2Journal of Neurology, Neurosurgery, and Psychiatry 1993;56:290-294

SHORT REPORT

Pathophysiological studies of neuromuscularfunction in subacute organophosphate poisoninginduced by phosmet

J L Good, R K Khurana, R F Mayer,W M Cintra, E X Albuquerque

AbstractA 51 year old man developed progressivecranial and proximal muscle weakness,hyperreflexia and mental change. Thedisorder progressed over 9 days followingthe fifth weekly spraying with the organo-phosphate (OP) insecticide, phosmet,with limited symptoms of acute toxicity.Marked decremental responses of 50-80%on slow and fast rates of stimulation wereimproved to 15% by edrophonium orneostigmine. Intracellular recordings atthe endplate region of intercostal musclerevealed small miniature endplate poten-tials (mepps), reduced mean acetylcho-line sensitivity and normal membranepotentials. Electronmicroscopy revealeddegeneration and regeneration of theendplates. This study demonstrates thatOP poisoning due to phosmet can producea subacute postsynaptic neuromuscularsyndrome without marked symptoms ofacute toxicity.

(7 Neurol Neurosurg Psychiatry 1993;56:290-294)

University ofMaryland School ofMedicine, Baltimore,Maryland, USADepartment ofNeurologyJ L GoodR K KhuranaR F MayerDepartment ofPharmacology andExperimentalTherapeuticsE X AlbuquerqueLaboratory ofMolecularPharmacology II,Institute of Biophysics"Carlos ChagasFilho", FederalUniversity of Rio DeJaneiro, Rio DeJaneiro, BrazilW M CintraCorrespondence to:Dr Good, Department ofNeurology, Room N4W46,University of MarylandHospital, 22 S GreeneStreet, Baltimore, MD21201, USAReceived 17 December 1991and in revised form11 March 1992.Accepted 16 March 1992

Organophosphate (OP) insecticide poisoningin humans can produce: 1) acute peripheraland central cholinergic block; 2) an inter-mediate syndrome with weakness, and 3) a

delayed distal polyneuropathy. l The acute syn-drome that occurs 12 to 72 hours afterexposure to OP involves inhibition of acetyl-cholinesterase (AChE) by the OP24 and directactions at the synapse involving a variety ofchemosensitive receptor ion channels.`- Cen-tral and peripheral cholinergic signs occur. Theneuromuscular block to repetitive nerve stim-ulation shows decremental responses that are

increased by edrophonium.8 The delayed neu-

ropathic syndrome occurs 1-3 weeks afterexposure to the OP and probably results frominhibition of the neurotoxic esterase.9 A distalaxonopathy usually occurs with sensory andmotor signs. More recently, an intermediatesyndrome has been described in OP poisoningthat occurs 1-4 days after the acute poison-ing.'0 The patients reported developed pro-gressive weakness that was often severe andinvolved cranial, respiratory and proximal limbmuscles which persisted for up to 18 days.Three of the patients had signs suggestingCNS dysfunction. Electromyographic studies

suggested a neuromuscular junction (NMJ)defect of the postsynaptic type, but the exactmechanism of the dysfunction was not deter-mined. 5oWe describe the pathophysiological changes

in a patient who presented with a subacuteprogressive neuromuscular syndrome withoutmarked symptoms of acute toxicity after multi-ple OP insecticide sprayings. The defect in thisneuromuscular syndrome occurred at theendplate-receptor sites, is similar to the inter-mediate syndrome previously reported,10 anddiffers from the acute and delayed syndromes. lThese results are important in the diagnosisand management of patients exposed toorganophosphates. A preliminary report ofthis study has been published in abstractform. "

