Intermediate syndrome (IMS) is a major cause of death from respiratory failure following acute organophosphate poisoning. The objective of this study was to determine repetitive nerve stimulation (RNS) predictors of IMS that would assist in patient management and clinical research.
Methods and Findings
Seventy-eight consenting symptomatic patients with organophosphate poisoning were assessed prospectively with daily physical examination and RNS. RNS was done on the right and left median and ulnar nerves at 1, 3, 10, 15, 20, and 30 Hz. The study was conducted as a prospective observational cohort study in the Central Province, Sri Lanka. IMS was diagnosed in ten out of 78 patients using a priori clinical diagnostic criteria, and five of them developed respiratory failure. All ten patients showed progressive RNS changes correlating with the severity of IMS. A decrement-increment was observed at intermediate and high frequencies preceding the onset of clinical signs of IMS. As the patient developed clinical signs of IMS, decrement-increment was progressively noted at low and intermediate frequencies and a combination of decrement-increment and repetitive fade or severe decrement was noted at high frequencies. Severe decrement preceded respiratory failure in four patients. Thirty patients developed forme fruste IMS with less severe weakness not progressing to respiratory failure whose RNS was characterized by decrement-increment or a combination of decrement-increment and repetitive fade but never severe decrements.
Characteristic changes in RNS, preceding the development of IMS, help to identify a subgroup of patients at high risk of developing respiratory failure. The forme fruste IMS with the characteristic early changes on RNS indicates that IMS is a spectrum disorder. RNS changes are objective and precede the diagnosis and complications of IMS. Thus they may be useful in clinical management and research.
Jayawardane and colleagues evaluate a cohort of 78 patients with organophosphate poisoning from Sri Lanka, and identify changes in repetitive nerve stimulation that precede, and may help predict, the onset of intermediate syndrome.
Each year, many thousands of deaths around the world are caused by pesticide poisoning. Often, the pesticide involved is an organophosphate. These highly toxic compounds, which are widely used in agriculture, particularly in developing countries, disrupt the transmission of messages from the brain to the body in insect pests and in people. The brain controls body movements by sending electrical impulses along nerve cells (neurons). At the end of the neurons, these impulses are converted into chemical messengers (neurotransmitters), which cross the gap between neurons and muscle cells (the neuromuscular junction) and bind to proteins on the surface of the muscle cells to pass on the brain's message. One important neurotransmitter is acetylcholine. This is used in the part of the nervous system that controls breathing and other automatic vital functions, at neuromuscular junctions, and in parts of the central nervous system. Normally, acetylcholine is quickly broken down after it has delivered its message, but organophosphates disrupt this process and, consequently, affect nerve transmission to muscles. Organophosphate poisoning causes three syndromes. The cholinergic syndrome, which can be fatal, happens soon after organophosphates are swallowed, inhaled, or absorbed through the skin. The intermediate syndrome (IMS), which results in muscle weakness in the limbs, neck, and throat, develops in some patients 24–96 hours after poisoning. Finally, long-term nerve damage sometimes develops 2–3 weeks after poisoning.
Why Was This Study Done?
Although IMS is a major contributor to the illness caused by organophosphate poisoning and can result in respiratory (breathing) failure and death, the functional changes that are associated with IMS (its pathophysiology) are poorly understood. With a better understanding of these changes, it might be possible to find ways to prevent or treat IMS or to predict which patients with IMS are likely to develop respiratory failure. In this study, the researchers make a set of measurements of nerve transmission in a large group of organophosphate-poisoned patients in Sri Lanka to discover more about the pathophysiology of IMS.
What Did the Researchers Do and Find?
Seventy-eight patients with organophosphate poisoning were assessed several times a day for clinical signs of IMS. In addition, nerve transmission was measured daily in the patients using an electrophysiological technique called repetitive nerve stimulation (RNS). For this, a series of small electrical shocks was applied to the certain nerves in the arm and the responses in the muscles that these nerves control were recorded. In the ten study participants who developed IMS, the researchers observed several characteristic changes in their muscle responses to RNS, some of which were seen before the clinical signs of IMS. Other changes in muscle responses to RNS correlated with the development of clear IMS. Most importantly, in the four patients with IMS who developed respiratory failure, an RNS response pattern called severe decrement (a reduced response to the first electrical shock and then no response to the subsequent shocks) was seen before respiratory failure. Finally, there were other characteristic changes in muscle responses to RNS in 30 patients with muscle weakness not severe enough for a diagnosis of IMS (incomplete or “forme fruste” IMS).
What Do These Findings Mean?
These findings indicate that changes in nerve transmission that can be objectively monitored using RNS evolve during the development of IMS. In other words, IMS is a “spectrum” disorder in which the weakness and neuromuscular junction problems caused by organophosphate poisoning gradually progress over time through a series of electrophysiological changes that will sometimes resolve quickly and only in the most severe cases will result in respiratory failure. These findings need to be validated in further studies, particularly since most of the patients in this study had been exposed to a single organophosphate (chlorpyrifos). However, they suggest that the RNS tests might be useful in the clinical management of patients with organophosphate poisoning, particularly since such tests could provide an early warning of impending respiratory failure.
Please access these Web sites via the online version of this summary at http://dx.doi.org/10.1371/journal.pmed.0050147.
This study is further discussed in a PLoS Medicine Perspective by Cynthia Aaron
The US Environmental Protection Agency provides information about all aspects of pesticides (in English and Spanish)
Toxtown, an interactive site from the US National Library of Science, provides information on environmental health concerns including exposure to pesticides (in English and Spanish)
The US National Pesticide Information Center provides objective, science-based information about pesticides
MedlinePlus also provides links to information on pesticides (in English and Spanish)
The International Programme on Chemical Safety has information on poisoning prevention and management; its INTOX databank has a description of the cholinergic syndrome
WikiTox is a clinical toxicology resource