Clinical neurophysiology in ALS

Amyotrophic lateral sclerosis (ALS) belongs to a group of disorders known as motor neuron diseases. Despite being one of the most devastating diseases known, there is little evidence for diagnosing and managing patients with ALS. Clinical neurophysiologic tests are essential, when no biological marker exists to aid early diagnosis, not only in relation to diagnosis, but also in the development of disease progression, and perhaps, in the future, in measuring patients' response to therapy. The electrophysiological features used in the diagnosis of ALS are based on Awaji-shima consensus recommendations for the application of electrophysiological tests, as applied to the revised El Escorial Criteria. Measurements of axonal excitability through nerve conduction study (ENG) is useful to evaluate axonal degeneration. Electromyography (EMG) recordings with needle examination are essential for confirming lower motor neuron involvement in the initial diagnosis of ALS. EMG abnormalities are frequent and these include fibrillation potentials or positive sharp wave potentials, or both, with fasciculation potentials in resting muscle, and an incomplete interference pattern, with abnormal motor unit potentials. Collateral or terminal nerve sprouting is common in ALS and is frequent large macro-motor unit potentials (MUPs). Motor unit number estimation (MUNE) may be useful in measuring loss of functioning motor units and is an attractive endpoint measure in clinical drug trials in ALS because it directly assesses loss of lower motor neurons and is sensitive to disease progression. Transcortical magnetic stimulation protocols, and cortical excitability may be useful to assess the involvement of upper motor neuron system. In this chapter the advantages, limitations and promise of these various methods are discussed, in order to indicate the direction for further neurophysiological studies in this disorder.