Download Lambert-Eaton Myasthenic Syndrome

Lambert-Eaton Myasthenic Syndrome
Definition: Lambert-Eaton Myasthenic Syndrome (LEMS) is a para-neoplastic syndrome characterized
by proximal muscle weakness that typically begins in the lower extremities, but may spread to involve the
upper limb, bulbar and respiratory muscles. Other symptoms such as dry mouth, male impotence and other
autonomic dysfunctions may occur as well. It may be seen in association with cancer or as an idiopathic
immune disease process. LEMS may also not be diagnosed prior to a surgical procedure and only realized
after the patient has unexpected responses to anesthetic interventions.
Pathophysiology: LEMS is classified as a pre-synaptic neuromuscular transmission disorder of
voltage gated Calcium Channels reducing the release of acetylcholine from the neuromuscular junction.
The number of calcium channels (the proteins attacked by autoantibodies) is reduced on the pre-synaptic
neuron which in turn reduces the mobilization of Ach to the NMJ for release. The number of acetylcholine
receptors on the post-synaptic membrane remains the same which is key. The crux of the issue for
competitive inhibition of NMB agents. The drug remains active longer. The reason small cell carcinoma
is seen in conjuction with LEMS is because the cancer cells express an identical voltage gated channel
triggering an immune response. As the body reacts to the cancer cells it also affects the NMJ of normal
cells.
*For the anesthesiologist LEMS patients are extremely sensitive to the effects of both
depolarizing and non-depolarizing NMBAs. Volatile agents alone are often sufficient to provide
muscle relaxation for intubation and most surgical procedures.
Lambert-Eaton Myasthenic Syndrome
Auto-antibodies against the pre-synaptic,
voltage-gated calcium channels
Myasthenia gravis
Auto-antibodies against the postsynaptic
acetylcholine receptor
Complications:
- Diminished respiratory muscle strength
- Respiratory aspiration
- Prolonged muscle relaxation
- extended ventilatory support requirements
Assessment and Management:
- Minimal pre-operative sedation and opioid use if NMJ disorder is known
- Pre-operative assessment of pulmonary function if NMJ disorder is known
- Dosing of NMBAs should be in small doses with frequent twitch monitoring
- Short acting NMBAs such as cisatricurim/rocuronium should be chosen
- Ventilatory function should be examined closely prior to extubation
To complete the topic mentioned above see chapter 37 in Morgan, Mikhail and Murray (whom we never
mention)
Lambert-Eaton Myasthenic Syndrome Question
173. Which of the following diseases is associated with increased resistance to neuromuscular
blockade with succinylcholine?
A. Myasthenia Gravis
B. Myasthenic Syndrome
C. Huntington’s Chorea
D. Polymyositis
E. Duchenne’s muscular dystrophy (pseudohypertrophic muscular dystrophy)
Two other questions can be found in Anesthesia, A Comprehensive Review. Look in index for
myasthenia/myasthenic crisis.
I know the answer to the question isn’t from the title of this page, but it shows how they will try to throw us
off to the answer… the narrow it to two or three trick and the subtle differences between them.
A. In order for depolarizing muscle relaxants such as succinylcholine to work, the drug
must interact with the receptor at the myoneural junction. Patients with myasthenia
gravis have fewer acetylcholine receptors on the muscle and are more resistant to
succinylcholine, but are much more sensitive to non-depolarizing muscle relaxants.
Patients with Myasthenic syndrome (LEMS) have a decreased release of acetylcholine at
the myoneural junction: however the number of receptors is normal. Patients with
myasthenic syndrome are more sensitive to both depolarizing and nondepolarizing
muscle relazants. Huntington’s chorea is a degenerative CNS disease that is associated
with decreased plasma cholinesterase activity and prolonged responses to sux use have
been seen. The response to depolarizing and nondepolarizing muscle relaxants appears to
be unchanged in patients with polymyositis. Sux is contraindicated in patients with
Duchenne’s MD because of the risks of rhabdo, hyperkalemia and cardiac arrest.
Nondepolarizing muscle relaxants have a normal response in patients with DMD,
although some patients have prominent coexisting skeletal muscle weakness.