Electrical Issues

All electrical devices, including EMG machines, require current to operate. Current is delivered from an electrical cord plugged into a wall receptacle (Figure 40–1). A typical electrical receptacle in the United States contains three inputs: a black “hot” lead that carries 120 volts (V) of 60 Hz alternating current, a white “neutral” lead near 0 V, and a green ground lead that is used to dissipate leakage currents. When a circuit is created, current flows from the hot lead to the EMG machine and then returns via the neutral lead, based on the amount of resistance between the two leads as determined by Ohm’s law (see Chapter 39). Every wire, including power cords, has some small resistance; thus, a small voltage develops on the neutral lead, which equals the current flowing multiplied by the resistance in the power cord (Figure 40–2). The voltage increases with the length of the power cord and increases further if extension cords are added to the power cord. In addition, small voltage leaks often are present on the machine chassis, caused by stray capacitance and inductance from internal electronics (Figure 40–3). Thus, leakage currents may be transmitted onto the patient either from stray voltages on the machine chassis or on the neutral (reference) lead. As the ground electrode is close to true electrical neutral, the ground lead allows a pathway for stray current leaks to harmlessly dissipate.

FIGURE 40–1 Standard electrical wall receptacle.

A safe electrical receptacle must contain three inputs: a black “hot” lead that carries 120 V of 60 Hz alternating current, a white “neutral” lead near 0 V, and a green ground lead that is used to dissipate leakage currents. It is essential that all electromyography machines use a three-input receptacle, which includes a ground, for electrical safety.

FIGURE 40–2 Stray leakage current: reference lead.

Because every wire, including power cords, has some small resistance (R_N), a small voltage (V_N) develops on the neutral or reference lead as determined by Ohm’s law (V = I × R, where I is the current). The voltage increases with the length of the power cord and increases further if extension cords are added to the power cord. Thus, the voltage on the reference electrode is not zero and is a potential source for a leakage current transmitted to a patient.

(Adapted from Kimura, J., 1983. Electrodiagnosis in diseases of muscle and nerve. FA Davis, Philadelphia, pp. 615–619, with permission.)

FIGURE 40–3 Stray leakage current: stray voltages from the electromyography machine.

Small voltage leaks often are present on the machine chassis, caused by stray capacitance and inductance from internal electronics. These stray voltages are another potential source for leakage current that can be transmitted to a patient.

The risk of electrical injury depends on the amount of leakage current and whether the circuit passes through the heart. A very small current [e.g., 200 microamperes (µA)] applied directly to the heart can result in ventricular fibrillation and death. However, the normal, healthy individual typically is well protected by two important mechanisms. First, dry and intact skin provides a high resistance. Second, the large volume of soft tissue that surrounds the heart dilutes any current applied to the body (e.g., a current applied from arm to arm degrades to 1/1000 of the original signal when it reaches the heart, due to the dissipation from surrounding tissues).

The risk of electrical injury from leakage current increases in the following situations

The latter two (multiple electrical devices attached to the patient and loss of the body’s normal protective mechanisms) result in the “electrically sensitive” patient, a common situation in the intensive care unit.

To prevent the possibility of an electrical injury during EDX studies, it is essential for equipment to be regularly maintained, to always use a ground electrode, and to follow simple guidelines when using electrical devices attached to the patient (Box 40–1). A wooden examining table is preferable to a metal table, as it does not conduct electricity. Machines should be turned on before attaching electrodes to the patient and turned off after disconnecting the patient, to minimize the risk of power surges. Equipment should be periodically inspected by a biomedical engineer to measure leakage current and verify proper grounding. In general, the maximum amount of acceptable leakage current is 100 µA or less, measured from chassis to ground, and 50 µA or less from any input lead to ground. Extension cords should be avoided to reduce the risk of voltages developing on the reference electrodes. Ground electrodes should always be used to avoid current flows from reaching the patient. The ground needs to be placed on the same limb as the active electrodes so that leakage currents cannot flow in a path through the heart (Figure 40–5A).

