Corticosteroids and neuromuscular transmission: electrophysiological investigation of the effects of prednisolone on normal and anticholinesterase-treated neuromuscular junction

Pathomechanisms and Clinical Implications of Myasthenic Syndromes Exacerbated and Induced by Medical Treatments

Myasthenic syndromes are typically characterized by muscle weakness and increased fatigability due to an impaired transmission at the neuromuscular junction (NMJ). Most cases are caused by acquired autoimmune conditions such as myasthenia gravis (MG), typically with antibodies against the acetylcholine receptor (AChR). Different drugs are among the major factors that may complicate pre-existing autoimmune myasthenic conditions by further impairing transmission at the NMJ. Some clinical observations are substantiated by experimental data, indicating that presynaptic, postsynaptic or more complex pathomechanisms at the NMJ may be involved, depending on the individual compound. Most robust data exist for the risks associated with some antibiotics (e.g., aminoglycosides, ketolides, fluoroquinolones) and cardiovascular medications (e.g., class Ia antiarrhythmics, beta blockers). Apart from primarily autoimmune-mediated disorders of the NMJ, de novo myasthenic manifestations may also be triggered by medical treatments that induce an autoimmune reaction. Most notably, there is growing evidence that the immune checkpoint inhibitors (ICI), a modern class of drugs to treat various malignancies, represent a relevant risk factor to develop severe and progressive medication-induced myasthenia via an immune-mediated mechanism. From a clinical perspective, it is of utmost importance for the treating physicians to be aware of such adverse treatment effects and their consequences. In this article, we aim to summarize existing evidence regarding the key molecular and immunological mechanisms as well as the clinical implications of medication-aggravated and medication-induced myasthenic syndromes.

Remission of myasthenia gravis following cervical adjustment

Myasthenia gravis (MG) is an autoimmune disorder, caused by circulating antibodies against the acetylcholine receptor (AChR) and associated proteins. Anticholinesterase medications and immunomodulating therapies are the mainstays of current treatment. Presented here is a case of a 51-year-old female who had been diagnosed with MG based on symptoms and continued elevation of antibody to AChR (anti-AChR) by her family physician. The patient's anticholinesterase medication was halted due to significant side effects affecting bowel function. She only received acupuncture treatment in the past 4 months prior to this presentation. Myasthenic symptoms deteriorated and the anti-AChR titer kept elevating after stopping medication. She originally came to us due to neck and back pain rather than myasthenic complaints. This case is interesting that her back pain and myasthenic symptoms went into complete remission within 1 month of initiating chiropractic adjustment. The concomitant recession of the myasthenic symptoms raises considerable interest for the mystery of MG, including the causal link between stress and autoimmune disease, the role of ACh in immune regulation, and the possible mechanisms of disease amelioration. Further studies would shed more light on the efficacy of various modalities in treating MG.

The clinical management of neuromuscular disorders in intensive care

Life-threatening neuromuscular disorders affect a small, but growing group of patients in the intensive care unit who present special management problems, as well as great therapeutic opportunities. In inflammatory conditions, a cure is often possible, and for chronic, genetic or degenerative conditions, achieving the previous level of function is the target. Neuromuscular experts and intensivists need to cooperate closely to achieve the best possible outcomes. They need to acquire a very specific set of skills, including both a thorough understanding of the mechanics of ventilation as well as familiarity with the diagnostic categories of genetic and of autoimmune diseases. This review of the clinical management of adult neuromuscular disease in the ICU aims to provide an overview of the most important conditions encountered in the ICU and a practical approach to their diagnosis, monitoring, and treatment.

Sustained Neuromuscular Blockade after Vecuronium Use in a Premature Infant

Background Prolonged use of neuromuscular blocking agents (NMBAs) is very common in critically ill children both in pediatric and neonatal intensive care units. There are no guidelines available for use of NMBAs in children or neonates in the US, and the data for their safety in this age group is limited. Case Description Our case describes prolonged neuromuscular blockade following concurrent use of a NMBA along with aminoglycosides and steroids in the setting of renal failure in a premature infant. Conclusion Prolonged use of NMBAs in preterm infants should be avoided if possible or should be restricted to the shortest possible duration and the smallest possible physiologically effective dose. Concurrent use of NMBAs with aminoglycoside and steroids should be avoided, especially in the setting of renal failure.

Opposing effects of dexamethasone, agrin and sugammadex on functional innervation and constitutive secretion of IL-6 in in vitro innervated primary human muscle cells

Neuromuscular junction development is the key process required for successful neuromuscular transmission and functional innervation of skeletal muscle fibres. Various substances can influence these processes, some of which are in common use in clinical practice. In the present study, the effects of the potentially new therapeutic agent agrin were followed, along with the widely used glucocorticoid dexamethasone. The in vitro experimental model used was functional innervation and constitutive interleukin 6 (IL-6) secretion of human muscle cells. Additionally, the selective relaxant binding agent sugammadex and its possible interaction with dexamethasone were followed. Dexamethasone impaired functional innervation while agrin had opposing effects. Furthermore, based on interference with IL-6 secretion, we show potential (chemical) interactions between dexamethasone and sugammadex. The physiological effects of this interaction should be taken into consideration under clinical conditions where these two drugs might be applied simultaneously.

