Motor neuron firing dysfunction in spastic patients with primary lateral sclerosis

Current challenges in primary lateral sclerosis diagnosis

INTRODUCTION

Primary lateral sclerosis (PLS) is a rare, adult-onset and slowly progressive motor neuron disorder whose clinical core is characterized by upper motor neuron (UMN) dysfunction. Its formal diagnosis is clinically based and disease duration-dependent. Differentiating PLS from other disorders involving UMN can be challenging, particularly in the early stages.

AREAS COVERED

Our review covers and discusses different aspects of the PLS field, including the diagnostic criteria and its limitations, its differential diagnosis and their major pitfalls, and the actual role of neurophysiology, neuroimaging, genetics, and molecular biomarkers. Symptomatic treatment of the different manifestations is also addressed. The authors searched MEDLINE and Scopus. They also searched the reference lists of articles identified by our search strategy and reviewed and selected those deemed relevant. They selected papers and studies based on the quality of the report, significance of the findings, and on the author's critical appraise and expertise.

EXPERT OPINION

It is important to investigate novel molecular biomarkers and plan multicenter clinical trials for PLS. However, this will require a large international project to recruit enough patients, particularly given the diagnostic uncertainty of the current clinical criteria. A better understanding of PLS pathophysiology is crucial for designing disease-targeted therapies.

Upper and lower motor neuron neurophysiology and motor control

This chapter considers the principles that underlie neurophysiological studies of upper motor neuron or lower motor neuron lesions, based on an understanding of the normal structure and function of the motor system. Human motor neurophysiology consists of an evaluation of the active components of the motor system that are relevant to volitional movements. Relatively primitive motor skills include locomotion, much dependent on the spinal cord central pattern generator, reaching, involving proximal and distal muscles activation, and grasping. Humans are well prepared to perform complex movements like writing. The role of motor cortex is critical for the motor activity, very dependent on the continuous sensory feedback, and this is essential for adapting the force and speed control, which contributes to motor learning. Most corticospinal neurons in the brain project to brainstem and spinal cord, many with polysynaptic inhibitory rather than excitatory connections. The monosynaptic connections observed in humans and primates constitute a specialized pathway implicated in fractional finger movements. Spinal cord has a complex physiology, and local reflexes and sensory feedback are essential to control adapted muscular contraction during movement. The cerebellum has a major role in motor coordination, but also consistent roles in sensory activities, speech, and language, in motor and spatial memory, and in psychological activity. The motor unit is the final effector of the motor drive. The complex interplay between the lower motor neuron, its axon, motor end-plates, and muscle fibers allows a relevant plasticity in the movement output.

Motor unit recruitment in myopathy: The myopathic EMG reconsidered

Motor unit recruitment is abnormal in myopathies. We have addressed this subject by recording motor unit potentials (MUPs) using a standard concentric needle electrode in tibialis anterior muscles of clinically normal strength in a group of patients with myopathy (15 with myositis and 4 with facioscapulohumeral muscular dystrophy Type 1). In each recording site, a minimal voluntary contraction was sought in order to activate only 2 MUPs. At least 5 pairs of MUPs were recorded in each muscle. We analysed the recruitment rate of the first activated MUP and the mean consecutive difference (MCD) of firing frequency between the individual MUPs of each recruited pair. Results were compared with 30 healthy control subjects. In myopathy the first recorded MUs fired at similar rates to controls (8.2 vs 8.0 Hz, respectively), but the MCD of the firing rate difference between the first two recruited MUPs was less than in controls (median difference 1.78 Hz vs median difference 2.47 Hz, p = 0.02). This change suggests increased lower motor neuron excitability as a functional adaptation, since muscle strength was normal in the studied muscles. These findings are consistent with spinal cord adaptation to the functional changes associated with myopathic muscle disease, although a primary muscle fibre feedback sensing mechanism could also be involved.

Modified motor unit number index (MUNIX) algorithm for assessing excitability of alpha motor neuron in spasticity

Objective

The understanding of the spasticity mechanism is still a problem in the literature, as its definition can be made on the basis of more than one parameter. Therefore, we studied alpha motor neuron excitability, dynamic changes based on force production, and patellar tendon (T) reflex in spasticity and healthy control groups.

