Anjop J. Venker-van Haagen, DVM, PhD, DECVS
Electromyography (EMG) is used for diagnosis in dysphagia, when no abnormalities are found by pharyngoscopy. The motor supply to the pharyngeal muscles is distributed by the pharyngeal plexus. The glossopharyngeal nerve and the pharyngeal branch of the vagus nerve contribute to this plexus. EMG examination of the pharyngeal muscles is performed in dogs with spontaneous pharyngeal dysfunction, to detect possible muscular diseases. EMG is also used in research to study the swallowing action in dogs.
Electromyography of the Pharyngeal Muscles
The animal must be anesthetized for electromyography. The instruments needed are a laryngoscope fitted with a blade suitable for the size of the animal, a long tissue forceps, a Senn retractor with blunt prongs, and several sizes of endotracheal tubes. After premedication with medetomidine, anesthesia is induced by intravenous administration of propofol to effect. For inspection of the oropharynx and the hypopharynx, the dog is placed in a sphinx posture with its head supported by an assistant standing at its side. The assistant opens the dog's mouth and extends its neck, using one hand to raise the upper jaw and the other to depress the lower jaw and flatten the tongue. The laryngoscope is introduced over the tongue and then the mouth, oropharynx, palatine tonsils, soft palate, and base of the tongue are inspected. Then the hypopharynx is inspected by using long forceps or a Senn retractor to extend visibility caudal to and above the soft palate. It is advantageous to perform pharyngoscopy without an endotracheal tube in place, but if the animal is dyspneic endotracheal intubation precedes pharyngeal inspection.
The appropriate level of anesthesia is that at which there is just a low degree of normal electromyographic activity in the pharyngeal muscles, synchronous with respiration. The bipolar needle electrode (Danica, 9013L0601) is inserted into the muscles of both halves of the tongue and soft palate and into the bilateral thyropharyngeal, hyopharyngeal, and cricopharyngeal muscles, to record the spontaneous muscle action potentials. Fibrillation potentials were found to predominate in denervated pharyngeal muscles. In dogs with histological evidence of muscular dystrophy in the pharyngeal muscles, there were fibrillation potentials, positive sharp waves, and, most characteristic, abundant complex repetitive discharges. Following electromyography of the pharyngeal muscles, recordings can be made from the cervical part of the esophagus with the aid of a long holder for the bipolar needle electrode.
EMG Studies Demonstrating the Swallowing Action in Dogs
Continuous EMG recordings were made from paired wire electrodes implanted in the left and right hyopharyngeal, thyropharyngeal, and cricopharyngeal muscles of 5 normal beagles to determine the sequence of activity in each muscle and the combined muscle activity, both at rest and during swallowing of food. The data were digitized during 30-second periods and stored on diskette for further analysis. In all 5 dogs the pattern of muscle activity during swallowing was distinct, in a constant sequence (hyopharyngeal, thyropharyngeal, cricopharyngeal), and bilaterally synchronous. During eating, there were 5 to 12 short periods of synchronous activity in each muscle between swallowing actions, tentatively interpreted as bolus formation. During the resting period, there were longer periods of activity that were synchronous with respiration. These EMG recordings were made to improve understanding of the swallowing mechanism in dogs.
Electromyography of the Intrinsic Laryngeal Muscles
The larynx acts as a sphincter at the cranial end of the tracheobronchial tree. In order of priority, its functions are to protect the lower airways, to regulate the respiratory airflow, and to vocalize. The cartilages of the larynx--the cricoid, thyroid, and arytenoid cartilages and the epiglottis--interact under the control of the neuromuscular system of the larynx to perform these functions. The cartilages of the larynx simultaneously support and respond to the activity of the extrinsic and intrinsic laryngeal muscles. These muscles move the larynx in swallowing and they open and close the vocal folds in order to protect the larynx and lower airways, and to facilitate respiration and vocalization. Knowledge of the neurophysiology of laryngeal innervation is necessary for understanding of laryngeal dysfunction. The motor innervation of the intrinsic laryngeal muscles is provided by the bilateral recurrent laryngeal nerves.
