Electrophysiologic techniques in critical illness-associated weakness

Differentiation of Critical Illness Myopathy and Critical Illness Neuropathy Using Nerve Ultrasonography

PURPOSE

Intensive care unit-acquired weakness occurs frequently in intensive care unit patients, including critical illness myopathy (CIM) and critical illness polyneuropathy (CIPN). The authors present a prospective study to assess the ultrasound pattern sum score to differentiate between confirmed CIM, sensory neuropathy, and CIPN cases.

METHODS

Cross-sectional areas of 12 predefined nerve segments in 16 patients were sonographically examined. Single-nerve cross-sectional areas and ultrasound pattern sum score values were compared; results are given as P -values and receiver operating characteristic area under the curve (AUC).

RESULTS

In neuropathy, significant single-nerve cross-sectional area enlargement was observed in the median ( P = 0.04), ulnar ( P = 0.04), and fibular nerves ( P = 0.0003). The ultrasound pattern sum score could reliably differentiate between pure CIM and neuropathy ( P = 0.0002, AUC 0.92), CIM and sensory neuropathy ( P = 0.001, AUC 0.88), and CIM and CIPN ( P = 0.007, AUC 0.92), but not between sensory neuropathy and CIPN ( P = 0.599, AUC 0.48).

CONCLUSIONS

Nerve ultrasonography reliably identifies neuropathy in intensive care unit-acquired weakness, yet cannot differentiate between sensory neuropathy and CIPN. A standardized ultrasound algorithm can serve as a fast bedside test for the presence of neuropathy in intensive care unit-acquired weakness.

Critical illness-associated weakness and related motor disorders

Weakness of limb and respiratory muscles that occurs in the course of critical illness has become an increasingly common and serious complication of adult and pediatric intensive care unit patients and a cause of prolonged ventilatory support, morbidity, and prolonged hospitalization. Two motor disorders that occur singly or together, namely critical illness polyneuropathy and critical illness myopathy, cause weakness of limb and of breathing muscles, making it difficult to be weaned from ventilatory support, commencing rehabilitation, and extending the length of stay in the intensive care unit, with higher rates of morbidity and mortality. Recovery can take weeks or months and in severe cases, and may be incomplete or absent. Recent findings suggest an improved prognosis of critical illness myopathy compared to polyneuropathy. Prevention and treatment are therefore very important. Its management requires an integrated team approach commencing with neurologic consultation, creatine kinase (CK) measurement, detailed electrodiagnostic, respiratory and neuroimaging studies, and potentially muscle biopsy to elucidate the etiopathogenesis of the weakness in the peripheral and/or central nervous system, for which there may be a variety of causes. These tenets of care are being applied to new cases and survivors of the coronavirus-2 disease pandemic of 2019. This chapter provides an update to the understanding and approach to critical illness motor disorders.

Early detection of evolving critical illness myopathy with muscle velocity recovery cycles

OBJECTIVE

To investigate the sensitivity of muscle velocity recovery cycles (MVRCs) for detecting altered membrane properties in critically ill patients, and to compare this to conventional nerve conduction studies (NCS) and quantitative electromyography (qEMG).

METHODS

Twenty-four patients with intensive care unit acquired weakness (ICUAW) and 34 healthy subjects were prospectively recruited. In addition to NCS (median, ulnar, peroneal, tibial and sural nerves) and qEMG (biceps brachii, vastus medialis and anterior tibial muscles), MVRCs with frequency ramp were recorded from anterior tibial muscle.

RESULTS

MVRC and frequency ramp parameters showed abnormal muscle fiber membrane properties with up to 100% sensitivity and specificity. qEMG showed myopathy in 15 patients (63%) while polyneuropathy was seen in 3 (13%). Decreased compound muscle action potential (CMAP) amplitude (up to 58%) and absent F-waves (up to 75%) were frequent, but long duration CMAPs were only seen in one patient with severe myopathy.

CONCLUSIONS

Altered muscle fiber membrane properties can be detected in patients with ICUAW not yet fulfilling diagnostic criteria for critical illness myopathy (CIM). MVRCs may therefore serve as a tool for early detection of evolving CIM.

SIGNIFICANCE

CIM is often under-recognized by intensivists, and large-scale longitudinal studies are needed to determine its incidence and pathogenesis.

