Fatigue brought on by malfunction of the central and peripheral nervous systems

Analysis of the rate of force development reveals high neuromuscular fatigability in elderly patients with chronic kidney disease

BACKGROUND

Chronic kidney disease (CKD) induces muscle wasting and a reduction in the maximum voluntary force (MVF). Little is known about the neuromuscular fatigability in CKD patients, defined as the reduction of muscle force capacities during exercise. Neuromuscular fatigability is a crucial physical parameter of the daily living. The quantification of explosive force has been shown to be a sensitive means to assess neuromuscular fatigability. Thus, our study used explosive force estimates to assess neuromuscular fatigability in elderly CKD patients.

METHODS

Inclusion criteria for CKD patients were age ≥ 60 years old and glomerular filtration rate (GFR) < 45 mL/min/1.73 m2 not on dialysis, and those for controls were GFR > 60 mL/min/1.73 m2 , age and diabetes matched. The fatigability protocol focused on a handgrip task coupled with surface electromyography (sEMG). Scalars were extracted from the rate of force development (RFD): absolute and normalized time periods (50, 75, 100, 150 and 200 ms, RFD50 , RFD75 , RFD100 , RFD150 and RFD200 , respectively), peak RFD (RFDpeak in absolute; NRFDpeak normalized), time-to-peak RFD (t-RFDpeak ) and the relative force at RFDpeak (MVF-RFDpeak ). A statistical parametric mapping approach was performed on the force, impulse and RFD-time curves. The integrated sEMG with time at 0-30, 0-50, 0-100 and 0-200 ms time intervals relative to onset of sEMG activity was extracted and groups were compared separately for each sex.

RESULTS

The cohort of 159 individuals had a median age of 69 (9IQR ) years and body mass index was 27.6 (6.2IQR ) kg/m2 . Propensity-score-matched groups balanced CKD patients and controls by gender with 66 males and 34 females. In scalar analysis, CKD patients manifested a higher decrement than controls in the early phase of contraction, regarding the NRFDpeak (P = 0.009; η2 p  = 0.034) and RFD75 and RFD100 (for both P < 0.001; η2 p  = 0.068 and 0.064). The one-dimensional analysis confirmed that CKD males manifest higher and delayed neuromuscular fatigability, especially before 100 ms from onset of contraction. sEMG was lower in CKD patients than controls in the 0-100 ms (at rest: P = 0.049, Cohen's d = 0.458) and 0-200 ms (at rest: P = 0.016, Cohen's d = 0.496; during exercise: P = 0.006, Cohen's d = 0.421) time windows. Controls showed greater decrease of sEMG than CKD patients in the 0-30 ms (P = 0.020, Cohen's d = 0.533) and 0-50 ms (P = 0.010, Cohen's d = 0.640) time windows. As opposite to females, males showed almost the same differences between groups.

CONCLUSIONS

Our study is the first to show that CKD patients have higher fatigability than controls, which may be associated with an impaired motor-unit recruitment, highlighting a neural drive disturbance with CKD. Further studies are needed to confirm these findings.

Neural Aspects of Fatigue

Fatigue brought about by intense muscular contraction typically is accompanied by a reduction in motor-unit firing rate. The decrease in motor-unit output with fatigue appears to be caused by two interacting processes: 1) a decline in the net excitatory drive to motoneurons and 2) adaptation in the responsiveness of motoneurons to synaptic input. Whether a reduction in motor-unit firing rate in itself contributes to force loss associated with fatigue, however, is an unresolved question. The neuromuscular wisdom hypothesis suggests that decreases in firing rate help to maintain force by optimizing the input to motor units as their contractile properties change. On the other hand, recent work indicates that mechanical function of some motor units is altered during prolonged activity such that diminished firing rate would augment force loss and, thereby, contribute to fatigue. Neural adaptations, therefore, may serve to limit the extent of muscular activity. NEUROSCIENTIST 2:203-206, 1996

Impact of exercise-induced fatigue on the strength, postural control, and gait of children with a neuromuscular disease

OBJECTIVE

Children with a neuromuscular disease are prone to early muscular fatigue. The objective of the present study was to evaluate the effects of fatigue induced by a walking exercise on the strength, postural control, and gait of children with a neuromuscular disease.

