Neuromuscular Disorders

Dwayne McClerklin, MDAssistant Professor

Department of Anesthesiology and Perioperative MedicineMUSC

Myasthenia Gravis

Path: Autoimmune destruction of the postsynaptic Ach receptor

S/Sx: muscle weakness, easy fatiguability ; symptoms worsen with exercise and improve with rest. With bulbar involvement, laryngeal and pharyngeal muscle weakness results in dysarthria, difficulty chewing/ swallowing, problems clearing secretions, or pulmonary aspiration.

Myasthenia Gravis

Type I ocular muscle weakness

Type II mild nonocular weakness

Type III Moderate nonocular muscle weakness

Type IV Severe nonocular muscle weakness

Type V Tracheal intubation and mechanical ventilation

Myasthenia Gravis

Rx: Acetylcholinesterase Inhibitors (AchEI) increase the amount of acetylcholine at the neuromuscular junction through inhibition of end-plate acetylcholinesterase

Cholinergic crisis results from excessive AchEI dosing and is characterized by increased weakness and excessive muscarinic effects

Edrophonium test- differentiates cholinergic from muscarinic crisis. Sx of muscarinic crisis worsen with edrophonium administration

Myasthenia Gravis

Anesthetic Considerations

Preoperative continuation of AchEI controversial (pros- potential decreased postop weaknes; cons- altered action of NMBAs, bowel hyperactivity)

Pts may be at increased risk of aspiration, respiratory depression (go easy with premedications)

Myasthenia Gravis

Deep anesthesia with a volatile agent may produce favorable conditions for tracheal intubation

Pts demonstrate resistance to depolarizing NMBAs, proneness to Phase II blockade

Pts sensitive to nondepolarizing NMBAs

Myasthenia Gravis

Patients who have myasthenia gravis are at great risk for postoperative respiratory failure

Disease duration of more than 6 years, concomitant pulmonary disease, a peakinspiratory pressure of < –25 cm H2O (ie, –20 cm H2O), a vital capacity < 4 mL/kg, and a pyridostigmine dose > 750 mg/d are predictive of the need for postoperative ventilation

Eaton Lambert Syndrome

Path: Caused by failure of presynaptic Ca2+ entry (which normally stimulates release of Ach); associated with small cell cancer of the lung

S/Sx: muscle weakness that improves with exercise; minimal improvement with AchEI

Rx: Aminopyridines (presynaptic K+ channel blockers which prolong the AP, increasing Ca2+ influx and Ach release)

Eaton Lambert Syndrome

Anesthetic Considerations

Pts with ELS are sensitive to both depolarizing and nondepolarizing NMBAs

Deep anesthesia with volatile agents is usually sufficient for intubation

Muscular Dystrophies

Duchenne’s Muscular Dystrophy

An X-linked recessive disorder, Duchenne's muscular dystrophy affects males almost exclusively

Affected individuals produce abnormal dystrophin, a protein found on the sarcolemma of muscle fibers.

Muscular Dystrophies

Duchenne’s Muscular Dystrophy

S/Sx: Progressive proximal muscle weakness (gait disturbance) and fatty infiltration of calf muscles; respiratory muscle weakness (atelectasis, restrictive lung disease, impaired cough); cardiomyopathy, papillary muscle dysfunction leading to MR

Muscular Dystrophies

Duchenne’s Muscular Dystrophy

Dx: Elevated serum creatine kinase (10-100x normal), elevated serum myoglobin levels; Muscle biopsy is definitive

Rx: Supportive care; corticosteroids; spinal rodding and fusion to improve mobility and comfort

Muscular Dystrophies

Duchenne’s Muscular Dystrophy

Anesthetic considerations: assess degree of weakness, multisystem involvement; pts may be an aspiration risk; avoid Sux (risk of hyperkalemic arrest, MH); sensitivity to nondepolarizing NMBAs may be increased

Muscular Dystrophies

Myotonic Dystrophy

Myotonia-slowing of relaxation after muscle contraction in response to electrical or percussive stimuli.

Myotonic Dystrophy (MD) is the most common cause of myotonia

Muscular Dystrophies

Myotonic Dystrophy

S/Sx:Myotonia is the principal manifestation early in the disease, but as the disease progresses, muscle weakness and atrophy become more prominent.

Other manifestations include cardiomyopathy, gastric hypomotility, and alveolar hypoventilation

Muscular Dystrophies

Myotonic Dystrophy

Anesthetic Considerations- Pts are aspiration risks; avoid premedication due to proneness to hypoventilation; avoid Sux; Cis is a good choice; Neostigmine can exacerbate myotonia; avoid postoperative shivering

Myotonias

Myotonia Congenita

Path:caused by mutations of a gene on chromosome 7q35 encoding a chloride channel of the skeletal muscle fiber surface membrane.

S/Sx:. The disease is confined to skeletal muscle and produces no, minimal, or nonprogressive weakness. There is no cardiac involvement

Myotonias

Paramyotonia Congenita

S/Sx:Symptoms of paramyotonia congenita include transient stiffness (myotonia) and, occasionally, weakness after exposure to cold temperatures;The stiffness worsens with activity, in contrast to true myotonia, thus the term "paramyotonia."

Myotonias

Anesthetic Considerations

Pts may demonstrate anabnormal response to Sux and troublesome intraoperative myotonic contractions (not associated with MH)

Avoid hypothermia.

NMBAs may paradoxically cause generalized muscle spasms, including trismus, leading to difficulty with intubation and ventilation

Periodic Paralysis

Hypokalemic Periodic Paralysis

Path: abnormalities in membrane transport of Na+ and K+, making muscle excitation difficult

S/Sx: autosomal dominant disorder manifesting as acute bouts of weakness/ paralysis of the extremities and trunk muscles (spares diaphragm) lasting about 3 hours

Periodic Paralysis

Hypokalemic Periodic Paralysis

Episodes are most common in the early morning and can be precipitated by strenuous exertion, cold weather, high carbohydrate meals, or glc and insulin administration. Mild exertion can actually prevent or delay paralysis.

Periodic Paralysis

Hyperkalemic Periodic Paralysis

Path:paralysis is triggered by abnormal inactivation of Na+ channels by a mild increase in K+. Na+ and water flow into the cells with prolonged depolarization; more frequent attacks than hypokalemic variant

S/Sx: weakness triggered by fasting, cold weather, rest following exercise K+ intake; bradycardia

Periodic Paralysis

Anesthetic Considerations

Anesthetic management is directed toward preventing attacks

Check K+ often

Glucose-containing intravenous fluids should not be used in patients with hypokalemic paralysis, whereas such solutions may benefit patients with hyperkalemic and normokalemic paralysis

Periodic Paralysis

Anesthetic Considerations

In patients with periodic paralysis, the response to NMBAs is unpredictable

Pts with hypokalemic variant are sensitive to nondepolarizing NMBAs

Sux is contraindicated in the hyperkalemic variant

Maintain normothermia