Fig. 1.1
For EMG-guided injection of botulinum toxin, the needle has to be hollow. The shaft of the needle is insulated except the tip. For a bipolar recording, the reference electrode (often a surface electrode) should be placed at a myoelectric inactive location of the body, e.g., the manubrium sterni
Fig. 1.2
The concentric needle electrode consists of a hollow steel needle with a steel, silver, or platinum wire running through the needle, which is fully insulated except for the tip
Comparing monopolar with concentric needle electrodes, potentials recorded by monopolar needle electrodes tend to be larger and longer with more phases than those recorded with concentric needle electrodes. This difference is based on the existence of more muscle fibers within the zone of detection, as well as on fewer cancelations due to potentials being recorded from the cannula of the electrode. The bipolar concentric electrode is sometimes called bifilar or double concentric. It is a hollow needle containing two platinum wires, each of which is insulated except for 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 concentric electrode is restricted to the area between the two tips of the wires within the shaft, which makes it too small for many routine clinical purposes. The potentials are shorter and lower in the voltage than those recorded with concentric needle electrodes [7]. For single-fiber EMG, a fine wire capable of recording a single muscle fiber action potential is embedded at the tip of the needle shaft acting as reference.
1.3.3 Hooked-Wire Electrodes
The other common type of indwelling electrodes used in LEMG is the hooked-wire electrode (Fig. 1.3a). This electrode consists of two separate fully isolated fine wires except of the hooks, each with a maximum diameter of 50 μm often made of platinum. They are inserted into the muscle after having been threaded through a hollow-core needle [8]. For reaching the larynx, a curved (Fig. 1.3b) and a strait application tool (Fig. 1.3c) is available, depending if the patient is awake in a sitting position or lying during a suspension laryngoscopy. When the needle is withdrawn, the hooks on the end of the wires act as a barb, stabilizing the position of the electrodes within the muscle. These electrodes cannot be repositioned once placed but, as they bend easily, can be removed smoothly. The field of recording can vary during measurement, depending on the distance between the bared wires in the moving muscle [8]. The advantage of hooked-wire electrodes is that they are well tolerated after correct placement, so that they can be left in place for longer periods of time than needle electrodes.
Fig. 1.3
(a) Hooked-wire electrodes are indwelling electrodes often used in LEMG. These electrodes consist of two separate fully isolated fine wires except of the hooks, each with a maximum diameter of 50 micrometers often made of platinum. They are inserted into the muscle after having been threaded through a hollow-core needle. The green rubber band holds back the hooks only for transport. (b) For reaching the larynx in an awake patient in a sitting position, a curved application tool is useful. (c) For reaching the larynx during a suspension laryngoscopy, a strait application tool is ideal
1.3.4 Selection of Electrode Type
For comparative or quantitative evaluation, the same electrode type must be used when obtaining reference values, due to the previously mentioned differences between electrode types. Filter settings must also be consistent. In general, monopolar electrodes pick up potentials from a larger spherical region than concentric electrodes [9].
For the transcutaneous approach to the laryngeal muscles, it is recommended to use concentric needle electrodes, providing a uniform field for motor unit action potential (MUAP) waveform analysis. They should have a length of 50 mm in order to reach even the posterior cricoarytenoid muscle in all patients and a thickness of at least 0.45 mm to provide rigidity for precise maneuvers behind the cricothyroid ligament [10]. The transoral approach with bipolar hooked wires allows an exact endoscopic controlled positioning into the target muscles and is useful for recording over a longer period of time or for recording multiple muscles simultaneously while the patient performs different tasks [11].
1.3.5 EMG Amplifier
Most clinical EMG amplifiers can be used with the differential configuration to obtain a cleaner signal: Two active electrode contacts are placed on the muscles being examined. Furthermore, a reference electrode is placed on the body. To reduce the risk of electrical injury, the patient must be grounded. The signal of interest is amplified as the difference of potentials (voltage) between each active electrode and the reference electrode. The frequencies of a muscle action potential range between 2 Hz and 10,000 Hz, and the frequency band of the electromyography machine is typically set from 10 Hz to 10,000 Hz.
