Lambert-Eaton Myasthenic Syndrome

Synonyms

5

Overview

Lambert Eaton myasthenic syndrome is a disorder of the neuromuscular junction. The neuromuscular junction is the site where nerve cells meet muscle cells and help activate the muscles. This syndrome occurs when antibodies interfere with electrical impulses between the nerve and muscle cells. It may be associated with other autoimmune diseases, or more commonly coincide with or precede a diagnosis of cancer such as small cell lung cancer.

Symptoms

 

Weakness or loss of movement that varies in severity:

  • Difficulty climbing stairs
  • Difficulty lifting objects
  • Need to use hands to arise from sitting or lying positions
  • Difficulty talking
  • Difficulty chewing
  • Drooping head
  • Swallowing difficulty, gagging, or choking
Vision changes:
  • Blurry vision
  • Double vision
  • Difficulty maintaining a steady gaze
Other symptoms may include blood pressure changes, dizziness upon rising, and dry mouth

Causes

Lambert Eaton myasthenic syndrome is the result of an autoimmune process which causes a disruption of electrical impulses between nerve cells and muscle fibers. In cases where Lambert Eaton myasthenic syndrome appears in association with cancer, the cause may be that the body's attempt to fight the cancer inadvertently causes it to attack nerve fiber endings, especially the voltage-gated calcium channels found there. The trigger for the cases not associated with cancer is unknown.

Diagnosis

The diagnosis is established by clinical and laboratory findings (chest x-ray for a possible lung malignancy, antibodies to calcium channels, incremental response in repetitive nerve stimulation). Incremental response is an increased response of muscle fibers to very high frequencies of electrical stimulation. Observed increase in the response of muscle fibers proves that there is a difficulty with the release of acetylcholine and this difficulty can be overwhelmed by intensive stimulation.

The diagnosis is usually made on electromyography (EMG), which is one of the standard tests in the investigation of otherwise unexplained muscle weakness. This involves the insertion of small needles into the nerves supplying several muscles, administering small electrical impulses through these needles, and measuring the electrical response of the muscle in question. Two EMG investigations can be characteristic in LEMS: compound motor action potentials (CMAP) and single-fiber examination.

CMAP shows small amplitudes but normal latency and conduction velocities. If repeated impulses are administered (two 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 ten 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. 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 (PET) scan of the body may also be performed to search for an occult tumour, particularly of the lung.

Prognosis

The prognosis for individuals with LEMS varies. The symptoms of Lambert-Eaton syndrome may improve with treatment of an underlying tumor and/or with suppressing the immune system. However, not all people respond well to treatment.

Treatment

Medications and therapies used to treat Lambert-Eaton myasthenic syndrome may include anticholinesterase agents (e.g., Pyridostigmine), guanidine hydrochloride, plasmapheresis (where blood plasma is removed and replaced with fluid, protein, or donated plasma) or IV immunoglobulins, steroids (e.g., prednisone), azathioprine or cyclosporine, and/or 3,4-diaminopyridine.

3,4-diaminopyridine is available in Europe and may be available in the U.S. on a compassionate use basis. While there has been some evidence that either 3,4-diaminopyridine or IV immunoglobulin can improve muscle strength and nerve to muscle cell communication, the degree of benefit (i.e., how much symptoms are improved) still needs to be determined.

Resources

  • NIH