Review Note

Last Update: 02/20/2025 02:12 AM

Current Deck: ACG Part 2::Thoracic SSU

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SS_TS 1.13 Discuss the anaesthetic management of the following procedures:
• Thymectomy, particularly the perioperative management of myasthenia gravis (also refer to the Perioperative medicine clinical fundamental)

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Myasthenia gravis:
Myasthenia gravis (MG) is the most common disorder of the neuromuscular junction (NMJ), characterised by fatigable weakness affecting the skeletal muscle.
It is a B-cell-mediated autoimmune disease, in which antibodies bind to acetylcholine (ACh) receptors (AChR) at the NMJ, or to functionally related molecules in the postsynaptic membrane, including muscle-specific kinase (MuSK), lipoprotein receptor-related peptide 4 (LRP4) or agrin.
Autoantibodies to AChR disrupt its function by receptor blockade, conformational change, complement activation and crosslinking, leading to increased degradation.
Decreased functional AChR density results in decreased motor end-plate potential amplitude and failure in the initiation of muscle fibre contraction. The alternative antibody targets disrupt the regulation and maintenance of normal synaptic function
Can also be a rare complication of novel cancer therapies e.g. pembrolizumab (melanoma, lung Ca, head and neck Ca, hodgkins lymphoma)

Clinical presentation:
Fatigable muscle weakness affecting the extraocular, bulbar, limb, axial and respiratory muscles, localised or generalised, mild to severe, worsened by repetitive movements, improved with rest
First symptoms usually occular, ptosis, diplopia
Thymic hyperplasia present in most patients with early disease onset, thymoma present in 15%
myasthenic crisis, a life-threatening condition - severe respiratory muscle insufficiency leads to respiratory failure. Crises are most commonly precipitated by infection. Other precipitants include surgery, residual neuromuscular block, pain, many drugs, hypo- and hyperthermia, reduction or withdrawal of treatment, pregnancy, stress and sleep deprivation.
diagnosed through clinical, serological and neurophysiological testing

Management of MG:
There are four main approaches to therapy.
(i) Symptomatic treatment with acetylcholinesterase inhibitors (e.g. pyridostigmine) increases the amount of ACh available at the NMJ. Muscarinic adverse effects may be controlled with oral anticholinergic drugs, including propantheline and mebeverine.
(ii) Chronic immunosuppressive therapy with corticosteroids (e.g. prednisolone) reduces the underlying autoimmune dysfunction. Non-steroidal immunosuppressive agents are introduced if remission is not achieved, initially azathioprine, which is escalated to mycophenolate mofetil, methotrexate, ciclosporin or rituximab.
(iii) Rapid, short-acting immunomodulating treatments (e.g. i.v. immunoglobulin [IVIg] and plasma exchange [PEX]), are used in combination with steroids for the management of significant bulbar and respiratory symptoms or progressive deterioration.
(iv) Thymectomy: patients with a thymoma should undergo a thymectomy to remove the tumour for prognostic benefit and cure. Thymectomy is also recommended in those without a thymoma with positive anti-AChR antibodies under the age of 45 yrs. This may reduce corticosteroid requirements, prevent generalisation and induce remission.
The goal of treatment is to achieve minimal manifestation status (MMS)

Pre-op:
Optimisation and timing:
Pre-op assessment by neurologist that condition is optimized, stable phase and for post-operative care planning
May need pre-op IVIg or PEX
Surgery should be scheduled in the morning
Anaesthetic assessment
Assess for bulbar dysfunction including dysphagia, dysarthria and dysphonia, dyspnoea and a history of myasthenic crises
Associated autoimmune diseases and their effects
Current and planned drug therapy
Anticholinesterases and glucocorticoids should continue perioperatively, as their discontinuation may cause deterioration of symptoms
Regular anticholinergics, such as propantheline, may continue, but should be noted if further doses of anticholinergic drugs are planned
Non-steroidal immunosuppressive agents have a long duration of action, and perioperative interruptions are unlikely to have a significant effect.
Regular glucocorticoid therapy may induce or exacerbate diabetes mellitus, and cause hypothalamoepituitaryeadrenal axis suppression and adrenal insufficiency, which requires perioperative cover with steroids.
Investigations:
Bloods: full blood count, electrolytes and renal and hepatic function (for patients taking non-steroidal immunosuppressive agents). Thyroid function tests should be performed to exclude associated thyroid disease
12-lead ECG to screen for evidence of cardiac involvement.
Available imaging should be reviewed. Large thymomas are rare, but may cause distortion and compression of the trachea
Lung function tests: for general anaesthesia + neuromuscular blocking drugs (NMBDs) or undergoing major surgery to assess the patient’s baseline and plan postoperative care.
Forced vital capacity (FVC) integrates inspiratory and expiratory muscle strength, and is the most reproducible and easily performed investigation.
Vital capacities of 65-79%, 50-64% and <50% predicted are considered mild, moderate and severe, respectively
Post-op ventilation:
Should be discussed with patient at pre-op assessment, risk factors:
Patient: chronic obstructive pulmonary disease, BMI >28 kg m2, Disease duration >2 yrs, bulbar or respiratory symptoms, generalised moderate weakness or greater, a history of previous myasthenic crises, pyridostigmine dose >750 mg/day , vital capacity <2-2.9 L, serum AChR antibody concentrations >100 nmol ml1 and a pronounced decremental response on repetitive nerve stimulation (>18e20%).
Surgical factors: Blood loss >1000ml, lung resection
Intra-op:

