Veterinary medicine has adopted many of the same diagnostic techniques that have been employed in the field of human medicine.  Among these are electrophysiological examinations such as electromyography and nerve conduction velocity determination.  In addition to most University veterinary teaching hospitals, many specialty clinics now have the equipment necessary to perform these tests.

To quote Kimura and Dimitru, “electromyography (EMG) strictly defined is the recording and study of insertion, spontaneous, and voluntary electric activity of muscle.” Nerve conduction measurements will be covered separately.Electromyography (EMG) is indicated in those patients suspected of having neuromuscular disease (where the lesion is presumed to involve peripheral nerves, neuromuscular junction or muscle).  A myriad of associated clinical signs can be observed, depending on the site and extent of involvement, as well as the etiology.  A partial list includes, muscle stiffness, paresis/paralysis, exercise intolerance, dysphagia, laryngeal dysfunction, dysphonia, muscle atrophy, and muscle pain.

In small animals, an EMG is typically performed with the patient under general anesthesia.EMGs are routinely performed as part of a complete neuromuscular work-up that can also include nerve conduction velocity determination (both motor and sensory), repetitive stimulation, and late wave testing (f-waves, H-reflexes), along with muscle and nerve biopsy collection.  Electromyography can also be utilized in conjunction with a myelogram/epidurogram, or other imaging technique (CT, MRI), when there is a question of whether clinical signs are the result of upper and/or lower motor neuron lesions.

Though there are different ways of performing the test, a commonly employed technique involves the use of a concentric needle electrode.  The electrode’s central wire (the active or exploring electrode) is insulated from the surrounding cannula (the reference electrode) so that the actual area being tested (between the two), is very small.  A ground electrode is used to minimize extraneous “noise”.  By varying the electrode’s depth and angle, multiple sites within a muscle can be examined using a single insertion through the skin (Fig. 1).  The size and thickness of the individual muscle will determine the number of areas within the muscle that can be sampled. Muscle selection may vary depending on the patient’s clinical signs and tentative diagnosis.  For a generalized condition, numerous muscles are tested.  Proximal and distal appendicular, as well as several axial muscles, are routinely examined.  In these patients testing is normally restricted to one side of the body to preserve the muscles on the opposite side for histopathologic evaluation.  In select cases, laryngeal, pharyngeal, tongue or anal sphincter muscle may also be examined.  Electrical activity, detected by the electrode, is amplified and displayed on a monitor or oscilloscope.  Modern EMG systems are also equipped with a speaker, allowing the operator to identify events by their characteristic sounds, in addition to, their appearance.  

  Fig 1 Clinician examining the interosseus m. in the left pelvic limb. Note the ground electrode near the hip.

The goal of the electromyographic examination is to determine whether the muscles of interest are electrically normal or abnormal (Go to Table 1). Placement of the electrode produces what is referred to as insertion activity, the result of mechanical disruption of myofibers.  In general, normal muscle at rest is electrically silent once the needle is no longer moving (Fig. 2).  An exception to this occurs when the electrode is positioned near the neuromuscular junction, at which time miniature endplate potentials (MEPPs) and/or endplate spikes can be recorded (Fig. 3).  These result from the ongoing release of minute amounts of acetylcholine, which induce localized muscle membrane depolarization, and the occasional myofiber action potential, respectively.  It is important to distinguish the latter from fibrillation potentials (Fig. 4), an abnormal finding.  Motor unit action potentials (MUAPs) can also be recorded from normal muscle that is not completely at rest.

	Fig 2. Normal Muscle at rest.
	Fig 3. Miniature endplate potentials with a single endplate spike.
	Fig 4. Fibrillation potentials.

Most of the EMG abnormalities described in people also occur in animals.  Several of these are conveniently lumped together under the term spontaneous activity.  This includes: 1) fibrillation potentials (fibs), 2) positive sharp waves (sharps, PSWs) and 3) complex repetitive discharges (CRDs) (Figs. 4, 5 and 6, refer to Table 1 for descriptions).  As the name implies, these events occur without any stimulation being applied to the muscle, other than the insertion of the electrode.  Fibs and sharps, frequently occur together (Fig. 5) and all three can occur in the same muscle (with different electrode placement).  A standardized rating system is used to grade the results from normal (0) to severe (4+) for each muscle tested (see Table 1).  An early abnormal finding is prolonged insertion activity, whereby activity continues for a short while after the electrode has stopped moving.  Its opposite, decreased insertion activity, is an indication that there is a significant (or complete) loss of myofibers, and may be the only EMG finding in some muscles with end stage disease (though it is often subtle and easily missed).  Myotonic potentials are another abnormality that can be recorded during an EMG.  Despite the name, they are not pathognomonic for myotonia, as they can also be seen in patients with radiculopathies and polyneuropathies.  Though similar to CRDs, they can be easily distinguished by their waxing and waning (CRDs do not change in frequency).

	Fig 5. Positive sharp waves and fibrillation potentials.
	Fig 6. Complex repetitive discharges.

EMG abnormalities are generally not specific; they may be the result of either primary muscle disease or damage to the motor nerve supplying that muscle. Giant Motor Unit Action Potentials are an exception.  They can be found in some chronic neuropathies and indicate that re-innervation has taken place (biopsies of these muscles usually contain substantial fiber-type grouping).  Due to this lack of specificity, additional tests (listed above) and muscle and peripheral nerve biopsy collection are usually indicated.  A lack of abnormal findings does not necessarily correlate with the absence of disease.  Some myopathies (i.e., dermatomyositis) can be very patchy in their distribution making it fairly easy to miss the signs of muscle damage.    Diseases in which demyelination is the primary insult (i.e., Niemann-Pick in cats) may initially have a normal EMG examination until a secondary axonopathy has developed.  EMG also cannot identify disorders restricted to sensory neurons.  It takes time, days to weeks, to find spontaneous activity in a muscle that has had its motor nerve acutely injured. This delay varies with the proximity of the insult to the site of recording (changes occur with distal lesions in a matter of days).

EMG can aid in the selection of tissue for biopsy collection.  Those muscles with moderate signs (2+) are ideal, since comparisons between affected and non-affected myofibers can be made.  As mentioned previously, it is best to biopsy the side that has not been examined.  In generalized disease it is assumed that findings would be symmetrical. 

Electromyography is a valuable diagnostic tool in spite of its limitations.  Used in conjunction with thorough physical and neurological examinations, bloodwork (including CK) and the previously listed techniques, it can help the clinician in understanding a patient’s neuromuscular status.