DESIGN

Maximal isometric knee strength (extension and flexion), quiet standing postural control, and gait were evaluated in 12 children (8.8 [1.4] yrs) with a neuromuscular disease before and after a walking exercise. The participants were asked to stop walking when they considered themselves "very fatigued."

RESULTS

After the exercise-induced fatigue, a significant increase in range of motion in pelvis obliquity, hip abduction and adduction, and ankle flexion and extension during gait was reported along with an increase in stride length variability. Fatigue also reduced the knee flexor strength and had a detrimental effect on postural control.

CONCLUSIONS

Fatigue affects the strength, postural control, and gait of children with a neuromuscular disease and could notably increase the risks of falling and the occurrence of serious injuries.

Functionality indexes assessed through a simple model of muscle activation, fatigue and recovery

A nonlinear dynamical system is proposed as a qualitative mathematical model with the twofold aim to reasonably describe the force behavior in a fatiguing sub-maximal contraction and to be possibly employed in assessing muscular activation indexes. The model's properties are studied in terms of its equilibria and their stability properties and the existence of the fatigue equilibrium is ensured as the only system's attractor in the feasibility range of the parameters. Suitable mathematical indicators — related to the dynamical properties of resilience and reactivity — are introduced to characterize the asymptotic and the transient system's behavior. The practical impact of the analytical results is elucidated and a connection is established between the introduced mathematical indicators and muscle functionality indexes as rate of force development, task failure time and complete restore time. Experimental validation with handgrip force signal at high load and possible practical applications are also presented.

Improved exercise performance and increased aerobic capacity after endurance training of patients with stable polymyositis and dermatomyositis

Introduction

This randomized, controlled study on patients with polymyositis or dermatomyositis was based on three hypotheses: patients display impaired endurance due to reduced aerobic capacity and muscle weakness, endurance training improves their exercise performance by increasing the aerobic capacity, and endurance training has general beneficial effects on their health status.

Methods

In the first part of this study, we compared 23 patients with polymyositis or dermatomyositis with 12 age- and gender-matched healthy controls. A subgroup of patients were randomized to perform a 12-week endurance training program (exercise group, n = 9) or to a non-exercising control group (n = 6). We measured maximal oxygen uptake (VO₂ max) and the associated power output during a progressive cycling test. Endurance was assessed as the cycling time to exhaustion at 65% of VO₂ max. Lactate levels in the vastus lateralis muscle were measured with microdialysis. Mitochondrial function was assessed by measuring citrate synthase (CS) and β-hydroxyacyl-CoA dehydrogenase (β-HAD) activities in muscle biopsies. Clinical improvement was assessed according to the International Myositis Assessment and Clinical Studies Group (IMACS) improvement criteria. All assessors were blinded to the type of intervention (that is, training or control).

Results

Exercise performance and aerobic capacity were lower in patients than in healthy controls, whereas lactate levels at exhaustion were similar. Patients in the exercise group increased their cycling time, aerobic capacity and CS and β-HAD activities, whereas lactate levels at exhaustion decreased. Six of nine patients in the exercise group met the IMACS improvement criteria. Patients in the control group did not show any consistent changes during the 12-week study.

Conclusions

Polymyositis and dermatomyositis patients have impaired endurance, which could be improved by 12 weeks of endurance training. The clinical improvement corresponds to increases in aerobic capacity and muscle mitochondrial enzyme activities. The results emphasize the importance of endurance exercise in addition to immunosuppressive treatment of patients with polymyositis or dermatomyositis.

Trial registration

ClinicalTrials.gov: NCT01184625

Fatigue in cerebral palsy: a critical review

OBJECTIVE

Fatigue contributes to the deterioration or cessation of walking ability in adults with cerebral palsy (CP). However, conflict exists as to its role. Studies involving functional tasks reported increased, and earlier onset of, fatigue in CP, whereas laboratory studies have reported individuals with CP to be more fatigue-resistant than their peers.

METHODS

A critical review of the literature related to fatigue in CP was conducted.

RESULTS

This review describes factors that contribute to the observed fatigue resistance in laboratory tasks and how a decreased force-production in CP can result in higher energy expenditure to perform the same amount of work as their peers.

CONCLUSION

More research regarding the process of fatigue and recovery for individuals with CP is needed; specifically studies that focus on functional movements requiring the integration of the whole body, thereby stressing the neuromuscular system in a different way than previously explored.

