Chest X-ray showing a tumor in the left lung (right side of the image) The diagnosis is usually made with nerve conduction study (NCS) and electromyography (EMG), which is one of the standard tests in the investigation of otherwise unexplained muscle weakness. EMG involves the insertion of small needles into the muscles. NCS involves administering small electrical impulses to the nerves, on the surface of the skin, and measuring the electrical response of the muscle in question. NCS investigation in LEMS primarily involves evaluation of compound motor action potentials (CMAPs) of effected muscles and sometimes EMG single-fiber examination can be used. CMAPs show small amplitudes but normal latency and conduction velocities. If repeated impulses are administered (2 per second or 2 Hz), it is normal for CMAP amplitudes to become smaller as the acetylcholine in the motor end plate is depleted. In LEMS, this decrease is larger than observed normally. Eventually, stored acetylcholine is made available, and the amplitudes increase again. In LEMS, this remains insufficient to reach a level sufficient for transmission of an impulse from nerve to muscle; all can be attributed to insufficient calcium in the nerve terminal. A similar pattern is witnessed in myasthenia gravis. In LEMS, in response to exercising the muscle, the CMAP amplitude increases greatly (over 200%, often much more). This also occurs on the administration of a rapid burst of electrical stimuli (20 impulses per second for 10 seconds). This is attributed to the influx of calcium in response to these stimuli. On single-fiber examination, features may include increased jitter (seen in other diseases of neuromuscular transmission) and blocking. Blood tests may be performed to exclude other causes of muscle disease (elevated creatine kinase may indicate a myositis, and abnormal thyroid function tests may indicate thyrotoxic myopathy). Antibodies against voltage-gated calcium channels can be identified in 85% of people with EMG-confirmed LEMS. Once LEMS is diagnosed, investigations such as a CT scan of the chest are usually performed to identify any possible underlying lung tumors. Around 50–60% of these are discovered immediately after the diagnosis of LEMS. The remainder is diagnosed later, but usually within two years and typically within four years. As a result, scans are typically repeated every six months for the first two years after diagnosis. While CT of the lungs is usually adequate, a positron emission tomography scan of the body may also be performed to search for an occult tumour, particularly of the lung.
Lambert–Eaton myasthenic syndrome
Botulism
Congenital myasthenic syndrome
Guillain–Barré syndrome
Tension myositis syndrome
De Quervain syndrome
Nemaline myopathy
Inclusion body myositis