Myasthaenia Gravis

 

Disease:

Pathophysiology

·      IgG antibodies against skeletal muscle nAChR, produced by abnormal immune response

·      Average 30% of normal number of nAChR

·      Very few spare receptors

Presentation

·      Women in their 20s, men in their 60s

·      Fatigueable weakness

·      Normal tendon reflexes

·      Oculobulbar commonest (diplopia, ptosis)

·      Craniocaudal spread

·      Myaesthaenic crisis (diaphragm) most likely during first 2 years

·      With use: brief potentiation then fade

Severity

1.    Ocular only

2.    Ocular + mild other

3.    Ocular + moderate other

4.    Ocular + severe other

5.    Intubation +/- IPPV

Subtypes for grades 2,3,4 – limb vs trunk vs resp muscles

Associated

·      Rheumatoid arthritis

·      Thyroid disease

Test

·      Edrophonium test: 10mg IV effective within 30 seconds, lasts 5 minutes (negative result does not exclude dx)

·      EMG: reduced compound muscle action potential (CMAP) amplitude i.e. fade!

·      Blood test: anti-nAChR antibody

Treatment

·      Anticholinesterase (note risk of cholinergic crisis cf. myasthaenic crisis)

·      Antibody removal: IVIg, plasma exchange

·      Immunosuppression: prednisolone, azathioprine

·      Thymectomy

 

Myasthaenia gravis vs Eaton-Lambert

 

ELMS

MG

Aetiology

Cancer (esp. SCLC)

T cell-mediated
(thymoma 10-15%)

Pathophysiology

Auto-antibody against pre-synaptic VDCC

Auto-antibody against muscle type nAChR

Presentation

Limb weakness (prox LL)

Symmetrical

Deterioration with use

Reduced tendon reflexes

PSNS dysfunction

Ocular, bulbar, facial weakness

Asymmetrical

Initial improvement with use

Normal tendon reflexes

Normal autonomic function

Time course

Fluctuation

Gradual onset

Testing

Reduced CMAP amplitude

Normal CMAP amplitude

Edrophonium test (Tensilon)

Treatment

Immunosuppression

Guanidine

3,4-diaminopyridine

Immunosuppression

Anticholinesterase

Thymectomy

 

 

Pre-op issues:

Location

·      Must have ICU available for prolonged intubation + IPPV

Assessment

·      Peri-op/neurology review

·      Respiratory function tests if possible

·      Risk stratification: Leventhal criteria ± refer to HDU/ICU

Optimisation

·      Medication planning:

o  Consider glucocorticoid, IVIg

o  Withhold anticholinesterase on morning of surgery

·      Chest physio

Risk stratification

Leventhal criteria: more likely prolonged ventilation if:

·      Severe disease (3 or 4)

·      Longstanding disease (>6 years)

·      Poor respiratory function (FVC <3L)

·      Other respiratory disease

 

 

Intra-op issues:

Mode

·      Prefer regional

Airway

·      Beware bulbar weakness -> risk of aspiration

·      Beware associated rheumatoid arthritis -> risk of C spine injury

Breathing

·      Prefer spont vent

Circulation

·      Nothing in particular

Drugs

·      Avoid muscle relaxant

o  Prefer rocuronium + sugammadex

o  Resistant to suxamethonium

o  Resistant to non-depolarising drugs

·      Avoid volatiles

·      Avoid excessive opioid; prefer non-opioids + neuraxial/regional

·      Give anticholinesterase + antimuscarinic pre-extubation

Monitoring

·      TOF: but how to interpret.

 

 

Post-op:

Extubation

·      Awake ++

·      Reversal ++

Monitoring

·      Expect high rate of failure if abdominal or thoracic surgery

·      Observe for longer than usual after extubation and in PACU

·      Low threshold for HDU

 

 

2014-2-9 A 30 year old patient with myasthenia gravis presents for orthopaedic procedure and refuses a regional anaesthetic technique
a. What are the signs and symptoms of myasthenia gravis? (30%)
b. How does the disease affect your anaesthetic management? (70%

 

Examiner’s report:

Key components of a basic answer for this question: Knowledge that is a disease of skeletal muscle manifest by fatigability and that both bulbar and respiratory muscles can be effected. A structured plan for anaesthetic management that acknowledges   Aspiration risk  Sensitivities to opiates and muscle relaxants  And includes a reasonable plan for post-operative management A plan for the management of the patients anticholinesterase therapy [and  steroids if applicable ] in the perioperative period

 

a.Signs and symptoms: see above

 

b.Anaesthetic management

 

Pre-op:

·         See above r.e. assessment and optimisation

·         Explore reason for refusal of regional

·         Discuss spinal + midazolam sedation

·         Discuss adductor canal block/catheter under GA at end of case

 

Intra-op:

·         Anaesthetic: propofol + remifentanil GA (minimize remi, use others for analgesia)

·         Airway: LMA if fasted and low aspiration risk. Beware rheumatoid arthritic C-spine, consider video laryngscopy.