From skeletal muscle weakness to functional outcomes following critical illness: a translational biology perspective

Intensive care unit acquired weakness (ICUAW) is now a well-known entity complicating critical illness. It increases mortality and in the critical illness survivor it is associated with physical disability, substantially increased health resource utilisation and healthcare costs. Skeletal muscle wasting is a key driver of ICUAW and physical functional outcomes in both the short and long term. To date, there is no intervention that can universally and consistently prevent muscle loss during critical illness, or enhance its recovery following intensive care unit discharge, to improve physical function. Clinical trials of early mobilisation or exercise training, or enhanced nutritional support have generated inconsistent results and we have no effective pharmacological interventions. This review will delineate our current understanding of the mechanisms underpinning the development and persistence of skeletal muscle loss and dysfunction in the critically ill individual, highlighting recent discoveries and clinical observations, and utilisation of this knowledge in the development of novel therapeutics.

Electrophysiologic alterations in the excitability of the sciatic and vagus nerves during early stages of sepsis

Background

Nonspecific and delayed diagnosis of neurologic damage contributes to the development of neuropathies in patients with severe sepsis. The present study assessed the electrophysiologic parameters related to the excitability and conductibility of sciatic and vagus nerves during early stages of sepsis.

Materials and methods

Twenty-four hours after sepsis induced by cecal ligation and puncture (CLP) model, sciatic and vagus nerves of septic (CLP group) and control (sham group) rats were removed, and selected electric stimulations were applied to measure the parameters of the first and second components of the compound action potential. The first component originated from fibers with motor and sensory functions (Types Aα and Aβ fibers) with a large conduction velocity (70-120 m/s), and the second component originated from fibers (Type Aγ) with sensorial function. To evaluate the presence of sensorial alterations, the sensitivity to non-noxious mechanical stimuli was measured by using the von Frey test. Hematoxylin and eosin staining of the nerves was performed.

Results

We observed an increase of rheobase followed by a decrease in the first component amplitude and a higher paw withdrawal threshold in response to the application of von Frey filaments in sciatic nerves from the CLP group compared to the sham group. Differently, a decrease in rheobase and an increase in the first component amplitude of vagal C fibers from CLP group were registered. No significant morphologic alteration was observed.

Conclusion

Our data showed that the electrophysiologic alterations in peripheral nerves vary with the fiber type and might be identified in the first 24 h of sepsis, before clinical signs of neuromuscular disorders.

Electrodiagnostic studies in the intensive care unit: A comparison study 2 decades later

INTRODUCTION

Since the late 1980s, critical illness myopathy (CIM) and critical illness polyneuropathy (CIP) have been increasingly recognized in the intensive care unit (ICU). We explored whether these causes of ICU weakness were now more likely to lead to electrodiagnostic studies (EDX) at our institution than they were 19-20 years earlier.

METHODS

We reviewed 100 consecutive ICU patients who underwent EDX from 2009 to 2015 and compared them to a previously reported study population from 1990-1995.

RESULTS

Thirty-seven (39%) had CIM, CIP, or both versus 55% in the previous study (P = 0.04). Thirty-four (36%) were diagnosed with "traditional" pre-ICU causes of weakness, such as motor neuron disease or Guillain-Barre syndrome, versus 29% in the earlier study (P = 0.3).

DISCUSSION

CIM and CIP continue to be common disorders that lead to ICU EDX, but their proportion declined compared with 19-20 years earlier, possibly due to the perceived role and selective use of EDX in the ICU. Muscle Nerve 57: 772-776, 2018.

Optimizing testing methods and collection of reference data for differentiating critical illness polyneuropathy from critical illness MYOPATHIES

INTRODUCTION

In severe acute quadriplegic myopathy in intensive care unit (ICU) patients, muscle fibers are electrically inexcitable; in critical illness polyneuropathy, the excitability remains normal. Conventional electrodiagnostic methods do not provide the means to adequately differentiate between them. In this study we aimed to further optimize the methodology for the study of critically ill ICU patients and to create a reference database in healthy controls.

METHODS

Different electrophysiologic protocols were tested to find sufficiently robust and reproducible techniques for clinical diagnostic applications.

RESULTS

Many parameters show large test-retest variability within the same healthy subject. Reference values have been collected and described as a basis for studies of weakness in critical illness.

CONCLUSIONS

Using the ratio of neCMAP/dmCMAP (response from nerve and direct muscle stimulation), refractory period, and stimulus-response curves may optimize the electrodiagnostic differentiation of patients with critical illness myopathy from those with critical illness polyneuropathy.

