The present study is the first prospective, randomized, double-blinded, placebo-controlled trial assessing the impact of IgM-enriched IVIG on CIPNM. To achieve a potentially optimal effect of IVIG, we included only severely ill patients with MOF, a SIRS/sepsis diagnosis and clinical signs of CIPNM.
However, IVIG did not mitigate CIPNM in the critically ill patients in the present trial. Neither CIP as determined by EPS of three nerves on days 0, 4, 7 and 14 nor CIM as assessed by the histological examination of muscle biopsies on days 0 and 14 were different in the IVIG group compared to the controls at any time point. Moreover, length of ICU stay and mortality were similar in both groups.
Mohr
et al. found some evidence in a retrospective chart analysis of IVIG being able to prevent CIPNM in critically ill patients using EPS [
15]. Based on the retrospective character of their analysis, the evidence has been regarded as weak and is in contrast with findings of our prospective, randomized, double-blinded placebo-controlled trial. However, Mohr
et al. started their IVIG treatment within 24 hours after onset of sepsis/multi organ failure and did not wait for the first clinical signs of CIPNM. As we administered IVIG only after the first clinical evidence of CIPNM at median five (three to seven) days after the start of the respective SIRS/sepsis episode, these two studies are not entirely comparable.
Rationale for the treatment strategy
The pathophysiologic rationale for using IVIG to treat CIPNM in the present study is based on the association of CIPNM with pro-inflammatory cytokines, such as TNF-α, IFN-γ, IL-1, and IL-12 accompanied by increased E-selection expression [
3,
17]. This is suggested to promote the adhesion of leukocytes to endothelial-cells and extravasation of activated leukocytes within the endoneurial space. The increased cytokine production leads to enhanced vascular permeability favoring the passage of neurotoxic factors into the endoneurium causing neuron damage [
18]. Furthermore, elevated cytokine levels directly induce muscle protein damage via activation of calpain and ubiquitine-proteasome [
14]. The anti-inflammatory and immunomodulating properties of IVIG are mediated by regulating the production, release and function of pro-inflammatory cytokines and have been successfully used in numerous autoimmune and inflammatory diseases [
16,
21,
22].
The use of IgM-enriched IVIG was based on the potential superiority over standard IVIG as seen in sepsis treatment and on the analysis of Mohr
et al., who suggested a beneficial effect of IgM-enriched IVIG in the prevention of CIPNM [
15,
23].
Standard IVIG has been safely administered intravenously at daily doses of 0.40 g/kg body weight over five days in patients with Guillain-Barré syndrome [
24]. Mohr
et al. administered IgM-enriched IVIG at doses of 0.3 g/kg body weight daily over three days [
15]. However, the manufacturer recommends that IgM-enriched IVIG be administered at a maximum dose of 0.25 g/kg body weight daily for three consecutive days, which is also the common dosage for the treatment of severe sepsis [
25]. Therefore, we decided to administer IgM-enriched IVIG at a dose of 0.25 g/kg body weight daily for three consecutive days. Nevertheless, we cannot rule out a potential benefit with higher doses of IgM-enriched IVIG regarding the treatment of CIPNM.
Strengths and limitations
It is desirable to have a clinical endpoint like the Medical Research Council (MRC) scale for muscle strength to assess the course of CIPNM. However, the MRC scale assessment depends on patient's cooperation and cannot be performed in patients who are not fully awake [
26]. Patients in our study were severely ill, represented by MOF, SIRS/sepsis and high SOFA/APACHE III scores. The vast majority was fully or partly sedated (87% at baseline; 50% on Day 14) and/or intubated/tracheotomized (95% at baseline; 84% on Day 14). Therefore, clinical assessment of the muscle weakness using the MRC scale was not feasible in the majority of our patients. At baseline only 5 of 38 patients (3 in the placebo, 2 in the IVIG group) were cooperative enough to allow the clinical assessment of muscle strengths using the MRC scale.
This is similar to Routsi
et al., who only could determine the MRC score in one-third of their patients although their patients were less ill than those in the present study [
26]. Thus, as a clinical endpoint was not feasible, we regarded the CIPNM severity sum score, based on serial EPS and two muscle biopsies, as the most appropriate method of assessing the course of CIPNM in critically ill patients who are not fully awake. A total of 106 patients fulfilling the screening criteria (SIRS/Sepsis and MOF) were evaluated by a neurologist in order to only randomize patients with clinical signs of CIPNM. This evaluation was challenging as the majority of the patients was not fully awake. However, unlike the MRC scale assessment, its aim was not to measure CIPNM using a metric scale but to select patients with an advanced stage of CIPNM. One-third (38 of 106) of patients met these criteria. CIPNM was confirmed in 97% (37 of 38) of patients at baseline based on EPS and muscle histology findings with relatively high CIPNM sum scores. Therefore, we regard the initial clinical evaluation as a valid tool to specifically select patients with an advanced stage of CIPNM, whereas the sensitivity of this evaluation may have been rather low [
1].