Box 40–1

Measures to Ensure Proper Grounding

• Always use a three-hole power receptacle with properly grounded outlet.

• Unnecessary electrical equipment should be kept outside the EMG examining room.

• Suspect improper grounding if

Equipment is wet or has been subjected to spillage of liquids

Equipment has been physically damaged or has loose parts

Equipment gives a tingling sensation when touched

Equipment becomes hot or gives off unusual odor or sound

There is damaged or cracked insulation in the power cable

• Use a wooden examining table if possible (metal conducts electricity) (Figure 40–4).

• Avoid patient contact with any metal objects or any part of the EMG machine.

EMG, electromyography.

From Al-Shekhlee, A., Shapiro, B.E., Preston, D.C., 2003. Iatrogenic complications and risks of nerve conduction studies and needle electromyography. Muscle Nerve 27, 517–526, with permission.

FIGURE 40–4 Wooden bed and electrodiagnostic studies.

Wood does not conduct electricity; therefore, wooden examining tables are preferable to metal tables to ensure safety during electrodiagnostic studies.

FIGURE 40–5 Leakage current and the risk of electrical injury.

A: Correct placement of the ground electrode on the limb being studied. If a stray leakage current develops, the ground allows a pathway for the current to dissipate safely. B: When a patient is connected to two electrical devices, leakage current present on one device potentially can flow to another. In the example shown here, device 1 has a faulty ground electrode. A leakage current from that device creates a circuit by flowing to the ground electrode of the second device. If the pathway traverses the heart and the current is large enough, potentially dangerous arrhythmias may result.

(Adapted from Starmer, C.F., McIntosh, H.D., Whalen, R.E., 1971. Electrical hazards and cardiovascular function. N Engl J Med 284, 181–186, with permission.)

The issue of an intact ground electrode and proper ground placement is most important when a patient is connected to other electrical devices. If the ground from the EMG machine is not functioning (i.e., ground fault), stray current from the EMG machine could flow to a ground electrode from a different electrical device. If the pathway included the heart and the amount of current was large enough, a cardiac arrhythmia could theoretically occur (Figure 40–5B).

Risk of Electrical Injury

Central Lines and Electrical Wires

One of the more common ways a patient can become electrically sensitive is when the normal protective function of the skin is breached by intravenous lines and wires. This danger increases if the lines are actually in contact with or in close proximity to the heart, as occurs in central intravenous catheters (Figure 40–6). Most dangerous is the presence of an external wire near or in the heart, such as occurs with placement of a temporary external pacemaker and during the use of a guidewire while placing or changing a central line. Skin resistance typically is several million Ohms (MΩ). A central catheter traversing the skin reduces this resistance to 300,000 Ohms (kΩ). Any fluid spill where a catheter enters the body decreases the resistance even further. If a catheter has an internal guidewire, the resistance drops to 70 Ohms (Ω). An external pacemaker wire essentially has no resistance. In situations where the resistance is so low, small leakage voltages may result in small leakage currents, known as microcurrents. Whereas microcurrents are completely harmless in a patient with intact skin, they are potentially very dangerous in an electrically sensitive patient (i.e., a patient with a central line, external pacemaker wires, etc.).

FIGURE 40–6 Risk of electrical injury from central lines: stimulation sites to avoid.

One of the more common ways that a patient can become electrically sensitive is when the normal protective function of the skin is breached by intravenous lines and wires that are in contact with or in close proximity to the heart, as occurs in central intravenous catheters. Nerve conduction studies can be performed safely in these patients, provided certain precautions are taken. Proximal stimulation sites should be avoided, most importantly at the axilla and Erb’s point.

