Electrophysiologic studies in critical illness associated weakness: myopathy or neuropathy--a reappraisal

Use of electrophysiologic testing

OBJECTIVE

To define the electrophysiologic tests to diagnose critical illness myopathy and critical illness polyneuropathy in intensive care unit patients.

DESIGN

Literature review.

MEASUREMENTS AND MAIN RESULTS

Critical illness myopathy and neuropathy are common complications in the critically ill patient. Myopathy and neuropathy are equally common, and often coexist. Electrophysiological alterations of peripheral nerves and muscle have an early onset in the first days of intensive care unit stay or shortly after sepsis, and precede the structural alterations. Conventional electrophysiologic evaluation can be performed easily on most intensive care unit patients, including patients with altered consciousness; in conjunction with direct muscle stimulation, it can differentiate myopathy from neuropathy, which might be important to define the long-term prognosis. However, electrophysiologic tests are not universally available; their interpretation requires special expertise; and their application is time consuming. A recently proposed simplified test of peroneal nerve stimulation could be used as a screening method to select patients who merit in-depth neurologic evaluation.

CONCLUSIONS

Early identification of neuromuscular alterations by means of electrophysiologic tests may be of value for targeted treatments and to anticipate the risk of short-term disability. Complete neurologic and electrophysiological evaluation is important to define the risk of long-term disability after intensive care unit discharge.

A framework for diagnosing and classifying intensive care unit-acquired weakness

Neuromuscular dysfunction is prevalent in critically ill patients, is associated with worse short-term outcomes, and is a determinant of long-term disability in intensive care unit survivors. Diagnosis is made with the help of clinical, electrophysiological, and morphological observations; however, the lack of a consistent nomenclature remains a barrier to research. We propose a simple framework for diagnosing and classifying neuromuscular disorders acquired in critical illness.

Nonexcitable muscle membrane predicts intensive care unit-acquired paresis in mechanically ventilated, sedated patients

OBJECTIVES

: To investigate the predictive value of electrophysiological measurements including validation of muscle membrane excitability on the development of intensive care unit (ICU)-aquired paresis.

DESIGN

: Prospective observational study.

SETTING

: University ICU.

PATIENTS

: Surgical ICU patients selected upon a simplified acute physiology score > or =20 on three successive days within 1 wk after ICU admission.

INTERVENTIONS

: We performed serial electrophysiological measurements with onset of critical illness including conventional electrophysiological parameters and compound muscle action potentials after direct muscle stimulation (dmCMAP). Patients' awareness and muscle strength were measured sequentially by Ramsay sedation scale and an additional questionnaire and by Medical Research Council score, respectively.

MEASUREMENTS AND MAIN RESULTS

: Among 56 sedated patients 34 patients revealed reduced dmCMAP values <3 mV indicating a myopathic process within 7.5 (5 of 11) days after admission to the ICU. Abnormal dmCMAP anticipated ICU-acquired paresis upon emergence from sedation with a sensitivity and specificity of 83.3% and 88.8%, respectively (positive predictive value of 0.91). Multivariate logistic regression analyses revealed that validating dmCMAP during early course of critical illness had significant diagnostic utility to anticipate ICU-acquired paresis (p = .004; odds ratio = .47; 95% confidence interval = .28-.79).

CONCLUSIONS

: Abnormal dmCMAP occurred within the first week after admission to the ICU and pointed towards a myopathic process as the primary cause of ICU-acquired paresis. Validation of dmCMAP with onset of critical illness allows an early prediction of ICU-acquired paresis and adds important information to clinical estimation of the patients' motor function.

Clinical review: Critical illness polyneuropathy and myopathy

Critical illness polyneuropathy (CIP) and myopathy (CIM) are major complications of severe critical illness and its management. CIP/CIM prolongs weaning from mechanical ventilation and physical rehabilitation since both limb and respiratory muscles can be affected. Among many risk factors implicated, sepsis, systemic inflammatory response syndrome, and multiple organ failure appear to play a crucial role in CIP/CIM. This review focuses on epidemiology, diagnostic challenges, the current understanding of pathophysiology, risk factors, important clinical consequences, and potential interventions to reduce the incidence of CIP/CIM. CIP/CIM is associated with increased hospital and intensive care unit (ICU) stays and increased mortality rates. Recently, it was shown in a single centre that intensive insulin therapy significantly reduced the electrophysiological incidence of CIP/CIM and the need for prolonged mechanical ventilation in patients in a medical or surgical ICU for at least 1 week. The electrophysiological diagnosis was limited by the fact that muscle membrane inexcitability was not detected. These results have yet to be confirmed in a larger patient population. One of the main risks of this therapy is hypoglycemia. Also, conflicting evidence concerning the neuromuscular effects of corticosteroids exists. A systematic review of the available literature on the optimal approach for preventing CIP/CIM seems warranted.

Electrophysiological tests in intensive care

Neuromuscular complications encountered in intensive care can be due to peripheral nerves (polyneuropathies), muscles (myopathies), neuromuscular end-plate (transmission deficit) or a combination of these. Neurophysiopathological tests that enable us to diagnose and maybe differentiate between these causes are essentially electroneurographs and electromyographs. These assess, either directly or indirectly, the integrity of the peripheral system, both sensory (centripetal afferent pathways) and motor (lower motor neurones, neuromuscular junction, muscle membrane and contractile substrate). Electrical activity in the muscle is measured using a special device (an electromyograph) comprising preamplifiers, a computer with special programmes and algorithms for analysing, digitalizing, amplifying and filtering the recorded signal and a monitor on which to visualize the on-line trace. The machine also has speakers. These serve to recognize and identify the physiological events being recorded as they make such unusual, individual sounds. An electric stimulator with constant current can be used to stimulate branches of superficial nerves (motor, sensory or mixed) so that electric conduction speed can be studied and the sensory or motor response analysed. The results can be saved on a hard disc or on any magnetic or digital support, viewed off-line or even printed.

