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Lambert Eaton Myasthenic Lambert-Eaton Myasthenic Syndrome (LEMS) is characterized as a rare autoimmune neuromuscular junction disorder that disrupts the communication between motor nerves and muscles.Lambert-Eaton myasthenic syndrome. Neurologic clinics, 2018. 36(2): p. 379.” style=”position:relative;color:#309b65;cursor:help;border-bottom:1px dotted #309b65;font-weight:bold”>[1] LEMS may present as a primary autoimmune disorder or paraneoplastic syndrome. In this condition, the body’s immune system produces antibodies that attack proteins at the presynaptic nerve terminal, impairing the release of acetylcholine and weakening the signal to the muscles. Progressive muscle weakness is the hallmark symptom of the disease. This weakness typically begins in your upper legs and then involves other muscle groups, including those in your arms and those responsible for breathing, speech, and swallowing. Most cases have associations withsmall-cell lung cancer (SCLC). LEMS is categorized as paraneoplastic and non-paraneoplastic (non-tumor LEMS). The true incidence of LEMS is unknown; however, estimates suggest that it affects approximately 2.8 people per million worldwide.[2]
Etiology of LEMS: Lambert Eaton Myasthenic
Clinical observations suggest an autoimmune etiology for LEMS. There are two main forms of LEMS:
Paraneoplastic LEMS:
Around 60% of the LEMS patients have an underlying malignancy (most commonly SCLC). It is the paraneoplastic type of LEMS. In this type, the immune system targets antigens shared by the tumor and presynaptic calcium channels, resulting in cross-reactive antibodies that impair acetylcholine release. Paraneoplastic LEMS may precede the diagnosis of cancer and can sometimes be the first clinical manifestation of an underlying malignancy.
Non-Paraneoplastic LEMS:
In non-paraneoplastic type LEMS (NT-LEMS), the condition occurs without detectable cancer. It sometimes has a link to certain genetic predispositions, such as specific human leukocyte antigen (HLA) genotypes like HLA-DR3-B8. NT-LEMS often develops at a younger age and accounts for about 35–40% of cases.[3]
LEMS can also coexist with other autoimmune or endocrine disorders, such as type 1 diabetes and hypothyroidism. Smoking is a strong risk factor for paraneoplastic LEMS because of its link with SCLC.
Pathophysiology of LEMS
LEMS affect the neuromuscular transmission at the presynaptic nerve terminal by impairing the release of acetylcholine (Ach). ACh is a neurotransmitter that is essential for the contraction of muscles. It happens through the following mechanism:
- In LEMS, the immune system produces autoantibodies against the P/Q-type voltage-gated calcium channels (VGCCs), which are located on the presynaptic membrane of motor nerve terminals at the neuromuscular junction.
- When a nerve impulse reaches the motor nerve terminal, VGCCs open. The opening of these channels allows calcium ions to enter the nerve terminal. This influx of calcium triggers the release of acetylcholine-containing vesicles into the synaptic cleft.
- In case of impairments, the antibodies bind and reduce the functional numbers of VGCCs. This diminishes or totally blocks the influx of calcium, leading to a reduced release of ACh into the synaptic cleft.
- Since ACh is necessary for the activation of muscle fibres, its insufficient release means fewer endplate potentials reach the threshold to generate muscle action potentials. This process causes muscle weakness (typically in the proximal). It may involve autonomic symptoms due to effects on their neural tissues. Additionally, reflexes become diminished or absent due to impaired neurotransmission.[4]
Symptoms of LEMS
The symptoms of LEMS involve muscle weakness and fatigue that gradually develop over weeks or months.[5] In paraneoplastic cases, symptoms often progress rapidly. The common symptoms include:
- Muscle fatigue, muscle ache, muscle stiffness, and tenderness.
- Proximal muscle weakness in legs, hips, arms, and shoulders.
- Difficulty with walking, climbing stairs, rising from a chair, pushing, or lifting.
- Feeling tired all the time.
- Difficulty in raising arms, walking, and lifting objects due to muscle weakness.
- Swallowing difficulties and sometimes trouble in speaking and breathing.
- Ptosis (drooping eyelids), blurred vision, dry eyes, or diplopia (double vision).
- Tingling sensations in hands and feet.
Autonomic symptoms include:
- Constipation
- Dry mouth
- Decreased sweating
- Erectile dysfunction
- Dizziness
A hallmark feature of LEMS is hyporeflexia or areflexia, which may temporarily improve after exercise.
Diagnosis of LEMS
The diagnosis of LEMS is a multistep process. It integrates clinical evaluation, laboratory tests, and electrodiagnostics.
History & Physical Assessment:
The first step in diagnosing LEMS is a thorough physical evaluation by your healthcare provider. A detailed history of your symptoms and a review of your medical and medication history are significant parts of the process. A neurological examination is necessary to identify patterns of weakness and symptoms consistent with the diagnosis of LEMS.