Epidural steroid therapy for back and leg pain: mechanisms of action and efficacy

BACKGROUND CONTEXT

Epidural steroid injection (ESI) is one of the most common nonsurgical interventions prescribed for back and leg pain symptoms. Although the use of ESI is widespread, proof of efficacy among the broad population of low back pain patients is lacking and use is predicated to a great extent on the cost and morbidity of the perceived "next step" in many patient's care-surgery.

PURPOSE

To review the relative indications and clinical features that predict success with ESI therapy, and to provide a physiological rationale to guide clinical decision-making.

STUDY DESIGN/SETTING

Review of literature and clinical experience.

RESULTS

Clinical studies have alternately supported and refuted the efficacy of ESI in the treatment of patients with back and leg pain. Steroid medications do benefit some patients with radicular pain, but the benefit is often limited in duration, making efficacy difficult to prove over time. Steroids appear to speed the rate of recovery and return to function, however, allowing patients to reduce medication levels and increase activity while awaiting the natural improvement expected in most spinal disorders. Fluoroscopic verification of needle placement, with contrast injection, greatly improves steroid delivery while reducing risks. Although it is assumed that the benefit of steroids is related to their effect on inflammation, that remains unproven, and it is possible that benefit is gained through an unrecognized action.

CONCLUSIONS

Randomized, controlled trials are needed to conclusively identify those patients most likely to benefit from ESI, and when and for how long. Until then, epidural steroids provide a reasonable alternative to surgical intervention in selected patients with back and/or leg pain, whose symptoms are functionally limiting. When appropriate goals are established and proper patients are selected, sufficient short-term benefit has been documented to warrant continued use of this tool.

Corticosteroid effects on diaphragm neuromuscular junctions

The effects of corticosteroid (CS) treatment (prednisolone continuously administered subcutaneously at a flow rate of 2.5 microl/h, daily dose 5.6 mg/kg, for 3 wk) on neuromuscular junction (NMJ) morphology and neuromuscular transmission in rat diaphragm muscle (Dimus) were compared with weight-matched (Sham) and ad libitum fed control (Ctl) groups. Fibers were classified on the basis of myosin heavy chain (MHC) isoform expression. CS treatment caused significant atrophy of fibers expressing MHC2X (type IIx), either alone or with MHC2B (type IIx/b). Fibers expressing MHCslow (type I) and MHC2A (type IIa) were unaffected by CS. The planar areas of nerve terminals and motor endplates at type IIx/b fibers were smaller in CS-treated Dimus compared with Sham and Ctl. However, CS-induced atrophy of type IIx/b fibers exceeded changes in NMJ morphology. Thus, when normalized for fiber diameter, NMJs were relatively larger in the CS-treated group compared with Ctl. Neuromuscular transmission failure, assessed in vitro by comparing force loss during repetitive (40 Hz) nerve vs. direct muscle stimulation, was less in CS-treated Dimus. These results indicate that alterations in NMJ morphology after CS treatment are dependent on fiber type and may contribute to improved neuromuscular transmission.

Long-term treatment with glucocorticoids increases synthesis and stability of junctional acetylcholine receptors on innervated cultured human muscle

We have studied the effect of long-term treatment with hydrocortisone on the expression of acetylcholine receptors (AChRs) at the neuromuscular junctions of human muscle cultured in monolayer and innervated de novo by fetal rat spinal cord motoneurons. Hydrocortisone increased accumulation of junctional AChRs in a dose- and time-dependent fashion. This increase was due to both decreased degradation and increased synthesis of AChRs. Other glucocorticoids, dexamethasone and prednisolone, exerted similar effects. Our study demonstrates a novel action of glucocorticoids on human junctional AChRs.

Effects of chronic corticosterone treatment on the morphology of rat neuromuscular junctions

This study examined the effects of chronic corticosterone (CORT) treatment on the morphology of two physiologically different muscles. Nerve terminals from the slow twitch soleus (SOL) and fast twitch extensor digitorum longus (EDL) of Fischer 344 rats were stained using the zinc iodide osmium (ZIO) technique. Nerve terminal area, perimeter, and longitudinal extent length were measured using computer-aided morphometry. Slow and fast muscle fibers from animals which received 5-10 mg CORT per day for 3 months were atrophied compared with controls. Treated animals failed to gain weight during the study, while controls gained 37%. Adrenal weights in treated animals were 30% less than controls, after correction for body weight. Morphological parameters for SOL nerve terminals were generally larger in the CORT group, while EDL nerve terminals from the CORT group did not differ significantly from controls. Soleus nerve terminal area was 43% greater, perimeter 14% longer, and longitudinal extent length 18% longer than the control nerve terminals. This study demonstrates a greater effect of CORT treatment on slow twitch muscle than has been demonstrated in previous studies. Changes in the nerve terminal morphology of the SOL were also greater than in previous studies and suggest that a functional adaptation or remodelling may occur following CORT treatment to maintain the neuromuscular interface during the enhanced catabolic effects of the steroid. These steroid-induced stress changes are similar in some respects to those observed in aging and disuse studies of the vertebrate neuromuscular junction. This suggests that glucocorticoid hormones may play an etiological role in the homeostasis of the neuromuscular junction in response to various stimuli.