Direct stimulation: a useful technique

Direct muscular stimulation is quick, simple, non-invasive and can be carried out at the patient's bedside. More importantly, it is reliable even when the patients cannot cooperate or are comatose. From the small amount of data available, it seems to be as accurate as neuromuscular biopsy in diagnosing critical illness myopathy. It would therefore be advisable to use it in everyday clinical practice.

Changes in intracellular action potential profile affect parameters used in turns/amplitude analysis

The influence of changes in the intracellular action potential (IAP) spatial profile on motor unit potentials (MUPs), number of turns per second (NTs), and mean turn amplitude were simulated and analyzed. We show why measurement of NTs was "the best indicator of neurogenic affection" and why the lower diagnostic yield of turns/amplitude analysis in myopathy could be due to changes in IAP shape caused by elevated free calcium concentration. The results explain the complications observed when interference electromyographic signals obtained during high levels of isometric contractions were analyzed. We show that, in contrast to earlier assumptions, the effect of increased IAP spike duration on NTs was stronger than that of a decrease in muscle fiber propagation velocity (MFPV). The decrease in the NTs could occur without a drop-out of MUs and/or a decrease in their firing rates, and without a change in MFPV and synchronous firing.

Mechanisms of neurologic failure in critical illness

Critical illness frequently is associated with neurologic failure that may involve the central and peripheral nervous systems. Central nervous system failure is associated with a spectrum of neurobehavioral changes including delirium, coma, and long-term cognitive dysfunction. Peripheral neurologic failure, or critical illness neuromuscular abnormalities, is suggested by diffuse arreflexic weakness and protracted respiratory insufficiency, and may also persist long after the acute hospitalization. While the burden of neurological disease complicating critical illness is considerable, preventive or therapeutic options are limited. This article provides an overview of research evaluating the relationship between critical illness and neurologic function, with a special emphasis on underlying mechanisms.

Mechanisms of neuromuscular dysfunction in critical illness

The development of neuromuscular dysfunction (NMD) during critical illness is increasingly recognized as a cause of failure to wean from mechanical ventilation and is associated with significant morbidity and mortality. At times, it is difficult to identify the presence of NMD and distinguish the etiology of the weakness in patients with critical illness, but subtle clinical findings and bedside electrophysiologic testing are helpful in establishing the diagnosis. This article describes the clinical spectrum of acquired neuromuscular weakness in the setting of critical illness, provides an approach to diagnosis, and discusses its pathogenesis. Finally, a defective sodium channel regulation as a unifying mechanism underlying NMD in critically ill patients is proposed.

The role of future longitudinal studies in ICU survivors: understanding determinants and pathophysiology of weakness and neuromuscular dysfunction

PURPOSE OF REVIEW

The goals of this review are to discuss the pathophysiology and determinants of muscle weakness and neuromuscular dysfunction after critical illness, and to offer thoughts regarding the role of future longitudinal studies in this area.

RECENT FINDINGS

While recent studies support the finding that neuromuscular dysfunction is common and important after critical illness, reversible risk factors and approaches to prevention and treatment remain unproven. Pathophysiologic studies implicate disease and treatment associated factors in the development of nerve and muscle damage during critical illness; these factors may provide targets for future studies.

SUMMARY

Additional studies with improved methodology that address epidemiology and that test interventions are needed to understand and to improve neuromuscular function after critical illness.

Critical illness myopathy: further evidence from muscle-fiber excitability studies of an acquired channelopathy

Recent studies have demonstrated acquired muscle inexcitability in critical illness myopathy (CIM) and have used direct muscle stimulation (DMS) techniques to distinguish neuropathy from myopathy as a cause of weakness in the critically ill. The mechanisms underlying weakness in CIM are incompletely understood and DMS is only semiquantitative. We report results from a series of 32 patients with CIM and demonstrate significant slowing of muscle-fiber conduction velocity (MFCV) and muscle-fiber conduction block during the acute phase of CIM, which correlates with prolonged compound muscle action potential (CMAP) duration, clinical severity, and course. We also used a paired stimulation technique to explore the excitability of individual muscle fibers in vivo. We demonstrate altered muscle-fiber excitability in CIM patients. Serial studies help define the course of these pathophysiological changes. Parallels are made between CIM and hypokalemic periodic paralysis. Our findings provide further evidence for muscle membrane dysfunction being the principal underlying abnormality in CIM.

Neuromuscular complications in the intensive care unit: critical illness polyneuromyopathy

Critical illness polyneuromyopathy, a complication of critical illness, is a primary axonal degeneration of motor and sensory fibers that leads to skeletal muscle weakness. It significantly contributes to the unexplained difficulty in weaning from mechanical ventilation and to their prolonged rehabilitation and poor quality of life after discharge. This article will discuss the diagnosis of critical illness polyneuromyopathy, identify risk factors, review several pathomechanisms that have been proposed, and discuss the implications for practice.

