Laryngeal electromyography (LEMG) evaluates the integrity of the neuromuscular system in the larynx by recording action potentials generated in the laryngeal muscles during voluntary and involuntary contraction. LEMG is particularly useful for helping to differentiate between disorders involving upper motor neurons, lower motor neurons, peripheral nerves, the neuromuscular junction, muscle fibers, and the laryngeal cartilages and joints. LEMG should be considered to be an extension of the physical examination, not an isolated laboratory procedure. A careful history and laryngeal evaluation determine the indication for LEMG and which muscles or muscle groups, in particular, are to be studied. Abnormalities detected by LEMG are always interpreted within the context of the clinical picture.
Laryngeal electromyogragphy (LEMG) is an invaluable adjunct to laryngologic assessment, diagnosis, and treatment of voice disorders. It is easy to perform, well-tolerated, and presents minimal risks to patients. It is useful in the evaluation of numerous laryngeal disorders, allowing clinicians to differentiate among upper motor neuron, lower motor neuron, peripheral nerve, neuromuscular junction, myopathic, and mechanical disorders. It is also useful in establishing prognosis in laryngeal nerve palsies and for guidance during the injection of botulinum toxin in the treatment of spasmodic dysphonia. Judgements regarding when to use LEMG, selection of muscles to be studied, and the choice of EMG techniques depend upon a comprehensive history and physical examination. LEMG requires expert interpretation, taking the clinical scenario into account. A skilled electromyographer is an immeasurable asset to the voice care team.
Basic neurophysiology
The interior of a muscle or nerve cell is electrically negative with respect to its exterior . This electrical potential difference is called the resting membrane potential. In muscles it is on the order of 90 millivolts; for lower motor neurons it is about 70 millivolts. The resting membrane potential reflects the ionic gradient of the cell membrane. The intracellular and interstitial fluids are in osmotic and electrical equilibrium with each other; however, the distribution of the ions between the two compartments is unequal. The intracellular compartment has a high concentration of potassium and the extracellular compartment has a high concentration of sodium and chloride, a gradient that is maintained by active transport over the cell membrane.
With the application of an appropriate stimulus, nerves and muscles generate action potentials. The action potential is a fast and transient reversal of the membrane potential caused by a temporary change in membrane permeability. The action potential is generated by depolarization of the cell membrane to the membrane threshold potential. This action potential is propagated along the fiber without decrement .
The motor unit consists of a single lower motor neuron and the muscle fibers that it innervates. It, therefore, includes the cell body of the lower motor neuron in the spinal cord, its axon with its terminal arborization, the neuromuscular junctions, and all the muscle fibers innervated by them. Every muscle unit has an innervation ratio, which is a measure of the total number of muscle fibers in the muscle to the total number of motor axons innervating that muscle. The innervation ratio in small muscles, such as the laryngeal muscles, external rectus oculi, and tensor tympani muscles, is approximately 25:1. The innervation ratio of the medial head of the gastrocnemius muscle, a large muscle, is approximately 1700:1. Muscles that perform fine motor movements typically require low innervation ratios. Muscles with high innervation ratios are typically involved in more gross motor movements. The individual muscle fibers belonging to a given motor unit are scattered diffusely in the muscles, without grouping .
There are two types of muscle fibers based on histochemical characteristics. Type 1 fibers are rich in mitochondrial oxidative enzymes but poor in myofibrillar adenosine triphosphatase (ATPase), whereas Type 2 fibers are rich in myofibrillar ATPase and low in mitochondrial oxidative enzymes. The muscle fibers of an individual motor unit are all of the same histochemical type. The lower motor neuron has trophic influence on the muscle fiber so that a muscle fiber may change its histochemical characteristics when reinnervated by motor neuron from a different motor unit type. Type 1 muscle fibers are best suited for producing sustained, low-intensity muscle contractions; Type 2 fibers are best suited for short bursts of high intensity muscle contractions . In the spinal cord, smaller motor neurons innervate Type 1 fibers and large motor neurons innervate Type 2 fibers. Smaller motor neurons are typically activated at low muscle tension; therefore, they are the first ones to be observed during the electromyographic (EMG) evaluation. Large motor neurons are recruited during high muscle tension and are therefore seen during maximal muscle contraction. Small motor neurons fire at a lower rate, typically less than 20 Hz; large motor neurons are capable of firing at rates as high as 100 Hz. With aging, there is a significant loss of motor neurons in the anterior horn cells, which causes an increase in the innervation ratio of the surviving units .
