Monitoring of neuromuscular transmission in organophosphate pesticide-poisoned patients

Abstract

Thirty-four adult patients with severe organophosphorus compounds (OP) poisoning requiring artificial ventilation were enrolled in a clinical study and received atropine and obidoxime (250 mg i.v., followed by 750 mg/24 h) as antidotal treatment. Here, we re-analyzed the cholinesterase status (red blood cell acetylcholinesterase (RBC-AChE) activity, reactivatability of RBC-AChE, and plasma butyrylcholinesterase (Pl-BChE) activity) in relation to the neuromuscular transmission (NMT) data. When RBC-AChE activity ranged between 100% and 30% NMT was unimpaired after tetanic stimulation with frequencies up to 50 Hz. A further decrease in RBC-AChE activity was accompanied by a marked disturbance of NMT, being strongly impaired at AChE activities <5% of normal. Higher stimulation frequencies (>30 Hz) facilitated the discrimination of the types of impairment. The neuromuscular transmission was the best quantified by using the ratio of the ninth to the first amplitude, while the standard method was less discriminative. At RBC-AChE levels higher than 40% of normal weaning from the ventilator may be considered. Completely aged RBC-AChE as indicated by loss of reactivatability loses its guidance function. Then, steadily increasing Pl-BChE activity suggests lack of circulating poison. One-week later, neuromuscular transmission may be largely normal and patients could be weaned from the respirator if other complications are not withstanding.

Introduction

Every year, hundreds of thousands of deaths occur due to poisoning with organophosphorus compounds (OP) (Eddleston and Phillips, 2004, Gunnell and Eddleston, 2003, Buckley et al., 2004). Inhibition of acetylcholinesterase is regarded as the major toxic mechanism, which leads to excess stimulation of muscarine and nicotine receptors in OP-poisoning. In the acute phase of severe poisoning, the typical clinical picture of cholinergic crisis develops fast (Lotti, 2001) being the basis for clinical diagnosis, which is confirmed by the history and demonstration of cholinesterase inhibition. In daily clinical practice activity of plasma butyrylcholinesterase (Pl-BChE, EC 3.1.1.8) is mostly used for diagnosis. However, it is well established that there is a wide physiological variation of Pl-BChE activity in dependence of age, sex and other parameters such as plasma lipids (Ostergaard et al., 1992). Furthermore, its activity may change during disease (e.g., chronic liver or renal disease, malignant tumors or burns) and drug administration (inhibition of the enzyme or reduction of its synthesis) (Ostergaard et al., 1992). In addition, it is known that a number of genetic mutants exist resulting in Pl-BChE levels from not measurable to higher than normal (Ostergaard et al., 1992). These influences limit the value of Pl-BChE as an indicator of OP anticholinesterase poisoning, particularly when baseline values are lacking.

In contrast, red blood cell (RBC) acetylcholinesterase (AChE, EC 3.1.1.7), which is encoded by a single gene in each animal species (Taylor and Radic, 1994), varies less, particularly when referred to hematocrit or hemoglobin (Augustinsson, 1955, Worek et al., 1997, Worek et al., 1999b). The enzyme is assumed to have an identical active site environment in all tissues at all ages (Mortensen et al., 1998, Eyer, 2003). In fact, similar kinetics of reactivation of OP-inhibited RBC- and muscle AChE have been confirmed experimentally (Eckert et al., 2008, Herkert et al., 2008).

Pl-BChE and AChE not only differ in their reactivity towards substrates but also in their interactions with OPs (inhibition) and post-inhibitory reactions (spontaneous reactivation, aging) (for review see (Eyer, 2003)) as well as in their behavior towards reactivators (Worek et al., 1999a, Worek et al., 1998, Lotti, 2001).

From this, it may be expected that red blood cell acetylcholinesterase (RBC-AChE) reflects more closely the functionally important synaptic enzyme and thus appears to be the more appropriate indicator of the severity of OP-poisoning compared to Pl-BChE. However, it is largely unknown how closely RBC-AChE activity mirrors synaptic activity and responds to therapy in the later course of poisoning. It would be of great help to know how long RBC-AChE activity is in fact a reliable surrogate parameter of the severity of OP-poisoning, which may guide the physician's decision when to withdraw analgosedation and to wean the patient from the respirator.

In clinical practice, the assessment of the critical parameters is not trivial when the patient is sedated, artificially ventilated and atropinized, which masks the guiding muscarinergic features. A less therapy-affected parameter would be the neuromuscular transmission (NMT), which can be assessed by non-invasive standard neurological techniques (Wadia et al., 1987, Besser et al., 1989). Repetitive discharges of compound muscle action potentials (CMAP) after single pulses and a progressive decrease of CMAPs after repetitive nerve stimulation with peculiar patterns were described as typical for OP-intoxications (Besser et al., 1989, Singh et al., 1998). It is not clear, however, at which frequency of muscle stimulation the responses would reflect OP-induced NMT disturbances most adequately.

We addressed these questions in a clinical study on therapeutic effectiveness of obidoxime in severe OP-poisoning (Eyer et al., 2009) and investigated the NMT and the RBC-AChE-status (Thiermann et al., 2009). In the manuscript presented, we examined the correlation of RBC-AChE activity with the specific patterns of NMT upon artificial stimulation at various frequencies and followed the recuperation of NMT in a few cases. To this end, we also assessed how to quantify best the different kinds of impairment of neuromuscular transmission.