Risk factors in critical illness myopathy during the early course of critical illness: a prospective observational study

ICU-acquired muscle weakness is a serious complication of critical illness. It has been recognized as the clinical manifestation of an ICU-acquired peripheral neuromuscular pathology [1] that, with regard to muscle pathology, is characterized by atrophy of type II muscle fibres and thick filament myopathy [2].

Diagnosis of critical illness myopathy (CIM) is either based on clinical proof of muscle weakness after awakening from analgesia and sedation, measurement of short duration low amplitude muscle unit potentials, depending on voluntary muscle contraction, or histological confirmation of muscle pathology [2]. As muscle biopsies are not routinely taken at the onset of critical illness neither approach is suitable to diagnose CIM at early critical illness.

Recent studies described measurements of muscle membrane excitability after direct muscle stimulation as a valid electrophysiological marker indicating CIM in critically ill patients [2‐5]. As the investigation of muscle membrane excitability is independent of voluntary muscle contraction, it enables the detection of CIM during early critical illness when clinical evaluation is generally not applicable. Attempts to determine predisposing risk factors for CIM have yielded mixed results [6‐8]. Illness severity [9], duration of immobility [10], systemic inflammation, hyperglycemia [8, 11, 12], and the use of corticosteroids or neuromuscular blocking agents [6] are disputed risk factors. Recent data suggest a relation between growth factor-mediated dysregulation of glucose and protein metabolism due to systemic inflammation and the development of myopathy [13, 14].

Measurement of muscle membrane excitability during early critical illness offers a unique opportunity to better understand and investigate early markers and potential risk factors for non-excitable muscle membrane.

The objective of this study is to investigate predisposing risk factors for the development of non-excitable muscle membrane during early critical illness, particularly considering concentration patterns of the insulin-like growth factor (IGF)-system prior to first proof of pathologically reduced muscle membrane excitability.

This study presents a subanalysis of 40 patients of a recent prospective observational study [5] that investigated the predictive value of certain electrophysiological measurements on the development of ICU-acquired weakness. Validating muscle membrane excitability at the onset of critical illness turned out to be most valuable for an early prediction of ICU-acquired weakness in immobile, sedated patients adding important information to clinical estimation of the patients' motor function upon emergence from sedation. Mechanically ventilated ICU patients on an operative ICU who featured simplified acute physiology (SAPS-II) scores of 20 or higher on three successive days within one week after ICU admission were included in the study. Sequential electrophysiological measurements including measurement of muscle membrane excitability had been performed at study enrollment and every three days until pathological findings were detected or clinical evaluation of muscle strength by Medical Research Council (MRC) score was possible.

Details of electrophysiological measurements are reported elsewhere [5], in brief we assessed the compound muscle amplitude with concentric needle electrodes after direct stimulation of the muscle. Comparable with measurement of compound muscle amplitude after nerve stimulation, this is a quantitative method and the normal data in critically ill patients are 3 mV or more [3]. Patients with non-excitable muscle membrane after direct muscle stimulation show reduced amplitudes of less than 3 mV, whereas patients with an acute neuropathy show normal amplitudes within the muscle after direct muscle stimulation. Measurement of muscle membrane excitability diagnoses myopathy but cannot exclude an additional axonal motor neuropathy. Here we focused on a risk factor analysis of non-excitable muscle membrane from the beginning of critical illness until first proof of non-excitable muscle membrane. Hence, we excluded patients being pretreated on other ICUs for more than 24 hours and only included values of risk factors of the first eight days of critical illness in the analysis. The study was approved by our local review board. Written informed consent was obtained from legal proxies.

Patients were treated following standard operating procedures of intensive care incorporating severe sepsis bundles [15]. Systemic inflammation, sepsis or severe sepsis [see Table E1 in Additional file 1] accompanied by organ dysfunction [see Table E2 in Additional file 1] was classified according to consensus conference criteria [16, 17].

Inflammatory cytokines (IL-6 and IL-10), IGF-I and its binding proteins (IGFBP-I, IGFBP-III) were analysed from blood samples, drawn between days 3 and 7 as well as between days 8 and 10 after ICU admission.

Hemodynamic parameters and blood glucose levels were recorded four times daily considering least favorable values within six-hour intervals. Illness severity, SAPS-II [18], sepsis-related organ failure assessment (SOFA) [19] and other clinical data were recorded on a daily basis. Methods are further described in Additional file 1.

In this observational study we investigated predisposing risk factors leading to non-excitable muscle membrane indicating CIM during early critical illness. The main finding was a significant relation between muscle membrane inexcitability, disease severity and IL-6 plasma levels.

In the absence of a reliable clinical parameter identifying patients at risk of developing CIM during early critical illness, when motor function is not assessable due to analgesia and sedation, current data on risk factors leading to CIM are derived mostly from prospective cohort studies relating data from ICU admission with patients' motor function once assessable [6]. One excellent study investigating risk factors for combined critical illness neuromyopathy [9] during early critical illness showed that illness severity, determined through acute physiology and chronic health evaluation (APACHE) III scores, predicted the later development of critical illness neuromyopathy. In our study, sequential measurements of muscle membrane excitability offers the opportunity to focus on CIM and to determine the time frame in which CIM originates during early critical illness, thereby improving stratification of predisposing risk factors for CIM.