Methods

Alpha motor neuron excitability, force production, and patellar T reflex were evaluated through three different test protocols. Motor Unit Number Index (MUNIX) measurement was applied for understanding motor neuron pool properties in the first protocol. Voluntary force production and patellar T reflex parameters were evaluated by voluntary force production and triggering patellar T reflex. Twenty spasticity and 20 healthy volunteers participated in the study.

Results

In the spasticity group, both MUNIX numbers and Motor Unit Size Index (MUSIX) numbers were lower than those in the control group. The results for the Ideal Case Motor Unit Count (ICMUC) parameter show that there is no significant difference between spasticity and healthy individuals for low-level contractions, whereas there is a significant difference for high-level contractions (p < 0.05). In the spasticity group, an increase was observed in the ratio of maximal voluntary force to the T reflex triggered force production (Tf/Vf).

Conclusion

Spasticity and healthy subjects can be distinguished easily and clearly by evaluating the changes in both kinesiological and electrophysiological findings and the decreasing threshold in the alpha motor neuron pool.

Significance

This study shows that such combined methods, which allow the evaluation of the alpha motor neuron pool, as well as kinesiological and electrophysiological parameters, are tools that cannot be overlooked in understanding spasticity.

Primary Lateral Sclerosis

Primary lateral sclerosis is characterized by insidious onset of progressive upper motor neuron dysfunction in the absence of clinical signs of lower motor neuron involvement. Patients experience stiffness; decreased balance and coordination; mild weakness; and, if the bulbar region is affected, difficulty speaking and swallowing, and emotional lability. The diagnosis is made based on clinical history, typical examination findings, and diagnostic testing negative for other causes of upper motor neuron dysfunction. Electromyogram is normal, or only shows mild neurogenic findings in a few muscles, not meeting El Escorial criteria. Treatment is largely supportive.

Motoneuron firing in amyotrophic lateral sclerosis (ALS)

Amyotrophic lateral sclerosis is an inexorably progressive neurodegenerative disorder involving the classical motor system and the frontal effector brain, causing muscular weakness and atrophy, with variable upper motor neuron signs and often an associated fronto-temporal dementia. The physiological disturbance consequent on the motor system degeneration is beginning to be well understood. In this review we describe aspects of the motor cortical, neuronal, and lower motor neuron dysfunction. We show how studies of the changes in the pattern of motor unit firing help delineate the underlying pathophysiological disturbance as the disease progresses. Such studies are beginning to illuminate the underlying disordered pathophysiological processes in the disease, and are important in designing new approaches to therapy and especially for clinical trials.

Myelin imaging in amyotrophic and primary lateral sclerosis

Primary lateral sclerosis (PLS) has been regarded as a rare, extreme form of amyotrophic lateral sclerosis (ALS). Like ALS, it is a clinical diagnosis without established biomarkers. We sought to explore loss of cerebral myelin in relation to clinical features, including cognitive impairment, in cases of both ALS and PLS.

A novel MRI sequence (mcDESPOT) sensitive to water pools within myelin and intra- and extra-cellular spaces was applied to 23 ALS patients, seven PLS patients and 12 healthy controls, with interval follow-up in 15 ALS and four PLS patients.

Results demonstrated that PLS patients were distinguished by widespread cerebral myelin water fraction reductions, independent of disease duration and clinical upper motor neuron burden. ALS patients showed a significant increase in intra- and extra-cellular water, indirectly linked to neuroinflammatory activity. Limited measures of cognitive impairment in the ALS group were associated with myelin changes within the anterior corpus callosum and frontal lobe projections. Longitudinal changes were only significant in the PLS group. In conclusion, in this exploratory study, myelin imaging has potential to distinguish PLS from ALS, and may have value as a marker of extramotor involvement. PLS may be a more active cerebral pathological process than its rate of clinical deterioration suggests.

