National Board Ex- 6 Book .pmd - National Board Of Examination

eral nerve can return to normal,while the pharyngeal and neckmuscles necessary to protect theairway can still be partially paralysed,and this difference might beexaggerated in patients of MGwho already have some degree ofbulbar and/or respiratory muscleweakness. Patients should be consideredpartially paralysed untilthey wake up and can lift theirhead for five seconds or generatean inspiratory force exceeding- 25 cm H 2O. Myasthenic patientsmay be at increased risk ofdeveloping postoperative respiratoryfailure, and following transsternal thymectomy, up to 50%of patients require prolongedpostoperative ventilation.Riskfactors predicting requirement ofprolonged post-operative ventilationinclude 10 :l Duration of myastheniagravis for longer than sixyears.l A history of chronicrespiratory disease other thanrespiratory dysfunctiondirectly due to MG.l A dose of pyridostigminegreater than 750 mg per day,48 hr before operation.l A pre-operative vital capacity< 2.9 L.l Clinical classification of MG(Ossermann classes 3 and 4).l A previous history ofrespiratory failure due to MG,and associated steroidtherapy.Thymectomy benefits nearly 96%of patients regardless of preoperativecharacteristics. Followingthymectomy, different therapeuticregimens have been recommendeddepending on the outcomeof surgery. Most early-onsetmyasthenics can be treatedwith thymectomy alone, whilelate-onset myasthenia as well asmyasthenia associated withthymoma needs additional postoperativeimmunosuppression.Some patients of myastheniagravis may come for emergencysurgeries before adequate medicalcontrol has been achieved, inwhich case, titrated doses ofmuscle relaxants should be usedwith neuromuscular monitoring,and arrangements for post-operativeventilation should bemade before taking up such apatient for surgeryMyasthenic crisis and Cholinergiccrisis -Myasthenic crisis isa medical emergency characterizedby respiratory failure fromdiaphragmatic weakness or severeoropharyngeal weaknessleading to aspiration. Crisis canoccur in the setting of surgery(postoperative), acute infection,or following rapid withdrawal ofcorticosteroids (though somepatients have no precipitating factors).Patients should be placedin an ICU setting and have forcedvital capacity (FVC) checked every2 hours. Changes in ABGoccur late in neuromuscular respiratoryfailure. Criteria for intubationinclude a drop in FVCbelow 15 ml/kg, severe oropharyngealweakness leading to aspirationor laboured breathingregardless of spirometric measurements.It can be extremelydifficult to distinguish too muchfrom too little anticholinergicmedication when a patient withknown myasthenia gravis presentswith rapidly increasing muscularweakness, with or withoutrespiratory difficulty. Featuressuggestive of a cholinergic crisis(too much medication) includemuscle fasciculation, pallor,sweating, hypersalivation andsmall pupils. If the diagnosis isnot clear-cut, it is advisable totemporarily discontinue cholinesterasemedication and to securethe airway with intubation,stabilize ventilation and then addressthe question of the underlyingdiagnosis.Cholinergic crisisresults from an excess of cholinesteraseinhibitors (ie, neostigmine,pyridostigmine, physostigmine)and resembles organophosphatepoisoning. In this case,excessive ACh stimulation of striatedmuscle at nicotinic junctionsproduces flaccid muscle paralysisthat is clinically indistinguishablefrom weakness due to MG.It may cause bronchospasm withwheezing, bronchorrhea, respiratoryfailure, diaphoresis, and cyanosis.Miosis and the SLUDGEsyndrome (ie salivation, lacrimation,urinary incontinence, diarrhea,GI upset and hypermotility,emesis) also may mark cholinergiccrisis. However, these findingsare not inevitably present. Ifin doubt, an edrophonium testcan be performed. Improvementsuggests too little medication i.e.myasthenic crisis and aggravationsuggests too much medication.This test should only be performedwith the necessary skillsand equipment ready for intubationand ventilation.References1. Vincent A, Palace J, Hilton-Jones D: Myasthenia gravis.Lancet 2001; 357: 2122-8.Journal of Postgraduate Medical Education, Training & Research73