Simplified electrophysiological evaluation of peripheral nerves in critically ill patients: the Italian multi-centre CRIMYNE study

Introduction

Critical illness myopathy and/or neuropathy (CRIMYNE) is frequent in intensive care unit (ICU) patients. Although complete electrophysiological tests of peripheral nerves and muscles are essential to diagnose it, they are time-consuming, precluding extensive use in daily ICU practice. We evaluated whether a simplified electrophysiological investigation of only two nerves could be used as an alternative to complete electrophysiological tests.

Methods

In this prospective, multi-centre study, 92 ICU patients were subjected to unilateral daily measurements of the action potential amplitude of the sural and peroneal nerves (compound muscle action potential [CMAP]). After the first ten days, complete electrophysiological investigations were carried out weekly until ICU discharge or death. At hospital discharge, complete neurological and electrophysiological investigations were performed.

Results

Electrophysiological signs of CRIMYNE occurred in 28 patients (30.4%, 95% confidence interval [CI] 21.9% to 40.4%). A unilateral peroneal CMAP reduction of more than two standard deviations of normal value showed the best combination of sensitivity (100%) and specificity (67%) in diagnosing CRIMYNE. All patients developed the electrophysiological signs of CRIMYNE within 13 days of ICU admission. Median time from ICU admission to CRIMYNE was six days (95% CI five to nine days). In 10 patients, the amplitude of the nerve action potential dropped progressively over a median of 3.0 days, and in 18 patients it dropped abruptly within 24 hours. Multi-organ failure occurred in 21 patients (22.8%, 95% CI 15.4% to 32.4%) and was strongly associated with CRIMYNE (odds ratio 4.58, 95% CI 1.64 to 12.81). Six patients with CRIMYNE died: three in the ICU and three after ICU discharge. Hospital mortality was similar in patients with and without CRIMYNE (21.4% and 17.2%; p = 0.771). At ICU discharge, electrophysiological signs of CRIMYNE persisted in 18 (64.3%) of 28 patients. At hospital discharge, diagnoses in the 15 survivors were critical illness myopathy (CIM) in six cases, critical illness polyneuropathy (CIP) in four, combined CIP and CIM in three, and undetermined in two.

Conclusion

A peroneal CMAP reduction below two standard deviations of normal value accurately identifies patients with CRIMYNE. These should have full neurological and neurophysiological evaluations before discharge from the acute hospital.

[The predominance of myopathy as a cause of intensive-care-unit-acquired paralysis: the diagnostic value of direct muscle stimulation]

INTRODUCTION

In the intensive care unit (ICU) patients sometimes develop diffuse neuromuscular deficit resulting in flaccid tetraparesia with a more or less severe prognosis.

STATE OF THE ART

ICU-acquired neuromuscular disorders have various possible origins, including necrotic or catabolic myopathies and sensori-motor axonal neuropathies. Electrophysiological testing determines these pathophysiological mechanisms better than clinical examination. The technique of direct muscle stimulation has been proposed, in addition to conventional electroneuromyographic methods, to improve the reliability of electrodiagnosis in ICU, but has been rarely studied. Using this technique, we recently showed that a majority of ICU-acquired pareses are of myopathic origin.

PERSPECTIVES

The technique of direct muscle stimulation could be fruitfully associated with usual electroneuromyographic methods to differentiate myopathic from neuropathic involvement at the origin of any severe weakness in ICU.

CONCLUSION

The contribution of myopathic processes in ICU-acquired paresis is probably underestimated. Direct muscle stimulation enables better understanding of the mechanisms underlying acquired motor deficit in ICU patients. However, it remains to be determined whether this refinement could have a significant impact on prognosis and treatment.

Occurrence of nervous system involvement in SIRS

Systemic inflammatory response syndrome (SIRS) is a medical condition in which the all-organ microcirculation is affected including nervous system. We describe neurological findings in 64 patients with SIRS at Hospital das Clínicas of Sao Paulo University School of Medicine; 45.3% were male and 54.7% female; their age ranged from 16 to 95 years old. SIRS was caused by infection in 68.8% of patients, trauma in 10.9%, burns in 7.8%, and elective surgery in 4.7%. The central nervous system involvement occurred in 56.3% of patients and was characterized as encephalopathy in 75%, seizures in 13.9%, non-epileptic myoclonus in 2.8%, and ischemic stroke in 8.3%. The magnetic resonance imaging, cerebrospinal fluid and electroencephalographic changes were unremarkable in encephalopathic patients. Neuromuscular disorders were diagnosed in 43.7%. Critical ill polyneuropathy was characterized in 57.1%, critical ill myopathy in 32.1%, demyelinating neuropathy in 7.2%, and pure motor neuropathy in 3.6%. Nerve and muscle pathological studies dismissed inflammatory abnormalities. The identification of these conditions has important economic implications and may change the critically ill patients' prognosis.

Dysregulation of sodium channel gating in critical illness myopathy

Critical illness myopathy (CIM) is the most common caused of acquired weakness in critically ill patients. While atrophy of muscle fibers causes weakness, the primary cause of acute weakness is loss of muscle excitability. Studies in an animal model of CIM suggest that both depolarization of the resting potential and a hyperpolarized shift in the voltage dependence of sodium channel gating combine to cause inexcitability. In active adult skeletal muscle the only sodium channel isoform expressed is Nav1.4. In the animal model of CIM the Nav1.5 sodium channel isoform is upregulated, but the majority of sodium current is still carried by Nav1.4 sodium channels. Experiments using toxins to selectively bock the Nav1.4 isoform demonstrated that the cause of the hyperpolarized shift in sodium channel inactivation is a hyperpolarized shift in inactivation of the Nav1.4 isoform. These data suggest that CIM represents a new type of ion channel disease in which altered gating of sodium channels is due to improper regulation of the channels rather than mutation of channels or changes in isoform expression. The hypothesis that dysregulation of sodium channel gating underlies inexcitability of skeletal muscle in CIM raises the possibility that there maybe dysregulation of sodium channel gating in other tissues in critically ill patients. We propose that there is a syndrome of reduced electrical excitability in critically ill patients that affects skeletal muscle, peripheral nerve, the heart and central nervous system. This syndrome manifests as CIM, critical illness polyneuropathy, reduced cardiac contractility and septic encephalopathy.