Degeneration of serotonergic neurons in amyotrophic lateral sclerosis: a link to spasticity

Spasticity is a common and disabling symptom observed in patients with central nervous system diseases, including amyotrophic lateral sclerosis, a disease affecting both upper and lower motor neurons. In amyotrophic lateral sclerosis, spasticity is traditionally thought to be the result of degeneration of the upper motor neurons in the cerebral cortex, although degeneration of other neuronal types, in particular serotonergic neurons, might also represent a cause of spasticity. We performed a pathology study in seven patients with amyotrophic lateral sclerosis and six control subjects and observed that central serotonergic neurons suffer from a degenerative process with prominent neuritic degeneration, and sometimes loss of cell bodies in patients with amyotrophic lateral sclerosis. Moreover, distal serotonergic projections to spinal cord motor neurons and hippocampus systematically degenerated in patients with amyotrophic lateral sclerosis. In SOD1 (G86R) mice, a transgenic model of amyotrophic lateral sclerosis, serotonin levels were decreased in brainstem and spinal cord before onset of motor symptoms. Furthermore, there was noticeable atrophy of serotonin neuronal cell bodies along with neuritic degeneration at disease onset. We hypothesized that degeneration of serotonergic neurons could underlie spasticity in amyotrophic lateral sclerosis and investigated this hypothesis in vivo using tail muscle spastic-like contractions in response to mechanical stimulation as a measure of spasticity. In SOD1 (G86R) mice, tail muscle spastic-like contractions were observed at end-stage. Importantly, they were abolished by 5-hydroxytryptamine-2b/c receptors inverse agonists. In line with this, 5-hydroxytryptamine-2b receptor expression was strongly increased at disease onset. In all, we show that serotonergic neurons degenerate during amyotrophic lateral sclerosis, and that this might underlie spasticity in mice. Further research is needed to determine whether inverse agonists of 5-hydroxytryptamine-2b/c receptors could be of interest in treating spasticity in patients with amyotrophic lateral sclerosis.

Motor unit firing in amyotrophic lateral sclerosis and other upper and lower motor neurone disorders

OBJECTIVE

Motor unit recruitment order and firing rate was investigated in healthy subjects, and in small numbers of patients 50years ago. We aimed to investigate firing rate in different disorders, testing the same target muscle with normal strength, to evaluate possible application in diagnosing upper motor neuron (UMN) lesion.

METHODS

We studied motor unit firing in the tibialis anterior muscle in six groups of subjects; normal subjects (n=45), patients with amyotrophic lateral sclerosis (ALS) (n=36), primary lateral sclerosis (PLS) (n=21), progressive muscular atrophy (PMA) (n=14), various upper motor neurone lesions (n=16) and polyneuropathy (n=42). In all these subjects the tibialis anterior muscle was of normal strength. Motor units were recruited during slight contraction in order to study 2-5 motor units at each recording site, using a standard concentric needle electrode, so that 20-22 motor units were recorded in each muscle. We analysed the coefficient of variation (CV) for amplitude, area, duration and firing rate in these motor units, and the correlation between motor unit potential size and recruitment order.

RESULTS

The mean MU firing rate in this task was similar in each group. No recruitment order was disclosed within the limits of the study task. The CV of firing rate was decreased in UMN and PLS groups. ALS patients with marked spasticity showed a lower CV of motor unit firing rate. The CV of amplitude, area and duration was similar between groups.

CONCLUSIONS

These results in tibialis anterior indicate that physiological modulation of lower motor neuron (LMN) firing rate is decreased in patients with lower limb spasticity. The variability of MU discharges tends to be greater in diseases affecting the LMN.

SIGNIFICANCE

These results suggest that, notwithstanding the simplicity of the task we have used, the physiological variability of motor unit firing may be a useful variable in assessing UMN involvement in motor system disorders.

Tongue's motor evoked potentials in the diagnosis of primary lateral sclerosis (PLS): preliminary report

BACKGROUND

Primary Lateral Sclerosis (PLS) is an adult-onset neurodegenerative disorder due to a selective loss of precentral pyramidal neurons. Our purpose was to evaluate preferential impairment of pyramidal tract to bulbar muscles in patients with PLS and identify a valuable electrophysiological method to help clinicians in the differential diagnosis from Amyotrophic Lateral Sclerosis (ALS).

MATERIALS AND METHODS

We recorded Motor Evoked Potentials (MEPs) from tongue's and anterior tibialis muscles in six patients with PLS and compared the results, in terms of Central Motor Conduction Time (CMCT), amplitude of MEPs and duration of controlateral silent period (cSP), with those obtained both from ten age-matched healthy volunteers and ten patients affected by ALS.