Thus, EDX studies should never be performed on patients with external wires in place (i.e., external pacing wire, guidewires, etc.) because the conductive pathway to the heart is so vulnerable. However, studies can be performed on patients with central lines provided certain precautions are followed. Equipment must be maintained. Ground electrodes must always be used. If an upper extremity must be studied, in general it is preferable and safer to study the upper extremity contralateral to the one with the central line. If that is not possible, one should refrain from proximal stimulation sites (i.e., axilla, Erb’s point, and root). Likewise, one should never proceed if there is a fluid spill where the central catheter enters the skin. It is important to note, however, that there is NO contraindication to performing routine nerve conduction studies on patients with peripheral IVs. Studies have been reported that specifically address this question and find that NCSs are completely safe in patients with peripheral IVs, regardless of whether they are infusing saline or any other solution.

Implanted Pacemakers and Cardioverter-Defibrillators

Patients with implantable cardiac pacemakers and cardioverter-defibrillators are at much lower risk from stray current leaks than patients with central lines or external wires in place, because these devices are implanted under the skin, which leaves the normal protective mechanism of the skin intact. Implantable pacemakers and cardioverter-defibrillators both have an electronic sensing as well as an electronic delivery function. Pacemakers are designed to treat bradycardia, as opposed to cardioverter-defibrillators which are primarily for tachyarrhythmias, especially ventricular fibrillation. In theory, stimulation delivered during NCSs might be mistaken as an abnormal cardiac rhythm. If the stimulator has a pulse duration greater than 0.5 ms and a stimulus rate greater than 1 Hz, a demand pacemaker might theoretically confuse such a stimulus with the ECG signal. There is only a single case report of an implantable pacemaker failure thought to be related to peripheral nerve stimulation. Other studies have shown no pacemaker inhibition or dysfunction with NCSs. Less is known about implantable automatic cardioverter-defibrillators (IACDs), which are now common. In theory, IACDs could be triggered by stimulation during NCSs, resulting in subsequent cardiac arrhythmias; however, there is no such reported case. One study directly addressed the safety of nerve conduction studies, including stimulating Erb’s point, in patients with IACDs. Schoeck et al. studied ten patients with pacemakers and five with IACDs. No electrical impulse was detected by either the atrial or ventricular amplifiers of the pacemakers or of the IACDs during median and peroneal nerve conduction studies. These studies included Erb’s point stimulation on the left. The authors emphasized that all modern pacemakers and IACDs use bipolar leads wherein both leads (active and reference for sensing, and cathode and anode for stimulating) are imbedded in the cardiac wall. This is in contradistinction to the pacemakers used 25 years ago wherein a single wire lead was placed in the heart, and the metal body of the pacemaker in the chest served as the reference. In modern pacemakers and IACDs, the bipolar leads are very close together in the heart, and very far away from the surface, making any electrical contamination from NCSs extremely unlikely. Although the number of patients in this study was small, the results are reassuring that NCSs can be safely performed in patients with pacemakers and IACDs.

If NCSs are performed in patients with implantable pacemakers or IACDs, several simple procedures are recommended to be followed in order to preserve safety (Box 40–2). Stimulation should not be performed near the actual implanted device. There should always be a minimum of 6 inches between the implanted device and the stimulator. Just as with NCSs performed in a patient with a central line, it is preferable to use the contralateral arm if possible. High stimulus intensities should be avoided and stimulus pulse duration should be 0.2 ms or less so that the stimulation is not misinterpreted as a QRS complex. Stimulation rates should be no greater than 1 Hz so as to prevent the theoretical risk that the stimulation is misinterpreted as a cardiac rhythm. Thus, the typical repetitive stimulation done during neuromuscular junction testing is best avoided.

Box 40–2

Guidelines for Pacemakers and Implantable Cardioverter-Defibrillators

• Do not perform studies on patients with external pacer wires.

• Ensure all ground electrodes are functional.

• Limit all electrodes, including the ground, to the extremity of interest, and keep all electrodes as far away from the heart as possible, without crossing cardiac devices or their wires.