Rehabilitation in practice: Management of lower motor neuron weakness

This series of articles for rehabilitation in practice aims to cover a knowledge element of the rehabilitation medicine curriculum. Nevertheless they are intended to be of interest to a multidisciplinary audience. The competency addressed in this article is ‘The trainee consistently demonstrates a knowledge of the pathophysiology of various specific impairments including lower motor neuron weakness’ and ‘management approaches for specific impairments including lower motor neuron weakness’.

This article explores weakness as a lower motor symptom. Weakness as a primary impairment of neuromuscular diseases is addressed, with recognition of the phenomenon of disuse atrophy, and how weakness impacts on the functional abilities of people with myopathy and neuropathy. Interventions to reduce weakness or address the functional consequences of weakness are evaluated with consideration of safety and clinical application.

Learning outcomes: This paper will allow readers to: (1) appraise the contribution of research in rehabilitation of lower motor neuron weakness to clinical decision making and (2) engage with the issues that arise when researching rehabilitation interventions for lower motor neuron weakness.

Aim of article: Impairments associated with neuromuscular conditions can lead to significant functional difficulties that can impact on a person’s daily participation. This article focuses on the primary impairment of weakness and explores the research evidence for rehabilitation interventions that directly influence weakness or address the impact of weakness on function.

Fatigue resistance during a voluntary performance task is associated with lower levels of mobility in cerebral palsy

OBJECTIVES

To investigate muscle fatigue of the knee flexors and extensors in people with cerebral palsy (CP) compared with those without motor disability during performance of a voluntary fatigue protocol and to investigate the relationship with functional mobility.

DESIGN

A case-control study.

SETTING

A biomechanics laboratory.

PARTICIPANTS

Ambulatory subjects with CP (n=18; mean age, 17.5 y) in Gross Motor Function Classification System (GMFCS) levels I, II, and III and a comparison group of age-matched subjects (n=16) without motor disability (mean age, 16.6 y).

INTERVENTIONS

Not applicable.

MAIN OUTCOME MEASURES

The voluntary muscle fatigue protocol consisted of concentric knee flexion and extension at 60 degrees a second for 35 repetitions on an isokinetic dynamometer. Peak torque for each repetition was normalized by the maximum peak torque value. Muscle fatigue was calculated as the rate of decline in normalized peak torque across all repetitions, represented by the slope of the linear regression. Self-selected and fast gait velocities were measured as well as the Pediatric Outcomes Data Collection Instrument (PODCI).

RESULTS

Greater fatigability (slope) was observed in the comparison group for both knee flexors and extensors than in the group with CP. Within CP, lower knee extensor fatigue (slope) was associated with lower functioning GMFCS levels and lower levels of activity and participation as measured by the PODCI transfers and basic mobility.

CONCLUSIONS

Even after adjusting for maximum peak torque, the knee flexors and extensors of participants with CP were observed to be less fatigable than age-matched peers without motor disability. The lower rate of muscle fatigue was also associated with lower functional mobility in CP. These results may be related to strength or activation differences and/or muscle property alterations. Future investigations are warranted.

Directed transfer function analysis of fMRI data to investigate network dynamics

In this work, we have adapted the directed transfer function (DTF) to fMRI for the analysis of cortical network dynamics. While modern fMRI sequences are capable of sampling at second or sub-second rates, the underlying hemodynamic response limits the true temporal resolution to the order of 6-12 seconds. Therefore, DTF analysis of fMRI is appropriate for characterizing dynamics in brain response which evolves more slowly than the fMRI response, such as those during learning, fatigue and habituation. In such cases, the response to repeated trials will change with time and a summary measure from each trial can be used as input to the DTF analysis because these summary measures are of appropriate sampling rates and are not affected by the sluggishness of the hemodynamic response. As an example, we investigated the dynamic effects of muscle fatigue on the motor network. Specifically, DTF was used as a multivariate measure of the strength and direction of information flow between the various nodes of the network. We found that the primary motor area had a causal influence on the supplementary motor area, pre-motor area and cerebellum, and this influence initially increased with time and diminished towards the end of the experiment, probably as a result of fatigue.