RESULTS

For lower limbs, CMCT resulted significantly increased in PLS and ALS samples compared with healthy subjects (p<0.01); we did not disclose any difference between ALS and PLS groups (p=0.417). Instead for tongue's recordings, CMCT, absolute amplitude of MEPs and cSP were significantly altered in PLS patients towards both ALS patients and healthy volunteers.

CONCLUSIONS

We showed that tongue's MEPs are selectively impaired in PLS. This technique could be helpful to differentiate patients with PLS from those affected by upper motor neuron-predominant variants of ALS. Tongue's MEPs could represent an interesting electrodiagnostic test, potentially useful for the diagnosis of PLS.

[Primary lateral sclerosis: the era of international diagnosis criteria]

Since Charcot's first description, primary lateral sclerosis (PLS) remains a rare clinical syndrome, a neuropathological phenotype of motor system degeneration. In turn, PLS has been described as belonging to the large spectrum of motoneuron diseases or to the diverse degenerative diseases of the nervous system. Clinically, it is characterized by progressive pyramidal involvement in patients who present insidiously progressive gait disorders and, on examination, have relatively symmetrical lower limb weakness, increased muscle tone, pathologic hyper-reflexia, and exaggerated extensor plantar responses. Pinprick, light touch, and temperature sensations are preserved. Viewed in another way, PLS mimicks progressive hereditary spastic paraparesis (HSP) and the "central" phenotype of amyotrophic lateral sclerosis (ALS). PLS is considered "idiopathic" and, depending on the presence or absence of similarly affected family members, the syndrome of idiopathic HSP and ALS are labeled "hereditary" or "apparently sporadic". The juvenile form of PLS and early age at onset in cases of HSP complicate our understanding of the relationship between these two disorders. Guidelines for diagnosis and genetic counseling have been published for HSP and ALS. Recently, since the first international workshop, guidelines for diagnosis of PLS propose a classification system, e.g. for heterogeneous HSP into "pure PLS", complicated or "plus PLS", symptomatic PLS and upper motor neuron-dominant ALS. However, when reviewing known cases of PLS drawn from the literature, rigorous retrospective application of these new PLS criteria raises an unanswered question: does pure PLS exist?

Primary lateral sclerosis

The spectrum of motor neuron diseases ranges from disorders that clinically are limited to lower motor neurons to those that exclusively affect upper motor neurons. Primary lateral sclerosis (PLS) is the designation for the syndrome of progressive upper motor neuron dysfunction when no other etiology is identified. Distinction between PLS and the more common amyotrophic lateral sclerosis (ALS) relies primarily on recognition of their symptoms and signs, as well as on ancillary, although non-specific, laboratory data. In this review, we survey the history of PLS from the initial descriptions to the present. We discuss the role of laboratory, electrodiagnostic, and imaging studies in excluding other diagnoses; the findings from major case series of PLS patients; and proposed diagnostic criteria. Consistent differences are evident in patients classified as PLS compared to those with ALS, indicating that, despite its limitations, this clinical designation retains important utility.

Movement-related cortical potentials in primary lateral sclerosis

OBJECTIVE

Some patients with primary lateral sclerosis (PLS) have a clinical course suggestive of a length-dependent dying-back of corticospinal axons. We measured movement-related cortical potentials (MRCPs) in these patients to determine whether cortical functions that are generated through short, intracortical connections were preserved when functions conducted by longer corticospinal projections were impaired.

METHODS

An electroencephalogram was recorded from scalp electrodes of 10 PLS patients and 7 age-matched healthy control subjects as they made individual finger-tap movements on a keypad. MRCPs were derived from back-averaging the electroencephalogram to the movement.

RESULTS

MRCPs produced by finger taps were markedly reduced in PLS patients, including components generated by premotor areas of the cortex as well as the primary motor cortex. In contrast, the beta-band event-related desynchronization from the motor cortex was preserved.

INTERPRETATION

These findings suggest that impairment in PLS is not limited to the distal axons of corticospinal neurons, but also affects neurons within the primary motor cortex and premotor cortical areas. The loss of the MRCP may serve as a useful marker of upper motor neuron dysfunction. Preservation of event-related desynchronization suggests that the cells of origin differ from the large pyramidal cells that generate the MRCP.