Univariate analysis indicated illness severity, IL-6, hemodynamic impairment, decreased insulin sensitivity as well as analgesia and sedation as predisposing risk factors. However, only IL-6 and dosage of analgesia emerged as independent risk factors from multivariate analysis. Interestingly, we did not observe any significant relation between development of non-excitable muscle membrane and application of low-dose hydrocortisone, aminoglycosides or neuromuscular blocking agents, which have frequently been incriminated as being involved in the development of CIM.

Large prospective randomized studies have shown that glycemic control is associated with the development of neuromuscular dysfunction [11, 12]. In our study, we did not observe a difference in blood glucose levels between dmCMAP normal and abnormal patients because we aimed for glycemic control below 150 mg/dl in all of our patients. However, despite comparable blood glucose levels between the groups, IGFBP-1 was significantly higher in abnormal dmCMAP patients. In agreement with other reports [21, 22] we consider increased IGFBP-1 as a parameter indicative of impaired insulin sensitivity. Hence, our data suggest that significantly impaired insulin sensitivity during early critical illness was related to development of abnormal muscle membrane excitability.

In parallel, dmCMAP abnormal patients revealed a significant hyperosmotic state within the first days of critical illness. Hyperosmolality is related to illness severity [23] and has been described as a risk factor for critical illness neuropathy [24]. It is worth noting that the study by Garnacho-Montero and colleagues [24] did not differentiate between myopathy and neuropathy. In our study, hyperosmolality during the first days after ICU admission was significantly related to electrophysiological-proven muscle pathology. It can be speculated that this hyperosmotic state may have led to osmotic stress-induced reduction of cellular insulin sensitivity in our patients, which has been shown in adipocytes under experimental conditions [25].

However, alteration of insulin sensitivity and plasma osmolality are most likely related to systemic inflammation in our study. Our data are in agreement with the general perception that systemic inflammation and sepsis-related organ dysfunction are major triggers for the development of CIM [6, 8, 9, 26]. Earlier experimental data inducing inflammation in rats by intravenous inoculation of endotoxin showed that IL-6 increased muscle fatigue [27] and decreased muscle contractility of the diaphragm [28]. Interestingly, recent clinical data indicate a relation between IL-6 and reduced muscle strength in elderly people [29]. Our data reveal a possible role of IL-6 in the development of non-excitable muscle membrane during early critical illness finally leading to muscle weakness.

IL-6 seems to be an important mediator leading to muscle protein breakdown [30]. One mechanism may lie in the inhibition of growth factor-mediated (e.g. IGF-I) intracellular signaling by IL-6 [13, 31]. IGF-I plays a central role in glucose uptake and protein synthesis, and was shown to be downregulated in inflammation and sepsis [13]. Impairment of IGF-I may be due to inflammation-induced upregulation of high-affinity IGFBP-1 which prevents IGF-I receptor binding [32]. In vitro [33] and later in vivo [14], it has been shown that an increase of IGFBP-1 reduced the IGF-I-mediated glucose uptake and reduced protein synthesis in skeletal muscle within an experimental setting. In line with this, our data suggest that impaired growth factor-mediated intracellular signaling due to systemic inflammation may be involved in the development of CIM.

Corticosteroids are controversially discussed as aggravating factors of CIM [6, 7, 9, 34]. It is well established that high-dose application of corticosteroids, for example in patients with chronic obstructive pulmonary disease, results in selective loss of thick myosin filaments in skeletal muscle fibers [35], a so-called steroid-induced myopathy [36].

Nevertheless, these reports refer to steroid myopathy as a result of high-dose steroid application. A link between 'low-dose hydrocortisone' treatment as adjunctive therapy during septic shock and development of CIM has been postulated [15], but never proven. Several data indicate that moderate doses of steroids do not prolong mechanical ventilation due to muscle weakness but are related to significantly more ventilator-free days and earlier spontaneous breathing capacity [37]. In an earlier study we did not observe an association between low-dose hydrocortisone application and development of paresis [5, 34]. In this study we were able to show that low-dose hydrocortisone application does not provoke impaired muscle membrane excitability, suggesting that steroid involvement in CIM development is dose dependent [8].

Furthermore, dosage of analgesics and sedatives was significantly associated with the development of non-excitable muscle membrane. Interpreting higher doses of analgesics and sedatives as higher degrees of immobilization, this finding is in line with recent studies describing that immobilization aggravated neuromuscular weakness in an experimental setting [38] and that early physical mobilization resulted in a better clinical outcome of motor function [10].

For clinicians it is difficult to estimate patients at risk for the development of CIM. The APACHE-III score has been cited as being able to identify patients at risk for critical illness neuromyopathy [9]. In our study we used the SOFA score because it is widely accepted in the ICU setting, and has been validated to monitor organ dysfunction-related to sepsis [19]. Our results indicate that a SOFA score of 10 or above and/or IL-6 plasma levels of 230 pg/ml or more at the onset of critical illness disclose high-risk patients for the development of non-excitable muscle membrane.

The following limitations of this study need to be addressed. Although we observed a statistically significant effect for IL-6 as a main risk factor for non-excitable muscle membrane, it has to be stressed that the overall effect was small, which may be due to small sample size. It also needs to be mentioned that blood samples were collected at two different time points only and that the course of inflammatory parameters was not followed daily. However, this was designed as a pilot study for hypothesis generation. The clinical significance has to be addressed in further studies.

Systemic inflammation during early critical illness turned out to be the main risk factor for the development of non-excitable muscle membrane indicating CIM. It may be hypothesized that inflammation-induced impairment of growth factor-mediated intracellular signaling is involved in the pathophysiology of CIM. Furthermore, adjunctive treatment with low-dose hydrocortisone during septic shock was not associated with development of CIM.