• Do not stimulate near the device (allow a minimum of 6 inches) and avoid ipsilateral proximal stimulation sites (i.e., axilla, Erb’s point, root stimulation).

• Use a stimulus duration of 0.2 ms or shorter and a stimulus rate of 1 Hz or slower. Thus, the typical repetitive stimulation done during neuromuscular junction testing is best avoided.

• Consult a cardiologist regarding performing studies in patients with an implantable automatic cardioverter-defibrillator.

• Laboratory emergency drugs should be available, including crash carts.

From Al-Shekhlee, A., Shapiro, B.E., Preston, D.C., 2003. Iatrogenic complications and risks of nerve conduction studies and needle electromyography. Muscle Nerve 27, 517–526, with permission.

Pneumothorax

Pneumothorax is the most potentially serious iatrogenic complication of needle EMG. At any time during or just after the EMG examination, unexpected chest pain, shortness of breath, or cyanosis in a patient should alert the electromyographer to the possibility of a pneumothorax. If such symptoms develop, a prompt chest X-ray film is indicated to confirm the diagnosis, followed by urgent consultation with a thoracic surgeon as to whether a chest tube or observation is required. Although rare, this complication has been reported when sampling the following muscles (Figure 40–7):

FIGURE 40–7 Needle electromyography (EMG) and the risk of pneumothorax.

One of the most potentially serious complications of needle EMG is pneumothorax. Right: Note the absence of normal lung markings due to pneumothorax. Arrows point to the collapsed right lung. Left: Although rare, this complication has been reported when sampling the following common muscles: (1) supraspinatus, (2) serratus anterior, (3) lower cervical paraspinal muscles, (4) rhomboids, and (5) thoracic paraspinal muscles.

FIGURE 40–8 Supraspinatus muscle and risk of pneumothorax.

The supraspinatus muscle lies in the supraspinous fossa. Needle electromyographic examination of this muscle may be complicated by pneumothorax if sampling is near the midpoint where the supraspinous fossa is narrowest (A). If the needle is placed deep above point A (area marked by *), there is a risk of pleural puncture. The muscle can be more safely sampled medially in the supraspinous fossa (B).

(Adapted from Reinstein, L., Twardzik, F.G., Mech, K.F. Jr., 1987. Pneumothorax: complication of needle electromyography of supraspinatus muscle. Arch Phys Med Rehabil 68, 561–562, with permission.)

FIGURE 40–9 Thoracic paraspinal muscles and the risk of pneumothorax.

Axial computed tomographic scan of a normal individual at the midthoracic level (left), with magnified view of the thoracic paraspinal muscles (right). Note the close proximity of the thoracic paraspinal muscles to the lungs. The correct location for sampling the paraspinal muscles is (B), just off the midline with the needle directed down and slightly medially. If the needle is placed too laterally (A) and directed deep and lateral, there is a risk of pneumothorax.

FIGURE 40–10 Lower cervical paraspinal muscles and the risk of pneumothorax.

Left: In some individuals, the lung apex rises above the clavicle where it may be punctured from a laterally placed electromyographic needle. Right: Axial computed tomographic scan of a normal individual at the C7–T1 vertebral level. Note that the correct location for sampling the paraspinal muscles is (B), just off the midline with the needle directed down and slightly medially. If the needle is placed too laterally (A) and directed deep and lateral, there is a risk of pneumothorax at this level in some individuals.

Bleeding

Needle EMG is generally well tolerated, with minimal or no bleeding. Some patients develop minor bruising that resolves within a few days. However, the possibility of bleeding and subsequent hematoma formation is a theoretic risk any time a needle punctures the skin, whether it occurs during phlebotomy, vaccination, aspiration, or needle EMG examination of a muscle. Clearly, the chance of bleeding increases if a patient has certain risk factors (discussed in the following section). However, bleeding can occur in the absence of any known risk factors or deviation from the usual performance of the examination.

