Neuroscience Clerkship at UH/VA
 

Electromyography

Median Motor Nerve Conduction Study - stimulating the median nerve at the wrist, recording the thenar muscles.

Electromyography (EMG) is the principal diagnostic test used in the evaluation of suspected disorders of the peripheral nervous system. EMG is conventionally a two part test:
 

1) Nerve Conduction Studies (NCS)

                  and

2) Needle Electromyography

During nerve conduction studies, a peripheral nerve is stimulated with an electrical impulse. That impulse propagates down the nerve where its response can be recorded over muscle and skin, for motor and sensory nerve conductions studies, respectively. Nerve conduction studies are able study a variety of motor and sensory nerves in the upper and lower extremities. Conduction velocity, reflecting myelin function, and amplitude, reflecting the number of axons, can be easily assessed. In needle electromyography, intramuscular electrical activity is measured in various muscles. Different responses occur depending on whether the disorder is acute vs. chronic; and neuropathic vs. myopathic. In addition, specialized nerve conduction studies can assess for neuromuscular junction disorders. In these studies, repetitive nerve stimulation is used to demonstrate a decrementing response, the hallmark of neuromuscular junction disorders.

 

INDICATIONS

Evaluation of Suspected Neuromuscular Disorders. These include:

Motor neuronopathy (e.g., Amyotrophic lateral sclerosis, polio)

Sensory neuronopathy (e.g., Paraneoplastic, autoimmune)

Radiculopathy (e.g., disk herniation, inflammatory)

Plexopathy (e.g., Neoplastic, entrapment)

Entrapment Neuropathy (e.g., carpal tunnel syndrome, ulnar neuropathy at the elbow)

Polyneuropathy (e.g., diabetes, toxic, inflammatory)

Neuromuscular junction disorders (e.g., Myasthenia gravis, botulism)

Myopathy (e.g., polymyositis, muscular dystrophy)

• For neuromuscular disorders, EMG studies can assess:

Localization

Severity

Temporal course


LIMITATIONS

EMG can assess nerve and muscle function, but can rarely lead to a specific etiologic diagnosis (e.g., a radiculopathy from a disk herniation will appear similar to one caused by a tumor)

Many of the abnormalities on nerve conductions and needle EMG are time dependent. In general, most nerve conduction abnormalities require a week to develop; many of the abnormalities on needle EMG requires several weeks. Thus, EMG studies may be insensitive if the disorder is too acute.

Routine studies are available for many but not all peripheral nerves. Some nerves are not possible to study with routine EMG studies.

A minority of patients are electrically sensitive and may have difficulty with the nerve conduction studies.

A minority of patients are intolerant of the needle examination and may have difficulty completing the study.

Much of the study, especially the needle EMG, requires patient cooperation. Thus, the study is limited in young children, and in cognitively impaired adults.

The study is individualized for the patient. Likewise, abnormalities must be determined on-line in real time as the study proceeds. The value of the information is highly dependent on the skill and ability of the EMG physician performing the examination.


CONTRAINDICATIONS AND RISKS

EMG studies are generally well tolerated and rarely thought to be associated with any significant side effects, other than mild discomfort at the time of the procedure and possibly minor bruising as a consequence of the needle EMG. However, the needle EMG is an invasive procedure and under certain situations has the potential to be associated with complications, including bleeding, infection, nerve injury, pneumothorax and other local trauma. In addition, like all other electrical devices and monitoring equipment connected to patients, nerve conduction studies and needle EMG carry the risk of stray leakage currents that under certain circumstances can result in electrical injury.

Relative contraindications include:

Coagulopathy. Patients on anticoagulation or who have marked thrombocytopenia are at higher risk for bleeding following needle EMG. In these patients, needle EMG needs to be limited to a few superficial muscles.

Electrically Sensitive Patients. The electrical impulses delivered during routine nerve conduction studies are safe and well tolerated. However, very small currents applied to the heart may result in arrhythmia and death. The skin forms a high resistance barrier protecting the heart from superficial electrical currents. In patients with central catheters and external pacer wires, the heart is at greater risk for stray currents reaching the heart. These patients are known as "electrically sensitive". Electrical stimulation during nerve conduction studies near these sites is contraindicated. Likewise, patients with pacemakers and other cardiac devices should not have nerve conductions performed near these devices.

Risk of Pneumothorax. Although very rare, pneumothorax has been reported following needle EMG of several proximal chest muscles. Among are the serratus anterior, supraspinatus, rhomboids and lower cervical/thoracic paraspinal muscles.