Electromyography or EMG is the segment of electrodiagnostic medicine that studies the electrical activity within muscles. It is the study of the motor unit representing the muscle and nerve as a unit. This anatomic element consists of a cell body, its axons, the neuromuscular junction and the corresponding muscle fibers. The EMG is a diagnostic tool used to detect abnormality along any portion of the motor unit. It is helpful to clinicians in evaluating patients with weakness, muscle wasting, pain, numbness and paraesthesia.

 

The EMG exam is divided into two components best performed in one setting: the needle EMG and the nerve conduction study or NCS. Total test time ranges from 30 minutes to over an hour, depending on study design. During the needle electromyogram, a fine sterile wire electrode is inserted within the skeletal muscle to record the variations of electrical potential from the motor unit. This potential or electrical activity is displayed as waveforms on a monitor for simultaneous auditory and visual analysis. A normal resting muscle has little or no electrical activity. Electrical activity appears when the muscle is voluntarily contracting. A diseased motor unit produces various types of electrical activity with action potential of abnormal forms and recruitment patterns. These abnormalities objectively help to characterize the nature of the disease process and its localization within the motor unit. A stimulator with a negatively charged cathode and a positively charged anode sends an electrical impulse that depolarizes the underlying nerve axon during the NCS. An action potential is generated down the length of the nerve which is then recorded as a waveform by surface electrodes. Electrodes are placed over the mid portion of the muscle to record motor signals. Sensory signals are recorded directly over the nerve. We ask patients to refrain from using skin moisturizers and to remove body oils before testing to avoid issues relating to electrode adhesiveness.

 

The benefits, complications, discomfort and associated anxiety of the EMG exam is not often appreciated. However, the degree of discomfort experienced by the patient can be controlled by the techniques and methods of a well trained electromyographer. Common complications of the needle EMG exam include transient minor soreness at the puncture site which is typically 3-20 sites per limb. There might be transient bleeding, bruising or hematoma formation. Most patients voice phobia for needles but their tolerance usually improve with proper relaxation. Other limiting factors include anticoagulant therapy, bleeding disorders, skin infections, soft tissue inflammation, tremors, spasticity and morbid obesity.

 

Likewise, the NCS can cause apprehension and transient discomfort from the brief electrical stimulus. The stimuli are usually 0.01-1.0 milliseconds in duration with current intensity between 0-100 milliamps. The electrical stimulus can be surprising and might cause a limb jerk from activation of a nearby muscle. It can be uncomfortable but well tolerated by most, with two notable exceptions: individuals who have suffered electrocution injuries and pacemaker/defibrillator wearers. Those that have been electrocuted tend to experience post traumatic stress symptoms from the brief electrical stimuli. Extra precautions should be taken in patients with cardiac devices as high frequency repetitive electrical stimuli may alter device settings. It is therefore best to check with the EMG lab to verify their protocol before scheduling patients.

 

The timing of the EMG exam is also important. The patient's symptoms may be too recent to detect pathology. Nerve degeneration may not appear for 10 – 14 days in many acute neurogenic cases. It is therefore best to wait at least 3 weeks after a traumatic event to perform the EMG exam or longer if there is significant soft tissue inflammation. One exception is the need to document a pre-exiting condition or to document baseline status.

 

The EMG exam is an unsurpassed leader in the diagnosis of motor unit disorders; however its superiority is not without limitations. Referring physicians and electromyographers may be puzzled by patients with apparent weakness or sensory loss with normal EMG/NCS findings. In this situation, the presenting complaint of "weakness" may be too vague or generalized. The patient may have behavioral issues or a central nervous system disorder, such as stroke or multiple sclerosis. The underlying symptoms may be a result of a small fiber neuropathy as current sensory NCS techniques are more specific to large nerve fibers. There may be pain in the region being tested or poor muscle activation from disuse or nutritional issues. Despite these limitations, the EMG exam remains an invaluable tool in the diagnosis of nerve and muscle disorders.

 

 

 

Paulette A. Smart-Mackey, MD

Board Certified in Physical Medicine & Rehabilitation

Specializing in Electromyography