Case reportA 51 year old man was admitted to the hospital5 days after the onset of diplopia, light headed-ness and a staggering gait. The patient devel-oped these symptoms 18 hours after the fifthweekly exposure to a liquid spray insecticidecontaining an organophosphate compound,phosmet, (phosphorodiothioic acid S-[(1,3-di-hydro-1 ,3-dioxo-2H-isoindol-2-yl) methyl] 0,0-dimethylester). He described his face andhands becoming wet from the insecticide andhe did not bathe until the following morning.Over the next few days, he noted progressiveunsteadiness of gait, dysphagia, change invoice tone and excessive diaphoresis. Five daysafter exposure, he noted jaw weakness anddroopy eyelids.The patient's general examination was nor-

mal. Profuse oral secretions, bilateral ptosis,and marked facial diaphoresis were noted. Hewas alert, mentation was normal, and speechwas dysarthric. Extraocular movements werelimited in all directions with the exception ofdowngaze. Pupils were 3 mm and reactive tolight. There was moderate symmetrical weak-ness of facial, jaw, tongue and neck muscles.There was mild weakness of distal muscles andmarked weakness of proximal muscles, es-pecially antigravity muscles that fatigued. Thetendon jerks were brisk throughout. BilateralBabinski responses were present. Sensoryexamination was normal.The patient was admitted with the presumed

290 on F

ebruary 26, 2021 by guest. Protected by copyright.

http://jnnp.bmj.com

/J N

eurol Neurosurg P

sychiatry: first published as 10.1136/jnnp.56.3.290 on 1 March 1993. D

ownloaded from

Endplate studies in OP poisoning

diagnosis of acute OP poisoning. Atropine(2 mg) and pralidoxime chloride (2-PAM,1 gm) infusions were started. Within severalminutes of 2-PAM infusion, the patient hadincreased systemic and ventilatory weaknessrequiring intubation. After the patient wasstabilised on a ventilator, 3 additional doses of2-PAM were given without side effects over thenext 24 hours. Atropine (1 0 mg initially andthereafter 0-5 mg) was continued 4 times dailyfor 6 days. Paralysis was maximum on day 9.At this time, he was observed to have visualhallucinations, disorientation, and myoclonicjerks of his extremities. These signs persistedfor approximately 10 days during which timethe EEC showed diffuse slow wave activity.The patient remained unchanged for the

following 18 days, and supportive care con-tinued. By day 28, there was improvement instrength of neck flexion, shoulder and anti-gravity muscles. By day 44, he no longerrequired mechanical ventilation or tube feed-ings. Babinski responses were absent. Exam-ination on day 64 (time of the muscle biopsy)showed that he had mild fatiguable proximaland antigravity muscle weakness in the upperand lower limbs. Facial and oropharyngealmovements were near normal. There was mildsubjective diplopia on extreme lateral gaze.Neuromuscular examination was normal by 5months.

Laboratory studies revealed normal routineblood and urine studies, creatine kinase, EKGand chest radiographs. MRI of the brain wasnormal. CSF was normal except for a totalprotein of 75 mg%. On admission red bloodcell cholinesterase level was within normallimits (6110 U/L); and follow up levels on days16 and 23 were 5420 and 4180 respectivelyand at 10 months (5114 U/L) remained nor-mal (normal range 6666-3590). Studies torule out other causes of neuromuscular diseasewere normal. ANA and acetylcholine receptorantibody titres were negative.

MethodsNerve conduction and repetitive stimulation-neuromuscular studies were performed inperipheral nerves of upper and lower extrem-ities using established techniques.'2 53 Record-ings were performed in the intensive care unit

Table 1 Nerve conduction and repetitive stimulation studies in median nerve afterphosmet exposure

% Decrement at 5 Hz

Before After TensilonTime CMAP DL MCVDays mV MS MIS 4th R 9th R 4th R 9th R

9 90 50 54 48 431 1 11-6 55 43 33 1912 13-0 58 46 10 719 190 4-7 56 42 26 7 032 18-0 4-2 53 39 27 16 647 22-0 4-2 62 16 9 2 295 19 0 3-9 55 ±6 ±6Normal subjectsMean (SD) 17-0 (4 8) 3 5 (0 5) 55 (4) 0 (10) 0 (10) 0 (10) 0 (10)

CMAP-Compound muscle action potential amplitude in millivolts, mV, from thenar muscles.DL-Distal motor latency in milliseconds, ms, from wrist to thenar muscles.MCV-Maximal motor conduction velocity in meters per second, m/s, between elbow andwrist.R-Motor responses 4th or 9th on repetitive nerve stimulation.

and care was taken to keep the patient warm(rectal temperature of 37°C) during the proce-dure. Recordings were obtained at 9 to 95 daysafter exposure to phosmet.

