand and forearm were recorded. CBZ (100 mg) or a placebo was then given orally, and 6 h later subjects were re-tested. One week separated CBZ and placebo experiments in each subject. Coherence averaged across subjects and muscles during the hold phase of the task was maximal at 21 Hz; it increased significantly (P < 0.05, Z-test) by 89% after CBZ administration. This was significantly greater than a much smaller increase following placebo, which itself may reflect an effect of the time of day when experiments were performed. There was no significant approximately 10-Hz coherence either before or after CBZ administration. CBZ did not significantly alter EEG power at either 10 or 20 Hz. Recently, we showed that diazepam markedly increases the power of approximately 20-Hz motor cortical oscillations with little effect on coherence. We show here that CBZ raises coherence without altering EEG power. This pharmacological dissociation may indicate an important role for corticomuscular coherence in motor control.

The effect of diazepam on motor cortical oscillations and corticomuscular coherence studied in man

EEG recordings from sensorimotor cortex show oscillations around 10 and 20 Hz. These modulate with task performance, and are strongest during periods of steady contraction. The 20 Hz oscillations are coherent with contralateral EMG. Computer modelling suggests that oscillations arising within the cortex may be especially dependent on inhibitory systems. The benzodiazepine diazepam enhances the size of GABA(A) IPSPs; its effects are reversed by the antagonist flumazenil. We tested the effect of these drugs on spectral measures of EEG and EMG, whilst eight healthy human subjects performed a precision grip task containing both holding and movement phases. Either an auxotonic or isometric load was used. EEG changes following electrical stimulation of the contralateral median nerve were also assessed. The EEG power showed similar changes in all task/stimulation protocols used. Power around 20 Hz doubled at the highest dose of diazepam used (5 mg), and returned to control levels following flumazenil. EEG power at 10 Hz was by contrast little altered. The peak frequency of EEG power in both bands was not changed by diazepam. Corticomuscular coherence at ca 20 Hz was reduced following diazepam injection, but the magnitude of this effect was small (mean coherence during steady holding in the auxotonic task was 0.062 in control recordings, 0.051 after 2.5 mg and 5 mg doses of diazepam). These results imply that 20 Hz oscillations in the sensorimotor cortex are at least partially produced by local cortical circuits reliant on GABA(A)-mediated intracortical inhibition, whereas 10 Hz rhythms arise by a different mechanism. Rhythms generated during different tasks, or following nerve stimulation, are likely to arise from similar mechanisms. By examining the formulae used to calculate coherence, we show that if cortical oscillations are simply transmitted to the periphery, corticomuscular coherence should increase in parallel with the ratio of EEG to EMG power. The relative constancy of coherence even when the amplitude of cortical oscillations is perturbed suggests that corticomuscular coherence itself may have a functional role in motor control.

Structure of the voltage-gated L-type Ca2+ channel by electron cryomicroscopy

Voltage-dependent L-type Ca(2+) channels play important functional roles in many excitable cells. We present a three-dimensional structure of an L-type Ca(2+) channel. Electron cryomicroscopy in conjunction with single-particle processing was used to determine a 30-A resolution structure of the channel protein. The asymmetrical channel structure consists of two major regions: a heart-shaped region connected at its widest end with a handle-shaped region. A molecular model is proposed for the arrangement of this skeletal muscle L-type Ca(2+) channel structure with respect to the sarcoplasmic reticulum Ca(2+)-release channel, the physical partner of the L-type channel for signal transduction during the excitation-contraction coupling in muscle.

Activation of cerebellar climbing fibres to rat cerebellar posterior lobe from motor cortical output pathways

1. The activation of climbing fibres projecting to the posterior lobe cerebellar cortex by focal stimulation of the cerebral corticofugal pathway was investigated in anaesthetised rats. Large climbing fibre responses were evoked in parts of crus II and paramedian lobule by stimulation of corticofugal fibres. Lesions of the pyramidal tract just rostral to the inferior olive substantially reduced these responses, suggesting that they were not mediated by relays in the rostral brainstem. 2. By comparison of latencies of climbing fibre responses evoked from different locations in the corticofugal pathway, the conduction velocities of the corticofugal fibres that mediate the responses were estimated to be 1.9 +/- 0.3 m s(-1) (mean +/- S.E.M.). The fastest conducting corticofugal fibres were estimated to conduct significantly faster (18.7 +/- 2.3 m s(-1)). 3. Climbing fibre responses with similar form and cerebellar distribution were evoked from sites in the pyramidal tract rostral and caudal to the inferior olive. This suggests that at least a proportion of the fibres that activate climbing fibres are corticospinal fibres. 4. Lesions of the dorsal column nuclei did not affect the climbing fibre responses evoked in crus II, and produced a relatively small reduction of the responses in the paramedian lobule. This implies that the climbing fibre responses were not exclusively mediated via the dorsal column nuclei. 5. Corticofugal evoked climbing fibre responses were mapped across the cerebellar hemisphere. At some sites they were co-localised with responses evoked by limb afferents. On the basis of limb afferent inputs and other work, these zones were tentatively identified as being functionally equivalent to the c1, c2 and d zones described in the cat.