Electrodiagnosis of Motor Neuron Disease

Key points

Clinical features of amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disorder of upper motor neurons (UMN) and lower motor neurons (LMN). It has a worldwide incidence of approximately 1.5 per 100,000, with a male/female ratio of approximately 1.5.¹ Although occasional patients present before the age of 25, the incidence increases after age 40 and does not clearly decline in the elderly population.² Approximately 10% of cases are familial and include autosomal recessive, X-linked, and

Electrodiagnosis

ALS is a clinical diagnosis, but is supported by electrophysiologic study, which can either help to rule out other possible diagnoses or show characteristic abnormalities in body areas not yet clinically affected. The electrophysiologic studies that are in common practice, such as needle electromyography (EMG) and nerve conduction studies (NCS), directly identify LMN pathology, and at best may suggest UMN pathology by the observation of decreased activation on EMG. How do needle EMG and nerve

Nerve conduction studies

Because the fundamental pathology in the disease is motor neuron loss resulting in retrograde axonal degeneration followed by reinnervation, features that are not seen on NCS in ALS include the following:

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  Evidence of demyelination or conduction block on motor nerve conduction. Evidence for demyelination suggests pathology at the level of the myelinated axon rather than the motor neuron. Demyelination is characterized by prolonged distal latencies or slowing of conduction velocity, with the

Recommendation for NCS

At a minimum, NCS of a patient with suspected ALS should include testing of at least one motor nerve with F wave study and one sensory nerve in an upper and lower extremity on the most symptomatic side. If suspicion is high for multifocal motor neuropathy with conduction block, multiple upper and lower extremity nerves should be studied, with stimulation as proximal as is feasible.

Needle EMG

The hallmark of ALS on needle EMG is chronic and active loss of LMNs innervating muscles with multiple nerve root innervation and spread within an initial body segment and to other body segments. Although NCS is used primarily to help rule out other causes of the same clinical symptoms, such as neuropathy and radiculopathy, needle EMG is primarily used to establish evidence of ongoing denervation and chronic compensatory reinnervation. EDX is particularly helpful in uncovering subclinical

Recommendations for needle EMG testing

At a minimum, needle EMG study of a patient with suspected ALS should include testing of at least three limbs, sampling muscles innervated by at least two different nerve roots, and peripheral nerves and proximal and distal muscles. Additionally, testing should be performed on at least one bulbar muscle, such as a facial muscle, the masseter muscle, or tongue. Finally, needle EMG should be done on at least two thoracic paraspinal muscles.

Other electrodiagnostic tools

Motor unit number estimation (MUNE), a sensitive technique for identifying lower motor unit loss, particularly before the onset of clinical weakness, is not yet widely performed and is used primarily in the research setting. MUNE can be used as marker for disease progression in ALS, because it can be linked to important outcomes, such as survival.39, 40, 41 A recently described standardized technique for multipoint incremental MUNE generates highly reproducible data, and can be rapidly

Summary

ALS is a disease diagnosed primarily on clinical grounds, because specific imaging abnormalities or other biomarkers have not been clearly identified. Clinical neurophysiology, as an extension of the neurologic examination, has proved useful in helping to establish a diagnosis, by eliminating possible disease mimics and providing evidence of abnormalities in body areas that may yet be clinically unaffected. Electrodiagnosis begins with an understanding of the clinical features of the disease,