Modeling heterogeneity and dependence for analysis of neuronal data

In this paper, we describe two types of neuroscience problems which challenge the typical statistical models assumed for analyzing neuronal data. This offers an opportunity for new modeling and statistical inference. In the first problem, the data are spatial neural counts which are often over-dispersed and spatially correlated so that a standard Poisson regression model is inadequate. In the second problem, the data are averaged electroencephalograph signals recorded during muscle fatigue, where a time series AR(1) regression model cannot fully capture all the variation and correlation structure in the data. It is shown that an additional parameter has to be included in the modeling of the correlation structure and that the role of the parameter differs from one channel to the other. We propose appropriate generalized models for these data, develop statistical procedures under the generalized models, and apply these procedures to the real data that motivated this paper. The effect of mis-specification of a correlation structure is also investigated.

Symptom burden in persons with spinal cord injury

OBJECTIVE

To determine (1) the frequency, severity, and reported course of 7 symptoms in persons with spinal cord injury (SCI) and (2) the association between these symptoms and patient functioning.

DESIGN

Postal survey.

SETTING

Community.

INTERVENTION

A survey that included measures of the frequency, severity, and recalled course of pain, fatigue, numbness, weakness, shortness of breath, vision loss, and memory loss, as well as a measure of community integration and psychologic functioning was mailed to a sample of persons with SCI. One hundred forty-seven usable surveys were returned (response rate, 43% of surveys mailed).

MAIN OUTCOME MEASURES

The frequency and average severity of each symptom was computed, and the frequencies of each type of reported course were noted. Analyses estimated the associations among the symptoms, and between symptom severity and measures of patient functioning.

RESULTS

The most common symptoms were pain, weakness, fatigue, and numbness. All symptoms were reported to remain the same or to get worse more often than they were reported to improve once they began. Pain, weakness, fatigue, and memory loss were the symptoms most closely associated with patient functioning.

CONCLUSIONS

Patients with SCI must deal with a number of secondary complications in addition to any disability caused by the injury itself. Of 7 symptoms studied, pain, weakness, and fatigue appeared to be most common and most closely linked to patient social and mental health functioning. Research is needed to identify the causal relationships between perceived symptoms and quality of life in patients with SCI and to identify effective treatments for those symptoms shown to impact patient functioning.

Possible therapeutic vaccines for canine myasthenia gravis: implications for the human disease and associated fatigue

Myasthenia gravis (MG) is caused by T cell-dependent antibodies reactive with acetylcholine receptors. These autoreactive antibodies cause muscle weakness by interfering with neuromuscular transmission via removal of acetylcholine receptors from the neuromuscular junction as well as changing the architecture of the junction itself. Consequently, muscle fatigue is a debilitating aspect of MG often leading to more general feelings of tiredness not directly due to muscle weakness. We have previously described two peptides that are mimetics of antigen receptors on certain autoreactive T and B cells that are involved in MG. When used as vaccines in the rat model of MG, these peptides prevented and ameliorated disease and muscle fatigue by blunting acetylcholine receptor antibody responses. Such disease protection resulted from vaccine-induced anergizing antibodies against acetylcholine receptor-specific T and B cell antigen receptors. The present study prospectively evaluated the efficacy of these two vaccines in spontaneous acquired MG in pet dogs. When compared to historical controls that were prospectively studied, the vaccines increased the proportion of remitted dogs from 17 to 75%. In comparison to retrospectively studied historical controls that spontaneously remitted from MG, the vaccines accelerated the rate of decline in acetylcholine receptor antibody titers which resulted in a 3-fold decrease in the mean time to remission. These results are suggestive of a new type of targeted therapy that can drive autoimmune responses into long-term remission and possibly afford a means of determining whether correction of a physical cause of muscle weakness also corrects the perception of chronic, generalized fatigue.

