Lambert-Eaton Myasthenic Syndrome

Lambert-Eaton myasthenic syndrome has a similar set of features to myasthenia gravis. It causes progressive muscle weakness with increased use as a result of damage to the neuromuscular junction. The symptoms tend to be more insidious and less pronounced than in myasthenia gravis.

Lambert-Eaton syndrome typically occurs in patients with small-cell lung cancer. It is a result of antibodies produced by the immune system against voltage-gated calcium channels in small cell lung cancer (SCLC) cells. These antibodies also target and damage voltage-gated calcium channels in the presynaptic terminals of the neuromuscular junction where motor nerves communicate with muscle cells.

These voltage-gated calcium channels are responsible for assisting in the release of acetylcholine into the synapse of the neuromuscular junction. This acetylcholine then binds to the acetylcholine receptors and stimulates a muscle contraction. When these channels are destroyed, less acetylcholine is release into the synapse.

 

Presentation

The symptoms of Lambert-Eaton syndrome tend to develop slowly. The proximal muscles are most notably affected, causing proximal muscle weakness. It most notably presents with proximal leg muscle weakness. It can also affect the intraocular muscles causing double vision (diplopia), the levator muscles in the eyelid causing eyelid drooping (ptosis) and the oropharyngeal muscles causing slurred speech and swallowing problems (dysphagia). Patients may also experience dry mouth, blurred vision, impotence and dizziness due to autonomic dysfunction.

Patients with Lambert-Eaton have reduced tendon reflexes. A notable finding is that these reflexes become temporarily normal for a short period following a period of strong muscle contraction. For example, the patient can maximally contract the quadriceps muscle for a period, then have their reflexes tested immediately afterwards, and display an improvement in the response. This is called post-tetanic potentiation.

 

Treatment

It is important to diagnose and manage any underlying malignancy. In older smokers with symptoms of Lambert-Eaton syndrome consider investigating for small cell lung cancer.

Amifampridine allows more acetylcholine to be released in the neuromuscular junction synapses. It works by blocking voltage-gated potassium channels in the presynaptic cells, which in turn prolongs the depolarisation of the cell membrane and assists calcium channels in carrying out their action. This improves symptoms in Lambert-Eaton syndrome.

Other options:

  • Immunosuppressants (e.g. prednisolone or azathioprine)
  • IV immunoglobulins
  • Plasmapheresis
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