Repetitive stimulation was performed atrates of 2, 5, 10, 20, and 50 Hz for 9 responsesin the median and peroneal nerves. Recordingsof the median nerve were made before andafter 10 mg of intravenous edrophonium atdays 11 to 47. Recordings were made 5 to 40minutes after 1-5 mg of intramuscular neo-stigmine on day 12. Measurements of themotor amplitudes were peak-to-peak and per-centage change (decrement) was calculated forthe fourth and ninth responses compared withthe initial one.

Concentric needle and single fibre (SFEMG) electromyography and fibre densitieswere performed in the extensor digitorumcommunis and first dorsal interosseus musclesusing established techniques. 14 SF EMGrecordings were obtained when the patient wasimproving and able to cooperate, 3-4 monthsafter exposure.A diagnostic intercostal muscle biopsy was

obtained 2 months after exposure because ofpersistent muscle weakness with informedconsent by the patient. Muscle was removedfrom the right sixth intercostal space in theanterior axillary line using standardised aneas-thesia and surgical technique. The muscle wasimmediately placed in oxygenated Krebs-Ringer solution and prepared for intracellularrecordings and ultrastructure as previouslydescribed.'5 Conventional microelectrodetechniques were used for recording fromhuman intercostal muscle. High resistancepipettes (approximately 150 MQ) filled with2 5 M acetylcholine (ACh) were used formicroiontophoretic application of ACh at theneuromuscular junctions. Results were com-pared statistically (Student t test) with controldata that we have obtained from normal adulthuman intercostal muscle biopsied duringthoracotomies for cardiopulmonary disease.The muscle fibres were fixed in ice-coldphosphate buffered 2-5% glutaraldehyde forthin sectioning and electron microscopicexamination of the endplate regions.'5 16

ResultsClinical electrophysiologyInitial recordings were performed 9 days afterexposure at which time the weakness wasmaximal. Electromyography at this time didnot show any spontaneous activity such asfasciculations or fibrillation potentials. Inmedian, ulnar and peroneal nerves, distalmotor responses were mildly reduced in ampli-tude and prolonged in latency. Motor conduc-tion velocities were normal. No nerveconduction block on proximal or distal stimu-lation or repe'titve motor responses wereobserved following single supramaximalshocks. Antidromic motor F responses werenormal in amplitude and latency. Serial studieswere performed in the median nerve-thenar muscles and these results are shown intable 1.

291 on F

ebruary 26, 2021 by guest. Protected by copyright.

http://jnnp.bmj.com

/J N

eurol Neurosurg P

sychiatry: first published as 10.1136/jnnp.56.3.290 on 1 March 1993. D

ownloaded from

2Good, Khurana, Mayer, Cintra, Albuquerque

Figure I Recordinrgs olcomtipound muscle acti'onpotenltials from the thenarmuscles on day 9 us'ingsurjfice electrodessti'mulatinlg the medi'annerve at the wrist at ratesof 50 Hz (A), 20 Hz (B1)and 2 Hz ((C). Note themarked decrement by thefourth response with somerecovery by the lastresponse at rates of 2 and20 Hz but not at 50 Hz.Amplitude calibration is2 mV B

I

11 tlA AJVJ vv1

ALl I. I I [I

1-1

Repetitive stimulation(2-10 Hz) in the median ar

No impulse blocking was observed. Fibredensities were normal. No spontaneous fasci-culations or denervation potentials wereobserved.

