Introduction

Lambert-Eaton Myasthenic Syndrome (LEMS) is a rare autoimmune disease that affects neuromuscular transmission. The neuromuscular junction plays an essential role in the communication between muscles and nerves. In LEMS, this communication pathway is disrupted, which consequently drives muscle weakness and reduced deep tendon reflexes. The symptoms observed in LEMS are driven by the development of autoantibodies that target voltage-gated calcium channels (VGCC). These channels play a role in muscle contraction by allowing for the triggering of an action potential. Individuals with LEMS often experience difficulty in walking, climbing stairs, or lifting objects due to weakness in their legs and arms.^1,2

Autoantibodies are a key feature of autoimmune diseases, where antibodies produced by an individual's immune system mistakenly target the body’s tissues. Unlike Myasthenia Gravis, a more common autoimmune neuromuscular disorder in which autoantibodies disrupt nerve signals to muscles at the neuromuscular junction, LEMS affects nerve signal transmission. In LEMS, these antibodies target nerve cells, specifically VGCCs at the neuromuscular junction, and directly interfere with their function in neurotransmission, causing reduced muscle contraction and muscle weakness. This article will explore the role of autoantibodies against VGCC, also referred to as anti-VGCC antibodies, in LEMS.^1,2,3

Understanding lambert-eaton myasthenic syndrome (LEMS)

Definition and classification

There are two distinct underlying causes of LEMS, the most common form being associated with small-cell cancers. LEMS was first discovered in patients with lung cancer, where fluctuating muscle weakness and fatigue were also experienced. Over 50% of LEMS patients have this form, most commonly associated with small-cell lung cancer. In these cases, the tumour expresses VGCCs on its surface and triggers an antibody response which has been shown to cross-react with normal calcium channels at the nerve terminals. Smoking has also been associated with a risk factor for developing LEMS in this case. The second cause of LEMS has been identified as a primary autoimmune disease without cancer. This occurs more frequently in younger individuals and has a strong genetic predisposition associated with HLA-B8, HLA-DR3, or HLA-DQ2.^1,2,4

Symptoms 

LEMS can be challenging to identify due to overlapping symptoms with other neuromuscular disorders, fluctuating symptoms, or symptoms that are attributed to cancer-associated effects. Nevertheless, symptoms of LEMS that distinguish it from other disorders, such as Myasthenia Gravis, include:^1,2,5,6

• Proximal muscle weakness and fatigue 
• Reduced deep tendon reflexes
• Hypotonic muscle stretch reflexes 
• Lower extremity symptoms such as difficulty rising from a chair, walking, or going up and down stairs 
• Autonomic dysfunction symptoms include dry mouth, constipation, and atypical pupillary responses

Interestingly, symptom improvement is observed with physical exertion. In other words, people with LEMS who frequently exercise and stimulate their muscles see improvements in both muscle strength and function.²

Mechanism of action of anti-VGCC antibodies

The role of calcium channels in neurotransmitter release 

Acetylcholine (ACh) is a neurotransmitter responsible for muscle contraction. It is stored in the presynaptic nerve terminals in droplets called vesicles. The action potential, which triggers an electrical spike of activity, triggers the opening of VGCCs. This allows entry of calcium into the nerve terminal, which activates the fusion of these vesicles carrying ACh with the membrane, and finally, the release of ACh at the neuromuscular junction. The binding of ACh to the ACh receptor in the postsynaptic membrane activates a cascade of events in the muscle cell, which then leads to muscle contraction. The efficiency of transmission is therefore dependent on the amount of ACh released.^1,4

Mechanism by which anti-VGCC antibodies disrupt normal function

In LEMS, autoantibodies bind to the VGCCs on the nerve endings, which leads to their destruction in nerve terminals. As a result of having fewer calcium channels, less calcium enters the nerve terminal, leading to a decrease in ACh release. A reduction in ACh levels in the neuromuscular junction leads to a reduction in muscle contraction. In summary, individuals with LEMS have a reduced ability to generate a high enough concentration of calcium to support neurotransmission, which leads to a degree of muscle weakness. Studies have detected IgG antibodies against the P/Q-subtype of VGCCs in approximately 90% of individuals with LEMS. This subtype is particularly seen in motor neuron nerve endings.^2,4,6

Clinical implications of anti-VGCC antibodies

Diagnosis

Diagnosis of LEMS includes antibody testing, tumour screening and electrodiagnostic testing, which includes nerve conduction studies. These procedures are considered after observing clinical features, including muscle weakness and autonomic dysfunction, which raise suspicion of LEMS.¹

A serology antibody test, which looks for the presence of antibodies in blood serum, is conducted against the P/Q-subtype of VGCCs, which is present in the vast majority of LEMS cases. In addition, up to 40% of people have also been shown to express the N-subtype. In cases that are associated with small-cell lung cancer, antibodies can also be detected against tumour-associated markers that are shared with LEMS.¹

Prognosis and clinical severity 

The muscle weakness and autonomic dysfunction observed in LEMS can severely impact an individual's quality of life. The prognosis of LEMS varies among individuals and depends on several factors, including the presence of any underlying tumours, such as small-cell lung cancer. Generally, patients respond well to symptomatic treatments, and improvements can often be observed following physical activity. Early diagnosis and intervention are crucial, as they can lead to significant enhancements in muscle function and overall quality of life. With appropriate management, many individuals with LEMS can experience improvements and maintain a good quality of life.^1,2,4

Treatment approaches

The approach to treatment relies on symptom control. Some of the treatment options include:^1,2,4,6

1. Amifampridine - This medication aims to prolong the action potential, allowing for greater calcium influx into the presynaptic nerve terminal, thereby enhancing neuromuscular transmission
2. Acetylcholinesterase inhibitors - These drugs inhibit the enzyme responsible for breaking down ACh at the neuromuscular junction, increasing ACh availability and improving muscle contraction
3. Intravenous immune globulin - This treatment functions by neutralising autoantibodies that target VGCCs, helping to reduce the autoimmune response

It is also important to treat the underlying cancer if present. In some cases, treatment directly against the cancer can lead to improvements in LEMS-related symptoms. Interestingly, studies have shown that those with LEMS and small cell lung cancer have greater overall survival rates compared to those with small cell lung cancer alone.^1,6

Summary

Lambert-Eaton Myasthenic Syndrome (LEMS) is a rare autoimmune disease driven by autoantibodies targeting voltage-gated calcium channels (VGCCs) at the nerve terminals. The consequent reduction in the neurotransmitter acetylcholine (ACh) release at the neuromuscular junction leads to impaired muscle contraction. Common symptoms observed in individuals with LEMS include muscle weakness, reduced deep tendon reflexes, and autonomic dysfunction. LEMS can arise either with an underlying small-cell cancer, which is associated with a poorer prognosis, or as a primary autoimmune disease. If LEMS is suspected from clinical features, antibody testing is one of the diagnostic tests considered to confirm the presence of the disease.

Over 90% of individuals with LEMS contain the P/Q-subtype of VGCCs, which can be detected through serologic antibody testing. Physical exertion, including exercise, has been associated with improved symptoms. In addition, treatment options including amifampridine, acetylcholinesterase inhibitors, and intravenous immune globulin are considered to enhance neuromuscular transmission, improve muscle contraction, and reduce the level of autoantibodies against VGCCs, respectively.