page_title: Lambert–Eaton myasthenic syndrome
section_title: Diagnosis
hierarchical_section_title: Lambert–Eaton myasthenic syndrome / Diagnosis
caption_attribution_description: Beschreibung: Konventionelles Röntgenbild des Thorax (der Lunge) mit rundlicher Verdichtung in der linken Lunge Quelle: selbst erstellt --de:Benutzer:Lange123 17:18, 11. Nov. 2004 (CEST)
context_page_description: Lambert–Eaton myasthenic syndrome is a rare autoimmune disorder characterized by muscle weakness of the limbs.
  Around 60% of those with LEMS have an underlying malignancy, most commonly small-cell lung cancer; it is therefore regarded as a paraneoplastic syndrome. It is the result of antibodies against presynaptic voltage-gated calcium channels, and likely other nerve terminal proteins, in the neuromuscular junction. The diagnosis is usually confirmed with electromyography and blood tests; these also distinguish it from myasthenia gravis, a related autoimmune neuromuscular disease.
  If the disease is associated with cancer, direct treatment of the cancer often relieves the symptoms of LEMS. Other treatments often used are steroids, azathioprine, which suppress the immune system, intravenous immunoglobulin, which outcompetes autoreactive antibody for Fc receptors, and pyridostigmine and 3,4-diaminopyridine, which enhance the neuromuscular transmission. Occasionally, plasma exchange is required to remove the antibodies.
  The condition affects about 3.4 per million people. LEMS usually occurs in people over 40 years of age, but may occur at any age.
context_section_description: The diagnosis is usually made with nerve conduction study (NCS) and electromyography (EMG), which is one of the standard tests in the investigation of otherwise unexplained muscle weakness. EMG involves the insertion of small needles into the muscles.  NCS involves administering small electrical impulses to the nerves, on the surface of the skin, and measuring the electrical response of the muscle in question. NCS investigation in LEMS primarily involves evaluation of compound motor action potentials (CMAPs) of effected muscles and sometimes EMG single-fiber examination can be used.
  CMAPs show small amplitudes but normal latency and conduction velocities. If repeated impulses are administered (2 per second or 2 Hz), it is normal for CMAP amplitudes to become smaller as the acetylcholine in the motor end plate is depleted. In LEMS, this decrease is larger than observed normally. Eventually, stored acetylcholine is made available, and the amplitudes increase again. In LEMS, this remains insufficient to reach a level sufficient for transmission of an impulse from nerve to muscle; all can be attributed to insufficient calcium in the nerve terminal. A similar pattern is witnessed in myasthenia gravis. In LEMS, in response to exercising the muscle, the CMAP amplitude increases greatly (over 200%, often much more). This also occurs on the administration of a rapid burst of electrical stimuli (20 impulses per second for 10 seconds). This is attributed to the influx of calcium in response to these stimuli. On single-fiber examination, features may include increased jitter (seen in other diseases of neuromuscular transmission) and blocking.
  Blood tests may be performed to exclude other causes of muscle disease (elevated creatine kinase may indicate a myositis, and abnormal thyroid function tests may indicate thyrotoxic myopathy). Antibodies against voltage-gated calcium channels can be identified in 85% of people with EMG-confirmed LEMS. Once LEMS is diagnosed, investigations such as a CT scan of the chest are usually performed to identify any possible underlying lung tumors. Around 50–60% of these are discovered immediately after the diagnosis of LEMS. The remainder is diagnosed later, but usually within two years and typically within four years. As a result, scans are typically repeated every six months for the first two years after diagnosis. While CT of the lungs is usually adequate, a positron emission tomography scan of the body may also be performed to search for an occult tumour, particularly of the lung.
caption: Chest X-ray showing a tumor in the left lung (right side of the image)
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