The symptoms of LEMS develop gradually over weeks or months.
The main symptoms are weakness in the legs, arms, neck and face, as well as problems with automatic body functions, such as controlling blood pressure.
Common symptoms include:
See your GP if you have a combination of these symptoms.
LEMS is caused by the body's natural defences (the immune system) mistakenly attacking and damaging the nerves.
Normally, nerve signals travel down the nerves and stimulate the nerve endings to release a chemical called acetylcholine. This chemical then helps activate the muscles.
If the nerve endings are damaged, the amount of acetylcholine they produce decreases, which means nerve signals don't reach the muscles properly.
It's not known what triggers the immune system to attack the nerves. It's often associated with lung cancer, but can occur in people without cancer.
LEMS is not inherited.
Your GP will first check your medical history, ask about your symptoms, carry out a physical examination, and test your reflexes.
If they think you have a problem with your nerves, they may refer you to a specialist called a neurologist for further tests to determine the cause.
Tests you may have include:
If initial scans don't find cancer, you may be advised to have regular scans every few months for a few years to check that it doesn't develop later on.
There's currently no cure for LEMS, but a number of treatments can help reduce the symptoms.
Medication is the main treatment, although immunoglobulin therapy and plasmapheresis may be recommended in the short term, or if muscle weakness is severe and other treatments haven't helped.
Some people respond well to treatment and are eventually able to stop taking medication, although this may not be for several years.
Others respond less well and find the condition affects their everyday activities and quality of life.
LEMS doesn't affect life expectancy if it's not associated with cancer. But people with lung cancer and LEMS tend to have a shorter life expectancy because it's often not diagnosed until the cancer has spread, and by this point it's very difficult to treat.
If you have LEMS, your clinical team may pass information about you on to the National Congenital Anomaly and Rare Diseases Registration Service (NCARDRS).
This helps scientists look for better ways to prevent and treat this condition. You can opt out of the register at any time.
Find out more about the register.