Electromyography, or EMG, is a powerful diagnostic tool used by the clinician to evaluate the function and the nature of the dysfunction of the peripheral nervous system.  The EMG evaluation serves as an extension of the neurological exam and must be taken in that context to be accurate and effective.  Careful history taking and neurological evaluation will lead to a differential diagnosis, which will lay the groundwork of the electromyography evaluation.  In this way, each EMG evaluation is different and is tailored to the specific needs and questions that arise in an individual patient assessment.

There are several components of the EMG evaluation.  The first is nerve conduction studies.  This is a method by which cathode stimulation of a nerve with recording at a distant site leads to a waveform that can be evaluated for how many nerves are working (amplitude and area under the curve) and at what speed (distal latency).  If the nerve is stimulated at two or more locations, the difference in the distance between two points divided by the difference in the onset latency at the same two points gives a conduction velocity.  In this manner, sensory, motor and mixed nerves can be evaluated.  Distal latencies and amplitudes at a given distance have been normalized by height and age to allow comparison to norms.  Conduction velocities have also been normalized in this way.  Normalization has occurred with nerves warmed to 32^o  Centigrade so the nerves must be tested warm.  Nerves can be compared to normal and the nature and degree of abnormality ascertained.  It is up the electromyographer to decide what nerves need to be studied and this will be determined by the differential diagnosis decided after the physical examination.  The pattern of abnormality is also critical in making the final electrophysiological interpretation.

There are special tests that can be conducted in the nerve conduction study part of the test.  The H-reflex is a monosynaptic reflex that occurs with orthodromic stimulation of the tibial sensory fibers at the knee with reflex arc at the alpha motor neuron and subsequent recording at the soleus.  This gives the electromyographer information of the integrity of the sciatic nerve, lumbosacral plexus, and S1 nerve root.  F-responses can be obtained with any motor nerve study and involves the antidromic stimulation of the alpha motor neuron with the signal bouncing off that alpha motor neuron and recording over a distal muscle.  Again, this gives information about the integrity of a specific nerve, plexus and nerve root, depending on which muscle and nerve is tested. 

Repetitive nerve stimulation is special test done to evaluate the integrity of the neuromuscular junction.  Abnormality there may result in a decrement in response to low frequency(2 Hz) stimulation of motor nerves with pick-up over a distal or proximal muscle.  The detailed use of this technique is an advanced topic which should be take up in separate monograph.  The same can be said about autonomic nervous system testing, especially as it involves testing galvanic skin reponses.

The needle examination is a test where the electrical activity of an individual muscle can be evaluated.  The needle examination is done when the muscle is at rest so that small movements of the needle can ascertain the irritability of the resting muscle.  This irritability is seen in the form of positive waves, fibrillation potentials, fasciculations, and other signals such as complex repetitive discharges, myokymic discharges, or myotonic discharges.  Positive waves and fibrillation potentials are abnormalities generated by individual muscle fibers.  The rest are abnormalities generated by motor units or groups of motor units.  The needle examination is also done with the muscle active.  In this manner, the motor unit can be evaluated for any sign of abnormality.  Abnormality is seen in the height of the unit, in the complexity(percent polyphasia) of the unit, in the duration of the unit, and in the rate of recruitment with respects to the amount of force generated, be it fast or slow.  This pattern of abnormality can tell you if the problem is with the nerve or muscle, whether the problem is old or new, and whether the problem is ongoing or resolving.  The pattern of muscles tested, as chosen by the electromyographer and based on the results of physical examination and nerve conduction testing, will give the electromyographer information about the pattern of abnormality, and therefore will allow the making of a diagnosis.

Single fiber electromyography is an advanced technique which allows the electrical evaluation of active muscle fibers, rather than evaluating at the larger motor unit level.  This technique should be addressed in a separate monograph to give it the attention it would require.