V V V V ' Endplate physiologyIntracellular recordings at the endplate regionof intercostal muscle fibres revealed that theresting membrane potentials were similar tocontrols (table 2). Spontaneous miniatureendplate potential (mepp) frequency wasreduced (p < 0-01) compared with controls.Mean mepp amplitude was reduced (p < 0 01)compared with control values. The shape of themepps was slightly altered as the potentialswere prolonged by 20%. As many of the meppswere small (< 0 5 mV), it is possible that somewere not observed above the noise level of therecording system accounting for the reducedfrqency.Microiontophoretic application of ACh to

the endplate region revealed decreased andmarked, mean (SD), variability of the AChsensitivity in different fibres [2730 (533)mV/nC to 448 (128)]. Mean ACh sensitivitywas reduced significantly (p < 0 01) compared

at slow rates with controls (table 2). Falling phase of thend peroneal nerves ACh potential was prolonged by 20%.

produced marked decremental responses(44-50%) at the fourth response with somerecovery by the last response (figure 1). At fastrates of stimulation (50 Hz) marked decre-ment (80%) was observed without recovery.Serial recordings were performed in themedian nerve and the results are shown intable 1. Response to intravenous edrophoniumchloride was studied from days 11 to 47 (table1) and showed a marked decrease in thedecrement (58-42% reduced to 33-7%) atslow and fast rates of stimulation. The decreasein decrement was associated with transientincrease in strength on muscle testing. Thebeneficial effect of edrophonium persistedthroughout the 6 week period of neuromus-cular weakness. Similar beneficial effects wereobtained with intramuscular neostigmine thatreduced the decremental response of 58% (4thresponse at 65 Hz) to 10% at 35 minutes afterthe injection.Three months after exposure when the

patient had mild proximal muscle weaknessand fatiguability, no decremental responseswere observed. However, SF EMG revealedincreased variability in the interpotential inter-vals (jitter) in all 20 potential pairs examined.

Table 2 Endplate recordings in intercostal nmuscles inpatient with phosmet exposure and nornmal subjects

Recordings Patient Normals

Resting membrane - 75 (2 73) - 76 (2 1)Potential, mV 45a 30

MEPP 0-56 (0-07)* 0-83 (0-22)Amplitude, mV 40/481 b 31/435

MEPP 0-25 (0-02)* 0-38 (0-08)Frequency, Sec' 40/581 30/870ACh sensitivity 1182 (216)* 3730 (102)mV/nC 11 20

Mean (SE)'Number of fibres studiedbNumber of fibres/number of potentials counted*Significance p < 0 01 from normalsMEPP-miniature endplate potentialACh-Acetylcholine

UltrastructureEndplate regions at low magnification showedthat the subjunctional myofibrils appearednormal without evidence of a myopathy. Neu-romuscular junctions contained numeroussynaptic vesicles, mitochondria and synapticclefts with well established basal lamina. Theendplate soleplasm contained an increase ofsmooth endoplasmic reticulum, ribosomes,polysomes and invaginated nuclei comparedwith controls suggesting high metabolic andregenerative activity (figure 2). Some endplatesshowed subsynaptic vesiculation and phago-cytic lysosomal activity suggesting degenera-tion. At higher magnification some of thesecondary synaptic clefts were widened andfilled with debris and junctional folds weresimplified suggesting endplate damage.

DiscussionThe patient described in this report had asubacute progressive neuromuscular symdromeassociated with CNS dysfunction following thefifth weekly spraying with phosmet, an OPinsecticide. Paralysis became maximum by 9days, continued for 30 days, but some weak-ness persisted for 4 months. CNS signs ofvisual hallucinations, disorientation andmyoclonic jerks occurred at the peak of thetoxicity and most likely reflected central chol-inergic dysfunction. These signs clearedrapidly without residual effects. Limited symp-toms and signs of acute OP toxicity werenoted. No myopathy or delayed peripheralneuropathy occurred. The neuromuscular dvs-function was associated with CNS involvementwhich was observed also in patients with theintermediate syndrome following exposure toOP insecticides."' Acute and delayed CNScholinergic dysfunction is well recognised in