Serology/Antibody Testing:
Blood tests are performed to detect the antibodies for P/Q-type voltage-gated calcium channels. The presence of these antibodies is highly specific for LEMS. This test is positive in approximately 85-95% of the LEMS patients. However, their absence does not exclude the diagnosis, as some patients are seronegative.[6]
It is noteworthy that P/Q-type VGCCs are not exclusive to LEMS. These have a link to other autoimmune or neurological conditions.
Additionally, 64% of the LEMS patients with SCLC were found to have antibodies against SOX1. It is an immunogenic tumor antigen in SCLC.[7]
Electrodiagnostics Testing (EDS):
EDS offers significant assistance in confirming the diagnosis of LEMS. EDS involves electromyography (EMG) and nerve conduction studies (NCS). NCS typically shows unaffected nerve conduction velocities but significantly reduced motor unit amplitudes.
Repetitive nerve stimulation (RNS) demonstrates a characteristic incremental response (>100% increase) with high-frequency stimulation, considered a hallmark of LEMS.
Needle EMG reveals the presence of unstable action potentials. Single-fibre EMG is the most sensitive test. It detects neuromuscular junction disorders. This test shows a significant increase in jitter and transmission block. SFEMG is more sensitive than RNS. However, RNS is a widely available and notably valuable method.
Imaging:
A diagnosis of LEMS also warrants an immediate and extensive investigation for underlying malignancy. Magnetic resonance imaging (MRI) and computed tomography (CT) are the initial recommended imaging studies for this condition. Doctors recommend a positron emission tomography (PET) scan when a CT scan is negative. When the initial assessment does not show any malignancy, then cancer screening should be done every 3 to 6 months for a minimum of two 2 years.[8]
Management & Treatment of LEMS
Treatment involves addressing the underlying cause and providing symptomatic therapy.
In cases of LEMS associated with SCLC, it is crucial to address the underlying cause by treating the malignancy. Cancer treatment includes chemotherapy, radiation therapy, and surgery based on the type of cancer. Managing symptoms is the key approach for LEMS treatment. There are various options to treat symptoms, but the most effective and theoretically sound intervention aims to improve presynaptic ACh.
3,4-diaminopyridine (3,4-diaminopyridine) or Amifampridine:
It is the first-line symptomatic therapy. It blocks presynaptic potassium channels, thereby prolonging the duration of the action potential. The elongation increases the presynaptic calcium influx and precedes the release of ACh. The recommended dosage of amifampridine for adults is 15 to 30 mg three times a day.[9]
Acetylcholinesterase Inhibitors:
Pyridostigmine is an acetylcholinesterase inhibitor that addresses weakness, and the recommended dose for adults is 30 to 120 mg every 3 to 6 hours.[10]
Guanidine enhances the release of acetylcholine (ACh) following a nerve impulse. Doctors consider it only when amifampridine is unavailable due to its significant side effects and potential renal toxicity. The recommended adult dosage of guanidine is 1 g daily.[11]
For patients who do not respond to symptomatic treatments, doctors consider immunosuppressive therapies or immunomodulating therapies.
Plasma Exchange:
Doctors consider plasma exchange for only some refractory cases. Plasma exchange holds benefits for LEMS.
Intravenous Immune Globulin (IVIG):
It is the primary treatment option for refractory cases. It functions by neutralizing autoantibodies and regulating autoreactive B cells. The recommended dosage involves 2g/kg over 2 to 5 days.[12]
Immunosuppressive Agents & Steroids:
These options include mycophenolate, cyclosporine, and azathioprine. But their side effects often limit the widespread use of these agents.
Rituximab:
It is effective against all B-cell-mediated disorders, including LEMS. However, its extensive utilization is reported to be insufficient.
Differential Diagnosis:
The differential diagnosis of LEMS includes:
- Myasthenia gravis (MG): Can be distinguished by antibody testing (anti-AChR or anti-MuSK positive in MG, anti-VGCC in LEMS) and by electrophysiology (MG shows a decremental response on low-frequency RNS, while LEMS shows an incremental response on high-frequency RNS).
- Polymyositis and other myopathies: Weakness is usually proximal but reflexes are preserved, and muscle enzyme levels (CK, aldolase) are elevated. EMG shows myopathic rather than neuromuscular junction features.
- Polyneuropathy: Sensory loss and distal weakness are prominent, unlike the proximal pattern of LEMS. Nerve conduction studies reveal sensory abnormalities, which are absent in LEMS.
- Motor neuron disease: Presents with both upper and lower motor neuron signs (spasticity, fasciculations, atrophy) and progressive weakness. Reflexes are exaggerated, unlike the reduced or absent reflexes in LEMS.
- Autonomic neuropathies: Share autonomic features with LEMS (dry mouth, constipation), but usually lack prominent muscle weakness and electrophysiology shows peripheral nerve involvement rather than a neuromuscular junction defect.
