Electromyography
Synonym(s): EMG
Introduction
- Electromyography (EMG) covers methods that allow objective investigation of peripheral nerve, neuromuscular junction and muscle functioning. Useful complement to neurologic examination in hypotonic paresis and paralysis and in muscle weakness.
- "Peripheral nerve" in its physiological sense includes nerve cell bodies in the brain stem, spinal cord and ganglions and nerve roots from which the peripheral nerve originates.
- Electromyographic examination is divided into several procedures. The most reliable and widely used in a clinical setting are:
- Detection (or needle) electromyography.
- Maximum motor nerve conduction velocity measurement.
- Maximum sensory/mixed nerve conduction velocity measurement.
- Repetitive motor nerve stimulation.
- In addition, ventral roots can be specifically investigated through F-wave studies.
- In detection EMG, muscles are investigated, looking for spontaneous electrical activity.
- In motor nerve conduction velocity studies, repetitive stimulations and F-wave studies, motor nerves are electrically stimulated and resulting electrical activity in their target muscles, evaluated.
- In sensory nerve conduction studies, nerves are stimulated and recordings obtained from the same nerves.
- Denervated muscle fibers undergo many biochemical and physiological changes including:
- Increase in sensitivity to acetylcholine.
- Drifts in membrane potential.
- These result in spontaneous potentials observed between 5-10 days after a nerve injury.
- Parts of muscle fibers isolated from the end plate by a muscle lesion can survive and behave like denervated muscle cells, exhibiting spontaneous electrical activity.
- In both cases, spontaneous activities can be recorded by detection EMG.
- Disease processes may affect nerve cell body or processes, myelin sheaths or both. As a general rule, the larger a nerve fiber, the faster its conduction velocity. A larger fiber also has higher metabolic requirements and usually is more susceptible to insults. Disappearance of largest fibers results in a slight maximum conduction velocity drop and a marked diminution of the amplitude of the resulting muscle potential.
- Myelin sheath damage induces dramatic conduction slowing and conduction blockade, both identifiable through nerve conduction studies.
Uses
- Diagnostic technique to investigate neuromuscular abnormalities if neurological examination is inconclusive.
- Gives picture of severity of lesion.
- May localize it more or less precisely.
- May discriminate whether nerve fibers or myelin sheaths in a nerve are most affected.
Advantages
- Highly sensitive.
- May evidence changes before they become clinically detectable.
Disadvantages
- Material expensive.
- Operator must be experienced.
- Method restricted to academic and referral institutions due to above.
- Technical problems:
- High amplifier gains require high quality grounding of the recording apparatus.
- Requires careful upkeep of electrodes and connecting wires.
- Requires skillful operation.
Alternative techniques
- No alternative for functional exploration of peripheral nervous and muscular systems although lesional information is best drawn from microscopic examination of nerve Biopsy: nerve and muscle biopsies Biopsy: muscle.
- Serum measurement of CK enzyme and antibodies against end plates may contribute to the diagnosis of myositis/myopathy Myopathies and myasthenia gravis Myasthenia gravis respectively.
Time required
Preparation
- Sedation.
- Anesthesia: 20 minutes.
Procedure
- 90-120 minutes needed for complete exploration but procedure may be tailored to individual cases and may take more/less time.
Decision taking
Criteria for choosing test
- Confirmation of, and discrimination among, peripheral nerve, nerve-muscle junction or muscle problems.
- When need refining site (local vs general, proximal vs distal etc) of nervous lesions.
Risk assessment
- No specific risk associated with the method.
- The only risk is related to the need for anesthesia. Extremely weak patients may need assisted ventilation.
- Patients with neuromuscular weaknesses, ie laryngeal paralysis and megaesophagus Megaesophagus may predispose to aspiration pneumonia Pneumonia.
Requirements
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Preparation
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Technique
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Aftercare
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Outcomes
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Further Reading
Publications
Refereed papers
- Recent references from PubMed and VetMedResource.
Motor nerve conduction velocity
- Pillai S R, Steiss J E & Wright J C (1991) Age-related changes in peripheral nerve conduction velocities of cats. Progress Vet Neurol 2 (2), 95-104 VetMedResource.
- Malik R & Ho S (1989) Motor nerve conduction parameters in the cat. JSAP 30 (7), 396-400 VetMedResource.
- van Nes J J (1986) An introduction to clinical neuromuscular electrophysiology. Vet Quart 8 (3), 233-9 PubMed.
- van Nes J J (1986) Clinical application of neuromuscular electrophysiology in the dog: a review. Vet Quart 8 (3), 240-250 PubMed.
- Farnbach G C (1980) Clinical electrophysiology in veterinary neurology. Part I: Electromyography. Comp Contin Edu Pract Vet 11, 791-797 ResearchGate.
- Sims M H & Redding R W (1980) Maturation of nerve conduction velocity and the evoked muscle potential in the dog. Am J Vet Res 41 (8), 1247-1252 PubMed.
- Brown N O & Zaki F A (1979) Electrodiagnostic testing for evaluation of neuromuscular disorders in dogs and cats. J Am Vet Med Assoc 174 (1), 86-90 PubMed.
- Lee A F & Bowen J M (1975) Effect of tissue temperature on ulnar nerve conduction velocity in the dog. Am J Vet Res 36 (9), 1305-1307 PubMed.
Repetitive stimulation
- Gödde T, Jaggy A, Vandevelde M et al (1993) Evaluation of repetitive nerve stimulation in young dogs. J Small Anim Pract 34 (8), 393-398 VetMedResource.
Sensory/mixed nerve conduction velocity
- van Nes J J (1985) Sensory action potentials in the ulnar and radial nerves of the dogs: effect of stimulation site and voltage. Am J Vet Res 46 (5), 1155-1161 PubMed.
- Redding R W & Ingram J T (1984) Sensory nerve conduction velocity of cutaneous afferents of the radial, ulnar, peroneal, and tibial nerves of the cat: reference values. Am J Vet Res 45 (5), 1042-1045 PubMed.
F-waves and H-reflexes
- Cuddon P A (1998) Electrophysiologic assessment of acute polyradiculoneuropathy in dogs: comparison with Guillain-Barré syndrome in people. J Vet Intern Med 12 (4), 294-303 PubMed.
- Poncelet L & Balligand M (1991) Nature of the late potentials and F-ratio values in dogs. Res Vet Sci 51 (1), 1-5 PubMed.
- Knecht C D, Redding R W & Wilson S (1985) Characteristics of F and H waves of ulnar and tibial nerves in cats: reference values. Am J Vet Res 46 (4), 977-979 PubMed.
- Gassel M M & Wiesendanger M (1965) Recurrent and reflex discharges in plantar muscles of the cat. Acta Physiol Scand 65 (1-2), 138-142 Wiley Online Library.