292

I -

C

A

on February 26, 2021 by guest. P

rotected by copyright.http://jnnp.bm

j.com/

J Neurol N

eurosurg Psychiatry: first published as 10.1136/jnnp.56.3.290 on 1 M

arch 1993. Dow

nloaded from

-Endplate studies in OP poisoning

~~ ~ ~ ~ ~ ~ ~ ~ ~ ~~-

Av~~ ~ ~ ~ ~ ~ ~

Figure 2 High magnification micrograph of a neuromuscularjunction of intercostalmuscle showing numerous synaptic vesicles and clefts. The soleplasm contains largeamounts of smooth endoplasmic reticulum and polysomes. Calibration bar = 2 um.

experimental animals exposed to OP.29

Although the paralysis in our patient did notoccur after overt cholinergic toxicity, pro-gressed over a longer period (9 days rather than4) and persisted longer (30 days rather than18) than those reported,'0 it is likely that OPtoxicity due to phosmet can produce a delayedprogressive neuromuscular syndrome inhumans. In this syndrome the endplate physio-logical studies clearly document postsynapticneuromuscular dysfunction. This is charac-terised by: mildly reduced CMAP amplitudes,decremental responses reversed by edrophon-ium and neostigmine, small mepps andreduced junctional ACh sensitivity. Theseresults support those of the previous report inwhich limited electromyographic studies sug-gested a postsynaptic defect.'0 Decrementalresponses at slow rates of stimulation observedin this patient but not in those reported in theintermediate syndrome'0 may reflect greaterneuromuscular blockade, desensitisation andloss of ACh receptors. This may account forthe prolonged duration of the syndrome. Thissubacute OP postsynaptic neuromuscularblock can be differentiated electrophysiolog-ically from acute OP cholinergic block thatresults in spontaneous repetitive firing of mus-cle action potentials and decremental-incre-mental responses."' In acute OP toxicity,edrophonium increased the decremental res-

ponses,8 while in subacute toxicity it reducedthe decremental responses toward normal.Edrophonium is recommended as a test in OPtoxicity to document the acute cholinergic

block. The acute syndrome may benefit fromtherapy with pralidoxime chloride, an AChEreactivator that may have varied actions,'8 butit should not be used in the subacute syndromewith postsynaptic dysfunction, since it mayincrease the neuromuscular block and weak-ness.The endplate physiological findings in this

patient with subacute neuromuscular weaknessare similar to those reported in myastheniagravis that have shown loss of functionalpostsynaptic ACh receptors.'5 9 The mechan-ism of receptor loss in subacute phosmettoxicity is unknown but is likely to result fromendplate degeneration as shown on the ultra-structural studies. The physiological findings ofmildly prolonged distal motor latency andreduced mepp frequency and the pathologicalfinding of endplate damage suggest that nerveterminals may be affected by OP toxicity.Experimental studies with chronic AChEinhibitor toxicity in rats have shown similarphysiological and morphological changes ofthe neuromuscular junctions.202' Postsynapticdysfunction was greater and persisted longerthan presynaptic changes, was prevented bychronic nerve section and was therefore possi-bly due to the prolonged increased activity ofsynaptic ACh. Prolonged action ofACh at theNMJ has been reported to produce a myopathythat is mediated by calcium activation of aprotease and requires intact ACh receptors.22A myopathy was not observed in our patientand this may reflect the endplate degenerationwith loss of receptors. Prolonged action ofsynaptic ACh accounts for the acute OPsyndrome, and it is possible that it may inducethe subacute neuromuscular syndrome by pro-longed receptor desensitisation and influx ofcalcium that damages the NMJ. The endplatedamage may be enhanced also by the directeffects of AChE inhibitors that act as AChRagonists and channel blockers.23 It is possiblethat similar changes may occur at CNS chol-inergic junctions. In addition, unpublisheddata from our laboratory suggest that organo-phosphates cause release of excitatory andinhibitory transmitters in the CNS.24Our patient was exposed to phosmet, an