In most settings, the electrodiagnostic signal can be heard in real time on a loudspeaker. Connected computers can then display the signals on-screen, as well as process, analyze, and store them on hard disks. In addition, electrical stimulators are incorporated into the system so that it cannot only record the electric activity of muscles but also stimulate muscles or nerves depending on the chosen parameters. For basic examinations of LEMG, a one-channel EMG amplifier is suitable. If the activity of several muscles is to be compared in synchrony, a multichannel EMG amplifier is required. Not only EMG activity of muscles can be recorded on additional channels but also behaviorally or physiologically relevant signals like voice production or respiration activity. This is essential for offline interpreting stored data afterward.
1.3.6 Microphone
When a microphone is connected to a channel of a multichannel EMG amplifier, a synchronic voice recording is possible. This allows a comparison of myoelectric activity with the patient’s sound production (Fig. 1.4). Additionally, the recorded instructions and comments of the examiner can be helpful for interpretation of the stored data. The upper filter cutoff frequency of the microphone should be set to half the sampling rate of the system.
Fig. 1.4
Three-channel synchronic recording of TA activity by needle EMG, phonation by microphone, and respiration by the thorax belt. During phonation, good to detect in channel 2 via microphone, there is strong EMG activity in channel 1 recorded in the thyroarytenoid muscle. [Courtesy of Dr. med. Gerd Fabian Volk and www.lemg.org]
1.3.7 Thermistor and Piezoelectric Thorax Expansion Belt
Respiration is another fundamental parameter of voice production. It can be measured by placing a thermistor in front of a patient’s nose or mouth. A thermistor detects the changes of air temperature caused by every breath and can thus be used to record respiration activity. Alternatively, the movement of the chest wall and/or the upper abdominal wall can be monitored by using thorax and/or abdomen expansion belts with piezoelectric sensors. This movement is related to effort and produces a low-frequency sinusoidal waveform as the patient inhales and exhales. For both the piezoelectric expansion belt and for the thermistor, the lower cutoff frequency should be set to 0.01 Hz (eliminating a possible drift of the signal) and the higher frequency from 10 Hz to 30 Hz for smoothing the signal.
1.3.8 Indirect Laryngoscopy
The vocal fold motion status can be determined before, during, and after the LEMG with transnasal flexible laryngoscopy. Thus, potential swellings or bleedings can be detected in time. The use of a rigid laryngoscope with magnification is recommended for placing hooked-wire electrodes.
1.4 EMG Monitoring and Documentation
Recording an EMG from the laryngeal muscles is technically difficult, even though suitable methods are well described in literature since the 1970s [12]. Although otolaryngologists who regularly inject botulinum toxin into thyroarytenoid muscles for the treatment of laryngeal dystonia/spasmodic dysphonia are skilled in performing LEMG, only few have had training in interpreting MUAP signals. And because of the neurophysiological differences between the EMG of big muscles and of muscles of the larynx, even well-trained neurophysiologists can have difficulties in interpreting LEMG. Thus, we recommend that a laryngologist and an experienced user with clinical neurophysiology training should work as a team to gain experience with the specific attributes of laryngeal muscles.
1.5 Requirements and Preparations for LEMG
Laryngeal electromyography is a medical procedure requiring active cooperation by the patient. Thus, not only oral consent but also a detailed explanation of the intended procedure and expected responses is required. The patient’s ability and willingness to cooperate has to be considered when planning an LEMG examination.
In addition, a detailed examination of the head and neck, including indirect or fiberoptic laryngoscopy and/or videostroboscopy, is important to detect altered anatomy, previous operations, or acute infections—conditions which can potentially hinder the electrode placement. Bleeding disorders caused, for example, by coagulopathy, warfarin, or acetylsalicylic acid are relative contraindications. Due to these rare but possible complications, emergency equipment including a cricothyrotomy set must be readily available. As with most medical procedures, LEMG should be carried out only if the medical information expected to be gained justifies the low risk of complications.
Special caution must be exercised in patients with narrow glottic gap due to bilateral vocal cord paralysis. In these cases, LEMG of the vocalis muscle should be performed only on one side at a time. During this test, the respiratory function of the patient must be monitored constantly to prevent breathing problems due to the LEMG. Electric stimulation studies should not be performed on patients with pacemakers without previous and subsequent cardiologic consultation.