Clinical and electrophysiological findings and long-term outcomes in paediatric patients with critical illness polyneuromyopathy

BACKGROUND

Neuromuscular weakness in paediatric patients with sepsis and multiple organ dysfunction is increasingly reported. However, many aspects of neuromuscular involvement in critically ill children are not completely understood. As more patients survive the critical illness, an understanding of the long-term outcomes of this condition is needed.

AIMS

To describe clinical and electrophysiological features and evaluate the long-term outcomes in critically ill paediatric patients with neuromuscular complications.

METHODS

A case series of five critically ill children was observed prospectively for a 1-month period. Selected clinical and laboratory parameters were evaluated. Electrophysiological studies were performed during the first week and then 1 month later in order to detect signs of critical illness polyneuromyopathy (CIPM). Patients with neuromuscular involvement completed a 1-year follow-up.

RESULTS

Electrophysiological abnormalities were detected in two patients. Flaccid quadriplegia was a clinical presentation. Both children had electromyographic evidence of chronic partial denervation at follow-up, findings indicative of a preceding axonal neuropathy. Marked but incomplete recovery within 1 year after the onset of the disease occured in both patients. With a mild residual functional handicap the health-related quality of life was not significantly impaired (Barthel Index > 80).

CONCLUSIONS

In both our patients with CIPM, the long-term clinical outcome is markedly better than we expected when electromyography in the 1-year follow-up demonstrated a persistent severe chronic partial denervation. These findings can have important implications for the management and rehabilitation of paediatric intensive care survivors.

The role of different EMG methods in evaluating myopathy

For the diagnosis of myopathy, EMG may have an important role along with blood tests, muscle biopsies and genetic testing. This review evaluates different EMG methods in the diagnosis of myopathy. These include manual analysis of individual motor unit potentials and multi-motor unit potential analysis sampled at weak effort. At high effort, turns-amplitude analyses such as the cloud analysis and the peak ratio analysis have a high diagnostic yield. The EMG can seldom be used to differentiate between different types of myopathy. In the channelopathies, myotonia, exercise test and cooling of the muscle are helpful. Macro-EMG, single-fibre EMG and muscle fibre conduction velocity analysis have a limited role in myopathy, but provide information about the changes seen. Analysis of the firing rate of motor units, power spectrum analysis, as well as multichannel surface EMG may have diagnostic potential in the future. EMG is of great importance in the diagnosing of patients with myopathy, preferably a needle electrode and quantitative analyses should be used. A combination of a method at weak effort as well as a method at stronger effort seems optimal.

Origin of ICU acquired paresis determined by direct muscle stimulation

BACKGROUND

Acquired diffuse paresis in an intensive care unit (ICU) can result from critical illness myopathy or polyneuropathy. Clinical examination and conventional neurophysiological techniques may not distinguish between these entities.

OBJECTIVE

To assess the value of direct muscle stimulation (DMS) to differentiate myopathic from neuropathic process in critically ill patients with diffuse severe muscle weakness.

METHODS

30 consecutive patients with ICU acquired diffuse motor weakness were studied. Responses of the right deltoid and tibialis anterior muscles to DMS and to motor nerve stimulation (MNS) were studied and compared with results of conventional nerve conduction studies and concentric needle electromyography (EMG). An original algorithm was used for differential diagnosis, taking into account first the amplitude of the responses to DMS, then the MNS to DMS amplitude ratio, and finally the amplitude of the sensory nerve action potentials recorded at the lower limbs.

RESULTS

Evidence of neuropathy and myopathy was found in 57% and 83% of the patients, respectively. Pure or predominant myopathy was found in 19 patients. Other results were consistent with neuromyopathy (n = 5) and pure or predominant neuropathy (n = 2). Four patients had normal results with stimulation techniques, but spontaneous EMG activity and raised plasma creatine kinase suggesting necrotic myopathy.

CONCLUSIONS

A neurophysiological approach combining DMS and conventional techniques revealed myopathic processes in a majority of ICU patients. Reduced muscle fibre excitability may be a leading cause for this. The diagnosis of myopathy in ICU acquired paralysis can be established by a combination of DMS, needle EMG, and plasma creatine kinase.

Critical care myopathy in a child

Critical care myopathy (CCM) is an important, often unrecognized cause of weakness in the intensive care unit (ICU). We report a child who developed severe myopathy in the ICU. Our patient was newly diagnosed to have autoimmune hepatitis that may have predisposed her to toxicity from corticosteroids and neuromuscular-blocking agents, but she recovered her strength completely. Risk factors that may have predisposed this patient to CCM include underlying liver dysfunction, presence of acute lung injury, use of corticosteroids, and use of neuromuscular blockade. Given the apparent prevalence and underreporting of CCM in the ICU, it is important for rheumatology consultants to include this entity in their differential diagnosis of weakness in critically ill children and adults, and to recommend appropriate evaluation.

