Electromyography

Japanese: 筋電図

Electromyogram (2)
a. Needle electromyography
i) Purpose This test involves inserting needle electrodes connected to an electromyograph into the muscle and recording muscle fiber discharges at rest and during voluntary contraction to determine the pathology of motor neurons, motor nerve fibers, and muscle tissue.
ii) Principle A single anterior horn motor neuron and the group of muscle fibers it controls are called a motor unit (MU). Muscle tissue is composed of many MUs, and the muscle fibers controlled by each MU are scattered in a mosaic pattern within the muscle. The sum of the electrical potentials of the muscle fibers controlled by a single motor neuron resulting from an impulse is called the motor unit potential (MUP) (Figure 15-4-4). During voluntary movement, a small number of MUs are mobilized during weak contractions, while a large number of MUs are mobilized during strong contractions, and these MUPs are recorded as electromyograms. Needle electromyography is a test that infers the pathology of motor neurons, motor nerve fibers, and muscle tissue from the presence or absence of spontaneous discharges at rest, as well as changes in the shape and mobilization pattern of the MUP.
iii) Methodology A coaxial needle electrode is used for the standard test. This is a syringe needle with an insulated inner wall, with a conductor wire of about 0.1 mm in diameter enclosed inside, with the tip exposed as the active electrode. Muscle fiber discharges within a range of about 1 mm around the active electrode are recorded. The test is performed in three stages: (1) at rest, (2) during weak contraction, and (3) during strong contraction.
iv) When interpreting findings:
In healthy people, when they are in a relaxed state, there is no muscle discharge (silent). However, the following a) and b) can be induced depending on the movement and position of the needle tip inserted into the muscle.
a) Insertion activity: A transient electrical potential lasting several tens of milliseconds observed when the needle tip penetrates the fascia and is inserted into the muscle. No abnormalities were observed.
b) Endplate noise and nerve potential: These are seen when the needle tip touches the neuromuscular junction. The former is a noise-like low-voltage sustained high-frequency potential, and the latter is a short-duration negative spike. No abnormalities are observed.
c) Denervation potential (Figure 15-4-5): A pathological potential emitted by denervated muscle fibers, it is an important indicator of progressive motor nerve degeneration. There are two types: fibrillation potential (muscle fiber potential) and positive sharp wave. The former is a spike-like potential similar to b), but can be differentiated by having an initial positive phase. Denervation potentials can also originate from muscle fiber fragments, and appear in myopathic diseases such as glycogen storage disease, myositis, and Duchenne muscular dystrophy.
d) Fascicle potential: A spontaneous MUP observed in association with muscle fasciculation contractions. Although it may also be observed in healthy individuals, high amplitude, multiphasic, and long duration fascicle potentials are characteristic of amyotrophic lateral sclerosis.
e) Myokimic potential: Spontaneous repetitive discharges of MUP populations, often resulting from ectopic discharges in peripheral nerves. It is also seen in tetany attacks.
f) Myotonic discharge: A spontaneous repetitive discharge that gradually increases and decreases in amplitude and frequency, seen in myotonic diseases including myotonic dystrophy. A dive-bomber sound is heard from the speaker of the electromyograph.
g) Complex repetitive discharge: High-frequency repetitive discharge similar to myotonic potentials, but it does not gradually increase and decrease, but begins and stops suddenly. It is thought to be due to pathological short-circuiting between muscle fibers. It is often seen in muscle diseases such as myositis and motor neuron diseases.
2) Weak contraction:
Individual MUPs are recorded separately using isometric weak contractions. Multiple MUPs can be observed by changing the position of the needle tip while performing the procedure. Normal limb muscle MUPs are 1 to 3 mV, last for a few milliseconds, and are often triphasic or less, as shown in Figure 15-4-4.
a) Polyphasic motor unit potential (polyphasic MUP): An abnormal MUP with 5 or more phases. Those seen in muscle diseases are accompanied by a decrease in amplitude and shortened duration (Figure 15-4-6, top), and are low amplitude spike-like potentials. In neurogenic diseases, the shape of the MUP is a combination of a normal type MUP and a muscle fiber reinnervation potential caused by regenerating nerves.
b) High amplitude MUP (giant MUP) (Figure 15-4-6 bottom): This refers to a high amplitude MUP exceeding 5 mV. It is often the result of increased synchronization of regenerating fiber conduction within polyphasic MUP and is seen in neurogenic diseases. It becomes larger the more denervation and reinnervation are repeated.
3) During strong contraction:
In healthy individuals, MUPs are gradually recruited as contraction becomes stronger, and at maximum contraction, an interference pattern is formed in which individual MUPs are indistinguishable.
a) Poor recruitment pattern of MUPs: In neurogenic diseases, the number of MUs is reduced, so even if voluntary contraction is strengthened, new MUP recruitment is limited. Therefore, interference waves are not easily formed (Figure 15-4-7 left). Poor recruitment of high-amplitude potentials is called a neurogenic finding.
b) Early recruitment pattern of MUPs: In myogenic diseases, individual MUs are weak, so many MUPs are recruited even during weak contractions. Early recruitment of low-amplitude spike-like MUPs due to myogenic changes forms extremely fine, excessive interference waveforms (Figure 15-4-7, right), and is called a myogenic finding.
b. Other electromyography techniques
i) Single fiber electromyogram (SF-EMG) This is a method to observe the electrical potential of muscle fibers in the same MUP separately. It is mainly used to measure the jitter of individual muscle fiber excitation in neuromuscular junction disorders.
ii) Surface electromyogram
This test records the muscle activity of multiple muscles using surface electrodes attached to the skin directly above the target muscles, and examines the interrelationships between muscle contractions. It is mainly used to analyze involuntary movements. [Masayuki Baba]