Contribution of central and peripheral factors to residual fatigue in Guillain–Barré syndrome

Many patients with Guillain-Barré syndrome (GBS) suffer from severe residual fatigue that has an uncertain basis. We determined the relative contribution of peripheral and central factors during a 2-min fatiguing sustained maximal voluntary contraction (MVC) in 10 neurologically well-recovered GBS patients and 12 age- and sex-matched healthy controls. Physiological fatigue was defined as the decline of voluntary force during an MVC of the biceps brachii. Relative amounts of peripheral fatigue and central activation failure were determined combining voluntary force and force responses to electrical stimulation. Surface electromyography was used to determine muscle-fiber conduction velocity. During the first minute of sustained MVC, peripheral fatigue developed more slowly in patients than in controls. Central fatigue only occurred in patients. The muscle-fiber conduction velocity was higher in patients. The initial MVC, decrease of MVC, initial force response, and initial central activation failure did not significantly differ between the groups. Although peripheral mechanisms cannot be excluded in the pathogenesis of residual fatigue after GBS, these results suggest that central changes are involved. This study thus provides further insight into the factors contributing to residual fatigue in GBS patients.

Experienced and physiological fatigue in neuromuscular disorders

OBJECTIVE

Fatigue has been described as a typical symptom of neurological diseases. It might be caused both by changes at the peripheral and at the central level. This study measured the level of experienced fatigue and physiological correlates of fatigue in three genetically defined neuromuscular disorders.

METHODS

Sixty-five facioscapulohumeral dystrophy (FSHD), 79 classical myotonic dystrophy (DM), 73 hereditary motor and sensory neuropathy type I (HMSN) patients and 24 age-matched healthy controls made a 2-min sustained maximal voluntary contraction of the biceps brachii muscle. Experienced fatigue at the current moment was assessed with the abbreviated fatigue questionnaire just before the physiological measurement. Peripheral fatigue was quantified by comparing the amplitudes of an initial and a final stimulated force response during rest. Muscle fibre conduction velocity was determined from a 5-channel surface EMG recording in order to show peripheral changes during the contraction. Central aspects of fatigue were measured using superimposed electrical endplate stimulation.

RESULTS

Patients showed an increased level of experienced fatigue. Total physiological and peripheral fatigue were smaller in patients compared to controls, and central fatigue was normal. The most interesting result of this study was the presence of a large central activation failure (CAF) in all groups of neuromuscular patients; they showed CAF values of 36-41% already directly at the start of sustained contraction, whereas the control group showed only 12%. CAF slightly correlated with the level of experienced fatigue just before the test.

CONCLUSIONS

The cause of the large CAF in patients is unclear. Reduced concentration, motivation or effort can lead to lower central activation. In neuromuscular patients especially fear of physical activity or fear to damage the muscle or nerve tissue may contribute. Besides, also physiological feedback mechanisms or changes at the motocortical level may be a cause of reduced central activation.

SIGNIFICANCE

For the clinician it is important to know that experienced fatigue is part of the clinical spectrum of neuromuscular patients. Besides, the weakness in these patients is aggravated by reduced central activation. Potentially, both problems could be subject of an intervention.

Pathophysiology of spastic paresis. I: Paresis and soft tissue changes

Spastic paresis follows chronic disruption of the central execution of volitional command. Motor function in patients with spastic paresis is subjected over time to three fundamental insults, of which the last two are avoidable: (1) the neural insult itself, which causes paresis, i.e., reduced voluntary motor unit recruitment; (2) the relative immobilization of the paretic body part, commonly imposed by the current care environment, which causes adaptive shortening of the muscles left in a shortened position and joint contracture; and (3) the chronic disuse of the paretic body part, which is typically self-imposed in most patients. Chronic disuse causes plastic rearrangements in the higher centers that further reduce the ability to voluntarily recruit motor units, i.e., that aggravate baseline paresis. Part I of this review focuses on the pathophysiology of the first two factors causing motor impairment in spastic paresis: the vicious cycle of paresis-disuse-paresis and the contracture in soft tissues.

Measurement and reproducibility of strength and voluntary activation of lower-limb muscles

Accurate measurement of muscle strength and voluntary muscle activation is important in the assessment of disorders that affect the motor pathways or muscle. We designed a multipurpose system to assess the variability and reproducibility of isometric torque measurements obtained during maximal voluntary efforts of the knee flexor, knee extensor, ankle dorsiflexor, and ankle plantarflexor muscles on each side. It used two isometric myographs mounted on an adjustable frame. Measurements of maximal voluntary torque (range, 25-188 Nm) displayed low variability within a testing session and over five testing sessions (coefficient of variation range, 5-11%). We used the same equipment to measure voluntary activation of the triceps surae muscles. Voluntary activation, measured with a sensitive twitch interpolation method, increased with increasing voluntary contraction torque (P < 0.001) and was very high during maximal efforts (mean, 97.8 +/- 2.1%; median, 98.5%). Furthermore, measurements of voluntary activation during maximal efforts were reproducible across testing sessions with very little variability (coefficient of variation, <2%). The myograph system and the testing procedures should allow accurate measurement of strength and voluntary drive in longitudinal patient studies.