·         Breathing: pressure supported breaths to avoid diaphragm weakness (occurs from 30 mins of apnoea)

·         Circulation: no problem really

·         Treatment drugs: consider stress dose corticosteroid e.g. hydrocortisone 50mg TDS

·         Analgesics: adductor canal catheter under GA, bolus 20mL 0.2% ropivacaine + paracetamol + NSAID + fentanyl 0.5mcg/kg PRN + ketamine bolus 0.5mg/kg and infusion 0.1-0.2mg/kg/h; paracetamol 15mg/kg; NSAID (if allowed); fentanyl boluses 0.5mcg/kg PRN

·         Other drugs: avoid volatile, avoid paralysis

·         NMBD: avoid paralysis altogether; avoid sux (resistant), half dose roc 0.3mg/kg (sensitive), sugammadex for reversal

Post-op:

·         Analgesia: paracetamol + NSAID + oxycodone 0.1-0.2mg/kg q3h prn +/- adductor canal catheter 0.2% ropiv 6mL/h (to be removed day 2 post-op) +/- ketamine IV infusion 0.1-0.2mg/kg/h
(I believe preferable to PCA because a) very painful surgery b) reduces opioid requirement c) gut function is intact

·         Longer stay in PACU

·         Follow-up: APS review daily

 

 

2006-1-5 A fifty year old man taking corticosteroid and pyridostigmine for myasthenia gravis is to have an elective right hemicolectomy under general anaesthesia. Discuss your management of his myasthenia pre and post operatively.

 

Examiner’s report:

This question dealt with the uncommon but important topic of myasthenia gravis.  The principal concerns from an anaesthetist’s standpoint are the degree of respiratory dysfunction present, the increased risk of aspiration, the altered doses of neuromuscular relaxants required, the timing of extubation post-operatively and the ongoing need to ensure adequate muscle power.  Most candidates were aware of the preoperative Leventhal criteria that are predictive of the need for postoperative ventilation but relatively few were able to coherently discuss the other important clinical aspects of this disease that need to be addressed before providing anaesthesia.  A significant minority of candidates did not appear to appreciate the factors involved in deciding when and where to extubate these patients, nor the continuing aspects of clinical care involved in their postoperative management.  A description of how to conduct the actual anaesthetic was not required nor expected.

 

Pre-op:

·         See above

·         Plan ICU/HDU admission post-op for resp monitoring

 

Intra-op:

·         Anaesthetic: propofol GA (avoid volatile)

·         Airway: ETT, consider RSI if aspiration risk. Beware rheumatoid arthritic C-spine, consider video laryngoscopy.

·         Breathing: paralysis + IPPV required

·         Circulation: less likely a problem

·         Treatment drugs: take AChEi pre-op; consider IVIG pre-op; stress corticosteroid e.g. hydrocortisone 50mg TDS

·         Analgesics: paracetamol + fentanyl 0.5mcg/kg PRN + (if large midline laparotomy) ketamine IV infusion 0.1-0.2mg/kg/h; wound infiltration catheter inserted at closure

·         NMBD: avoid paralysis altogether; avoid sux (resistant), half dose roc 0.3mg/kg (sensitive), sugammadex for reversal; TOF monitoring

Post-op:

·         Attempt extubation in OR: airway suctioned, sugammadex reversal for TOF ratio >0.9, sitting up, pressure support 10/5cmH2O, obeying commands

·         Communicate re-intubation plan to assistant

·         If failed, then transfer to ICU for slower extubation

·         If success, then longer stay in PACU before transfer to HDU

·         Analgesia: wound infiltration catheter 0.2% ropivacaine 20mL bolus + 6mL/h; to be removed at day 3-5

·         Follow-up: APS review daily