Laryngeal dysfunction is often an indication for EMG examination of the intrinsic laryngeal muscles. EMG can be performed routinely in the dog but the cat is not a good candidate because it has a small larynx and is to prone to develop laryngeal edema after the larynx is touched. When a dog has signs of laryngeal dysfunction and laryngoscopic examination does not produce a diagnosis, EMG of the intrinsic laryngeal muscles can be helpful, for it can distinguish among normal activity, neurogenic paralysis, ankylotic paralysis, and muscular disease.
When laryngoscopy is performed for diagnosis of laryngeal disease, the dog or cat is usually in a certain state of dyspnea. The laryngoscope is fitted with a blade suitable for the size of the animal and lubricated endotracheal tubes of several sizes are prepared. The anesthetic is then administered to effect, preferably by intravenous injection. Propofol is satisfactory and may be used after premedication with medetomidine. Medetomidine premedication is given to cats intramuscularly and to dogs intravenously. If laryngeal function is to be investigated the anesthesia should be superficial, for if it is too deep the activity of the intrinsic laryngeal muscles, abduction and adduction, is absent. When the laryngeal movements are absent and the depth of anesthesia may be the cause, the short half-life of propofol is advantageous because after a short pause there is sufficient recovery for the inspection to proceed. For diagnostic laryngoscopy the animal is placed in a sphinx posture with its head supported by an assistant standing at its side. The mouth is then opened and the neck extended by the assistant, using one hand to hold the upper jaw and the other to hold the lower jaw and flatten the tongue. The laryngoscope is introduced over the tongue and the mouth, oral pharynx, and ventral side of the epiglottis are inspected before the epiglottis is depressed for inspection of the glottis. The size of the laryngeal opening (rima glottidis) is the first concern, for spontaneous breathing requires an adequate laryngeal opening and if the opening is obstructed, endotracheal intubation must be performed immediately. Artificial ventilation via the tube will then be needed, at least for a short while. If intubation is not required, the glottis (the paired arytenoid cartilages dorsally and the paired vocal folds ventrally) is inspected, the movement of the glottis and the color of the laryngeal mucosa are evaluated, and the cartilages are inspected for deformities.
The epiglottis is depressed with the blade of the laryngoscope, giving access to several of the intrinsic laryngeal muscles. Recordings are made via a bipolar needle electrode (Danica, 9013L0601) which is fixed in a long, rigid holder that enables the tip of the electrode to be inserted through the mucosa into separate laryngeal muscles under visual guidance. The thyroarytenoid, ventricular, and dorsal cricoarytenoid muscles are accessible beneath the mucosa in this way). In the paired abductors (dorsal cricoarytenoid muscles) of normal dogs action potentials are observed predominantly during inspiration. In the paired thyroarytenoid and vocal muscles, visible as the paired vocal folds, action potentials are observed mainly but not exclusively during expiration. In the paired ventricular muscles action potentials are synchronous with expiration. No action potentials are observed if the level of anesthesia is too deep, but abnormal potentials such as fibrillation potentials and complex repetitive discharges (CRDs) will be observed irrespective of the level of anesthesia. Fibrillation potentials are the result of denervation, which can be due to recurrent laryngeal nerve trauma or progressive neurogenic laryngeal paralysis. CRDs are also a common finding in the latter and they are also abundant in muscular disease.
1. Venker-van Haagen AJ. The Pharynx. In: Ear, Nose, Throat, and Tracheobronchial Diseases in Dogs and Cats. Hannover: Schlütersche Verlagsgesellschaft, 2005: 83-116.
2. Venker-van Haagen AJ. The Larynx. In: Ear, Nose, Throat, and Tracheobronchial Diseases in Dogs and Cats. Hannover: Schlütersche Verlagsgesellschaft, 2005: 121-161.