Acquired peripheral neuropathy

The element of time is a crucial factor in the electrodiagnostic presentation of PN. The characteristic changes seen in various neuropathies evolve over time. If testing is performed very early in the course of the disease, abnormalities may not yet be present, in part, because the range of normal values for NCS parameters is broad. In addition, if the process is asymmetric, the affected nerves may not be sampled. Very late in the course of the disease, a multifocal process may appear diffuse and symmetric as the areas of focal involvement coalesce. A primarily, demyelinating process may begin to demonstrate secondary axonal involvement. In very severe neuropathies, it may become difficult to evoke any NCS responses to characterize the neuropathy. It is essential to place the electrodiagnostic findings within the context of the clinical progression to avoid drawing erroneous conclusions. Sometimes, the true nature of the neuropathy is clear only after sequential testing. The etiologic diagnoses of acquired polyneuropathy are vast. A well-crafted electrodiagnostic evaluation can categorize neuropathies into more specific diagnostic groups by identifying the descriptive diagnosis, significantly narrowing the list of possible etiologic diagnoses. Electrodiagnostic testing, which always starts with a pertinent history and physical examination, should always be viewed as a continuation and quantification of the physical examination. Only by knowing the extent and pattern of the clinical involvement is it possible to formulate a thorough electrodiagnostic evaluation. This knowledge is especially important in cases in which the presentation is multifocal, because the neuropathy can be missed entirely if the affected nerves are not evaluated. When evaluating a neuropathy, there are three important questions to answer: (1) Is the process diffuse or multifocal? (2) Is it demyelinating or axonal? (3) Does it predominantly involve the motor or sensory nerves? By assessing the neuropathy along these three axes, the large number of possible etiologic diagnoses becomes much more manageable.

Physical activity, health impairments, and disability in neuromuscular disease

Reduced physical activity is a consequence of progressive neuromuscular diseases, which negatively impacts quality of life and health outcomes. Reduced functional muscle mass is common to all neuromuscular diseases and results from both atrophy of disuse secondary to a sedentary lifestyle and muscle degeneration secondary to the disease itself. This review summarizes current concepts relating to the impact of reduced physical activity on health and fitness, potential determinants of physical activity levels in neuromuscular diseases, and new approaches to the quantitative measurement of physical activity in neuromuscular disease populations. The interrelationship of disease pathophysiology, impairment, functional limitation, disability, and societal limitation in the determination of physical activity in the community in neuromuscular diseases is discussed using Duchenne muscular dystrophy as an example. Future research pertaining to physical activity in neuromuscular disease will need to focus on the development of scientifically based recommendations concerning optimal exercise approaches with both disease-specific and general guidelines.

Spinal and supraspinal factors in human muscle fatigue

Muscle fatigue is an exercise-induced reduction in maximal voluntary muscle force. It may arise not only because of peripheral changes at the level of the muscle, but also because the central nervous system fails to drive the motoneurons adequately. Evidence for "central" fatigue and the neural mechanisms underlying it are reviewed, together with its terminology and the methods used to reveal it. Much data suggest that voluntary activation of human motoneurons and muscle fibers is suboptimal and thus maximal voluntary force is commonly less than true maximal force. Hence, maximal voluntary strength can often be below true maximal muscle force. The technique of twitch interpolation has helped to reveal the changes in drive to motoneurons during fatigue. Voluntary activation usually diminishes during maximal voluntary isometric tasks, that is central fatigue develops, and motor unit firing rates decline. Transcranial magnetic stimulation over the motor cortex during fatiguing exercise has revealed focal changes in cortical excitability and inhibitability based on electromyographic (EMG) recordings, and a decline in supraspinal "drive" based on force recordings. Some of the changes in motor cortical behavior can be dissociated from the development of this "supraspinal" fatigue. Central changes also occur at a spinal level due to the altered input from muscle spindle, tendon organ, and group III and IV muscle afferents innervating the fatiguing muscle. Some intrinsic adaptive properties of the motoneurons help to minimize fatigue. A number of other central changes occur during fatigue and affect, for example, proprioception, tremor, and postural control. Human muscle fatigue does not simply reside in the muscle.