Prognosis
LEMS patients experience a low quality of life due to weakness. The disease typically responds positively to symptomatic and immunosuppressive therapies. The life expectancy of LEMS depends on its type. Survival rates for patients with NT-LEMS are similar to those of the general population. The prognosis for the paraneoplastic type relies on the underlying cancer. nterestingly, patients with LEMS and SCLC may survive longer than SCLC patients without LEMS, possibly due to earlier cancer detection.[13]
LEMS versus Myasthenia Gravis (MG)
LEMS and MG are both rare autoimmune neuromuscular disorders. However, the key differences are summarized in Table 1.
Table 1: Summary of the Key Differences between LEMS and MG
| Features | LEMS | MG |
|---|---|---|
| Definition | It occurs due to antibodies targeting presynaptic voltage-gated channels. It redcues the neurotransmitter release necessary for the contraction of muscles. | Dry mouth, Constipation, Erectile dysfunction, etc. |
| Muscle group affected | Proximal muscle weakness in the lower limb. Eye muscle involvement is rare or mild. | Weakness in the eye muscles can then progress to facial, neck, and limb muscles. Severe respiratory muscle weakness, which is rare in LEMS. |
| Weakness pattern | Tends to start in the legs and then move upward. | Tends to start in the eyes and face and then move downward. |
| Automnoic symptoms | Dry mouth, Constipation, Erectile dysfunction, etc. | Rare or absent. |
| Respiratory Involvement | Less commonly involves severe symptoms. | Can cause severe respiratory muscle weakness (myasthenia crisis). |
| Diagnostic antibodies | Anti-voltage-gated calcium channel antibodies | Anti-acetylcholine receptor antibodies. |
| Treatment | Treatment involves addressing the underlying cause and managing LEMS. | Immunotherapy, thymectomy, acetylcholinesterase inhibitors. |
| Prognosis | The prognosis depends upon the type of LEMS. | The condition is manageable, and most patients enjoy a near-normal life expectancy with modern therapy and careful management. |
A Quick Review
In LEMS, your nerve cells do not send enough messages to tell your muscle fibers to contract properly. It typically presents with a gradual onset and progressively advancing disease course. A high index of suspicion is essential for clinicians to make an on-time diagnosis. A prompt and precise diagnosis will lead to a careful search for underlying malignancy. The primary objective of the treatment is to elevate ACh levels. In refractory cases, the recommended first-line immunosuppressive method includes IVIG, and other alternative treatments include rituximab and plasma exchange. However, it is noteworthy that complete recovery is usually unattainable.
References
[1] Kesner, V.G., et al., Lambert-Eaton myasthenic syndrome. Neurologic clinics, 2018. 36(2): p. 379.
[2] Viveiros, L., et al., Paraneoplastic Lambert-Eaton myasthenic syndrome: a diagnostic challenge. BMJ Case Reports CP, 2023. 16(1): p. e250947.
[3] Khadilkar, S.V., R.S. Yadav, and B.A. Patel, Lambert–Eaton Myasthenic Syndrome, in Neuromuscular Disorders: A Comprehensive Review with Illustrative Cases. 2024, Springer. p. 433-450.
[4] Jayarangaiah, A., F. Lui, and P.T. Kariyanna, Lambert-Eaton myasthenic syndrome, in StatPearls [Internet]. 2023, StatPearls Publishing.
[5] Pascuzzi, R.M. and C.L. Bodkin, Myasthenia gravis and Lambert-Eaton myasthenic syndrome: New developments in diagnosis and treatment. Neuropsychiatric Disease and Treatment, 2022: p. 3001-3022.
[6] Jayarangaiah, A., F. Lui, and P.T. Kariyanna, Lambert-Eaton myasthenic syndrome, in StatPearls [Internet]. 2023, StatPearls Publishing.
[7] Vabanesi, M., et al., SOX1 antibody-related paraneoplastic neurological syndromes: clinical correlates and assessment of laboratory diagnostic techniques. Journal of Neurology, 2023. 270(3): p. 1691-1701.
[8] Jayarangaiah, A., F. Lui, and P.T. Kariyanna, Lambert-Eaton myasthenic syndrome, in StatPearls [Internet]. 2023, StatPearls Publishing.
[9] Meisel, A., et al., The European Lambert–Eaton myasthenic syndrome registry: long-term outcomes following symptomatic treatment. Neurology and Therapy, 2022. 11(3): p. 1071-1083.
[10] Jayarangaiah, A., F. Lui, and P.T. Kariyanna, Lambert-Eaton myasthenic syndrome, in StatPearls [Internet]. 2023, StatPearls Publishing.
[11] Pascuzzi, R.M. and C.L. Bodkin, Myasthenia gravis and Lambert-Eaton myasthenic syndrome: New developments in diagnosis and treatment. Neuropsychiatric Disease and Treatment, 2022: p. 3001-3022.
[12] Pascuzzi, R.M. and C.L. Bodkin, Myasthenia gravis and Lambert-Eaton myasthenic syndrome: New developments in diagnosis and treatment. Neuropsychiatric Disease and Treatment, 2022: p. 3001-3022.
[13] Lipka, A.F., et al., Long‐term follow‐up, quality of life, and survival of patients with Lambert‐Eaton myasthenic syndrome. Neurology, 2020. 94(5): p. e511-e520.