Medicine

MG and Related Disorders

This post originally appeared on MedLine Plus.

Ocular myasthenia gravis is a form of MG in which the muscles that move the eyes and control the eyelids are easily fatigued and weakened. Common symptoms include double vision, drooping eyelids and/or eye closure. Approximately 15% of people with MG have ocular problems only. Ocular symptoms are often the first symptoms of MG, and many people may develop other generalized muscle weakness issues later on. In approximately 50% of people diagnosed with ocular myasthenia gravis, autoantibodies against acetylcholine receptors (AChR) can be detected with a blood test. Learn more about this type of MG.

Generalized myasthenia gravis is a form of MG with generalized muscle weakness. Symptoms may include droopy eyelids (ptosis) and/ or double vision, difficulty speaking, difficulty breathing, problems chewing and swallowing, trouble performing everyday tasks, or generalized muscle weakness. Approximately 85% of people diagnosed with MG have acetylcholine receptor (AChR) autoantibodies present in their blood tests.

About 6% of people with MG will test positive for anti-muscle-specific kinase (MuSK) antibodies. In very rare cases, both AChR and MuSK antibodies can be detected. Most patients who are positive for MuSK antibodies have generalized myasthenia gravis. 
 

The presence and type of antibody present in blood can help guide treatment choices for people with MG. People with anti-MuSK antibodies may respond differently to medications than people with AChR antibodies.

Seronegative myasthenia gravis describes those with ocular or generalized myasthenia gravis who do not have anti-AChR and anti-MuSK autoantibodies detectable in the blood. Symptoms and response to treatments are often the same as those with AChR and MuSK antibodies. Specialized diagnostic testing may be required to confirm an MG diagnosis (see the Seronegative Resource Center for more).

Approximately 10% of people with MG are considered seronegative. Sometimes, antibodies for other proteins such as agrin or LRP4 antibodies are present, signaling that these autoantibodies may be biomarkers of MG. As newer antibodies are discovered for MG, fewer patients are likely to be classified as seronegative.

Learn more about seronegative MG.

Learn more about the different types of MG

Congenital myasthenic syndromes (CMS): This term is reserved for a group of uncommon, hereditary disorders of the neuromuscular junction.  There are many different types of CMS, each the result of a genetic mutation in a specific protein component of the neuromuscular junction. A thorough diagnostic evaluation is worthwhile in patients with suspected CMS because of the different types and different treatment options.

What is the difference between CMS and autoimmune MG?

  • CMS is rarer than autoimmune MG.
  • CMS usually manifests early in life, often in infancy.
  • CMS symptoms tend to be lifelong and/or relatively stable with generalized fatigue and weakness, with less frequent or less significant exacerbations.
  • CMS is not associated with antibodies against the components of the neuromuscular junction.
  • All CMS disorders result from genetic mutations in components of the neuromuscular junction.
  • CMS symptoms do not require immunotherapy.

Transient neonatal myasthenia: This disorder occurs in 10-15% of infants born to mothers with autoimmune MG. Maternal antibodies that cross the placenta in late pregnancy cause transient neonatal myasthenia. Symptoms include a weak cry or suck and generalized weakness at birth. As the maternal antibodies degrade in infant’s body, symptoms gradually disappear within a few weeks. Infants with transient neonatal myasthenia gravis do not have an increased risk for long-term or future MG.
 

To learn more, see information from Andrew G. Engel, MD, courtesy of the National Organization for Rare Disorders

Lambert-Eaton Myasthenic Syndrome (LEMS)*: This is another autoimmune disease in which the immune system attacks the body’s own neuromuscular junction. Different from autoimmune MG in which the autoimmune system attacks the muscle aspect of the neuromuscular junction, LEMS is caused by autoimmune attacks to the nerve aspect, interfering with the ability of nerve cells to send signals to muscle fibers.

There are two forms of LEMS:

  • One form (50-60% of cases) is associated with small cell lung cancer and has an older age of onset.
  • The other form (40-50% of cases) is not associated with cancer and has a younger age of onset.

* Information is courtesy of the Muscular Dystrophy Association

This post originally appeared on MedLine Plus.