Effects of acute organophosphorus or paraquat exposure on neuromuscular function and efficacy of antioxidant therapy in acute paraquat poisoning

Abstract

Organophosphates (OP) are the most frequently involved pesticides in acute poisoning. Paraquat (PQ) poisoning has the highest case fatality. As the mechanism of PQ toxicity includes free radical generation, antioxidants have been tried as a treatment. Neurotoxic effects of acute OP/PQ poisoning (OP/PQ-P) have been hitherto underexplored. The aims of the study were to assess the effects of acute OP/PQ poisoning on somatic, autonomic nerves, neuromuscular junction (NMJ), brain stem, cognitive function and psychological status. Further, aims were to evaluate adherence to existing guidelines on the management of OP/PQ poisoning and to find out the efficacy of antioxidant therapy in acute PQ poisoning. In a cohort study we evaluated the function of peripheral nerves (somatic and autonomic nerves) with the Neuropack MEB-9400A/K EMG/EP (Nihon Koden). Motor and sensory nerve function was tested with nerve conduction studies. Electromyography (EMG) studies were performed on the deltoid and the first dorsal interosseous muscle on the dominant side. Cardiovascular reflexes based autonomic function tests and sympathetic skin response (SSR) was used to evaluate autonomic function. NMJ function was assessed with slow repetitive supramaximal stimulation of the median nerve of the dominant upper limb. Brain stem function, cognitive function and psychological status were assessed with Brain Stem Evoked Response Audiometry (BERA), Mini Mental State Examination (MMSE) and General Health Questionnaire (GHQ) respectively. The data of the patients were compared with age, gender and occupation matched controls. A cross sectional survey was conducted to evaluate adherence to existing guidelines on the management of OP/PQ poisoning. The details of administration of atropine, pralidoxime and Fuller’s earth were collected. A randomized double blind placebo controlled clinical trial was conducted to determine the efficacy of antioxidant therapy in acute PQ poisoning. Both arms received intravenous vitamin C (IV vit C) 100 mg, 500 mg, 1000 mg, 3000 mg/day and 3000 mg/8 h for 5 consecutive days. One arm received Nacetylcysteine (NAC) 20 mg/kg in 200 mL of 5% dextrose over 15 minutes followed by 50 mg/kg in 500 mL over 12 hours ( ^ 4 mg/h/kg) twice per day for 3 days whereas, the other arm received 200 mL of 5% dextrose over 15 minutes followed by 500 mL over 12 hours twice per day for 3 days as placebo. These were compared to 24 historical and 80 parallel controls who received standard supportive treatment. The survival of the test individuals was compared to the historical and parallel controls by Log Rank (Mantel-Cox) and Tarone-Ware test. There were 70 OP patients (70 controls) and 28 PQ patients (56 controls). In OP patients, motor nerve conduction velocity (MCV), amplitude and area of compound muscle action potential on distal stimulation (CMAP-D), sensory nerve conduction velocity (SCV) and F-wave occurrence were significantly reduced. At one week the significant impairment in autonomic function were change of diastolic blood pressure 3 min after standing, heart rate variation during deep breathing (HR-DB), SSRamplitude, post-void urine volume and size of pupil. All except HR-DB were reversed at six weeks. No significant impairment of NMJ function, BERA,distress was significantly higher among the patients. In PQ patients, amplitude of ulnar nerve CMAP-D, median nerve area of CMAP-D and F-wave occurrence of median, ulnar and tibial nerves, blood pressure variation after standing and SSR-amplitude were significantly reduced at the first assessment. All but F-wave occurrence remained impaired at six weeks. A significant decrement response in RNS was observed following exercise in both assessments. No significant impairment of BERA, MMSE were noted. GHQ showed high prevalence of psychological distress among the patients. None of OP/PQ patients showed spontaneous activity, fibrillation potentials, high amplitude of the motor units, polyphasia or reduced interference pattern in EMG. Atropine was commenced in 44% of patients without cholinergic features. 73% of patients developed atropine toxicity. None of the patients received the maintenance therapy ofpralidoxime for the recommended duration. Ninety percent of PQ poisoned patients received Fuller's earth, but 15% of them did not receive it in adequate amounts. There were 40 test, 24 historical-controls and 80 parallelcontrols in the clinical trial. The median survival time was longer in the patients given antioxidants than in the historical controls [8 days (95% Cl 1.8- 14.2) vs 1 day (95% Cl 0.53- 1.47)]. This difference was significant by both the Log Rank (Mantel-Cox) (p=0.034) and Tarone-Ware tests (p=0.012). The absolute risk reduction is 18.33% (95% Cl: - 2.66% to 39.33%, Number Needed to Treat was 6). The median survival time of parallel controls were 7 days. There was no statistically significant survival difference between the tests vs parallel controls and the individuals who received IV vit C + placebo vs IV vit C+NAC. The Proportional Hazard assumption appeared to hold. Stratified Cox Proportional Hazard model was done as variable sex violated the Proportional Hazard assumption (Chi-square value = 66.6, p<0.01). There were no statistically significant changes of antioxidant levels within the groups over five consecutive days. Some of the effects of OP/PQ on peripheral nervous system persist at least for six weeks. NMJ function is not affected in OP patients. However impairment of NMJ function was observed following exercise in patients with PQ poisoning even at 6 weeks of the exposure. Brain stem and cognitive function were not affected in both groups of patients. High prevalence of psychological distress may indicate need for psychological support, diagnosis of depression and treatment. Adherence to guidelines was not optimal regarding the administration of antidotes in OP/PQ poisoning. Antioxidant therapy did not show promising effects on acute PQ poisoning. This study was performed in University o f Ruhuna, Sri Lanka and the results were included in a thesis with ten research papers in peer reviewed index journals. Further. 21 abstracts were presented in national and international forums including 10 platform presentations. An oration was done at inauguration o f Annual Academic Sessions o f the Galle Mediccd association in 2015, based on the findings o f organophosphate neurotoxic study. The study received an Academic award by Asian and Oceaninan congress o f clinical neurophysiology’, an international Travel award by the American academy o f clinical toxicology’, the best poster presentation at the annual academic sessions o f the Galle Medical association and President's award fo r highly rated scientific publications. The thesis was defended on 10'h o f May 2012. MMSE were noted. The prevalence of psychological