The Sick and the Weak: Neuropathies/Myopathies in the Critically Ill

Critical illness polyneuropathies (CIP) and myopathies (CIM) are common complications of critical illness. Several weakness syndromes are summarized under the term intensive care unit-acquired weakness (ICUAW). We propose a classification of different ICUAW forms (CIM, CIP, sepsis-induced, steroid-denervation myopathy) and pathophysiological mechanisms from clinical and animal model data. Triggers include sepsis, mechanical ventilation, muscle unloading, steroid treatment, or denervation. Some ICUAW forms require stringent diagnostic features; CIM is marked by membrane hypoexcitability, severe atrophy, preferential myosin loss, ultrastructural alterations, and inadequate autophagy activation while myopathies in pure sepsis do not reproduce marked myosin loss. Reduced membrane excitability results from depolarization and ion channel dysfunction. Mitochondrial dysfunction contributes to energy-dependent processes. Ubiquitin proteasome and calpain activation trigger muscle proteolysis and atrophy while protein synthesis is impaired. Myosin loss is more pronounced than actin loss in CIM. Protein quality control is altered by inadequate autophagy. Ca(2+) dysregulation is present through altered Ca(2+) homeostasis. We highlight clinical hallmarks, trigger factors, and potential mechanisms from human studies and animal models that allow separation of risk factors that may trigger distinct mechanisms contributing to weakness. During critical illness, altered inflammatory (cytokines) and metabolic pathways deteriorate muscle function. ICUAW prevention/treatment is limited, e.g., tight glycemic control, delaying nutrition, and early mobilization. Future challenges include identification of primary/secondary events during the time course of critical illness, the interplay between membrane excitability, bioenergetic failure and differential proteolysis, and finding new therapeutic targets by help of tailored animal models.

Critical illness polyneuropathy and myopathy: a systematic review

Critical illness polyneuropathy and critical illness myopathy are frequent complications of severe illness that involve sensorimotor axons and skeletal muscles, respectively. Clinically, they manifest as limb and respiratory muscle weakness. Critical illness polyneuropathy/myopathy in isolation or combination increases intensive care unit morbidity via the inability or difficulty in weaning these patients off mechanical ventilation. Many patients continue to suffer from decreased exercise capacity and compromised quality of life for months to years after the acute event. Substantial progress has been made lately in the understanding of the pathophysiology of critical illness polyneuropathy and myopathy. Clinical and ancillary test results should be carefully interpreted to differentiate critical illness polyneuropathy/myopathy from similar weaknesses in this patient population. The present review is aimed at providing the latest knowledge concerning the pathophysiology of critical illness polyneuropathy/myopathy along with relevant clinical, diagnostic, differentiating, and treatment information for this debilitating neurological disease.

Muscle ultrasound for early assessment of critical illness neuromyopathy in severe sepsis

INTRODUCTION

Muscle ultrasound is emerging as a promising tool in the diagnosis of neuromuscular diseases. The current observational study evaluates the usefulness of muscle ultrasound in patients with severe sepsis for assessment of critical illness polyneuropathy and myopathy (CINM) in the intensive care unit.

METHODS

28 patients with either septic shock or severe sepsis underwent clinical neurological examinations, muscle ultrasound, and nerve conduction studies on days 4 and 14 after onset of sepsis. 26 healthy controls of comparable age underwent clinical neurological evaluation and muscle ultrasound only.

RESULTS

26 of the 28 patients exhibited classic electrophysiological characteristics of CINM, and all showed typical clinical signs. Ultrasonic echogenicity of muscles was graded semiquantitatively and fasciculations were evaluated in muscles of proximal and distal arms and legs. 75% of patients showed a mean echotexture greater than 1.5, which was the maximal value found in the control group. A significant difference in mean muscle echotexture between patients and controls was found at day 4 and day 14 (both p < 0.001). In addition, from day 4 to day 14, the mean grades of muscle echotexture increased in the patient group, although the values did not reach significance levels (p = 0.085). Controls revealed the lowest number of fasciculations. In the patients group, fasciculations were detected in more muscular regions (lower and upper arm and leg) in comparison to controls (p = 0.08 at day 4 and p = 0.002 at day 14).

CONCLUSIONS

Muscle ultrasound represents an easily applicable, non-invasive diagnostic tool which adds to neurophysiological testing information regarding morphological changes of muscles early in the course of sepsis. Muscle ultrasound could be useful for screening purposes prior to subjecting patients to more invasive techniques such as electromyography and/or muscle biopsy.

TRIAL REGISTRATION

German Clinical Trials Register, DRKS-ID: DRKS00000642.