Approach to neuromuscular disorders in the intensive care unit

Neuromuscular disorders increasingly are recognized as a complication in patients in the intensive care unit (ICU) and represent a common cause of prolonged ventilator dependency. The distinct syndromes of critical illness myopathy, prolonged neuromuscular blockade, and critical illness polyneuropathy (CIP) may arise as a consequence of sepsis, multi-organ failure, and exposure to various medications—notably, intravenous corticosteroids and neuromuscular blocking agents—but the pathophysiology of these disorders remains poorly understood. More than one syndrome may occur simultaneously, and the distinctions may be difficult in a particular patient, but a specific diagnosis usually can be established after careful clinical, electrodiagnostic, and, when necessary, histological evaluation. For example, asthmatics requiring treatment with corticosteroids and neuromuscular blocking agents may develop an acute myopathy characterized by generalized weakness, preserved eye movements, elevated creatine kinase levels, and myopathic motor units on electromyography (EMG). Muscle biopsy demonstrates distinctive features of thick (myosin) filament loss on ultrastructural studies. Conversely, those with a prolonged ICU course that is complicated by episodes of sepsis with failure to wean from the ventilator, distal or generalized flaccid limb weakness, and areflexia probably have CIP. EMG in these patients demonstrates reduced or absent motor and sensory potentials with neurogenic motor units. Prolonged neuromuscular blockade most commonly occurs in patients with renal failure who have received prolonged infusions of neuromuscular blockers. There is severe flaccid, areflexic paralysis with normal sensation, facial weakness, and ophthalmoparesis that persists for days or weeks after the neuromuscular blockers have been discontinued. Repetitive nerve stimulation shows a decrement of the compound muscle action potential and, in most cases, establishes a disorder of neuromuscular transmission. With the recent epidemic of West Nile virus infection, a clinical syndrome of acute flaccid paralysis with several features indistinguishable from poliomyelitis has emerged. This article critically examines the clinical, electrophysiological, and pathological features of these and other acute neuromuscular syndromes that arise in the context of ICU care and summarizes the current understanding of the pathophysiology and treatment of these disorders.

Electrophysiologic techniques in critical illness-associated weakness

Neuromuscular disorders are increasingly recognized in the critically ill but conventional electrodiagnostic techniques often provide non-specific results or are hampered by local conditions that prevent adequate disease classification. Muscle fiber inexcitability is a common phenomenon in critical illness myopathy possibly secondary to disordered sodium channel fast inactivation and associated with loss of myosin staining. Direct muscle stimulation techniques, measuring evoked response amplitudes and comparison of nerve and muscle stimulated responses, are recognized methods of demonstrating this phenomenon. Other measures studied in this population include increased compound motor action potential duration, motor unit number estimates and mean step area of individual motor unit potentials during motor unit number estimate studies. An electrophysiologic approach to the study of patients with critical illness associated weakness is proposed.

Les anomalies neuromusculaires acquises de reanimation

Critical illness neuromuscular abnormalities (CINMA) are found in 25 percent of ITU patients who recover consciousness and are characterized by a bilateral and symmetric weakness that involves the four limbs but spares the facial muscles. Electrophysiological testing shows an axonal sensory motor polyneuropathy and/or myopathy. The main risk factors of CINMA are prolonged durations of multiple organ failure and mechanical ventilation, use of corticosteroids and hyperglycaemia. CINMA contribute also to increase the duration of mechanical ventilation, this effect being mediated by diaphragm weakness. The median duration of limb weakness is 21 days, although it can exceed several months in some patients. Few preventive measures have been assessed. Whether the benefit of strict blood glucose control in ITU patients recovering from heart surgery on CINMA incidence can be extended to medical ICU patients needs to be determined.

Neuromuscular manifestations of critical illness

Critical illness, more precisely defined as the systemic inflammatory response syndrome (SIRS), occurs in 20%-50% of patients who have been on mechanical ventilation for more than 1 week in an intensive care unit. Critical illness polyneuropathy (CIP) and myopathy (CIM), singly or in combination, occur commonly in these patients and present as limb weakness and difficulty in weaning from the ventilator. Critical illness myopathy can be subdivided into thick-filament (myosin) loss, cachectic myopathy, acute rhabdomyolysis, and acute necrotizing myopathy of intensive care. SIRS is the predominant underlying factor in CIP and is likely a factor in CIM even though the effects of neuromuscular blocking agents and steroids predominate in CIM. Identification and characterization of the polyneuropathy and myopathy depend upon neurological examination, electrophysiological studies, measurement of serum creatine kinase, and, if features suggest a myopathy, muscle biopsy. The information is valuable in deciding treatment and prognosis.

Critical illness myopathy and neuropathy

PURPOSE OF REVIEW

To present the major pathophysiological and diagnostic features of critical illness myopathy (CIM) and polyneuropathy (CIP), and to discuss problems concerning the risk factors for CIM and CIP.

RECENT FINDINGS

The pathophysiology of critical illness myopathy and critical illness polyneuropathy is complex, involving metabolic, inflammatory, and bioenergetic alterations. This review cites new evidence supporting several pathogenetic mechanisms. These include microvascular changes in peripheral nerves (with increased endothelial expression of E-selectin), the possible role for an altered lipid serum profile in promoting organ dysfunction (including nerve dysfunction), the damage or inhibition of complex I of the respiratory chain as a cause of muscle ATP depletion and bioenergetic failure, and the activation of specific intracellular proteolytic systems causing myofilament loss and apoptosis in CIM. The diagnostic role of direct muscle stimulation and the rapid quantification of myosin/actin ratio based on electrophoresis are also presented.

