[Myasthenia gravis and myasthenic syndromes].

Neuromuscular transmission is compromised in a variety of disorders due to immunological, toxic or congenital mechanisms. Myasthenia gravis (MG) is the most frequent among these disorders. In about 15% of cases, MG is associated with a second autoimmune disorder mainly seen in rheumatologists. Some of the drugs used in rheumatology can exacerbate MG or even trigger immunologically the occurrence of MG. In most MG patients, antibodies to the acetylcholine receptor (AChR) are present, but around 10% have AChR antibodies that are only identified by novel methods, and up to 5% have muscle-specific kinase antibodies which define a different subgroup of myasthenia. Among those MG patients with anti-AChR antibodies, a number of clinical subtypes can be identified including early-onset MG (onset <or= 40 years), late-onset MG (onset after 40 years) and thymoma-associated MG. Even though less common, it is important to recognize Lambert Eaton myasthenic syndrome (LEMS). The abnormality in LEMS is a presynaptic failure to acetylcholine release caused by antibodies to voltage-gated calcium channels. More than half of LEMS patients have small-cell lung cancer.