[ICU acquired neuromyopathy]

ICU acquired neuromyopathy (IANM) is the most frequent neurological pathology observed in ICU. Nerve and muscle defects are merged with neuromuscular junction abnormalities. Its physiopathology is complex. The aim is probably the redistribution of nutriments and metabolism towards defense against sepsis. The main risk factors are sepsis, its severity and its duration of evolution. IANM is usually diagnosed in view of difficulties in weaning from mechanical ventilation, but electrophysiology may allow an earlier diagnosis. There is no curative therapy, but early treatment of sepsis, glycemic control as well as early physiotherapy may decrease its incidence. The outcomes of IANM are an increase in morbi-mortality and possibly long-lasting neuromuscular abnormalities as far as tetraplegia.

[Critical illness polyneuropathy and critical illness myopathy]

Critical illness polyneuropathy (CIP) and critical illness myopathy (CIM) are frequent complications in critically ill patients and both are associated with sepsis, systemic inflammatory response syndrome (SIRS) and multiorgan failure. Major signs are muscle weakness and problems of weaning from the ventilator. Both CIP and CIM lead to elongated times of ventilation, elongated hospital stay, elongated times of rehabilitation and increased mortality. Electrophysiological measurements help to detect CIP and CIM early in the course of the disease. State of the art sepsis therapy is the major target to prevent the development of CIP and CIM. Although no specific therapy of CIP and CIM has been established in the past, the diagnosis generally improves the therapeutic management (weaning from the ventilator, early physiotherapy, etc.). This review provides an overview of clinical and diagnostic features of CIP and CIM and summarizes current pathophysiological and therapeutic concepts.

[Intensive care unit-acquired weakness in the critically ill : critical illness polyneuropathy and critical illness myopathy]

Intensive care unit-acquired weakness (ICUAW) is a severe complication in critically ill patients which has been increasingly recognized over the last two decades. By definition ICUAW is caused by distinct neuromuscular disorders, namely critical illness polyneuropathy (CIP) and critical illness myopathy (CIM). Both CIP and CIM can affect limb and respiratory muscles and thus complicate weaning from a ventilator, increase the length of stay in the intensive care unit and delay mobilization and physical rehabilitation. It is controversially discussed whether CIP and CIM are distinct entities or whether they just represent different organ manifestations with common pathomechanisms. These basic pathomechanisms, however, are complex and still not completely understood but metabolic, inflammatory and bioenergetic alterations seem to play a crucial role. In this respect several risk factors have recently been revealed: in addition to the administration of glucocorticoids and non-depolarizing muscle relaxants, sepsis and multi-organ failure per se as well as elevated levels of blood glucose and muscular immobilization have been shown to have a profound impact on the occurrence of CIP and CIM. For the diagnosis, careful physical and neurological examinations, electrophysiological testing and in rare cases nerve and muscle biopsies are recommended. Nevertheless, it appears to be difficult to clearly distinguish between CIM and CIP in a clinical setting. At present no specific therapy for these neuromuscular disorders has been established but recent data suggest that in addition to avoidance of risk factors early active mobilization of critically ill patients may be beneficial.

Maximum inspiratory pressure, a surrogate parameter for the assessment of ICU-acquired weakness

BACKGROUND

Physical examination has been advocated as a primary determinant of ICU-acquired weakness (ICU-AW). The purpose of the study is to investigate ICU-AW development by using Maximum Inspiratory Pressure (MIP) as a surrogate parameter of the standardized method to evaluate patients' peripheral muscle strength.

METHODS

Seventy-four patients were recruited in the study and prospectively evaluated in a multidisciplinary university ICU towards the appearance of ICU-AW. APACHE II admission score was 16 ± 6 and ICU stay 26 ± 18 days. ICU-AW was diagnosed with the Medical Research Council (MRC) scale for the clinical evaluation of muscle strength. MIP was measured using the unidirectional valve method, independently of the patients' ability to cooperate.

RESULTS

A significant correlation was found between MIP and MRC (r = 0.68, p < 0.001). Patients that developed ICU-AW (MRC<48) had a longer weaning period compared to non ICU-AW patients (12 ± 14 versus 2 ± 3 days, p < 0.01). A cut-off point of 36 cmH2O for MIP was defined by ROC curve analysis for ICU-AW diagnosis (88% sensitivity,76% specificity). Patients with MIP below the cut-off point of 36 cmH2O had a significant greater weaning period (10 ± 14 versus 3 ± 3 days, p = 0.004) also shown by Kaplan-Meier analysis (log-rank:8.2;p = 0.004).

CONCLUSIONS

MIP estimated using the unidirectional valve method may be a potential surrogate parameter for the assessment of muscle strength compromise, useful for the early detection of ICU-AW.