The electrodiagnostic apparatus
Bioelectrical potentials from the muscles or nerves being examined are detected by an active recording electrode connected to a differential amplifier with a typical common mode rejection ratio of 100,000:1 and a high input impedance of at least 100,000. The frequencies of muscle action potentials range between 2 Hz and 10,000 Hz and the frequency band of the EMG machine is typically set at 10 Hz to 10,000 Hz. The reference electrode is also connected to the amplifier. The signal of interest is measured as the potential difference between the active and reference electrodes. The patient must be grounded to reduce the risk for electrical injury and 60 Hz interference. The electrodiagnostic signal is displayed on a cathode ray oscilloscope in real time and can be heard through a loudspeaker. The amplified signal can then be monitored visually and acoustically. The signal can be stored permanently on magnetic tape, a computer disk, or paper. In addition to the qualitative analysis used most commonly, quantitative EMG assessment is also possible. In modern systems, the amplifier signal is also connected to an analog-to-digital converter, a microprocessor, and a video monitor for a digital display of the signal. This connection permits rapid mathematic manipulation of the raw data. In addition, there is an electrical stimulator incorporated into the system that is connected to the microprocessor and the oscilloscope so that it can trigger the recording system when stimulation is provided. The ability of an amplifier to reject common mode signals is indicated by its common mode rejection ratio (CMRR). The higher the ratio, the greater the ability of the amplifier to reject common mode potentials. In clinical EMG, amplifiers with a CMRR of 10,000 are preferred, which means that unequal potential differences between the two inputs of the amplifier is amplified 10,000 times more than potentials equal to both inputs .
In most EMG laboratories, sophisticated, multichannel systems are used. There are several excellent systems available commercially. They have many advantages, including permitting simultaneous, multichannel recording, but EMG systems are fairly expensive. For otolaryngologists who plan to use laryngeal EMG for needle guidance when injecting botulinum toxin or for occasional diagnostic purposes, less expensive, conveniently portable systems are now available, such as the device manufactured by Xomed (Jacksonville, Florida). In its basic form, this EMG unit provides only auditory information and single-channel recording; but it can be connected to a computer to provide a visual display. Such compact devices are also valuable for bedside, in-patient testing of patients who have laryngeal trauma in whom differentiation between arytenoid injury and vocal fold paralysis is necessary. They are especially convenient during evenings and weekends, when formal EMG laboratory facilities may not be available. Another cost-effective option for otolaryngologists is the use of the brainstem-evoked response (ABR) audiometer found in many offices. Most ABR units can be used (sometimes with minor modifications) for single-channel EMG recordings. Although such devices can be used for specific clinical indications when formal EMG cannot be performed, they should be used in addition to, not in place of, a sophisticated, multichannel EMG system for diagnostic testing.
The electrodiagnostic apparatus
Bioelectrical potentials from the muscles or nerves being examined are detected by an active recording electrode connected to a differential amplifier with a typical common mode rejection ratio of 100,000:1 and a high input impedance of at least 100,000. The frequencies of muscle action potentials range between 2 Hz and 10,000 Hz and the frequency band of the EMG machine is typically set at 10 Hz to 10,000 Hz. The reference electrode is also connected to the amplifier. The signal of interest is measured as the potential difference between the active and reference electrodes. The patient must be grounded to reduce the risk for electrical injury and 60 Hz interference. The electrodiagnostic signal is displayed on a cathode ray oscilloscope in real time and can be heard through a loudspeaker. The amplified signal can then be monitored visually and acoustically. The signal can be stored permanently on magnetic tape, a computer disk, or paper. In addition to the qualitative analysis used most commonly, quantitative EMG assessment is also possible. In modern systems, the amplifier signal is also connected to an analog-to-digital converter, a microprocessor, and a video monitor for a digital display of the signal. This connection permits rapid mathematic manipulation of the raw data. In addition, there is an electrical stimulator incorporated into the system that is connected to the microprocessor and the oscilloscope so that it can trigger the recording system when stimulation is provided. The ability of an amplifier to reject common mode signals is indicated by its common mode rejection ratio (CMRR). The higher the ratio, the greater the ability of the amplifier to reject common mode potentials. In clinical EMG, amplifiers with a CMRR of 10,000 are preferred, which means that unequal potential differences between the two inputs of the amplifier is amplified 10,000 times more than potentials equal to both inputs .
In most EMG laboratories, sophisticated, multichannel systems are used. There are several excellent systems available commercially. They have many advantages, including permitting simultaneous, multichannel recording, but EMG systems are fairly expensive. For otolaryngologists who plan to use laryngeal EMG for needle guidance when injecting botulinum toxin or for occasional diagnostic purposes, less expensive, conveniently portable systems are now available, such as the device manufactured by Xomed (Jacksonville, Florida). In its basic form, this EMG unit provides only auditory information and single-channel recording; but it can be connected to a computer to provide a visual display. Such compact devices are also valuable for bedside, in-patient testing of patients who have laryngeal trauma in whom differentiation between arytenoid injury and vocal fold paralysis is necessary. They are especially convenient during evenings and weekends, when formal EMG laboratory facilities may not be available. Another cost-effective option for otolaryngologists is the use of the brainstem-evoked response (ABR) audiometer found in many offices. Most ABR units can be used (sometimes with minor modifications) for single-channel EMG recordings. Although such devices can be used for specific clinical indications when formal EMG cannot be performed, they should be used in addition to, not in place of, a sophisticated, multichannel EMG system for diagnostic testing.
