Myasthenia gravis can be difficult to diagnose and you may need to have several tests.
First your GP will ask about your medical history and symptoms. Alternatively, your optician may have noticed problems such as double vision or eyelid droop.
If they think you could have a problem with your brain or nerves, they may refer you to a specialist for tests in hospital to help diagnose myasthenia gravis and rule out other conditions with similar symptoms.
The main test for myasthenia gravis is a blood test to look for a type of antibody (produced by the immune system) that stops signals being sent between the nerves and muscles.
A high level of these antibodies usually means you have myasthenia gravis.
But not everyone with the condition will have a high level of antibodies, particularly if it's only affecting the eye muscles (ocular myasthenia).
The blood test may be repeated at a later date if the result is normal but your symptoms continue or get worse.
If the result of your blood test is normal but your doctor still thinks you could have myasthenia gravis, they may suggest an electrical test of your nerves and muscles.
These tests, known as electromyography, involve inserting very small needles into your muscles to measure the electrical activity in them.
These needles are typically inserted around the eyes, in the forehead or possibly in the arms.
The electrical recordings can show whether the signals sent from the nerves to the muscles are being disrupted, which may be a sign of myasthenia gravis.
You may also have a CT scan or MRI scan of your chest to check if your thymus gland is bigger than usual or has grown abnormally (a thymoma).
The thymus gland is a small gland in the chest that forms part of the immune system. Problems with the gland are closely associated with myasthenia gravis.
Sometimes an MRI brain scan may also be carried out to check that your symptoms aren't being caused by a problem in your brain.
If it's still not clear what's causing your symptoms, your doctor may recommend a test called an edrophonium test.
The test involves having an injection of a medicine called edrophonium chloride. If you have a sudden but temporary improvement in muscle strength after the injection, it's likely you have myasthenia gravis.
But this test is rarely done these days because there's a risk it could cause potentially serious side effects, such as a slow heartbeat and breathing problems.
It's only done if absolutely necessary and in a hospital setting where treatment for any side effects is readily available.