Fatigue and its impact on patients with myasthenia gravis

We have examined fatigue in myasthenia gravis (MG) by administering a measure of cognitive and physical fatigue to patients and control subjects before and after administration of a lengthy cognitive battery. Subjects also completed a scale that assessed the impact of fatigue on physical, social, and cognitive function. Results of the study revealed that MG patients experience significantly more cognitive and physical fatigue than do control subjects, and the patients' perceptions of both cognitive and physical fatigue increased significantly following completion of demanding cognitive work. Control subjects reported no significant change in fatigue. Furthermore, MG patients reported that fatigue produced mild to moderate effects on cognitive and social function and moderate effects on physical function. Results from this study indicate that cognitive fatigue is an important symptom of MG and that fatigue produces pervasive impairments in important aspects of patients' lives. Additional studies are needed to understand the neurobehavioral determinants of cognitive fatigue in this population.

Modalities of fatigue in multiple sclerosis: correlation with clinical and biological factors

Although different factors are probably involved in the etiology of fatigue in multiple sclerosis patients, no definite mechanism has been proposed. We have proposed that fatigue is a complex symptom that includes three clinical different entities (asthenia, fatigability and worsening of symptoms with effort). The goal of this study is to demonstrate if there is a peculiar mechanism for each of the different varieties of fatigue. A control sample of 155 patients (105 women, 50 men) with clinically definite MS was studied. Fatigue was measured using the Fatigue Descriptive Scale (FDS) and the Fatigue Severity Scale (FSS). Treatment, depression, anxiety, sleep and cellular immune status were studied too. Fatigue was a symptom in 118 patients (76.13%); 26 patients (22.03%) described it as asthenia (fatigue at rest); 85 patients (72.03%) as fatigability (fatigue with exercise), and seven patients (5.9%) as worsening of symptoms. The severity of pyramidal involvement was significantly more severe in patients suffering from fatigue; some immunological parameters were associated with fatigue as well. The discriminant analysis of the data shows that some of the immunoactivation parameters are associated with asthenia (F=21.5, P<0.001), and pyramidal tract involvement is associated with fatigability (F=10.5, P<0.001). Sleep disorders, anxiety and depression were linked with fatigue in a few patients. No relationship with treatment was proven. In conclusion, fatigue in MS seems to be a heterogeneous entity. Asthenia and fatigability may be different clinical entities. Certain immunoactivation parameters correlate with the presence of asthenia while pyramidal involvement is associated with fatigability.

The Fatigue Descriptive Scale (FDS): a useful tool to evaluate fatigue in multiple sclerosis

Although fatigue is common among multiple sclerosis (MS) patients, evaluation of this symptom is difficult due to the subjectivity and variability of the complaint. We proposed the Fatigue Descriptive Scale (FDS) as a tool to evaluate the severity and quality of fatigue in a group of patients suffering from MS. As a way to demonstrate the usefulness of this scale we applied the FDS in a group of 155 patients (105 women and 50 men) with clinically-definite multiple sclerosis, as outlined according to Poser's criteria. Age was 36.2 +/- 11.1 years (range 12-62) and time of evolution was 8.3 +/- 9.4 years (range 1-44). The Fatigue Severity Scale (FSS) was also used. Descriptive statistics techniques and techniques for nonparametric distribution (Spearman Rank, Kruskal-Wallis ANOVA) were used. One hundred and eighteen patients reported fatigue (73 spontaneously, 45 when questioned). All descriptions of fatigue were ranked according to FDS categories. Eighty-five patients defined the symptom as fatigue with exercise, 26 as asthenia and seven as the worsening of other symptoms. Fatigue by itself produced limited or disrupted activity in 78 patients; work-related functions were limited in 48 patients; social relations were limited in 29 patients; and self-care difficult for one patient. Fifty-six patients suffered fatigue daily. FDS score was 4.9 +/- 3.9 (range 0-13). FSS was 3.1 +/- 1.7 (range 0.2-6.6). FDS and FSS of Krupp were highly correlated (R = 0.87, p < 0.001). Therefore, in comparison with other scales, the FDS shows remarkable usefulness in classifying, periodicity, and severity of fatigue in MS patients. The high correlation with the FSS implies that it is a valid method to measure the severity of fatigue, as was demonstrated in our paper proposing the FDS. The importance of this new scale is its ability to inform the clinicians in a very quick, easy, and at the same time complete way, how severe the fatigue really is and how it affects the patient.