Figure 15-4-4
Motor unit potentials (MUPs)

Figure 15-4-4

Figure 15-4-5
Positive sharp waves and fibrillation potentials ">

Figure 15-4-5

Figure 15-4-6
Low amplitude polyphasic potentials (top two rows) and giant potentials (bottom three rows)

Figure 15-4-6

Figure 15-4-7
MUP Mobilization Test Poor (left) and excessive interference ">

Figure 15-4-7

Source : Internal Medicine, 10th Edition About Internal Medicine, 10th Edition Information

Japanese:

筋電図（electromyogram）（2）
a. 針筋電図検査（needle electromyography）
　i）目的
　筋電計に接続した針電極を筋内に刺入し，安静時と随意収縮時の筋線維放電を記録して，運動ニューロン，運動神経線維，筋組織の病態を知る検査である．
　ii）原理
　1個の前角運動ニューロンとそれに支配される筋線維群を運動単位（motor unit：MU）とよぶ．筋組織は多数のMUから構成され，個々のMU支配筋線維は筋内にモザイク状に散在する．1個の運動ニューロンのインパルスから生じた支配下筋線維電位の総和を運動単位電位（motor unit potential：MUP）（図15-4-4）とよぶ．随意運動では弱収縮では少数の，強収縮では多数のMUが動員され，そのMUPが筋電図として記録される．安静時自発放電の有無，ならびにMUPの形状変化と動員様式の変化から，運動ニューロン，運動神経線維，筋組織の病態を推察する検査が針筋電図検査である．
　iii）方法
　標準的検査には同心針電極（coaxial needle）を用いる．これは内壁を絶縁した注射針に直径0.1 mmほどの導線を封入し，先端を活性電極として露出させたものである．活性電極の周囲約1 mm範囲以内の筋線維放電が記録される．検査は，①安静時，②弱収縮時，③強収縮時の3段階で行う．
　iv）所見の解釈時：
　健康人の場合，力を抜いたリラックス状態では筋放電がない（silent）．ただし，筋に刺入した針先の動きや位置によって次のa），b）が誘発される．
　a）刺入電位（insertion activity）：針先が筋膜を貫通して筋内に刺入されたときにみられる数十msecの一過性電位である．異常性なし．
　b）終板雑音と神経電位：針先が神経筋接合部に触れたときにみられる．前者はノイズ様の低電位持続性高周波電位，後者は持続時間の短い陰性棘波である．異常性なし．
　c）脱神経電位（denervation potential）（図15-4-5）：脱神経筋線維が発する病的電位で，進行性運動神経変性の重要な指標である．フィブリレーション電位（筋線維電位）（fibrillation potential）と陽性鋭波（positive sharp wave）の2つがある．前者はb）類似の棘波だが，初期陽性相を有することで鑑別される．脱神経電位は筋線維断片が発生源の場合もあり，糖原病，筋炎，Duchenne型筋ジストロフィ症など筋原性疾患でも出現する．
　d）筋線維束電位（fasciculation potential）：筋線維束性攣縮に伴ってみられる自発性MUPである．健常者でもみられる場合があるが，高振幅，多相性，長持続時間の筋線維束電位は筋萎縮性側索硬化症の特徴である．