Iontophoretic application of glucocorticoids inhibits identified neurones in the rat paraventricular nucleus

In an electrophysiological study designed to examine the negative feedback effects of glucocorticoid hormones, we have recorded the electrical activity of 147 neurones in the paraventricular nucleus of the rat hypothalamus. 37 (25%) of the neurones were antidromically identified as projecting to the median eminence and were located at a mean depth of 2.35 +/- 0.08 mm from the base of the brain, corresponding with the corticotropin-releasing factor-rich region of the nucleus. The mean firing rate of the identified cells was 4.7 +/- 0.6 Hz which was not significantly different from that of adjacent, unidentified cells (5.6 +/- 0.6 Hz). Most (17/18, 94%) of these cells tested responded to painful somatosensory stimuli and 26 (74%) of the identified cells were inhibited by iontophoretic application of corticosterone and/or hydrocortisone, whereas only one cell was excited and 8 unaffected. Of the identified cells, only 18 (20%) were inhibited, 36 (41%) were excited and 34 (39%) were non-responsive. The proportion of inhibitory responses was thus greater for the identified cells (P less than 0.005; chi 2-test). For the identified cells, whose spontaneous activity was unaffected by glucocorticoid application, glutamate-evoked responses could usually be depressed by the application. The time course of all responses usually showed an immediate onset, increasing in magnitude and continuing for extended periods following cessation of iontophoresis. Electrophysiologically identified magnocellular neurones were also tested and the majority (7/12, 58%) of vasopressin-secreting neurons were also found to be inhibited, whilst all (8/8, 100%) of the oxytocin-secreting neurones were excited by the glucocorticoid application. These results may represent an electrophysiological correlate of the negative feedback control of adrenocortical secretion and are discussed within this context.

Cortisol alters firing rate and synaptic responses of limbic forebrain units

Single units antidromically identified as projecting to the hypothalamic paraventricular nucleus (PVN) from the lateral septum (LS) and preoptic area (POA) were recorded in adrenalectomized male rats. Eight of the 15 POA units identified as projecting to the PVN were affected by reciprocally projecting pathways also showing either orthodromic excitation or inhibition. A variety of responses from POA units following cortisol injection suggested a heterogeneity of PVN afferent projections arising from the POA and possibly involved in the regulation of adrenocortical secretion. Cortisol injection had variable effects on the reciprocal POA axon pathways demonstrated and also revealed the presence of reciprocal projections to four of the LS cells. Most of the 10 LS cells tested showed an increase in firing rate following the steroid injection. It is suggested that the reciprocal pathways demonstrated may provide a steroid-dependent, 'ultra-short loop' negative feedback circuit serving to regulate the neural control of adrenocortical secretion. Unidentified cells in both regions were also affected by cortisol injection, the majority of such cells tested being excited, some also showing steroid-sensitive synaptic effects following PVN stimulation. The latter results may be related to the involvement of the structures examined, and of adrenal glucocorticoids, in central mechanisms of arousal.

Effects of corticosteroids on the myoneural junction. A morphometric and electrophysiological study

The mean size of the synaptic vesicles in the nerve endings of the isolated rat diaphragm was significantly increased after incubation with the glucocorticoids prednisolone (10(-5) mol/l) and dexamethasone (2 X 10(-7) mol/l). The shape of the vesicles was also changed, i.e. they became rounder. Similar increases in the mean size of the diaphragm synaptic vesicles were seen when the rats were injected with prednisolone (i.p., 2 mg/kg) and dexamethasone (i.p., 50 micrograms/kg) 2-4 h previously. The amplitude of the miniature endplate potentials was also significantly increased in diaphragms isolated from rats which had been injected with prednisolone or with dexamethasone. Changes in frequency of the MEPPs were less marked after injection of prednisolone, but a significant increase was seen after injection of dexamethasone. It is concluded that relatively low concentrations of glucocorticoids have direct effects on the motor endplate in rat diaphragm both in vitro and in vivo. We tentatively suggest that presynaptic effects may contribute to the beneficial effect of corticosteroids in deficient neuromuscular transmission, e.g. in myasthenia gravis.

Disorders of neuromuscular transmission other than myasthenia gravis

Disorders of neuromuscular transmission in humans are caused by a wide variety of agents including systemic diseases, drugs, environmental toxins, animal envenomation, cations, and hormones. Some are genetically determined. Many are of known etiology. All such disorders interfere with one or more events in the sequence whereby a nerve impulse excites a muscle action potential. In many disorders of neuromuscular transmission, abnormal fatigue occurs, and some cases respond dramatically to treatment. Investigation of the microphysiology, microanatomy, and pharmacology of both normal and diseased neuromuscular junctions has increased our knowledge of these disorders.