eMedicine Specialties > Neurology > Neurotoxicology

Organophosphates: Differential Diagnoses & Workup

Author: Frances M Dyro, MD, Chief of the Neuromuscular Section, Associate Professor, Department of Neurology, New York Medical College, Westchester Medical Center
Contributor Information and Disclosures

Updated: Oct 12, 2009

Differential Diagnoses

Acute Inflammatory Demyelinating Polyradiculoneuropathy
HIV-1 Associated Neuromuscular Complications (Overview)
Assessment of Neuromuscular Transmission
Myasthenia Gravis
Chronic Inflammatory Demyelinating Polyradiculoneuropathy
Organic Solvents
Diabetic Neuropathy
Toxic Neuropathy
Essential Tremor
HIV-1 Associated Distal Painful Sensorimotor Polyneuropathy
HIV-1 Associated Multiple Mononeuropathies

Workup

Laboratory Studies

  • In the acute care setting, laboratory studies should include glucose, BUN, electrolytes, prothrombin time, liver function studies, and cholinesterase measurements.
  • Levels of plasma and/or RBC cholinesterase enzyme may be measured by any of several existing methods. Most frequently used, because it is most readily available, is the test for plasma (or pseudo) cholinesterase. Because this can be affected by other disorders, it does not confirm the diagnosis. Mild intoxication is diagnosed when RBC cholinesterase inhibition is less than 50% of normal.
    • Depression of this value by 25% or more is confirmatory; this test may be used to follow progress.
    • Workers exposed to organophosphates used in agriculture should have a baseline level recorded.

Imaging Studies

The only imaging study that may be useful in acute management is a chest radiograph because of the danger of aspiration pneumonia in a confused patient with vomiting and compromised respiration.

Other Tests

  • Electrocardiography should be done on admission, since many patients develop cardiac irregularities following acute exposure.
  • Electrophysiologic studies
    • Acute organophosphate toxicity producing weakness is due to sustained endplate depolarization. In vivo microelectrode studies reported by Maselli and Soliven demonstrated no reduction in the amplitude of miniature endplate potentials or of the quantal content of the endplate potentials. Repetitive stimulation produced a decremental response as the endplate was "flooded" with acetylcholine.16
    • Rutchik and Rutkove found a temperature-dependent response; cooling to 32°C produced a normal-amplitude compound action potential (CMAP). A smaller spontaneous repetitive response was elicited in a man with organophosphate intoxication. Warming the limb to 39°C caused these responses to decrease in amplitude; the authors likened the effect to that seen in myasthenia gravis.17
    • De Luca et al have described a technique for evaluating neuromuscular involvement by measuring changes in the firing pattern of surface electromyography. Studies in rhesus monkeys showed gaps in the motor unit recruitment following a dose of OP. This pattern may reflect the tremor related to the degree of OP exposure.18
    • Singh et al examined the phrenic nerve conduction of 29 patients with organophosphate toxicity admitted to the hospital in 1997, 14 of whom required mechanical ventilation. They found that reduction in CMAP correlated with need for ventilatory assistance. By following patients with daily studies, they were able to predict successful weaning.19
  • Neuropsychological testing: Workers exposed over a long period to pesticides have been reported to manifest irritability, fatigue, headache, and difficulties with memory and concentration.
  • Evidence of neuropsychological abnormalities has been found in 4-9% of those who have had long-term exposure.8,9

Histologic Findings

Nerve biopsy in late-onset neuropathy reveals a primary axonopathy with secondary demyelination. CNS myelin may be lost as well.

More on Organophosphates

Overview: Organophosphates
Differential Diagnoses & Workup: Organophosphates
Treatment & Medication: Organophosphates
Follow-up: Organophosphates
Multimedia: Organophosphates
References
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References

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  2. Sung JJ, Kim SJ, Lee HB, et al. Anticholinesterase induces nicotinic receptor modulation. Muscle Nerve. Sep 1998;21(9):1135-44. [Medline].

  3. Trojan DA, Collet JP, Shapiro S, et al. A multicenter, randomized, double-blinded trial of pyridostigmine in postpolio syndrome. Neurology. Oct 12 1999;53(6):1225-33. [Medline].

  4. Abou-Donia MB. Organophosphorus ester-induced chronic neurotoxicity. Arch Environ Health. Aug 2003;58(8):484-97. [Medline].

  5. Eddleston M, Buckley NA, Eyer P, Dawson AH. Management of acute organophosphorus pesticide poisoning. Lancet. Feb 16 2008;371(9612):597-607. [Medline].