　e）ミオキミア電位（myokimic potential）：MUP集団の自発性反復放電で，多くは末梢神経の異所性放電に由来する．テタニー発作などでもみられる．
　f）ミオトニー電位（myotonic discharge）：振幅・周波数が漸増漸減する自発性反復放電で，筋強直性ジストロフィ症を含むミオトニー疾患にみられる．筋電計のスピーカーから急降下爆撃音（dive-bomber sound）が聴かれる．
　g）複合反復放電（complex repetitive discharge）：ミオトニー電位類似の高周波反復放電だが漸増漸減せず，突然始まり突然止まる．筋線維間に生じた病的短絡によると推定される．筋炎などの筋疾患や運動ニューロン疾患でしばしばみられる．
2）弱収縮時：
　等尺性弱収縮で個々のMUPを分別記録する．刺入した針先の位置を変えながら施行すれば，複数のMUPを観察できる．正常四肢筋MUPは，図15-4-4のように，1～3 mV，持続時間数msecで，3相性以下が多い．
　a）多相性運動単位電位（polyphasic MUP）：5相性以上の異常MUPである．筋疾患でみられるものは，振幅低下と持続時間短縮を伴い（図15-4-6上），低振幅棘波様電位（low amplitude spiky MUP）である．神経原性疾患では通常型MUPに再生神経による筋線維再支配電位が加わった形状となる．
　b）高振幅電位（high amplitude MUP）（巨大電位，giant MUP）（図15-4-6下）：5 mVをこす高振幅MUPを指し，多くは多相性MUP内の再生線維伝導の同期化が進んだ結果であり，神経原性疾患でみられる．脱神経と再支配を繰り返すほど巨大になる．
3）強収縮時：
健常者では，収縮を強めるにつれてMUPが徐々に動員され（recruitment），最大収縮時，個々のMUPが識別不能の干渉波形（interference pattern）が形成される．
　a）MUP動員不良所見（poor recruitment pattern）：神経原性疾患ではMU数減少があるため，随意収縮を強めても新たなMUP参入が限られる．したがって，干渉波が形成されにくい（図15-4-7左）．高振幅電位の動員不良所見を指して神経原性所見とよぶ．
　b）MUP早期動員所見（early recruitment pattern）：筋原性疾患では個々のMUの筋力低下があるため，弱収縮に際しても多数のMUPが動員される．筋原性変化による低振幅棘波様MUPの早期動員は，極度に細かな干渉過多波形を形成し（図15-4-7右），筋原性所見とよばれる．
b.その他の筋電図手法
i）単一線維筋電図
（single fiber electromyogram：SF-EMG）　同一MUP内の筋線維電位を分離観察する手法である．おもに神経筋接合部疾患で個々の筋線維興奮のばらつき（jitter）を測定するために行われる．
　ii）表面筋電図（surface electromyogram）
　目的筋直上の皮膚に添付した表面電極によって複数筋の筋活動を記録し，筋収縮の相互関係をみる検査である．おもに不随意運動の分析に用いられる．［馬場正之］

図15-4-4
運動単位電位（MUP）">

図15-4-4

図15-4-5
陽性鋭波とフィブリレーション電位">

図15-4-5

図15-4-6
低振幅多相電位（上2 段）と巨大電位（下3 段）">

図15-4-6

図15-4-7
MUP 動員検査不良（左）と干渉過多">

図15-4-7

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