For the LEMG, the patient is asked to adopt a comfortable position with a slightly reclined head. If necessary, the larynx can be anaesthetized locally with lidocaine, either applied transorally to the mucosal surface or with a transcutaneous intraluminal injection. For transcutaneous insertion with a concentric needle, the patient can be either sitting or supine, depending on the patient’s needs and the examiner’s experience. Hooked-wire electrodes are inserted transorally under visual control with an indirect laryngoscope. Thus, a sitting position of the patient is recommended. A detailed comparison between a transcutaneous and a transoral LEMG can be found in Table 1.1.
Table 1.1
Comparison between transcutaneous and transoral LEMG
Transcutaneous | Transoral |
|---|---|
Inexpensive needles; one needle can be used for several muscles in one patient | Expensive hooked-wire electrodes; one electrode can be inserted only once in one muscle |
Constant signal quality | Variable signal quality because tips of hooked-wire electrodes can move inside the muscle during activation |
Position of needle can be corrected | Position of hooked-wire electrodes cannot be changed after insertion |
Simultaneous measurements of several muscles are difficult due to the same approach of the needles to the laryngeal muscles | Multichannel measurements of several muscles possible |
Placing the electrodes is guided by palpation, acoustic, and visual EMG feedback. Visual control with an endoscopy is possible, but only rarely necessary, e.g., when controlling the mucosal penetration or the transluminal PCA EMG to test stimulation. Transcutaneous EMG in combination with endoscopy is often stressful for the patient | Placing the electrodes occurs under direct visual control, but assessment of signal quality is possible only after removal of the application device. Thus, correction is only possible by discarding the electrodes and reinserting another pair |
1.6 Insertion into the Laryngeal Muscles
1.6.1 Transcutaneous Insertion of the Needle Electrodes
At the beginning of the examination, main structures of the anterior neck are palpated to identify the midline, the cricoid cartilage, the lower border of the thyroid cartilage, the thyroid notch, and the hyoid bone. Figure 1.5 is a schematic drawing of the laryngeal cartilage, the laryngeal muscles, landmarks, and virtual planes of orientation between the landmarks. In obese patients or after neck surgery, palpating these landmarks can be difficult. With ultrasound, the landmarks can be localized and marked with a felt pen. If local anesthesia is used, excessive injections should be avoided to allow continued palpation of structures after injection. If the patient has a tracheotomy, it is usually necessary to remove the tracheotomy tube to be able to place the needle. If the patient does not tolerate a short-term removal, a nasal speculum may dilate the tracheostomy and maintain the airway open during the examination.
Fig. 1.5
Schematic representation of the laryngeal cartilage, laryngeal muscles, landmarks, and virtual planes of orientation. (a) Fronto-superior view; (b) inner view of the left hemilarynx: M, thyroid notch; N, center of inferior border of the thyroid cartilage; O, center of the anterior arc of the cricoid cartilage; [Sigma], midsagittal plane containing M, N, and O; [Omega], plane lying at right angle to [Sigma] containing M and N; [Phi], plane lying at right angle to [Sigma] containing N and O; [Pi], plane perpendicular to [Omega] and [Sigma]; [Delta], plane perpendicular to [Phi] and [Sigma]. (c) Superior view with laryngeal muscles: CT, right cricothyroid muscle; TA, right thyroarytenoid muscle; IA, right and left interarytenoid muscles (the muscles of the two sides intertwine); LCA, right lateral cricoarytenoid muscle; PCA, right posterior cricoarytenoid muscle
It is recommended to begin the LEMG with the thyroarytenoid muscle (TA) (Fig. 1.6), followed by an examination of the posterior cricoarytenoid (PCA) muscle (Fig. 1.7) if there is a good level of tolerance. Both muscles are innervated by the inferior recurrent laryngeal nerve (ILN). Finally, the needle should be inserted into the cricothyroid muscle (CT) (Fig. 1.8) (Table 1.2) to examine the function of the superior laryngeal nerve (SLN).