SUMMARY

Basic and clinical research is unraveling the pathophysiological mechanisms of critical illness myopathy and polyneuropathy, and methods for rapid diagnosis are actively investigated. Future studies should better define the population at risk of developing CIM and CIP. In fact, although sepsis, multi-organ failure and steroids are often cited as risk factors, uncertainty remains due to the poor methodological quality of studies, or because of inferences that are exclusively based on animal studies. New simplified diagnostic techniques and machines for electrophysiological investigations of peripheral nerves and muscles in the intensive-care unit (ICU) patient would also be welcome.

Critical illness polyneuropathy and myopathy in patients with acute respiratory distress syndrome

OBJECTIVE

Critical illness polyneuropathy/myopathy (CIP/CIM) is frequently described in critically ill patients who survive severe sepsis. Clinically relevant paresis is major symptom of CIP/CIM. We aimed at determining risk factors and diagnostic value of electrophysiologic testing for CIP/CIM in patients with acute respiratory distress syndrome (ARDS).

DESIGN

Single-center, retrospective analysis, using charts.

SETTING

University medical center.

PATIENTS

Fifty consecutive ARDS patients in our intensive care unit.

INTERVENTIONS

Patient characteristics and clinical course were analyzed. All patients received early electrophysiologic testing. CIP/CIM was diagnosed by the presence of clinical relevant paresis.

MEASUREMENTS AND MAIN RESULTS

Clinically relevant paresis was confirmed in 27 ARDS patients (60%), whereas in 18 patients no paresis was determined (controls); five patients died before clinical assessment of paresis was feasible. Patients with paresis were older, had more days on mechanical ventilation, and had increased intensive care unit length of stay compared with controls. Patients who developed paresis had elevated daily peak blood glucose levels during 28 days of intensive care unit treatment: 166 (134, 200) mg/dL in CIP/CIM patients vs. 144 (132, 161) mg/dL in controls (median, quartiles). Twenty-five of 27 patients with paresis revealed reduced motor unit potentials, fibrillation potentials, or positive sharp waves on early electrophysiologic testing indicating CIP/CIM, whereas 16 of 18 control patients did not.

CONCLUSIONS

In ARDS patients, paresis is a frequent complication causing prolonged mechanical ventilation and intensive care unit length of stay. An association between hyperglycemia and CIP/CIM has been found. However, since this is a retrospective survey, a causal relation is not clearly supported. In this study, the use of early electrophysiologic testing in ARDS patients was a valuable diagnostic tool for detecting CIP/CIM.

Risk factors for critical illness polyneuromyopathy

Although numerous clinical, laboratory, and pharmacological variables have been reported as significant risk factors for critical illness polyneuromyopathy (CIPM), there is still no consensus on the aetiology of this condition. Objectives of the study were to assess the clinical and electrophysiological incidence and risk factors for CIPM.A cohort of critically ill patients was observed prospectively for a one-month period and the association between neuromuscular involvement and various potential risk factors was evaluated. Sixty one critically ill patients completed the follow-up (30 women, 31 men, median age 59 years).CIPM development was detected clinically in 17 patients (27.9 %) and electrophysiologically in 35 patients (57.4 %). CIPM was significantly associated with the presence and duration of systemic inflammatory response syndrome and the severity of multiple, respiratory, central nervous, and cardiovascular organ failures. The median duration of mechanical ventilation was significantly longer in patients with CIPM than in those without (16 vs 3 days, p<0.001). Independent predictors of CIPM obtainable within the 1(st) week of critical illness were the admission sequential organ failure assessment score (odds ratio [OR], 1.15; 95% confidence interval [CI], 1.02-1.36), the 1(st) week total sequential organ failure assessment scores (OR, 1.14; 95 % CI, 1.06-1.46) and the 1(st) week duration of systemic inflammatory response syndrome (OR, 1.05; 95% CI, 1.01-1.15). They were able to correctly predict the development of CIPM at the end of the 1(st) week in about 80% of critically ill cases.In conclusion, the presence and duration of systemic inflammatory response syndrome and the severity of multiple and several organ failures are associated with increased risk of the development of CIPM.

The utility of muscle biopsy

Despite major advances in molecular genetics, histopathologic evaluation of muscle biopsy specimens continues to provide important diagnostic information in patients with suspected muscle diseases and in patients with vasculitic neuropathies. Muscle biopsy specimens are used in diagnosing many inherited as well as inflammatory and toxic myopathies. Furthermore, the study of muscle histopathology can also enhance our understanding of disease pathogenesis.

Electrophoretic determination of the myosin/actin ratio in the diagnosis of critical illness myopathy

OBJECTIVE

To develop a rapid method to quantify myosin in muscle biopsy specimens from patients with critical illness myopathy (CIM).

DESIGN

Percutaneous muscle biopsy specimens at different stages of CIM were examined by light microscopy and transmission electron microscopy (TEM) and by horizontal pore gradient SDS electrophoresis (SDS-PAGE). The myosin/actin ratio was calculated densitometrically. Neurophysiological examinations were performed at various times during the course of CIM.

SETTING

All patients were treated in intensive care units at Karolinska Hospital.

PATIENTS AND PARTICIPANTS

We studied 11 patients with CIM, 5 patients with axonal neuropathies, and 42 control individuals.