insecticide recommended for the control offruit tree pests.25 It is less toxic as an AChEinhibitor than other insecticides such as para-thion.26 Phosmet has very low water solubilityand is lipid soluble.26 It is possible thatrepeated weekly exposure to a lipid soluble OP,like phosmet, spray by inhalation and skincontact resulted in the subacute progressiveneuromuscular and CNS syndrome in ourpatient. There are no other reports of subacutetoxicity due to phosmet but fenthion, whichalso has high lipid solubility, produced 4 of the10 reported cases of the intermediate syn-drome.'o Proposed mechanisms by whichsome of the OP may produce subacute effectsrange from higher lipid solubility to delayedconversion to a more active metabolite.227Although it is possible that the insecticidewhich the patient used contained a toxicproduct of phosmet, there are no reports ofother toxic substances in the compound.25

293 on F

ebruary 26, 2021 by guest. Protected by copyright.

http://jnnp.bmj.com

/J N

eurol Neurosurg P

sychiatry: first published as 10.1136/jnnp.56.3.290 on 1 March 1993. D

ownloaded from

Good, Khurana, Mayer, Cintra, Albuquerque

Supported in part by the Neuromuscular Fund at the Uni-versity of Maryland School of Medicine (RFM) and a Grantfrom the US Army Medical Research and DevelopmentCommand Contract DAMD17-88-C-8119 (EXA).

1 Wadia RS, Sadagopan C, Amin RB, Sardesai HV. Neuro-logical manifestations of organophosphorus insecticidepoisoning. J Neurol Neurosurg Psychiatry 1974;37:841-7.

2 Russell RW, Overstreet DH. Mechanisms underlying sensi-tivity to organophosphate anticholinesterase compounds.Prog Neurobiol 1987;28:97-129.

3 Namba T, Nolte CT, Jackrel J, Grob D. Poisoning due toorganophosphate insecticides. Am J Med 1971;50:475-92.

4 Wadia RS, Chitra S, Amin RB, Kiwalkar RS, Sardesai HV.Electrophysiological studies in acute organophosphatepoisoning. J Neurol Neurosurg Psychiatry 1987;50:1442-8.

5 Albuquerque EX, Aracava Y, Idriss M, Schoenenberger B,Brossi A, Deshpande SS. Activation and blockade of thenicotinic and glutamatergic synapses by reversible andirreversible cholinesterase inhibitors. In: Dun NJ andPerlman RL, eds. Neurobiology and acetylcholine. NewYork: Plenum, 1987:301-28.

6 Albuquerque EX, Alkondon M, Deshpande SS, CintraWM, Brossi A. The role of carbamates and oximes inreversing toxicity of organophosphorus compounds: aperspective into mechanisms. In: Lunt GG, ed. Neurotox'88: molecular basis ofdrug and pesticide action. Cambridge,UK: Elsevier, 1988:349-73.

7 Albuquerque EX, Aracava Y, Cintra WM, Brossi A, Schoe-nenberger B, Deshpande SS. Structure-activity relation-ship of reversible cholinesterase inhibitors: activation,channel blockade and stereospecificity of the nicotinicacetylcholine receptor-ion channel complex. Brazilian JMed Biol Res 1988;21:1173-96.

8 Maselli R, Jacobsen JH, Spire J-P. Edrophonium: An aid inthe diagnosis of acute organophosphate poisoning. AnnNeurol 1986;19:508-10.

9 Abou-Donia MB, Lapodula DM. Mechanisms of organo-phosphorus ester-induced delayed neurotoxicity: Type Iand Type II. Ann Rev Pharmacol Toxicol 1990;30:405-40.

10 Senanayake N, Karalliede L. Neurotoxic effects of organo-phosphorus insecticides. An intermediate syndrome. NEngl _' Med 1987;316:761-3.