Fig. 1.6
EMG approaches to TA, LCA, and IA. Left fronto-superior view, right inner view of left hemilarynx
Fig. 1.7
EMG approaches to PCA. Upper row midline approach, left fronto-superior view, right inner view of left hemilarynx, lower row lateral approach, left superior view, right left lateral view
Fig. 1.8
EMG approach to CT. Left superior view, right lateral view of left cricoid
Table 1.2
Implementation of laryngeal EMG
Muscle | Transcutaneous needle EMG | Transoral EMG | Identification |
|---|---|---|---|
Thyroarytenoid (TA) innervated by the inferior recurrent laryngeal nerve (ILN) | Pierce the skin in midline in sagittal direction. Penetrate the cricothyroid ligament directly under the lower border of the thyroid cartilage in an angle of 30° laterally and 15° superiorly. Alternatively, to reduce the risk of entering the airway, the cricothyroid ligament can be penetrated around 5 mm laterally to the side to be examined. In this case, the needle should be angled 20° laterally and 15° superiorly. After 15 mm behind the ligament, the thyroarytenoid muscle should be reached | Spray lidocaine transorally onto the mucosa of the soft palate, the pharynx, and the larynx to decrease the pharyngeal reflex. Inject glycopyrrolate i.v. to reduce salivation. Visual control of insertion by flexible fiberoptic or rigid laryngoscopy or directly in the operating room. Hooked-wire electrodes are inserted laterally to the free edge of the vocal fold by using the curved application device by INOMED©. If the needle is placed too laterally, recording the activity of the lateral cricoarytenoid muscle (LCA), EMG activity increases and drops rapidly during phonation | Validation of position by phonating /a:/ or holding breath in a glottal stop. Both procedures increase EMG activity strongly and sustainably. Swallowing causes a short, strong thyroarytenoid activity during the glottal stop. While breathing deeply in and out, the resting activity drops periodically during expiration. If the needle is placed too laterally and records the activity of the lateral cricoarytenoid muscle (LCA), EMG activity increases and rapidly drops during phonation |
Posterior cricoarytenoid (PCA) innervated by the inferior recurrent laryngeal nerve (ILN) | Midline approach: Pass a needle posteriorly through the cricothyroid membrane, airway, and cricoid cartilage. Only possible in non-ossified larynges. Insertion of the electrode too cranially is a common reason for difficulties in locating the PCA. Pierce the skin and penetrate the cricothyroid ligament in midline in sagittal direction. A burst of sine waves modulated by phonation indicates that the electrode tip is vibrating freely in the airway. Approximately 5–10 mm lateral to the midline, the posterior mucosa of the airway is penetrated. After drilling through the lamina of the cricoid cartilage by slowly rotating the needle, the tip of the needle should be right in the muscle. Pushing any further will cause a penetration of the cricopharyngeal muscle Lateral approach: Rotate the larynx manually and pass the electrode posterior to the thyroid lamina. This requires a good mobilization of the whole larynx during the procedure, which can cause discomfort to the patient | Spray lidocaine transorally onto the mucosa of the soft palate, the pharynx, and the larynx to decrease the pharyngeal reflex. Inject glycopyrrolate i.v. to reduce salivation. Visual control of insertion by flexible fiberoptic or rigid laryngoscopy or direct laryngoscopy in the operating room. Insert the hooked-wire electrodes from the pharyngeal side 10 mm caudally to the interarytenoid area by using the curved application device by INOMED©. If the electrode is inserted too cranially, the arytenoid muscle instead of the cricoarytenoid muscle will be recorded. If the needle is placed too caudally, the activity of the cricopharyngeal muscle will be recorded | Position is confirmed through detection of increased EMG activity during sniffing and with a considerably weaker EMG activity during swallowing and phonation of the sound /a:/. If the needle is placed too dorsally (by transcutaneous insertion) or too caudally (by transoral insertion), the activity of the cricopharyngeus muscle is recorded. The strong constant EMG activity of the cricopharyngeus muscle will decrease during swallowing |
Cricothyroid (CT) innervated by the superior laryngeal nerve (SLN) | 5 mm insertion lateral to the midline on level of the cricothyroid notch. Movement tangential to the cricothyroid membrane (around 50°) until the electrode enters the muscle after 15–20 mm. If no activity is recorded, the needle must be withdrawn a few millimeters and aimed more laterally. Too laterally, the needle will pierce the sternohyoid muscle | Not possible | To validate the position of the electrode, the patient is asked to phonate /a:/ at a low pitch and then to raise the pitch. If the electrode is in a normal cricothyroid muscle, the EMG activity will increase sharply. The activity of the sternohyoid muscle can be tested by raising the head of the lying patient or pressing your fingers against his forehead if patient is sitting |