Sedation:
short-acting agents could be given in small, titratable doses under close supervision
Regional and local:
Spares GA and sedation and potential respiratory affects
Amide local anaesthetics should be used, as the metabolism of esters is theoretically impaired because of the action of the patients’ regular anticholinesterase medications
Caution should be exercised when considering neuraxial and brachial plexus blocks in those with significant respiratory or bulbar weakness
Neuraxial blockade to mid-thoracic levels paralyse the accessory muscles of breathing, which may not be well tolerated.
Interscalene, supraclavicular and infraclavicular blocks paralyse the ipsilateral phrenic nerve in up to 100%, 50% and 25% of cases, respectively, although ultrasound and low volume techniques may improve upon this
GA:

Induction:
Short-acting agents promote early recovery and minimise respiratory depression.
Total i.v. anaesthesia (TIVA) with propofol and remifentanil can provide satisfactory conditions for tracheal intubation, rapid recovery of consciousness and psychomotor function and an enhanced recovery profile without the use of NMBDs
Induction of anaesthesia with i.v. propofol may be combined with short-acting agents, including opioids, lidocaine or esmolol, to facilitate intubation without NMBDs
I.V. lidocaine is a useful analgesic adjunct, but can potentiate the effect of NMBDs through presynaptic and postsynaptic effects.
Relaxant:
Avoid NMBD’s where able as all patients with MG respond unpredictably to NMBDs and reversal with anticholinesterases, including those in remission and after thymectomy.
Residual paralysis carries the risk of precipitating a myasthenic crisis, whilst excess reversal may lead to a cholinergic crisis.
If it is not possible to avoid NMBD’s:
Roc + sugammadex
Because of reduced AChR density at the NMJ, patients are resistant to depolarising NMBDs the ED95 of suxamethonium is approximately 2.5 times greater in patients with MG
Increased risk of phase 2 block
Very sensitive to non-depolarising relaxants - given in 1/10th usual dose incrementally with quantitative neuromuscular monitoring device
Regular anti-cholinesterase use prolongs action of sux and may require a larger dose of non-depolarising relaxant
Neostigmine effect is unpredictable and may indice cholinergic crisis
Sugammadex: demonstrated to be superior to neostigmine for reversing moderate-to-deep neuromuscular block, and may reduce the risk of perioperative myasthenic crises and postoperative pneumonia
Ventilation:
Spontaneous ventilation with pressure support minimises respiratory fatigue for short procedures, avoid excess respiratory effort
IPPV for longer/bigger surgeries will optimise respiratory function for extubation
Monitoring:
Routine
A-line + CVL for thymectomy/larger surgeries as induction
Neuromuscular function monitoring
Temperature monitoring - maintaining normothermia to avoid myasthenic crisis
Analgeisa
Multi-modal, opioid sparing where able
Emergence:
Can usually be extubated post-op
Sitting upright, pre-oxygenated, adequate analgesia
Spontaneously ventilating, own airway reflexes, return of muscle strength (Quantitative)
A TOFR >0.9 should be confirmed as a minimum
Drugs to avoid (exacerbate muscle weakness) table 3

Surgery:
Thymectomy has been traditionally performed via median sternotomy.
Minimally invasive procedures via the transcervical, transthoracic and subxiphoid approaches are thought to have a number of benefits:
decreased risk of respiratory and cardiac complications,
less intraoperative blood loss,
less requirement for blood products,
decreased inflammatory cytokine response,
less postoperative pain,
early removal of chest drains,
shorter hospital stay and superior cosmesis.
No significant difference has been found in long-term complications and survival rate.
Post-op care:
Surgery under sedation, local or regional anaesthesia and general anaesthesia without NMBDs may be suitable for day surgery.
Routine critical care admission after thymectomy has been shown to not be necessary with good perioperative preparation and management.
Any evidence of a myasthenic crisis necessitates critical care admission.

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