MEASUREMENTS AND RESULTS

The histopathological changes included in all cases muscle fiber atrophy, degeneration, regeneration, nuclear changes, and reduction in myofibrillar ATPase activity in both type I and II fibers. In severely affected muscles fiber type differentiation was lost. On TEM preferential loss of thick filaments was the dominant finding. In some patients changes were present only in parts of the specimen. The neurophysiological examinations indicated myopathy in five patients and combined myopathy and neuropathy in five and suggested neuropathy in one. The SDS-pore PAGE used showed a technical variation of 4-5%. Quantitative results were obtained within 1 day and night. The mean value of the myosin/actin ratio in controls was 1.37+/-0.21 and in CIM patients 0.37+/-0.17, without overlapping with the control values.

CONCLUSIONS

Considering the diagnostic difficulty using morphological and neurophysiological methods, especially in early stages of CIM, we suggest including SDS-pore PAGE to determine the myosin/actin ratio for rapid diagnosis of CIM.

Critical illness polyneuromyopathy: the electrophysiological components of a complex entity

OBJECTIVE

To evaluate the spectrum and time profile of electrophysiological parameters in the detection of neuromuscular involvement in critically ill patients and establish their correlation with biopsy findings.

DESIGN

Prospective clinical and neurophysiological study.

SETTING

One general and one neurological intensive care unit in a university hospital.

PATIENTS

Forty-six critically ill patients with failure of at least two organ systems were enrolled and completed the 1-month follow up.

INTERVENTIONS

Detailed clinical and electrophysiological evaluation including direct muscle stimulation was performed in all cases on entry and at the end of the follow-up. Muscle biopsy was performed in 11, and sural nerve biopsy in 5, cases.

MEASUREMENTS AND RESULTS

Electrophysiological signs of new or progressing neuromuscular involvement at the end of the first month were detected in 26 patients (56%) and could be classified into three groups: "pure motor syndrome" (12 cases), combined motor syndrome and sensory polyneuropathy (13 cases) and isolated sensory polyneuropathy (1 case). Direct muscle stimulation showed decreased muscle membrane excitability in 11 of these abnormal cases. Muscle biopsy disclosed various myopathic abnormalities in all 11 cases examined with motor syndrome, in 7 of them in association with denervation/re-innervation changes.

CONCLUSIONS

Electrophysiological and histological examinations showed significant overlapping of several pathogenic components of neuromuscular involvement in critically ill patients, namely decreased muscle excitability, myopathy, axonal motor neuropathy and sensory neuropathy. The characterisation of the electrophysiological components of a complex polyneuromyopathy is preferred to the strict categorisation of abnormalities into critical illness myopathy and polyneuropathy.

Myopathies

Electrodiagnostic studies are an important adjunct to the clinical examination of a patient with a suspected myopathy; however, the clinical examination is crucial in making an accurate diagnosis, because electrodiagnostic studies have only a limited role in delineating with certainty the underlying myopathic disorder. Hereditary and acquired myopathies are reviewed in this article, with particular emphasis on distinguishing clinical and electrodiagnostic features. The hereditary myopathies that are discussed include the muscular dystrophies and the congenital distal mitochondrial, and metabolic myopathies. Acquired myopathies, including inflammatory, endocrine, and toxic myopathies, as well as those associated with systemic illness, are briefly reviewed.

Continuous nervous system monitoring, EEG, the bispectral index, and neuromuscular transmission

In critically ill patients, the central nervous system remains vulnerable to multiple insults including ischemia, hemorrhagic events, and encephalopathy. The peripheral nervous system is vulnerable in the setting of neuro-muscular blockade (NMB), related drug-drug interactions, and drug-clinical state interactions. Optimal assessment of the nervous system is done by means of the clinical neurological examination. In this manner, orientation, arousal, and responsiveness to stimulation provide feedback on focal and global stability of the central nervous system. Where clinical evaluation is compromised, such as with deep sedation and NMB, risk of undetected seizure activity, and/or progression of neurological injury increases dramatically. A patient receiving NMB risks breakthrough awareness and pain. Long-term complications of NMB including prolonged weakness or paralysis as well as post-traumatic stress dramatically increase morbidity and length of stay. Technologies such as electroencephalogram (EEG) and bispectral index (BIS trade mark ) monitoring are effective for assessing cerebral function as well as level of sedation or arousal, respectively, in patients with a compromised neurological assessment. Neuromuscular transmission (NMT) monitoring by means of peripheral nerve stimulation and assessment of the evoked response may be utilized, within the context of clinical assessment, to determine level of chemical paralysis and minimize dosing of NMB agents. This article explores utilization and differentiates technologies such as EEG, BIS, and NMT monitoring. Monitoring parameters are illustrated using a case study approach.

Critical illness myopathy and polyneuropathy

Neuromuscular weakness commonly develops in the setting of critical illness. This weakness delays recovery and often causes prolonged ventilator dependence. An axonal sensory-motor polyneuropathy, critical illness polyneuropathy (CIP), is seen in up to one third of critically ill patients with the systemic inflammatory response syndrome (usually due to sepsis). An acute myopathy, critical illness myopathy (CIM), frequently develops in a similar setting, often in association with the use of corticosteroids and/or nondepolarizing neuromuscular blocking agents. These patients are often difficult to evaluate due to the limitations imposed by the critical care setting and may be further complicated by the presence of both CIP and CIM in varying degrees. This paper reviews the clinical and electrophysiologic features of these disorders, as well as the putative pathophysiology. In the case of CIM, an animal model has provided evidence that weakness in this disorder is caused by muscle membrane inexcitability due to altered membrane sodium currents and loss of myosin thick filaments.

