[Electrodiagnosis of ALS]

Electrodiagnostic methods are crucial for the diagnosis of ALS. In nerve conduction studies, normal sensory conduction and absence of the sign of demyelination is required. Here, false-positive signs for demyelination should be paid special attention. Loss of motor units in ALS alone can cause significant slowing and absence of F-waves. Needle electromyography shows active neurogenic changes with denervation potentials (fibrillation potentials and positive sharp waves) and polyphasic and unstable motor unit potentials (MUPs). High-amplitude or giant MUPs may not be evident in rapidly progressing cases. Complex repetitive discharges are frequently seen. World Federation of Neurology established criteria for the diagnosis of ALS at El Escorial workshop in 1994, which was later revised (Airlie House criteria; AHC, 1998). The problem of AHC is its low sensitivity, especially in early cases. Specificity should also be cautioned because widespread fibrillations and frequent high-amplitude MUPs in chronic myopathies, such as inclusion body myositis, may easily fulfil the electrodiagnostic criteria in AHC. The diagnostic value of fasciculation potentials (FPs) is devaluated in AHC because of its "low specificity". However, our investigation using EMG database actually revealed high specificity of FPs for ALS, which may be a key point in considering the pathophysiology of ALS.