  6. Compston JE, Vedi S, Stephen AB, et al. Reduced bone formation after exposure to organophosphates. Lancet. Nov 20 1999;354(9192):1791-2. [Medline].

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  8. Eskenazi B, Maizlish NA. Effects of Occupational Exposure to Chemicals on Neurobehavioral Functioning. In: Tarter RE, Thiel DHV, Edwards KL, eds. Medical Neuropsychology: The Impact of Disease on Behavior. New York, NY: Plenum Press; 1988.

  9. Rosenstock L, Keifer M, Daniell WE, et al. Chronic central nervous system effects of acute organophosphate pesticide intoxication. The Pesticide Health Effects Study Group. Lancet. Jul 27 1991;338(8761):223-7. [Medline].

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  16. Maselli RA, Soliven BC. Analysis of the organophosphate-induced electromyographic response to repetitive nerve stimulation: paradoxical response to edrophonium and D-tubocurarine. Muscle Nerve. Dec 1991;14(12):1182-8. [Medline].

  17. Rutchik JS, Rutkove SB. Effect of temperature on motor responses in organophosphate intoxication. Muscle Nerve. Jul 1998;21(7):958-60. [Medline].

  18. De Luca CJ, Buccafusco JJ, Roy SH, et al. The electromyographic signal as a presymptomatic indicator of organophosphates in the body. Muscle Nerve. 2006;33(3):369-76. [Medline].

  19. Singh G, Sidhu UP, Mahajan R, et al. Phrenic nerve conduction studies in acute organophosphate poisoning. Muscle Nerve. Apr 2000;23(4):627-32. [Medline].

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  24. Ludomirsky A, Klein HO, Sarelli P, et al. Q-T prolongation and polymorphous ("torsade de pointes") ventricular arrhythmias associated with organophosphorus insecticide poisoning. Am J Cardiol. May 1982;49(7):1654-8. [Medline].

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  26. Savage EP, Keefe TJ, Mounce LM, et al. Chronic neurological sequelae of acute organophosphate pesticide poisoning. Arch Environ Health. Jan-Feb 1988;43(1):38-45. [Medline].

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Further Reading

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Keywords

organophosphates, nerve agent, pesticides, organophosphate insecticides, diazinon, disulfoton, azinphos-methyl, fonofos, therapeutic use of organophosphates, toxic nerve agents, sarin, neurotoxicity

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Contributor Information and Disclosures

Author

Frances M Dyro, MD, Chief of the Neuromuscular Section, Associate Professor, Department of Neurology, New York Medical College, Westchester Medical Center
Frances M Dyro, MD is a member of the following medical societies: American Academy of Neurology, American Association of Neuromuscular and Electrodiagnostic Medicine, and Muscular Dystrophy Association
Disclosure: Nothing to disclose.

Medical Editor

Jonathan S Rutchik, MD, MPH, Assistant Professor, Department of Occupational and Environmental Medicine, University of California at San Francisco
Jonathan S Rutchik, MD, MPH is a member of the following medical societies: American Academy of Neurology, American Association of Neuromuscular and Electrodiagnostic Medicine, American College of Occupational and Environmental Medicine, and Society of Toxicology
Disclosure: Nothing to disclose.

Pharmacy Editor

Francisco Talavera, PharmD, PhD, Senior Pharmacy Editor, eMedicine
Disclosure: eMedicine Salary Employment

Managing Editor

Neil A Busis, MD, Chief, Division of Neurology, Department of Medicine, Head, Clinical Neurophysiology Laboratory, University of Pittsburgh Medical Center-Shadyside
Neil A Busis, MD is a member of the following medical societies: American Academy of Neurology and American Association of Neuromuscular and Electrodiagnostic Medicine
Disclosure: Nothing to disclose.

CME Editor

Selim R Benbadis, MD, Professor, Director of Comprehensive Epilepsy Program, Departments of Neurology and Neurosurgery, University of South Florida School of Medicine, Tampa General Hospital
Selim R Benbadis, MD is a member of the following medical societies: American Academy of Neurology, American Academy of Sleep Medicine, American Clinical Neurophysiology Society, American Epilepsy Society, and American Medical Association
Disclosure: Nothing to disclose.

Chief Editor

Stephen A Berman, MD, PhD, Professor, Department of Internal Medicine, Section of Neurology, Dartmouth Medical School; Chief, Neurology Service, White River Junction Veterans Medical Center
Stephen A Berman, MD, PhD is a member of the following medical societies: Alpha Omega Alpha, American Academy of Neurology, and Phi Beta Kappa
Disclosure: Nothing to disclose.

 
 
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