Electrodes
The flow of current in biologic tissues occurs as a result of the movement of ions. In electronic systems, it is caused by the movement of electrons. The conversion of ionic activity into electron movements occurs at the electrode–tissue interface, using electrodes that conduct electricity well, these may include surface or needle electrodes.
Surface electrodes are placed on the skin or mucosa and do not penetrate the surface. Although they are noninvasive, they are the least selective electrode type. Surface electrodes are used in the study of nerve conduction velocity and neuromuscular transmission. The potential that is recorded represents the sum of all individual potentials produced by the nerve or muscle fibers that are activated. These electrodes are not suitable for recording details of electrical events associated with individual motor units.
There are several types of needle electrodes: monopolar, bipolar, concentric, hooked wire, and single-fiber ( Figs. 1 and 2 ). The concentric needle electrode consists of a hollow steel needle; a silver, steel, or platinum wire runs through the needle, which is insulated fully except at the tip. The potential difference between the outer shaft of the needle, which serves as a local reference electrode, and the tip of the wire is measured by connecting it to one side of the differential amplifier. Because the electrode cannula acts as a shield, the electrode has directional recording characteristics that are controlled by the angle and position of the bevel. Simple rotation of the electrode may alter significantly which individual motor units are recorded.
The monopolar needle electrode is a solid stainless steel needle that is insulated except at its tip. The recording area from this electrode is circular. Potentials therefore tend to be larger and longer and have more phases than those recorded with concentric needle electrodes, primarily because more muscle fibers are within the zone of detection and there is less cancellation because of potentials being recorded from the cannula of the electrode. The reference electrode is at a remote location on the body and may be a surface electrode.
The bipolar electrode is a hollow needle containing two platinum wires, each of which is insulated except at its tip. The outer shaft is grounded and the two internal wires are each connected to one side of the differential amplifier so that the potential difference between the two wires is measured. The recording range of the bipolar electrode is restricted to the area between the two wires within the shaft, which makes it unsatisfactory for many routine clinical purposes. The potentials are shorter and lower in voltage than those recorded with concentric needle electrodes .
With single-fiber EMG, a fine wire that is capable of recording a single muscle fiber action potential is embedded at the tip of a needle shaft that acts as the reference electrode. A hooked wire electrode is completely insulated except at the tip, which is hooked. A needle is used to insert the electrode. When the needle is withdrawn, the hook on the end of the wire acts as a barb, stabilizing the position of the electrode in the muscle. Obviously, these electrodes cannot be repositioned once they have been placed, but they bend easily and can thus be withdrawn without difficulty. Hooked wire electrodes are extremely well tolerated and can be left in place for long periods of time (hours, or even days).
Technical considerations
The authors use percutaneous monopolar needle electrodes routinely. The patient is placed in the supine position, with the neck extended. Because the procedure is generally not very painful, and because local anesthesia may alter results (especially in the cricothyroid muscle), local anesthesia is not used. A surface electrode is used as the ground electrode, and a reference (also surface) is placed on the cheek. For diagnostic purposes, routinely we test cricothyroid, thyroarytenoid, and posterior cricoarytenoid muscles. In some cases, additional muscles are tested also. If there are questions regarding hysteria, malingering, or synkinesis, simultaneous recordings of abductors and adductors are obtained. In cases of laryngeal dystonia, electrical recordings may be coordinated with acoustic data. Blitzer and colleagues observed that the normal delay between the onset of the electrical signal and the onset of the acoustic signal (0–200 milliseconds) can be increased to a delay of 500 milliseconds to 1 second in patients who have spasmodic dysphonia .
After cleaning the skin with alcohol, the needle electrode is inserted into the muscle belly. The cricothyroid (CT) notch is the anatomic reference for needle insertion. To locate the CT notch, the patient’s neck is extended and the cricoid cartilage is identified. Immediately above the cricoid cartilage is a small depression, which is the CT notch (also known as the CT space) and the CT membrane region. The CT notch may be difficult to find in obese patients or those who have had a tracheotomy. The CT muscles are evaluated by inserting the needle approximately 0.5 cm from the midline and angled laterally 30° to 45° ( Fig. 3 ). The needle first passes through the sternohyoid muscle. The CT muscle is approximately 1 cm deep. To validate the position of the electrode, the patient is asked to phonate /i/ at a low pitch and then asked to raise the pitch. If the electrode is in a normal CT muscle, the EMG activity increases sharply.