The differentiation between CIP and CIM is often not possible in critically ill patients by EPS alone. This shortcoming also could be compensated for by using the CIPNM severity sum score. Routsi
et al., who suggested that electrical muscle stimulation may prevent CIPNM, used only a clinical score for muscle strength to assess CIPNM and, therefore, could not differentiate between CIP and CIM [
26]. Another method to make a distinction between CIP and CIM is direct muscle stimulation [
27]. However, as muscle biopsy is regarded as the gold standard, we did not use direct muscle stimulation in our study [
4].
Van den Berghe
et al. found a reduced incidence of CIPNM in a pre-planned subgroup analysis of critically ill patients treated with IIT compared to conventional insulin therapy. Similarly, no differentiation between CIP and CIM was feasible in their study, as no histological assessment was done [
13].
It has been controversially discussed if discrimination between CIP and CIM is reasonable. However, exact differential diagnosis between these two entities leads to better prognostic information regarding long term disability [
1,
28]. CIM in combination with CIP is associated with a more severe weakness and longer ICU length of stay than CIM alone [
29]. Moreover, CIM has a better long-term prognosis than CIP [
30].
The main limitation of the present trial is the relatively small number of critically ill patients included in our trial prone to type II errors. Although we did not see any differences in the outcomes between the groups, this cannot entirely rule out a (small) effect of IVIG on CIPNM.
Furthermore, not all EPS or muscle biopsy evaluations could be performed as scheduled. The recruitment period of 4.5 years is rather long for a single-center trial and this potentially influenced the results. The slow recruiting is attributed to the very specific inclusion criteria, based on which only patients with a two-or-more organ failure, SIRS/sepsis, and clinical evidence for CIPNM could be included. To minimize the potential bias of the relatively long recruitment period we ensured that all procedures were carried out by the same team throughout the study period.
Although EPS and muscle biopsy are the methods of choice of assessing nerve and muscle damage in CIPNM [
4], a combination of both (CIPNM sum score) as used in the present study has never been validated to be superior. Therefore, we also provide separate results of EPS and muscle biopsy assessment which do not differ from the CIPNM sum score (Figure
3). The CIPNM sum score should be further validated in future trials for determining the specificity and sensitivity of CIPNM in critically ill patients.
Another limitation may have been the use of the “CIM score” based on the histological assessment of muscle biopsies. Although histological assessment is the diagnostic method of choice to evaluate myopathy in critically ill patients the grading of the “CIM score” is only semi-quantitative and has not been validated before. Muscle biopsy is regarded as safe and well tolerated in critically ill patients but it is still an invasive procedure [
31]. Therefore, we suggest that muscle biopsies should primarily be used in clinical trials. Unclear muscle weakness and inconclusive electrophysiological findings may justify muscle biopsy in the clinical routine.
Ultrasound has been successfully used to reliably measure muscle mass in critically ill patients [
32]. However, at the start of enrollment (December 2004), this information was not yet available. Furthermore, the patients included in our trial were more severely ill than in the trial of Gruther
et al. As tissue edema is common in severely ill patients, the assessment of the muscle mass using ultrasound may be challenging. Nevertheless, ultrasound examination should be considered as additional outcome in future trials.
Patients with clinical unapparent polyneuropathy or mild polyneuropathy were eligible for enrollment, as we did not expect an effect on the primary outcome.
We hypothesize that the following circumstances may be responsible for the lack of effect of IVIG. First, we decided to include patients that were already presenting with clinical signs of CIPNM at an early stage to achieve a maximal effect of IVIG. However, the application of IVIG at an even earlier time point - when the first signs of CIPNM can be verified only using electrophysiology measures may result in improved effects of IVIG. This was similarly observed in patients with severe sepsis, who had a significantly improved survival rate when IVIGs were administered early compared to at a more advanced phase of sepsis [
33].
Thus, earlier or even prophylactic application of IVIG may show better effects of IVIG regarding the prevention or mitigation of CIPNM, since a short, albeit crucial, time period may pass between first nerve and/or muscle fiber damage and first demonstrable electrophysiological changes, not to mention the first clinical signs, chosen as inclusion criterion in our study. However, a prophylactic treatment had required the inclusion of a lot more patients.
Potentially, a beneficial effect of IVIG on CIPNM may only be seen months after ICU discharge and was still concealed on Day 14 when we assessed the primary outcome. Due to patients lost to follow-up, this requires the inclusion of a higher number of patients.
Furthermore, the pathophysiology of CIPNM is complex and a multimodal cause is postulated. This includes alterations of the local immunity, decreased microcirculation of peripheral nerves, increased generation and deficient scavenging of reactive oxygen species, enhanced permeability for neurotoxic factors into the endoneurium, direct muscular protein breakdown and acquired channelopathy [
34]. However, IVIG has only a relatively limited point of action by modulating the local immunity [
16]. Thus, a multimodal therapy approach may be necessary to improve CIPNM.