Introduction
The neuromuscular junction disorder that can be present as a paraneoplastic phenomenon or a primary autoimmune disorder is known as lambert-eaton myasthenic syndrome. The main clinical manifestation of this syndrome is muscle weakness. Half of the cases or more than that are associated with small cell lung cancer. It is a rare neuromuscular disorder. Lambert-Eaton myasthenic syndrome is a rare presynaptic misalignment of neuromuscular transmission in which the scheduled release of acetylcholine is disturbed causing a different set of clinical characteristics, that include proximal weakness of the muscles, depressed tendon reflexes, post-tetanic potentiation, and autonomic changes.
The beginning presentation of the disorder may be similar to that of myasthenia gravis, but after the progression, these two disorders are distinct from each other. Myasthenia gravis can be defined as a chronic autoimmune, neuromuscular disease that may lead to muscle weakness of the skeletal muscles. Here the primary stage symptoms of lambert-eaton myasthenia syndrome can be similar to the myasthenia gravis symptoms. The pathology of lambert-eaton myasthenic syndrome is due to the generation of antibodies that act against the voltage-gated calcium channels on presynaptic nerve terminals leading to a decreased acetylcholine.
What Is Lambert-Eaton Myasthenic Syndrome?
Lambert-Eaton myasthenic syndrome is an autoimmune disease. The attack appears at the connection between the nerve and muscle also known as the neuromuscular junction and meddles with the function of the nerve cells to transmit signals to muscle cells. Particularly, the immune system attacks the nerve endings of calcium channels that are essential for the release of the neurotransmitter acetylcholine. As the digit of calcium channels decreases, the nerve endings become incapable of releasing low amounts of acetylcholine. Acetylcholine is a neurotransmitter that is accountable for initiating muscle contraction. In people suffering from this syndrome, muscle weakness is seen due to insufficient levels of acetylcholine which is unable to cause normal muscle contractions. Muscle weakness is the primary clinical sign of the disorder. Lambert-Eaton myasthenic syndrome classifies as a paraneoplastic or non-paraneoplastic phenomenon. It is also known as a non-tumor lambert-eaton myasthenic syndrome. Most of the cases are associated with small-cell lung cancer. There are two forms of the Lambert-Eaton myasthenic syndrome, one is associated with small-cell lung cancer, and the other is not related to cancer and can be associated with the genetic component linked with autoimmunity.
What Is the Etiology and Pathophysiology of Lambert-Eaton Myasthenic Syndrome?
Etiology:
Lambert-Eaton myasthenic syndrome generally classifies as paraneoplastic or non-paraneoplastic, which can be referred to as non–tumor lambert-eaton myasthenic syndrome. Non-tumor lambert-eaton appears in the absence of malignancy. Most of the patients having lambert-eaton myasthenic syndrome are usually diagnosed with an underlying tumor. Paraneoplastic lambert-eaton myasthenic syndrome is typically seen along with small-cell lung cancer. Other cancerous conditions that had been associated with the lambert-eaton myasthenic syndrome include non-small cell and mixed lung carcinoma, prostate cancer, thymoma, and many more. One of the risk factors is a history of smoking. The genetic association with autoimmunity is present in more than 60 percent of young patients.
Pathophysiology:
Lambert-Eaton myasthenic syndrome is defined as a condition of lowered release of acetylcholine from the presynaptic nerve terminals as the action of antibodies to voltage-gated calcium channels in the presynaptic neuronal cell membrane.
The following points describe the normal mechanism of the release of the neurotransmitter acetylcholine and its interaction:
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Acetylcholine is a neurotransmitter that is synthesized and stored in the vesicles at the motor nerve terminal.
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Acetylcholine is released on stimulation by an action potential that travels down the motor nerve. Acetylcholine after its release binds to the acetylcholine receptors on the postsynaptic neuron which leads to the immediate entry of cations that delivers depolarization at the end plate region of the muscle fiber and provokes an action potential along with subsequent muscle contraction.
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The enzyme acetylcholinesterase quickly breaks down acetylcholine within the synaptic cleft.
In lambert-eaton myasthenic syndrome, the Voltage-gated calcium channel gets downsized because of the IgG antibody-mediated cross-linking of the channels. To be particular, the antibodies are directed toward the P/Q subtype of voltage-gated calcium channels. Around 80 to 85 % of patients with this syndrome show antibodies against the P/Q-type voltage-gated calcium channels. The antibodies against the N-type voltage-gated calcium channels were found rarely in malignancy-associated lambert-eaton myasthenic syndrome.
In small-cell lung cancer autoimmunity, patients with lambert-eaton myasthenic syndrome, the tissue expresses voltage-gated calcium channels. Autoantibody production is induced by the expression of antigens on the tumor cells and the autoantibodies cross-react with presynaptic voltage-gated calcium channels antigens.
Non-tumor lambert-eaton myasthenic syndrome in association with HLA-B8, HLA-DR3, and -DQ2. Some other autoimmune conditions such as myasthenia gravis are also associated with these HLA genotypes. These HLA genotypes association does not appear in lambert-eaton myasthenic syndrome with small-cell lung cancer.
What Are the Clinical Manifestations of Lambert-Eaton Myasthenic Syndrome?
The most common clinical manifestations of the Lambert-Eaton myasthenic syndrome are autonomic dysfunction, proximal muscle weakness, and absent deep tendon reflexes. Symptoms are gradual in onset and progress more quickly in small-cell lung cancer-lambert-eaton myasthenic syndrome. The following are some of the symptoms:
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Muscle weakness.
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Oculobulbar weakness.
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Autonomic dysfunction.
What Is the Treatment of Lambert-Eaton Myasthenic Syndrome?
Lambert-Eaton myasthenic syndrome associated with small-cell lung cancer includes the treatment of underlying cancer. The primary focus in the initial management or treatment of Lambert-Eaton myasthenic syndrome with or without malignancy is focused on increasing the levels of acetylcholine. Acetylcholinesterase inhibitors like pyridostigmine are included in the treatment of weakness. Yet, the effects are not as prominent as with patients of myasthenia gravis. There is another therapeutic option where 3,4-diamino pyridine binds to voltage-gated calcium channels leading to the prolongation of depolarization of the action potential increasing the open time of the voltage-gated calcium channel resulting in an increased presynaptic influx of calcium and increased acetylcholine release. For other patients with refractory weakness, immunosuppression with intravenous immune globulin being the first-line agent is recommended. Some other drugs such as Prednisone, Rituximab, Azathioprine, or plasma exchange are also prescribed.
Conclusion
An interprofessional team is required for the diagnosis and management of patients with lambert-eaton myasthenic syndrome due to its diverse presentation. The disorder is commonly seen in the presence of malignancy and hence the team should have an oncologist, surgeon, hematologist, ophthalmologist, neurologist, primary care provider, and nurses. If the primary malignancy is controlled in time, some improvement can occur but full recovery is not possible.
