What is Lambert Eaton myasthenic syndrome (LEMS)?
Lambert Eaton myasthenic syndrome is an antibody-mediated autoimmune disease.
The target of the aberrant immune response is the presynaptic P/Q voltage-gated calcium channel (VGCC) at the neuromuscular junction.
The cardinal symptom of Lambert Eaton myasthenic syndrome is weakness, usually in the proximal muscles and more prominent in the lower extremities.
In about 50% to 60% of cases, the syndrome is paraneoplastic, associated mostly with small-cell lung cancer.
Onset of symptoms may precede detection of tumor by 1 to 4 years.
LEMS is frequently associated with autonomic symptoms, including dry mouth, impotence, and constipation.
Less common symptoms include orthostatic hypotension, micturition difficulties, dry eyes, and impaired sweating.
Variations in heart rate and blood pressure may occur with the Valsalva maneuver or deep breathing, and sweat tests also may be abnormal.
7 Interesting Facts of Lambert Eaton myasthenic syndrome
- Some cases are associated with carcinoma, particularly small cell carcinoma of the lung; other cases are true autoimmune disorders
- Weakness with diminished or absent muscle stretch reflexes
- Proximal lower extremity muscles are affected most; ocular and bulbar muscles are mildly affected
- Transient strength improvement with exercise
- Poor response to pyridostigmine or other cholinesterase inhibitors
- Autonomic dysfunction is possible
- Differentiated with repetitive nerve stimulation studies that confirm diagnosis
How is LEMS treated?
When an underlying neoplasm is identified, removal or treatment of the tumor usually results in substantial improvement in all symptoms, including those of autonomic dysfunction.
In patients who do not have an underlying malignancy, the treatment is directed toward the enhancement of cholinergic function and immunosuppression.
The first treatment of choice is 3,4-diaminopyridine.
Previously, it was thought that blocking voltage-gated potassium channels, resulting in prolongation of action potential at the motor nerve junctions and lengthening the opening time of the VGCC, mediates this agent’s effect.
Recent studies show that it may also target the VGCC β-subunit directly and result in potentiating neuromuscular transmission.
In cases when this agent is not available, guanidine or pyridostigmine may provide benefit.
When the symptoms persist, immunosuppression with corticosteroids, azathioprine, or IVIG may be needed.