Neuromuscular disorders in the intensive care unit

Neuromuscular disorders encountered in the ICU can be categorized as muscular diseases that lead to ICU admission and those that are acquired in the ICU. This article discusses three neuromuscular disorders can lead to ICU admission and have a putative immune-mediated pathogenesis: the Guillian-Barré syndrome, myasthenia gravis, and dermatomyositis/polymyositis. It also reviews critical care polyneuropathy and ICU acquired myopathy, two disorders that, alone or in combination, are responsible for nearly all cases of severe ICU acquired muscle weakness.

Nerve conduction studies in selected peripheral nerve disorders

PURPOSE OF REVIEW

The physiological properties of nerve and muscle are influenced by pathological changes and the aim of this review is to discuss recent contributions of electrophysiological studies to the understanding and diagnosis of selected peripheral nerve disorders. The relationships between pathology and physiology emphasize the close interdependence between electrophysiological studies, clinical deficits and other laboratory information. Attention should be paid to the strengths and limitations of electrophysiological methods, considering their impact on diagnosis and treatment of patients.

RECENT FINDINGS

Several studies have shown particular pathophysiological profiles associated with different antibody subtypes in autoimmune peripheral neuropathies and this association further supports the suggestion of pathological specificity in both acute and chronic neuropathy. The sensitivity and specificity of physiological profiles therefore become increasingly important since some of these neuropathies are accessible to treatment. On the other hand, the pathophysiological and clinical profiles may be heterogeneous in patients with some disorders. This could be related to a more indistinct division between different types of pathology with increased understanding of pathogenetic mechanisms. Moreover, new insights into disturbed axonal function have stimulated attempts to develop methods to explore normal and diseased human nerve function.

SUMMARY

The exploration of axonal membrane and ion-channel function has become accessible using studies of excitability and are of potential value where conventional studies only provide nonspecific evidence of the number of fibers and the integrity of myelin. These studies will presumably become increasingly important in the years ahead considering the lack of understanding of the functional disturbances in axonal neuropathies.

Critical illness myopathy

Acute myopathy is a common problem in intensive care units. Those at highest risk for developing critical illness myopathy are exposed to intravenous corticosteroids and paralytic agents during treatment of various illnesses. Diffuse weakness and failure to wean from mechanical ventilation are the most common clinical manifestations. Serum creatine kinase levels are variable. Electrodiagnostic studies reveal findings of a myopathic process, often with evidence of muscle membrane inexcitability. Based on animal model studies, the loss of muscle membrane excitability may be related to inactivation of sodium channels at the resting potential. In addition, human and animal pathologic studies reveal characteristic loss of myosin with relative preservation of other structural proteins. In some patients, there is also upregulation of proteolytic pathways, involving calpain and ubiquitin, in conjunction with increased apoptosis. Fortunately, the disorder is reversible, but there may be considerable morbidity.

Critical illness polyneuropathy

Critical illness polyneuropathy (CIP) is a syndrome that was first extensively described in the early 1980s, mainly in patients with failure to wean from mechanical ventilation. The syndrome is further characterized by limb muscle weakness, usually more pronounced distally than proximally, and is often accompanied by atrophy. The facial musculature is often strikingly spared. Reduced or absent deep-tendon reflexes and loss of peripheral sensation to light touch and pin prick often accompany the syndrome. Involvement of the phrenic nerve has been shown to further contribute to delayed weaning from the ventilator in many patients. The electrophysiologic studies are consistent with a predominantly motor and, often to a lesser extent, sensory axonal polyneuropathy. The incidence of CIP is high, with often more than 50% of patients in major medical and surgical critical care units suffering from the syndrome. The systemic inflammatory response syndrome (SIRS) is strongly associated with CIP and, among the multiorgan failure often seen in SIRS, CIP is thought to represent a neurologic manifestation of SIRS. The neurologic effects of SIRS are thought to be mediated by released mediators like cytokines and free radicals, affecting the microcirculation of the central and peripheral nervous system. Examination of the peripheral nervous system is often unreliable, and the only way to establish a definitive diagnosis is by performing electrophysiologic studies. Morbidity and mortality rates are high. If the underlying problem causing sepsis and/or SIRS can be treated successfully, full recovery from CIP can occur. This recovery often occurs in a matter of weeks in milder cases and in months in more severe cases. Knowledge of CIP is essential for intensivists and other specialists who care for critically ill patients. This review summarizes the current available literature on this topic.

Motor Unit Estimate Number in the Anterior Tibial Muscle: Normative Data versus Findings in Critically Ill Patients in Intensive Care Units

OBJECTIVES

To determine the number of motor units (MUNEs) in the anterior tibial muscle of normal subjects for comparison with those of severely paretic or paralytic muscles of critically ill patients in intensive care units.

RESULTS

The mean MUNE for 24 normal subjects (194 +/- 5; mean +/- standard deviation) was similar to that of the 22 patients with critical illness (184 +/- 10). However, both the mean amplitude of the evoked compound muscle action potential (CMAP) and of the single motor unit action potential (S-MUAP) among patients were approximately one third of those in normal subjects.

CONCLUSION

Critically ill patients in this study demonstrated normal MUNEs with reduced CMAP and S-MUAP amplitudes in the setting of severe clinical weakness, suggestive of predominantly myopathic injury. MUNE may provide a valuable tool for distinguishing between neuropathy and myopathy in critically ill patients.