1.6.2 Transcutaneous Thyroarytenoid Muscle Recording
The cricothyroid notch, also known as cricothyroid space or cricothyroid membrane region, is the anatomic reference for the needle insertion. First, the needle pierces the skin in midline in sagittal direction and will be placed directly under the lower border of the thyroid cartilage (N). Second, the needle tip will be angled laterally (30° to [Sigma]) and superiorly (15° to [Pi]) and penetrates through the cricothyroid ligament without entering the airway (Fig. 1.6). It is also possible to place the needle tip around 5 mm laterally of the midline. In this case, the needle should be angled 20° laterally and 15° superiorly. Depending on the thickness of the neck and the entry angle, the thyroarytenoid muscle should be reached after pushing the needle 15 mm through the ligament. Coughing of the patient generally indicates that the needle has penetrated the airway and is causing irritation of the mucosa. A burst of sine waves modulated by phonation also indicates that the electrode tip has entered the airway. In both cases, the needle should be withdrawn to the cricothyroid ligament and reinserted more laterally.
The position of the needle is confirmed by asking the patient to say /a:/ (“aa”) or holding his breath by a glottal stop (Table 1.3). During both procedures, EMG activity sharply and sustainably increases. Also swallowing causes a short strong thyroarytenoid activity during the glottal stop. While deeply breathing in and out, the resting activity drops periodically during expiration. If the needle is placed too laterally and records the activity of the lateral cricoarytenoid muscle (LCA) (Fig. 1.6), EMG activity increases and rapidly drops during phonation (Table 1.3). Viewed from the cricothyroid space, the TA and LCA are nearly on the same axis and in direct contact to each other. It is thus not always possible to reliably differentiate between these two muscles. However, this is not necessary for most clinical questions.
Table 1.3
Identification of laryngeal muscles
Muscle | Physiological reaction/identification features |
|---|---|
Thyroarytenoid (TA) | – Patient phonates /a:/: a sudden sustained increase in EMG activity while phonating – Forced sniffing inspiration: decrease in EMG activity – Swallowing: during swallowing, short maximal activity with decrease in activity at the end of the swallowing |
Posterior cricoarytenoid (PCA) | – Forced sniffing inspiration: increase in EMG activity – Patient phonates /a:/: a decrease in EMG activity while phonating – Swallowing: during swallowing, short maximal activity with decrease in activity at the end of the swallowing |
Lateral cricoarytenoid (LCA) | – Patient phonates /a:/: a short initial increase in EMG activity followed by a decrease while phonating – Swallowing: during swallowing short maximal activity with decrease in activity at the end of the swallowing |
Cricothyroid (CT) | – Patient phonates a low note on /a:/ and then increases pitch: with increasing pitch, the interference patterns become dense |
Cricopharyngeal part of inferior pharyngeal constrictor | – Constant resting tone; during swallowing decrease in activity; no respiration-dependent activity |
Using a fiberoptic scope, moving the needle within the substance of the thyroarytenoid muscle and observing the vocal fold movement confirms the position of the electrode. This procedure may cause the patient to swallow or cough.
1.6.3 Transcutaneous Posterior Cricoarytenoid Muscle Recording
The posterior cricoarytenoid (PCA) muscle can be accessed by passing a needle posteriorly through the cricothyroid membrane, airway, and cricoid cartilage or by rotating the larynx manually and passing the electrode posteriorly to the thyroid lamina (Fig. 1.7) (Table 1.2). The lateral approach requires a good mobilization of the whole larynx during the procedure, which can cause discomfort to the patient. The midline approach is only successful in not totally ossified cricoids. However, even in older patients, the cricoid cartilage is rarely completely ossified. Insertion of the electrode too cranially is a common reason for difficulties in locating the PCA. This is often due to the fact that the posterior cricoarytenoid muscle lies more caudally than assumed by many physicians.
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