Electrodiagnostic studies are used to measure certain types of electrical activity, particularly action potentials of muscle, nerve, and brain tissue. An action potential is a “spike” of electrical discharge that travels along the membrane of a cell and carries information between cells such as muscles and neurons. Electrodiagnostic studies can measure communication between these cells suggesting particular types of nervous systemdisease including muscle (myopathy), nerve (neuropathy), and brain(encephalopathy). In addition, certain electrodiagnostic tests can be used to screen for deafness.
Types of Electrodiagnostic Studies:
Electromyography measures the electrical discharges produced in muscles. During the test, two thin needle electrodes are placed in different regions of the body and a third is placed in the muscle to be studied. An instrument records the electrical activity in the muscle at rest and as the muscle is stimulated to contract. The size, duration and frequency of these signals helps determine if there is damage to the muscle or to the nerves leading to the muscle. Ideally, an electromyography study is performed on an awake patient since the muscles communicate best when the muscle cells are “warm”. The patient must remain still to eliminate unwanted voluntary action that may cause artifacts. Therefore, certain patients must be sedated in order to obtain accurate results. The test is painless and takes approximately 20 minutes. EMG’s are often performed in conjunction with nerve conduction studies. EMG is used to evaluate disorders such as myopathies, peripheral neuropathies, and other lower motor neuron diseases.
Nerve Conduction Velocity (NCV) Studies
Nerve conduction velocity studies are performed by placing thin needle electrodes on the skin over particular nerves and the muscles that areinnervated by those nerves. Electrical stimulation is applied to activate the nerve and see how it functions as the signal is sent along thenerve. An instrument records the velocity of the signal traveling along the nerve. This test is often performed after an EMG and is good for evaluating neuropathies
caused by demyelination or axonal degeneration. Repetitive nerve stimulation may also lead to reduction of evoked potential as seen in myasthenia gravis.
Brainstem Auditory Evoked Response (BAER) Testing
During brainstem auditory evoked response (BAER) testing, earphones deliver a series of clicks to each ear. The BAER test records the electrical activity inthe auditory (hearing) nerve pathway from the inner ear receptors through the brainstem to the cerebral cortex. Brainstem auditory evoked responses are evoked potentials used to detect, localize, and monitor auditory and neurological deficits. No response is seen in auditory nerve disorders associated with hearing loss. Brainstem disorders may also alter the BAER.
Electroencephalography (EEG) is performed by placing electrodes on the scalp to record the electrical activity of the cerebral cortex, which is influenced by subcortical structures. The test is painless and takes about 90 minutes. The EEG is sometimes abnormal in patients withhydrocephalus, meningoencephalits, head trauma, or cerebralneoplasia. An EEG may also determine whether seizure discharges are focal or diffuse. The EEG is often normal in idiopathicepilepsy, unless seizures are not well controlled and interictal spikes are present. EEG’s are not always warranted for determining brain disorders since superior diagnostic methods (e.g. MRI) provide more definitive diagnosis. EEG’s are occasionally performed at the VNC for research purposes.
Brainstem Auditory Evoked Response (BAER) evaluation is a non-invasive hearing test that allows us to confirm the existence of congenital or acquired hearing loss. This specialized test for dogs and cats takes approximately 10-15 minutes and tests each ear individually. BAER testing can be performed at any point in a dog or cat’s life. If you are interested in testing puppies or kittens, it can be performed after 8 weeks of age.
The BAER test is completed by using a specialized computer that is connected to multiple electrodes placed just under the skin at specific points on the patient’s head. The placement of these small electrodes is not painful to the patient. Once the electrodes are in place, the patient will then have a foam earphone placed in each ear which will make a computer-generated click between 70 and 105 decibels. After the
sound has been transmitted, the electrodes will detect any activity in the hearing pathways of the inner ear and brainstem. This activity will be transmitted to the computer and will show as waveforms on a graph, much like an EKG shows electrical activity in the heart. Peaks in the waveform represent activity in the cochlea and other auditory pathways, while a flat waveform represents inactivity (deafness).