Neuromuscular disorders in critically ill patients: review and update

Neuromuscular disorders that are diagnosed in the intensive care unit (ICU) usually cause substantial limb weakness and contribute to ventilatory dysfunction. Although some lead to ICU admission, ICU-acquired disorders, mainly critical illness myopathy (CIM) and critical illness polyneuropathy (CIP), are more frequent and are associated with considerable morbidity. Approximately 25% to 45% of patients admitted to the ICU develop CIM, CIP, or both. Their clinical features often overlap; therefore, nerve conduction studies and electromyography are particularly helpful diagnostically, and more sophisticated electrodiagnostic studies and histopathologic evaluation are required in some circumstances. A number of prospective studies have identified risk factors for CIP and CIM, but their limitations often include the inability to separate CIM from CIP. Animal models reveal evidence of a channelopathy in both CIM and CIP, and human studies also identified axonal degeneration in CIP and myosin loss in CIM. Outcomes are variable. They tend to be better with CIM, and some patients have longstanding disabilities. Future studies of well-characterized patients with CIP and CIM should refine our understanding of risk factors, outcomes, and pathogenic mechanisms, leading to better interventions.

Management of critical illness polyneuropathy and myopathy

A syndrome of generalized weakness, areflexia, and difficulty with weaning from a ventilator is a common clinical presentation in the critically ill patient, especially in the setting of sepsis, multiorgan failure, and hyperglycemia. At first believed to be a manifestation of nerve (critical illness neuropathy, CIN) or muscle (critical illness myopathy, CIM) dysfunction, our current conceptualization is as a spectrum (critical illness neuromuscular abnormalities, CINMA) that varies in extent and site(s) of involvement, but often a similar clinical presentation. Signs and symptoms of CINMA must be identified early to foster recovery and limit morbidity and mortality. The medical history is crucial in excluding preexisting neuromuscular conditions and electrodiagnostic testing helps to establish the diagnosis and prognostication. A stepwise approach to the management of a patient with CINMA is outlined, but avoiding potential medications, and ensuring supportive care are the primary interventions to consider. Recently intensive insulin therapy for hyperglycemia has been shown to lower the risk of CINMA and decrease the time of ventilatory support, but with a greater risk of hypoglycemia. Future therapeutic interventions will require a better understanding of disease pathogenesis, but may target proinflammatory cytokine and free-radical pathways, muscle gene expression, ion channel function, or proteolytic muscle protein mechanisms. Rehabilitation is an equally essential component in a patient's management. Although prognosis depends on the extent of the underlying muscle and nerve damage, mild persistent deficits are common and severe disability may be persistent.

Critical illness polyneuropathy and myopathy in the intensive care unit

Critical illness polyneuropathy (CIP) and critical illness myopathy (CIM) are major complications that occur in severely ill patients who require intensive care treatment. CIP and CIM affect the limb and respiratory muscles, and, as a consequence, they characteristically complicate weaning from the ventilator, increase the length of stay on the intensive care unit, and prolong physical rehabilitation. The basic pathophysiology of both disorders is complex and involves metabolic, inflammatory and bioenergetic alterations. It is unclear at present whether CIP and CIM are distinct entities, or whether they just represent different 'organ' manifestations of a common pathophysiological mechanism. This article provides an overview of the clinical and diagnostic features of CIP and CIM and discusses current pathophysiological and therapeutic concepts relating to these neuromuscular disorders.

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.

ICU-Acquired Weakness

Observational studies of patients receiving prolonged mechanical ventilation and other forms of critical care support have determined acquired neuromuscular disorders to be extremely common. Early studies used electrophysiologic investigations to diagnose critical illness polyneuropathy (CIP) and muscle biopsy to confirm critical illness myopathy (CIM). More recent approaches seek to obviate these invasive techniques and build on a standardized bedside neuromuscular examination to identify patients with acquired weakness syndromes. Serial examination in the alert patient may serve as a reasonable prognosticator for most patients. The importance of ICU-acquired weakness syndromes is supported by the observation that muscle wasting and weakness are among the most prominent long-term complications of survivors of ARDS. In addition, a strong association appears to exist between acquired weakness and protracted ventilator dependence, an important determinant of ICU length of stay. Multivariate analysis has identified several risk factors associated with increased incidence for ICU-acquired weakness, including severe systemic inflammation, medications (specifically, corticosteroids and neuromuscular blocking agents), glycemic control, and immobility. We advocate an approach to this common syndrome that identifies risk factors early in the hope of minimizing their impact.