11 Good JL, Khurana R, Mayer RF, Albuquerque EX.Endplate degeneration in organophosphate (Phosmet)poisoning. Neurology 1990;40(Suppl 1):431.

12 Mayer RF. Nerve conduction studies in man. Neurology(Minneap) 1963;13:1021-30.

13 Mayer RF, Williams IR: Incrementing responses in myasthe-nia gravis. AMA Arch Neurol 1974;31:24-6.

14 Stalberg E, Ekstedt J. Single fibre EMG and micro-physiology of the motor unit in normal and diseased

human muscle. In: Desmedt JE, ed. New development inelectromyography and clinical neurophysiology. vol 1. Basel:Karger, 1973;1 13-29.

15 Albuquerque EX, Rash JE, Mayer RF, Satterfield JR. Anelectrophysiological and morphological study of theneuromuscular junction in patients with myastheniagravis. Exp Neurol 1976;51:536-63.

16 Rash JE, Albuquerque EX, Hudson CS, Mayer RF,Satterfield JR. Studies of human myasthenia gravis:electrophysiological and ultrastructural evidence compat-ible with antibody attachment to the acetylcholine recep-tor complex. Proc NadAcad Sci USA 1976;73:4584-8.

17 Besser R, Gutmann L, Dillmann U, Weilemann LS, HopfHC. End-plate dysfunction in acute organophosphateintoxication. Neurology 1989;39:561-7.

18 Alkondon M, Albuquerque EX. The nonoxine bispyr-idinium compound SAD-128 alters the kinetic propertiesof the nicotinic acetylcholine receptor ion channel: Apossible mechanism for antidotal effects. J Pharmacol ExpTher 1989;250:842-52.

19 Cull-Candy SG, Miledi R, Trautmann A. End-plate cur-rents and acetylcholine noise at normal and myasthenichuman end-plates. J Physiol (Lond) 1979;287:247-65.

20 Tiedt TN, Albuquerque EX, Hudson CS, Rash JE. Neo-stigmine-induced alteration at the mammalian neuro-muscular junction. I. Muscle contraction and electro-physiology. J Pharmacol Exp Ther 1978;205:326-39.

21 Hudson CS, Rash JE, Tiedt TN, Albuquerque EX. Neo-stigmine-induced alterations at the mammalian neu-romuscular junction. II. Ultrastructure. J Pharmacol ExpTher 1978;205:340-56.

22 Leonard J, Salpeter M. Agonist-induced myopathy at theneuromuscular junction is mediated by calcium. J CellBiol 1979;82:811-9.

23 Pascuzzo GJ, Akaiki A, Maleque MA, Shaw K-P, AronstamRS, Rickett DL, Albuquerque EX. The nature of theinteractions of pyridostigmine with the nicotinic acetyl-choline receptor-ionic channel complex. I. Agonist,desensitizing and binding properties. Mol Pharmacol1984;25:92-101.

24 Pereira EFR, Aklondon M, Albuquerque EX. Effects oforganophosphate compounds and physostigmine (PHy)on nicotinic acetylcholine receptors (AChR) in themammalian central nervous system (CNS). Proc 1991Medical Defense Bioscience Review 1991:229-33.

25 MacNeil J, Hikichi M. Phosmet residues in an orchard andadjacent recreational area. J Environ Sci Health 1986;21:375-85.

26 Dedek W, Grahl R, Schmidt R. A comparative study ofguanine N-alkylation in mice in vivo by the organophos-phorus insecticides trichlorphon, dimethoate, phosmet,and bromophos. Acta Pharmacol et Toxicol 1984;55:104-9.

27 SakamotoT, SawadaY, Kazuyuki N et al. Delayed neurotox-icity produced by an organophosphorus compound(Sumithion). Arch Toxicol 1984;56: 136-8.

294 on F

ebruary 26, 2021 by guest. Protected by copyright.

http://jnnp.bmj.com

/J N

eurol Neurosurg P

sychiatry: first published as 10.1136/jnnp.56.3.290 on 1 March 1993. D

ownloaded from