Orthostatic intolerance in adolescent chronic fatigue syndrome

OBJECTIVES

To demonstrate the association between orthostatic intolerance and the chronic fatigue syndrome (CFS) in adolescents and to delineate the form that orthostatic intolerance takes in these children.

STUDY DESIGN

We investigated the heart rate and blood pressure (BP) responses to head-up tilt (HUT) in 26 adolescents aged 11 to 19 years with CFS compared with responses in adolescents referred for the evaluation of simple faint and to responses in 13 normal healthy control children of similar age.

RESULTS

A total of 4/13 of the controls and 18/26 simple faint patients experienced typical faints with an abrupt decrease in BP and heart rate associated with loss of consciousness. One CFS patient had a normal HUT. A total of 25/26 CFS patients experienced severe orthostatic symptoms associated with syncope in 7/25, orthostatic tachycardia with hypotension in 15/25, and orthostatic tachycardia without significant hypotension in 3/25. Acrocyanosis, cool extremities, and edema indicated venous pooling in 18/25. None of the control or simple faint patients experienced comparable acral or tachycardic findings.

CONCLUSIONS

We conclude that chronic fatigue syndrome is highly related to orthostatic intolerance in adolescents. The orthostatic intolerance of CFS often has heart rate and BP responses similar to responses in the syndrome of orthostatic tachycardia suggesting that a partial autonomic defect may contribute to symptomatology in these patients.

Neurally mediated hypotension and autonomic dysfunction measured by heart rate variability during head-up tilt testing in children with chronic fatigue syndrome

Recent investigations suggest a role for neurally mediated hypotension (NMH) in the symptomatology of chronic fatigue syndrome (CFS) in adults. Our previous observations in children with NMH and syncope (S) unrelated to CFS indicate that the modulation of sympathetic and parasympathetic tone measured by indices of heart rate variability (HRV) is abnormal in children who faint during head-up tilt (HUT). In order to determine the effects of autonomic tone on HUT in children with CFS we performed measurements of HRV during HUT in 16 patients aged 11-19 with CFS. Data were compared to 26 patients evaluated for syncope and with 13 normal control subjects. After 30 minutes supine, patients were tilted to 80 degrees for 40 minutes or until syncope occurred. Time domain indices included RR interval, SDNN, RMSSD, and pNN50. An autoregressive model was used to calculate power spectra. LFP (.04-.15 Hz), HFP (.15-.40Hz), and TP (.01-.40Hz). Data were obtained supine (baseline) and after HUT. Thirteen CFS patients fainted (CFS+, 5/13 pure vasodepressor syncope) and three patients did not (CFS-). Sixteen syncope patients fainted (S+, all mixed vasodepressor-cardioinhibitory) and 10 did not (S-). Four control patients fainted (Control+, all mixed vasodepressor-cardioinhibitory) and nine did not (Control-). Baseline indices of HRV were not different between Control+ and S+, and between Control- and S-, but were depressed in S+ compared to S-. HRV indices were strikingly decreased in CFS patients compared to all other groups. With tilt, SDNN, RMSSD, and pNN50 and spectral indices decreased in all groups, remaining much depressed in CFS compared to S or control subjects. With HUT, sympathovagal indices (LFP/HFP, nLFP, and nHFP) were relatively unchanged in CFS, which contrasts with the increase in nLFP with HUT in all other groups. With syncope RMSSD, SDNN, LFP, TP, and HFP increased in S+ (and Control+), suggesting enhanced vagal heart rate regulation. These increases were not observed in CFS+ patients. CFS is associated with NMH during HUT in children. All indices of HRV are markedly depressed in CFS patients, even when compared with already low HRV in S+ or Control+ patients. Sympathovagal balance does not shift toward enhanced sympathetic modulation of heart rate with HUT and there is blunting in the overall HRV response with syncope during HUT. Taken together these data may indicate autonomic impairment in patients with CFS.