Electrodiagnostic studies are essential tools for the assessment of patients with neuromuscular disorders. These studies, along with clinical examination, help diagnose and manage nerve, muscle, and neuromuscular junction diseases. Ultrasound has provided additional anatomical and diagnostic information in selected clinical presentations.
Electrodiagnostic studies include nerve conduction studies, repetitive nerve stimulation, late responses (F response and H reflex), needle electromyography, and other specialized examinations. These tests are valuable additions to the clinical examination and should be planned accordingly based on the physician’s clinical assessment.
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• Nerve conduction studies in conjunction with EMG can be used to objectively confirm the presence of a lesion, characterize the underlying pathophysiology, and determine the degree of severity.
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• Late responses (F waves, A waves, H reflex, and blink reflex) help determine the presence of neuropathy and axonal damage and suggest the presence of an acoustic neuroma or a demyelinating polyneuropathy.
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• Needle EMG provides information that cannot be obtained through nerve conduction studies alone about the function and distribution of muscle fibers and motor units.
Historical note and terminology
Nerve conduction studies and EMG are well-established electrophysiological techniques that have been instrumental in diagnosing neuromuscular disorders. Their early development has been closely linked to the discovery of electricity.
The interest in electricity increased after the Leyden jar was invented by Dean Von Kleist and Petrus van Musschenbroek in Leiden in 1745 to 1746. The Leyden jar provided physiologists a way to store and generate electricity in a reliable manner. It has been considered the precursor of current-day capacitors or condensers.
Luigi Galvani (1737-1798), a professor of anatomy at the University of Bologna in Italy, was dissecting a frog while other colleagues in the lab were playing with the Leyden jar. When Galvani touched the frog’s nerve with his metal knife, a spark thrown from the conductor provoked a violent convulsion of the frog’s body. He initially thought that the mechanical nerve stimulation made the frog shake, but the contraction never occurred with repeated stimulations. Galvani believed that the electricity was generated by the body and conducted through the nervous tissue. He used the term “animal electricity” to explain this phenomenon.
Alessandro Volta (1755-1832) doubted the biological nature of the phenomenon and emphasized physics. He considered the current generated in Galvani’s experiment was essentially flowing between two different metals. Volta showed the muscle twitching was elicited if the metal arc connecting the muscle and the nerve was comprised of two different metals.
In 1794, Galvani and his nephew Aldini showed that a muscle twitch could happen even with no metal involved if he brought the cut end of the sciatic nerve into contact with leg muscles.
Galvani’s work was overshadowed by Volta’s when Napoleon invaded North of Italy in 1796.
In 1838, Carlo Matteucci revived the concept of “ animal electricity” with modification of the experiment. He used a complete frog’s leg, severed below the knee, with only the isolated nerve above it. He noted the ability of the frog’s muscle contraction to stimulate the nerve at another muscle-nerve set-up.
Guillaume Benjamin Duchenne (1806-1875) was able to demonstrate that muscles can be stimulated percutaneously. He developed the stimulators and electrodes needed for his experiments and called the technique “localized electrization.” He noted that muscles would briskly respond if stimulated by an electrical current at certain spots on their surface.
In 1861, Wilhelm Erb used faradic and galvanic currents for electrical stimulation. He identified the points at which the brachial plexus can be proximally stimulated (Erb’s point).
Edgar Douglas Adrian (1889-1977) worked with United States physiologist Detlef Bronk to measure the action potential generated from a single nerve fiber. They also used a concentric needle electrode and a loudspeaker to record the electrical activity of muscle fibers supplied by a single nerve fiber, later known as motor unit potential.
Joseph Erlanger (1874-1965) and Herbert Gasser (1888-1963) managed to graph the action potentials from a frog’s sciatic nerve. They discovered that nerve fibers with different diameters have different conduction velocities. The 3 types of nerve fibers that they proposed were type alpha (NCV 100 m/s), type beta (NCV 2 to 14 m/s), and type gamma C (2 m/s).
Derek E Denny-Brown (1901-1981) first used the term “fasciculations” for the spontaneous muscle twitching in patients with amyotrophic lateral sclerosis. He emphasized the presence of fibrillation potentials in weak muscles due to axonal degeneration and described other EMG phenomena, such as myotonia and myokymia. He explained the reduced amplitude and duration of motor unit potentials in patients with muscular dystrophy. Denny-Brown described the long-duration polyphasic motor unit potentials in patients with chronic neurogenic duration due to axonal sprouting and grouping of the reinnervated muscle fibers.
Fritz Buchtal (1907-2003) reported the currently used normative data of amplitudes and durations of motor unit potentials in different muscles at different ages. Buchtal described the different nerve conduction velocities in axonal versus demyelinating neuropathies.
Martin Glover Larrabee (1910-2003) measured the combined potential of individual muscle fibers from the surface of muscle after supramaximal stimulation of the supplying nerve percutaneously, later known as the compound muscle action potential. Larrabee’s group reported that the normal conduction velocity of the fastest nerve fibers supplying the hands and foot muscles ranges between 46 and 67 m/s.
Roger W Gilliatt (1922-1991) established normative values for median and ulnar sensory studies. Gilliatt is known for describing the clinical and electrophysiologic findings seen in true neurogenic thoracic outlet syndrome (preferential wasting of thenar, rather than hypothenar, eminence), frequently referred to as “Gilliatt hand.”
Friederich Jolly (1844-1904) described the fatigability of the orbicularis oculi muscle in myasthenia gravis patients when exposed to repetitive faradic currents. Harvey and Masland reported the repetitive nerve stimulation and positive decrement seen in myasthenia gravis patients in 1941. Bernard Katz (1911-2003) described the miniature endplate potentials, the role of presynaptic voltage calcium channels, and the quantal release of acetylcholine.
In 1957 Edward H Lambert (1915-2003) and Lee Eaton (1905-1958) described a series of patients with a disorder resembling myasthenia gravis and concurrent intrathoracic malignancy. They noted that a series of electrical stimuli resulted in a transient decrease followed by a remarkable increase in CMAPs, with similar muscle strength increases after repetitive voluntary activity. Eaton and Lambert reported that these patients did not respond to neostigmine. Dr. Lambert continued to work with his wife and colleague Dr. Vanda Lennon to further characterize the clinical, electrophysiological, and serological aspects of Lambert Eaton myasthenic syndrome.
Erik Stalberg (1936- present) isolated a reproducible sharp single-spike, sometimes multiple-spike, signal after multiple recording trials on his adductor pollicis muscle. He discovered the variation in peak latencies between the two spikes was increased in myasthenia gravis patients, calling it “jitter.” Stalberg’s revelation led to the discovery of single-fiber EMG, the most sensitive test to diagnose neuromuscular junction disorders.
James Golseth (1912-2003), with the help of physicist James A Fizzel, developed the current impulse stimulator. They collaborated with Dr. Herbert Jasper (1906-1999), who at McGill University was performing EMGs on Canadian war victims using monopolar needle electrodes; as a team, they built the prototype EMG machine in 1948 (36).
The earlier historical evolution of nerve conduction studies and EMG was coupled to the modern discovery of electricity in mid 18th century. Significant progress did not occur until the early 20th century when the equipment capable of amplifying and recording the fine bioelectric potentials was invented. As with any scientific accomplishment, the development of nerve conduction studies and EMG has been a product of human intellect, hard work, and perseverance as well as chance and politics.