15.4 Coexisting Disease II: Neuromuscular and Neurological Disease

15.4 Coexisting Disease II: Neuromuscular and Neurological Disease

Principles

Neuromuscular diseases may affect the motor neurone, peripheral nerve, neuromuscular junction or muscle. The site determines the response to drugs:

Myasthenia Gravis

An autoimmune disease with antibodies against postsynaptic nicotinic acetylcholine receptors (about 85%) or muscle-specific kinase (MuSK). It causes fatigable weakness of ocular, bulbar, limb and respiratory muscles. Thymoma is present in about 10-15%, and thymic hyperplasia is common.

Treatment: pyridostigmine (anticholinesterase), immunosuppression (prednisolone, azathioprine), intravenous immunoglobulin or plasma exchange before surgery in poorly controlled patients, and thymectomy.

Anaesthetic management:

• Assess bulbar function and respiratory reserve (vital capacity); optimise before elective surgery.
• Continue pyridostigmine (it may slightly prolong suxamethonium and reduce response to neostigmine).
• Suxamethonium: relative resistance because fewer receptors are available; about 1.5-2 mg/kg may be needed.
• Non-depolarising blockers: greatly increased sensitivity; avoid if possible, or use about 10-20% of the normal dose with quantitative monitoring. Sugammadex is preferred for reversing rocuronium because neostigmine is unreliable.
• Volatile agents and magnesium potentiate weakness; aminoglycosides, some antiarrhythmics and fluoroquinolones can worsen the disease.
• Postoperative ventilation is more likely with long disease duration, high pyridostigmine doses, bulbar involvement and low vital capacity.

Myasthenic versus Cholinergic Crisis

Lambert-Eaton Myasthenic Syndrome

Antibodies against presynaptic voltage-gated calcium channels reduce acetylcholine release. About half of cases are paraneoplastic, usually with small-cell lung cancer. Weakness affects proximal limbs, often improves with repeated effort, and is accompanied by autonomic dysfunction (dry mouth, postural hypotension). Patients are sensitive to both depolarising and non-depolarising blockers. Treatment includes amifampridine (3,4-diaminopyridine), which prolongs presynaptic depolarisation.

Muscular Dystrophies

Duchenne muscular dystrophy (X-linked, absent dystrophin) causes progressive weakness from early childhood, cardiomyopathy and respiratory failure. Becker dystrophy is milder.

• Suxamethonium is contraindicated: it can cause acute rhabdomyolysis and hyperkalaemic cardiac arrest.
• Volatile agents can cause rhabdomyolysis and hyperkalaemia (anaesthesia-induced rhabdomyolysis), so total intravenous anaesthesia is generally preferred. This is distinct from malignant hyperthermia.
• Assess cardiac function (echocardiography) and respiratory function; risk of postoperative respiratory failure and aspiration.

Myotonic Dystrophy

An autosomal dominant multisystem disorder (CTG repeat expansion) with myotonia (delayed relaxation), weakness, cardiac conduction defects, cardiomyopathy, cataracts, diabetes, frontal balding and central sleep apnoea.

• Triggers of myotonia: suxamethonium, neostigmine, cold, shivering, surgical manipulation and diathermy. The resulting contracture is not relieved by non-depolarising blockers or regional anaesthesia because it originates in the muscle membrane; local anaesthetic infiltration into the muscle, phenytoin or quinine may help.
• Sensitivity to sedatives, opioids and anaesthetic agents, with a high risk of postoperative respiratory failure and aspiration.
• Short-acting non-depolarising blockers with sugammadex reversal are preferred; avoid neostigmine.
• Monitor for arrhythmias and conduction block; pacing may be needed.

Multiple Sclerosis

A demyelinating disease of the central nervous system with relapses and progression.

• Relapses may follow surgery, infection, stress and hyperthermia (even small rises in temperature worsen conduction in demyelinated nerves).
• General anaesthesia is not known to cause relapse. Epidural anaesthesia with low concentrations is considered acceptable; spinal anaesthesia has been associated with relapse in older reports, but evidence is weak, and the decision should be discussed with the patient.
• Avoid suxamethonium in patients with significant weakness or immobility (hyperkalaemia).
• Autonomic dysfunction can cause cardiovascular instability.

Parkinson's Disease

Loss of dopaminergic neurones in the substantia nigra causes bradykinesia, rigidity, tremor and autonomic dysfunction.

• Continue antiparkinsonian drugs without interruption; give levodopa by nasogastric tube or use a rotigotine patch if the patient cannot swallow. Omission can cause severe rigidity, dysphagia, aspiration and a neuroleptic malignant-like syndrome.
• Avoid dopamine antagonists: metoclopramide, prochlorperazine, droperidol and haloperidol.
• Risks: aspiration (bulbar dysfunction), postural hypotension, postoperative confusion, and hypotension with propofol in patients with autonomic failure.
• MAO-B inhibitors (selegiline, rasagiline) interact with pethidine.

Epilepsy

• Continue antiepileptic drugs; give intravenous alternatives (levetiracetam, sodium valproate, phenytoin, lacosamide) when oral intake is interrupted.
• Pro-convulsant drugs to avoid or use with care: high-dose enflurane (historical), pethidine (norpethidine), large doses of local anaesthetics, tramadol and hypocapnia. Sevoflurane at high concentrations can produce epileptiform EEG activity.
• Enzyme-inducing drugs increase requirements for opioids and non-depolarising blockers.

Spinal Cord Injury

• Acute phase: spinal shock with flaccid paralysis and, in high cervical and upper thoracic injuries, neurogenic shock (hypotension and bradycardia from loss of sympathetic outflow above T6). Injuries at C3-C5 affect the diaphragm.
• Suxamethonium: safe in the first 24-48 hours; avoid from about 72 hours until at least 6-12 months (some advise longer) because extrajunctional acetylcholine receptors cause dangerous hyperkalaemia.
• Autonomic dysreflexia: in chronic lesions above about T6, stimulation below the lesion (bladder distension, surgery) causes massive sympathetic discharge with severe hypertension and reflex bradycardia. Prevent it with adequate general or neuraxial anaesthesia even when the patient cannot feel the surgery; treat with deepening anaesthesia and vasodilators.

Guillain-Barré Syndrome

An acute inflammatory demyelinating polyneuropathy, often after infection, causing ascending weakness, areflexia and autonomic instability.

• Monitor vital capacity: below about 20 mL/kg (or a fall of 30%), inability to cough or bulbar weakness indicates the need for ventilation.
• Treatment: intravenous immunoglobulin or plasma exchange; corticosteroids are not effective.
• Avoid suxamethonium (hyperkalaemia); autonomic instability causes exaggerated responses to vasoactive drugs and to induction.