In one report from Caress et al., a patient with a large asymptomatic paraspinal hematoma was discovered incidentally on magnetic resonance imaging (MRI) just after needle examination of the lumbar paraspinal muscles, which had been performed for evaluation of lumbar radiculopathy. By happenstance, the patient had an MRI of the lumbar spine scheduled immediately after the EMG. The patient was not anticoagulated and had no risk factors for increased bleeding. Following this case, a retrospective review of patients referred to the EMG laboratory followed by MRI the same day revealed four other patients with radiologically proven paraspinal muscle hematomas, presumably as a result of the needle EMG examination. All patients were asymptomatic and had no history of anticoagulation or other known risk factors for bleeding.

However, in a recent large study from Gertken et al., 370 patients who underwent EMG studies that included the paraspinal muscles and who then had MRI scans at the concordant spinal level (168 MRIs were completed the same day as the EMG, and the remaining were completed within 7 days) were examined. A combined total of 431 spine segments were studied. No paraspinal hematoma was observed in any patient, including 139 patients taking aspirin, ten on warfarin (INRs between 1.2 and 2.9), eight on clopidogrel, and four patients who were on heparin, enoxaparin, or dalteparin.

In a prospective study by Lynch et al., EMG examination of the tibialis anterior muscle was followed by ultrasound to evaluate for the presence of a hematoma. Two of 101 patients on warfarin (INR values of 1.5 or above) had small, subclinical hematomas. Of 57 patients taking clopidogrel and/or aspirin, one patient was found to have a small, subclinical hematoma on ultrasound. None of the 51 control patients, who were not taking warfarin, aspirin, or clopidogrel, were found to have a hematoma by ultrasound. A recent prospective study by Boon et al. examined the incidence of hematoma, using ultrasound examination, after needle EMG of potentially “high risk” muscles (cervical, thoracic, and lumbar paraspinals; tibialis posterior; flexor digitorum longus; flexor pollicis longus; iliopsoas). A total of 205 patients were studied: 58 on warfarin, 78 on aspirin/clopidogrel; and 70 control patients taking none of these medications, with a minimum of 100 muscles per patient group. One patient in the aspririn/clopidogrel group had a subclinical hematoma in the tibialis posterior muscle, and one patient in the warfarin group had a subclinical hematoma in the flexor pollicis longus (INR 2.3). No patient in the control group had a hematoma.

In addition, there are two case reports of EMG needle-induced laceration or injury to nearby blood vessels that resulted in bleeding and a subsequent compartment syndrome requiring urgent fasciotomy and surgical evacuation of the hematoma. In one case, the compartment syndrome occurred in the superficial posterior compartment of the lower leg, presumably as a result of puncturing a small vessel. In the other case, needle EMG of the flexor carpi radialis inadvertently injured the ulnar artery, resulting in a compartment syndrome of the forearm. In neither of these cases was the patient anticoagulated or regularly taking any anti-platelet agents.

There are also some reports of bleeding following needle EMG in anticoagulated patients. In one anticoagulated patient (INR 2.5), a hematoma developed in the posterior calf along with a pseudoaneurysm of the posterior tibial artery. She improved with supportive care and holding the anticoagulation. In another case, a patient taking warfarin developed a large subcutaneous hemorrhage near an EMG needle insertion point.

Infection

Electrodes and needles used for EDX studies carry the possible risk of transmitting infection between patients or between the electromyographer and the patient. Although this risk is higher during the needle EMG portion of the examination, skin preparation occasionally may abrade the skin, resulting in minor oozing or bleeding, potentially contaminating surface electrodes used for NCSs. As learned from the human immune deficiency virus (HIV) epidemic, one should always assume that infection is possible and follow universal precautions. Hand washing before and after a patient encounter is essential. Gloves should always be worn during potential exposure to blood, which occurs during every needle EMG examination. After every NCS, surface electrodes should be cleaned with a 1:10 dilution of bleach or 70% isopropyl alcohol. If reusable needle electrodes are used (i.e., single-fiber needle electrodes), they should be autoclaved after every use, similar to any other surgical instrument. Note that standard autoclaving does not neutralize Jacob–Creutzfeldt disease infection, and any reusable electrode used on such a suspected patient should be discarded.

Inadvertent needle sticks are a risk during needle EMG. Transmissible diseases include HIV as well as other infections, especially viral hepatitis, underscoring the importance of hepatitis B vaccinations for all electromyographers. Similar to precautions used with any needle, the EMG needle should not be recapped using the contralateral hand. The risk of a needle stick is markedly reduced if the needle is placed safely out of the way when it is not being used (e.g., in between sampling muscles or when explaining the next muscle movement to the patient). In our laboratory, we successfully use a foam rubber block attached to the preamp arm of the EMG machine (Figure 40–11). The block holds the needle cap so that the needle can be recapped safely with one hand.

FIGURE 40–11 Reducing the risk of a needle stick.

The risk of a needle stick can be markedly reduced if the needle is not recapped using two hands, and is placed safely out of the way when not in use. One successful approach is to use a foam rubber block attached to the preamp arm of the electromyographic machine. The needle cap can be placed in the block, so that the needle can be recapped safely with one hand.

Fortunately, there appears to be little risk of transmitting infection to the patient during NCSs and needle EMG. With the modern use of sterilized, single-use needle electrodes, this complication has not been reported. However, there are several conditions wherein the risk of infection with needle EMG is theoretically higher. An EMG needle should never be placed through an infected space (e.g., skin ulcer), to prevent the spread of infection into deeper tissues. Unresolved is whether needle EMG is contraindicated in the feet of patients with diabetic neuropathy or vascular insufficiency. Such patients are commonly advised by their physician to inspect their feet and to avoid minor infections that could potentially threaten the limb if the infection became severe. Although there are no such reported cases, it is reasonable to be very cautious when performing needle EMG on intrinsic foot muscles in patients with diabetes or significant peripheral vascular disease. Similarly, patients who have undergone axillary lymph node dissections (usually in the context of breast cancer surgery) are cautioned against blood drawing and similar procedures in the ipsilateral extremity because of the possibility that an infection could spread quickly in the setting of lymphedema and a reduced number of lymph nodes proximally. Although there are no reported cases of infection in such patients following needle EMG, reasonable caution should be exercised in such patients.

Finally, the issue of using prophylactic antibiotics in patients at high risk for endocarditis should be addressed. Antibiotic prophylaxis is not recommended by the American Heart Association in patients undergoing needle EMG, where the risk is considered similar to phlebotomy.

Local Injury

Needle-induced local injury occurs rarely. Theoretically, an EMG needle could directly injure a nerve from direct intraneural puncture. To our knowledge, there is no reported case of such an injury. In near-nerve studies and during local anesthetic blocks, needle electrodes are intentionally placed very close to nerves, with intraneural placements not infrequently occurring without any sequelae. During routine needle EMG, there are several areas where nerves travel near or through the muscle of interest. Most important among these are the following:

Needle-induced paresthesias of these nerves are encountered occasionally during routine needle EMG. When this occurs, the needle should immediately be withdrawn from the muscle, and one should wait for the paresthesias to resolve before continuing to sample an alternative muscle or the original muscle at a different site.

Although there are no reported cases of EMG needle-induced nerve trauma, there are reports of nerve trauma from other types of needles, most often occurring during venipuncture as well as other procedures. The median, lateral antebrachial, medial antebrachial, and superficial radial sensory nerves are the ones most often reported to be damaged during venipuncture.

Summary

EDX studies as routinely performed often yield useful diagnostic information with minimal risks. However, it is essential that the electromyographer appreciate the known and theoretical complications discussed and follow the recommendations to minimize the chance of complications. Like all diagnostic tests, the electromyographer must always weigh the potential benefits of any EDX procedure versus the risks to the individual patient and use his or her best judgment.